Patent Publication Number: US-2013252310-A1

Title: Novel aldolase and production process of substituted alpha-keto acids

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is continuation of U.S. Ser. No. 13/533,317, filed Jun. 26, 2012, which is a divisional of U.S. Ser. No. 13/250,359, filed Sep. 30, 2011, which is a divisional of U.S. Ser. No. 11/960,116 (now U.S. Pat. No. 8,058,038), filed Dec. 19, 2007, which is a divisional of U.S. Ser. No. 11/066,542 (now U.S. Pat. No. 7,351,569), filed on Feb. 28, 2005, which is a continuation of PCT/JP03/10750, filed on Aug. 26, 2003, and claims priority to JP 2002-245980, filed on Aug. 26, 2002, and JP 2003-171299, filed on Jun. 16, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an aldolase and a process for producing substituted α-keto acid, and more particularly, to an aldolase that may be preferably used in the synthesis of 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (hereinafter, “IHOG”), which is useful as an intermediate in monatin synthesis, and a process for producing substituted α-keto acids. 
     2. Discussion of the Background 
     Monatin, which has the structure shown in formula (5) below, a naturally sweet amino acid that is isolated and extracted from the roots of shrubs in Southern Africa. It has a potent sweet taste equivalent to several ten to several thousand times that of sucrose, and is expected to be useful as a sweetener. However, the usefulness of monatin has only been recently discovered, and a process for synthesizing monatin as the level of industrial production has yet to be established. 
     
       
         
         
             
             
         
       
     
     Under these circumstances, the inventors of the present invention developed a novel process for producing monatin composed of the following reactions (1) and (2) by using indole pyruvic acid and pyruvic acid which may be acquired as reagents. 
     (1) A reaction step of synthesizing precursor keto acid (IHOG) by aldol condensation of indole pyruvic acid and pyruvic acid (and/or oxaloacetic acid); and,
 
(2) a reaction step of aminating the second position of IHOG.
 
     
       
         
         
             
             
         
       
     
     There have been no previous reports of examples of synthesizing precursor keto acid (IHOG) from indole pyruvic acid and pyruvic acid (or oxaloacetic acid) using a microbial enzyme system for the aldol condensation reaction of (1) in the aforementioned synthesis route of monatin. 
     Examples of microbial enzymes that catalyze aldol condensation using two molecules of α-keto acid (or substituted α-keto acid) as a substrate reported thus far include 4-hydroxy-4-methyl-2-oxoglutarate aldolase derived from bacteria belonging to the genus  Pseudomonas , and 4-hydroxy-2-oxoglutarate aldolase present in  E. coli, B. subtilis , and so forth. 
     The former 4-hydroxy-4-methyl-2-oxoglutarate aldolase has been reported to catalyze a reaction in which 4-hydroxy-4-methyl-2-oxoglutarate (4-HMG) is formed from two molecules of pyruvic acid, and a reaction in which one molecule of oxaloacetic acid and one molecule of pyruvic acid are formed from 4-oxalocitramalate (Kiyofumi Maruyama, Journal of Biochemistry, 108, 327-333 (1990)). Furthermore, the latter 4-hydroxy-2-oxoglutarate aldolase is known to catalyze a reaction in which 4-hydroxy-2-oxoglutarate (4HG) is formed from one molecule of glyoxylic acid and one molecule of pyruvic acid. 
     However, there have been no reports or findings indicating that any of these microorganisms are associated with activity that cleaves 4-phenylmethyl-4-hydroxy-2-oxoglutarate (PHOG) or activity that synthesizes a precursor keto acid (IHOG) of monatin from indole pyruvic acid and pyruvic acid (or oxaloacetic acid), and whether or not the aldolases produced by these microbial strains may be used in the aforementioned synthesis route of monatin is unknown. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an aldolase that may be appropriately used in the synthesis of IHOG, which is useful as an intermediate in the synthesis of monatin, and a process for producing substituted α-keto acids. 
     As a result of extensive research conducted in consideration of the aforementioned problems, the inventors of the present invention found that an aldolase that may be preferably used in the synthesis of the desired IHOG is present in certain microbial species, thereby leading to completion of the present invention. 
     Namely, the present invention is as described below. 
     [1] A DNA of following (a) or (b):
 
(a) A DNA comprising the nucleotide sequence of SEQ ID No: 1 or the nucleotide sequence of base numbers 444 to 1118 or base numbers 456 to 1118 of the same sequence;
 
(b) A DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 1 or a nucleotide sequence of base numbers 444 to 1118 or 456 to 1118 of the same sequence, and encodes a protein having aldolase activity.
 
[2] A DNA of following (c) or (d):
 
(c) A DNA that encodes a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(d) A DNA that encodes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence, and has aldolase activity.
 
[3] A DNA of following (e) or (f):
 
(e) A DNA comprising the nucleotide sequence of SEQ ID No: 15 or a nucleotide sequence of base numbers 398 to 1141 of the same sequence;
 
(f) A DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 15 or a nucleotide sequence of base numbers 398 to 1141 of the same sequence, and encodes a protein having aldolase activity.
 
[4] A DNA of following (g) or (h):
 
(g) A DNA that encodes a protein comprising the amino acid sequence of SEQ ID No: 16;
 
(h) A DNA that encodes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity.
 
[5] A recombinant DNA obtainable from ligating the DNA according to any of [1] to [4] with a vector DNA.
 
[6] A cell transformed by the recombinant DNA according to [5].
 
[7] A process for producing a protein having aldolase activity comprising: cultivating cells according to [6] in a medium, and accumulating a protein having aldolase activity in the any one of medium and cells or both.
 
[8] A protein of following (i) or (j):
 
(i) a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(j) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence, and has aldolase activity.
 
[9] A protein of following (k) or (l):
 
(k) a protein comprising the amino acid sequence of SEQ ID No: 16;
 
(l) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity.
 
[10] A protein which:
 
(A) has at least one of activities that catalyze reactions of:
     4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate is formed by aldol condensation of indole-3-pyruvic acid and pyruvic acid, and   4-phenylmethyl-4-hydroxy-2-oxoglutarate is formed by aldol condensation of phenyl pyruvic acid and pyruvic acid,
 
(B) the optimum pH of the activity according to (A) is about 9 at 33° C., and
 
(C) the molecular weight as measured by gel filtration is about 146 kDa, and the molecular weight per subunit as measured by SDS-PAGE is about 25 kDa.
 
[11] A protein which:
 
(A) has aldolase activity,
 
(B) is derived from  Pseudomonas  species,
 
(C) has pH stability at pH 6 and above,
 
(D) has temperature stability at 70° C. or lower, and
 
(E) the molecular weight as measured by gel filtration is about 146 kDa, and the molecular weight per subunit as measured by SDS-PAGE is about 25 kDa.
 
[12] A protein according to [11], wherein the aldolase activity is improved by containing inorganic phosphate in the enzyme reaction mixture.
 
[13] A composition having aldolase activity obtainable from cultivating  Pseudomonas  species bacteria in a medium, accumulating any of the proteins of following (i) to (l) in any one of the medium and cells or both, and purifying any one of the medium and cells or both:
 
(i) a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(j) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence, and has aldolase activity;
 
(k) a protein comprising the amino acid sequence of SEQ ID No: 16;
 
(l) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity.
 
[14] A process for producing a substituted α-keto acid represented by the following general formula (3):
   

     
       
         
         
             
             
         
       
     
     (wherein R 1  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof, and R 1  may be substituted with at least one substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms and an amino group; and R 2  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof, and R 2  may be substituted with at least one substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms and an amino group; and when R 1  represents a hydrogen atom, a methyl group or a carboxymethyl group in general formula (1), R 2  does not represent a hydrogen atom) comprising a reaction of the substituted α-keto acid represented by the following general formula (1): 
     
       
         
         
             
             
         
       
     
     (wherein R 1  has the same meaning as R 1  in general formula (3)), with the substituted α-keto acid represented by the following general formula (2): 
     
       
         
         
             
             
         
       
     
     (wherein R 2  has the same meaning as defined in general formula (3)); 
     wherein the reaction is performed in the presence of a protein that catalyzes the reaction. 
     [15] A process for producing a substituted α-keto acid according to [14], wherein R 2  represents a hydrogen atom or a carboxyl group.
 
[16] A process for producing a substituted α-keto acid according to [15], wherein R 2  represents a hydrogen atom.
 
[17] A process for producing a substituted α-keto acid according to [14], wherein R 1  represents a benzyl group or a 3-indolylmethyl group, and R 2  represents a hydrogen atom or a carboxyl group.
 
[18] A process for producing a substituted α-keto acid represented by the following formula (4):
 
     
       
         
         
             
             
         
       
     
     from oxaloacetic acid or pyruvic acid, and indole-3-pyruvic acid wherein the reaction is performed in the presence of a protein that catalyzes the reaction.
 
[19] A process for producing a substituted α-keto acid according to [14] to [18], wherein the protein that catalyzes the reaction is derived from a microorganism selected from the group consisting of  Pseudomonas  species,  Erwinia  species,  Flavobacterium  species and  Xanthomonas  species.
 
[20] A process for producing a substituted α-keto acid according to [19], wherein the microorganism is  Pseudomonas taetrolens, Pseudomonas coronafaciens, Pseudomonas desmolytica, Erwinia  sp.,  Flavobacterium rhenanum  or  Xanthomonas citri.  
 
[21] A process for producing a substituted α-keto acid according to [20], wherein the microorganism is  Pseudomonas taetrolens  ATCC4683 or  Pseudomonas coronafaciens  AJ2791.
 
[22] A process for producing a substituted α-keto acid according to [14] to [21], wherein the protein that catalyzes the reaction is a protein according to any of the following (i) to (l):
 
(i) a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(j) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition, or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence, and has aldolase activity;
 
(k) a protein comprising the amino acid sequence of SEQ ID No: 16;
 
(l) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition, or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity.
 
[23] A process for producing a substituted α-keto acid represented by the following general formula (3′):
 
     
       
         
         
             
             
         
       
     
     (wherein R 1  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof, R 3  represents a hydrogen atom or a carboxyl group, and R 1  may be substituted with at least one substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms, and an amino group; and R 4  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof, and R 4  may be substituted with at least one type of substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms, and an amino group) comprising a reaction of the compound represented by the following general formula (1′): 
     
       
         
         
             
             
         
       
     
     (wherein R 1  and R 3  have the same meanings as defined in general formula (3′)), with the substituted α-keto acid represented by the following general formula (2′): 
     
       
         
         
             
             
         
       
     
     (wherein R 4  has the same meaning as defined in general formula (3′)); 
     wherein the reaction is performed in the presence of a protein according to [8] or [9]. 
     [24] A process for producing a substituted α-keto acid according to [23], wherein R 4  is a hydrogen atom or a carboxyl group. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a flowchart of the process for producing aldolase of the present invention. 
         FIG. 2  shows the results of measuring the aldolase activity of aldolase derived from  Pseudomonas taetrolens  ATCC4683 (PtALD) versus PHOG concentration. 
         FIG. 3  shows the results of measuring the aldolase activity of PtALD versus MgCl 2  concentration. 
         FIG. 4  shows the results of measuring the aldolase activity of PtALD versus KPi concentration. 
         FIG. 5  shows the results of measuring the pH stability of PtALD. 
         FIG. 6  shows the results of measuring the temperature stability of PtALD. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following provides a detailed explanation of [I] aldolase and [II] a process for producing substituted α-keto acid using aldolase of the present invention with reference to the accompanying drawings. 
     [I] Aldolase 
     According to research by the inventors of the present invention, bacterial strains were confirmed to exist that form aldolase having the ability to cleave 4-phenylmethyl-4-hydroxy-2-oxoglutarate (PHOG) in the genus  Pseudomonas, Erwinia, Flavobacterium , and  Xanthomonas.    
     Since the aldolase produced by these microorganisms catalyzes a reaction that forms one molecule of phenyl pyruvic acid and one mole of pyruvic acid by cleaving one molecule of PHOG, the inventors of the present invention thought that the aldolase could catalyze a reaction in which 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG) is formed from indole pyruvic acid and pyruvic acid (or oxaloacetic acid). On the basis of this approach, in order to clarify the existence of a novel aldolase, together with purifying and isolating aldolase from cultivated microbial cells of the microorganisms, the inventors of the present invention found that this enzyme synthesizes IHOG by aldol condensation of indole pyruvic acid and pyruvic acid (or oxaloacetic acid). 
     Furthermore, the inventors of the present invention determined the amino acid sequence of the aldolase by purifying aldolase derived from  Pseudomonas taetrolens  ATCC4638 (which may also be abbreviated as PtALD). Furthermore, the inventors of the present invention also synthesized a DNA molecule of about 30 base pairs deduced from the amino acid sequence of the aldolase, isolated and obtained a fragment of DNA that encodes the aldolase by PCR using this DNA molecule, and succeeded in isolating the entire length of DNA that encodes PtALD from a chromosome library of this microorganism by using the DNA fragment as a probe. 
     The DNA that encodes the PtALD of the present invention that is identified according to the aforementioned method is shown in SEQ ID No: 1 in the Sequence Listing. Furthermore, the amino acid sequence of PtALD encoded by the nucleotide sequence of SEQ ID No: 1 is shown in SEQ ID No: 2 and SEQ ID No: 3. SEQ ID No: 2 is the amino acid sequence of PtALD encoded by the nucleotide sequence of base numbers 456 to 1118 in the nucleotide sequence of SEQ ID No: 1. Furthermore, SEQ ID No: 3 is the amino acid sequence of PtALD encoded by the nucleotide sequence of base numbers 444 to 1118 in the nucleotide sequence of SEQ ID No: 1, and is equivalent to the amino acid sequence of residue numbers 5 to 225 in the amino acid sequence of SEQ ID No: 2. Both of the PtALD of SEQ ID No: 2 and SEQ ID No: 3 have aldolase activity, and catalyze the reaction in which 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG) shown in the following formula (4) is synthesized from one molecule of indole pyruvic acid and one molecule of oxaloacetic acid (or pyruvic acid). 
     Furthermore, the inventors of the present invention also succeeded in isolating the entire length of DNA that encodes aldolase derived from  Pseudomonas coronafaciens  ATCC4683 (which may also be abbreviated as PcALD) from a chromosomal DNA library of the microorganism by using an obtained DNA fragment that encodes PtALD as a probe. A DNA that encodes the PcALD of the present invention identified by the aforementioned method is shown in SEQ ID No: 15. The amino acid sequence of PcALD encoded by the nucleotide sequence of SEQ ID No: 15 is shown in SEQ ID No: 16. SEQ ID No: 16 is an amino acid sequence of PcALD encoded by the nucleotide sequence of base numbers 398 to 1141 of the nucleotide sequence of SEQ ID No: 15. The PcALD of SEQ ID No: 16 also has aldolase activity and catalyzes the reaction in which 4-(indol-3-ylmetyyl)-4-hydroxy-2-oxoglutarate (IHOG) shown in the following formula (4) is synthesized from one molecule of indole pyruvic acid and one molecule of oxaloacetic acid (or pyruvic acid). 
     
       
         
         
             
             
         
       
     
     Next, a detailed explanation is provided of (1) a DNA encoding aldolase, (2) properties of aldolase and (3) a process for producing aldolase in that order. 
     (1) A DNA Encoding Aldolase 
     The aldolase gene of the present invention having the nucleotide sequence of SEQ ID No: 1 was isolated from a chromosomal DNA of  Pseudomonas taetrolens  strain ATCC4683 as described above. The nucleotide sequence of SEQ ID No: 1 demonstrates homology of 29% with known 4-hydroxy-4-methyl-2-oxoglutarate aldolase (gene name: proA) (Biosci. Biotechnol. Biochem., 65(12), 2701-2709 (2001), Maruyama, K. et al.) derived from the bacterium,  Pseudomonas  ochraceae, at the amino acid sequence level. 
     The aldolase gene of the present invention having the nucleotide sequence of SEQ ID No: 15 was isolated from a chromosomal DNA of  Pseudomonas coronafaciens  strain AJ2791 as described above. The amino acid sequence encoded by the nucleotide sequence of SEQ ID No: 15 demonstrates homology of 28% with known 4-hydroxy-4-methyl-2-oxoglutarate aldolase (gene name: proA) (Biosci. Biotechnol. Biochem., 65(12), 2701-2709 (2001), Maruyama, K. et al.) derived from the bacterium,  Pseudomonas  ochraceae. Furthermore, the amino acid sequence encoded by the nucleotide sequence of SEQ ID No: 15 demonstrates homology of 41% with the aldolase derived from the strain  Pseudomonas taetrolens  ATCC 4683 described above. 
     Here, analysis of homology indicates the value obtained by calculating the parameter as the initial set value using the “Genetyx Ver. 6” genetic analytical software (Genetyx). 
     The following provides an explanation of the method for obtaining a DNA encoding aldolase from aldolase-producing bacteria. 
     First, the amino acid sequence of the purified aldolase is determined. At this time, the amino acid sequence may be determined by using the method of Edman (Edman, P., Acta Chem. Scand. 4, 227 (1950)). Furthermore, the amino acid sequence may be determined using the Sequencer made by Applied Biosystems. Following limited hydrolysis with protease of the aldolase of the present invention derived from  Pseudomonas taetrolens  strain ATCC4683, peptide fragments were fractionated with reverse phase HPLC. Determination of the internal amino acid sequences of two of these fragments clearly revealed them to be the sequences shown in SEQ ID No: 4 and SEQ ID No: 5. 
     The nucleotide sequence of a DNA that encodes these amino acid sequences is then able to be deduced based on the amino acid sequences that are determined. Universal codons are employed to deduce the nucleotide sequence of the DNA. 
     DNA molecules of about 30 base pairs were then synthesized based on the deduced nucleotide sequence. The method used to synthesize the DNA molecules is disclosed in Tetrahedron Letters, 22, 1859 (1981). Furthermore, the DNA molecules may also be synthesized using the Synthesizer made by Applied Biosystems. The DNA molecules may be used as a probe when isolating the entire length of DNA that encodes aldolase from a chromosomal DNA library of a microorganism that produces aldolase. Alternatively, they may also be used as a primer when amplifying a DNA that encodes aldolase of the present invention by PCR. However, since the DNA amplified using PCR does not contain the entire length of DNA that encodes aldolase, the entire length of DNA that encodes aldolase is isolated from a chromosomal DNA library of a microorganism that produces aldolase using the DNA amplified by PCR as a probe. 
     The procedure employed for PCR is described in publications such as White, T. J. et al., Trends Genet. 5, 185 (1989). The method for isolating a chromosomal DNA, as well as the method for isolating a desired DNA molecule from a gene library using a DNA molecule as a probe, are described in publications such as Molecular Cloning, 2nd edition, Cold Spring Harbor Press (1989). 
     A method for determining the nucleotide sequence of isolated a DNA that encodes aldolase is described in A Practical Guide to Molecular Cloning, John Wiley &amp; Sons, Inc. (1985). Furthermore, the nucleotide sequence may be determined by using the DNA Sequencer made by Applied Biosystems. A DNA encoding aldolase derived from  Pseudomonas taetrolens  strain ATCC4683 is shown in SEQ ID No: 1, while a DNA encoding aldolase derived from  Pseudomonas coronafaciens  strain AJ2791 is shown in SEQ ID No: 15. 
     A DNA that encodes aldolase which catalyzes the reaction in which IHOG is formed from indole pyruvic acid and pyruvic acid (or oxaloacetic acid) is not only the DNA shown in SEQ ID Nos: 1 and 15. This is because there ought to be differences in nucleotide sequences observed for each species and strain among  Pseudomonas  species that form aldolase which catalyzes the reaction in which IHOG is synthesized from indole pyruvic acid and pyruvic acid (or oxaloacetic acid). 
     The DNA of the present invention not only includes the isolated DNA that encodes aldolase, but a DNA in which mutations have been artificially added to a DNA that encodes aldolase isolated from a chromosomal DNA of an aldolase-producing microorganism is also included in the DNA of the present invention when it encodes aldolase. Methods for artificially adding mutations include commonly used methods such as the method for introducing site-specific mutations described in Method. in Enzymol., 154 (1987). 
     A DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 1, and encodes a protein having aldolase activity is also included in the DNA of the present invention. As used herein, the “stringent conditions” refer to those conditions under which a specific hybrid is formed whereas an unspecific hybrid is not formed. Although it is difficult to numerically express these conditions explicitly, by way of example, mention is made of those conditions under which DNA molecules having higher homology e.g. preferably 50% or more, more preferably 80% or more, still more preferably 90% or more, and particularly preferably 95% or more homology, hybridize with each other, while DNA molecules having lower homology do not hybridize with each other, or those conditions under which hybridization occurs under usual washing conditions in Southern hybridization, that is, at a salt concentration corresponding to 0.1×SSC and 0.1% SDS at 37° C., preferably 0.1×SSC and 0.1% SDS at 60° C., and more preferably 0.1×SSC and 0.1% SDS at 65° C. Furthermore, “aldolase activity” may be sufficient for the activity that synthesizes HOG from indole pyruvic acid and pyruvic acid (or oxaloacetic acid). However, in the case of a nucleotide sequence that hybridizes under stringent conditions with a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 1, it preferably retains aldolase activity of 10% or more, preferably 30% or more, more preferably 50% or more, and still more preferably 70% or more, of protein having the amino acid sequence of SEQ ID No: 2 or 3 under conditions of 33° C. and pH 9. 
     A DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 15, and encodes a protein having aldolase activity is also included in the DNA of the present invention. As used herein, the “stringent conditions” refer to those conditions under which a specific hybrid is formed whereas an unspecific hybrid is not formed. Although it is difficult to numerically express these conditions explicitly, by way of example, mention is made of those conditions under which DNA molecules having higher homology e.g. preferably 50% or more, more preferably 80% or more, still more preferably 90% or more, and particularly preferably 95% or more homology, hybridize with each other, while DNA molecules having lower homology do not hybridize with each other, or those conditions under which hybridization occurs under usual washing conditions in Southern hybridization, that is, at a salt concentration corresponding to 0.1×SSC and 0.1% SDS at 37° C., preferably 0.1×SSC and 0.1% SDS at 60° C., and more preferably 0.1×SSC and 0.1% SDS at 65° C. Furthermore, “aldolase activity” may be sufficient for the activity that synthesizes IHOG from indole pyruvic acid and pyruvic acid (or oxaloacetic acid). However, in the case of a nucleotide sequence that hybridizes under stringent conditions with a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 15, it preferably retains aldolase activity of 10% or more, preferably 30% or more, more preferably 50% or more, and still more preferably 70% or more, of protein having the amino acid sequence of SEQ ID No: 16 under conditions of 33° C. and pH 9. 
     Furthermore, a DNA that encodes a protein which is substantially identical to the aldolase encoded by the DNA of SEQ ID No: 1 or 15 is also included in the DNA of the present invention. Namely, the following DNAs are also included in the DNA of the present invention: 
     (a) a DNA comprising the nucleotide sequence of base numbers 444 to 1118 or 456-1118 in the nucleotide sequence of SEQ ID No: 1;
 
(b) a DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of base numbers 444 to 1118 or 456 to 1118 in the nucleotide sequence of SEQ ID No: 1, and encodes a protein having aldolase activity;
 
(c) a DNA that encodes a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(d) a DNA that encodes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence, and has aldolase activity;
 
(e) a DNA comprising the nucleotide sequence of base numbers 398 to 1141 in the nucleotide sequence of SEQ ID No: 15;
 
(f) a DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of base numbers 398 to 1141 in the nucleotide sequence of SEQ ID No: 15, and encodes a protein having aldolase activity;
 
(g) a DNA that encodes a protein comprising the amino acid sequence of SEQ ID No: 16; and,
 
(h) a DNA that encodes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity. Here, “one or several” refers to the range over which the steric structure of a protein of amino acid residues or aldolase activity is not significantly impaired, and more specifically, a range of 1 to 50, preferably 1 to 30 and more preferably 1 to 10. Furthermore, “aldolase activity” refers to the activity that synthesizes IHOG from indole pyruvic acid and pyruvic acid (or oxaloacetic acid) as described above. However, in the case of an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2, 3, or 16, it preferably retains aldolase activity of 10% or more, preferably 30% or more, more preferably 50% or more, and still more preferably 70% or more, of protein having the amino acid sequence of SEQ ID No: 2, 3, or 16 under conditions of 33° C. and pH 9.
 
     (2) Properties of Aldolase 
     Next, an explanation is provided of the properties of purified aldolase derived from  Pseudomonas taetrolens  strain ATCC4683 (PtALD) and purified aldolase derived from  Pseudomonas coronafaciens  strain AJ2791 (PcALD). 
     The PtALD of the present invention has the amino acid sequence of SEQ ID No: 2 or 3 as is clearly determined by the previously described gene isolation and analysis. However, the present invention includes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or 3, which also has aldolase activity. 
     The PcALD of the present invention has the amino acid sequence of SEQ ID No: 16 as is clearly determined by the previously described gene isolation and analysis. However, the present invention includes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, which also has aldolase activity. 
     Namely, the aldolase of the present invention consists of the proteins indicated in (i) to (l) below. 
     (i) a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(j) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or a several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 in the same sequence, and has aldolase activity;
 
(k) a protein comprising the amino acid sequence of SEQ ID No: 16; and,
 
(l) a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity.
 
     Here, the definitions of “several” and “aldolase activity” are the same as defined in section (1), DNA Encoding Aldolase. 
     The aldolase of the present invention catalyzes the reaction that synthesizes 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG) by aldol condensation from indole pyruvic acid and pyruvic acid (or oxaloacetic acid). 
     The aldolase activity of the aldolase of the present invention may be measured by measuring the amount of IHOG formed from indole pyruvic acid and pyruvic acid (or oxaloacetic acid) by high-performance liquid chromatography (HPLC). 
     More specifically, aldolase activity may be estimated according to the following steps of adding aldolase to a reaction solution composed of 100 mM buffer, 50 mM indole-3-pyruvic acid, 250 mM pyruvic acid, 1 mM MgCl 2  and 1% (v/v) toluene; and shaking while reaction at 33° C. for 4 hours; and then quantifying the amount of IHOG formed by HPLC. 
     IHOG may be quantified by HPLC analysis using the “Inertsil ODS-2” (GL Sciences Inc., 5 μm, 4.6×250 mm). The following indicates an example of the analysis conditions. 
     Mobile phase: 40% (v/v) acetonitrile/5 mM dihydrogen phosphate tetrabutyl ammonium solution 
     Flow rate: 1 mL/min 
     Column temperature: 40° C. 
     Detection: UV 210 nm 
     The aldolase of the present invention is able to catalyze the reaction that synthesizes IHOG by aldol condensation from indole pyruvic acid and pyruvic acid (or oxaloacetic acid). Although two examples of microbial enzymes capable of catalyzing aldol condensation using 2 molecules of α-keto acid (or substituted α-keto acid) as a substrate have been reported thus far consisting of 4-hydroxy-4-methyl-2-oxoglutarate aldolase derived from  Pseudomonas  species bacteria and 4-hydroxy-2-oxoglutarate aldolase present in microorganisms such as  E. coli  and  B. subtilis , there have been no findings or reports describing the former acting on PHOG or IHOG, and it is completely unknown as to whether it is possible to synthesize PHOG (and IHOG) by using this enzyme. Furthermore, PHOG cleaving activity has not been observed for the latter, and it has not been possible to synthesize PHOG (and IHOG) using this enzyme as well. Namely, the aldolase of the present invention differs from aldolases that have been reported thus far in that it has the characteristic of being able to catalyze the reaction in which IHOG is synthesized by aldol condensation of indole pyruvic acid and pyruvic acid (or oxaloacetic acid). 
     Next, the following provides a description of the enzymatic properties investigated for purified PtALD. 
     PtALD catalyzes the reaction that forms a substituted α-keto acid represented by the following general formula (3′): 
     
       
         
         
             
             
         
       
     
     (wherein R 1  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof; R 3  represents a hydrogen atom or a carboxyl group, and R 1  may be substituted with at least one substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms and an amino group; and R 4  represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof, and R 4  may be substituted with at least one substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms and an amino group) from the compound represented by the following general formula (1′): 
     
       
         
         
             
             
         
       
     
     (wherein R 1  and R 3  has the same meaning as defined in general formula (3′)), and the substituted α-keto acid represented by the following general formula (2′): 
     
       
         
         
             
             
         
       
     
     (wherein R 4  has the same meaning as defined in general formula (3′)). Thus, a process whereby the compound of general formula (3′) is produced from the compounds of general formula (1′) and general formula (2′) by using the PtALD of the present invention also belongs to the present invention. Here, R 4  is preferably a hydrogen atom or a carboxyl group. 
     The optimum pH of PtALD is nearly about 9 at 33° C. Furthermore, the PtALD of the present invention has pH stability at pH 6 or higher, and has particularly high pH stability within the range of pH 6 to pH 11. Furthermore, the PtALD of the present invention has temperature stability at 70° C. or lower, and has particularly high temperature stability within the range of 20 to 60° C. Furthermore, the PtALD of the present invention has the property of aldolase activity being improved by addition of inorganic phosphoric acid such as KPi to the enzyme reaction mixture. 
     Since the molecular weight of PtALD is about 146 kDa as measured by gel filtration and about 25 kDa as measured by SDS-PAGE, It was suggested that PtALD has a hexameric structure consisting of six subunits having a molecular weight of about 25 kDa. 
     (3) Process for Producing Aldolase 
     Next, an explanation is provided for the process of producing the aldolase of the present invention. There are two ways to produce the aldolase of the present invention. These consist of (i) a process of cultivating an aldolase-producing microorganism to form and accumulate aldolase, and (ii) a process of preparing a transformant to form aldolase by a recombinant DNA technology and cultivating the transformant to accumulate aldolase. 
     (i) Process for Forming and Accumulating Aldolase by Microbial Cultivation 
     Examples of microorganisms serving as acquisition sources of aldolase in a process for forming and accumulating aldolase by cultivating aldolase-producing microorganisms include microorganisms belonging to the genus  Pseudomonas, Erwinia, Flavobacterium , and  Xanthomonas.    
     Any microorganisms belonging to the genus  Pseudomonas, Erwinia, Flavobacterium , or  Xanthomonas  may be used in the present invention provided they are microorganisms that form aldolase which catalyzes a reaction that synthesizes a precursor keto acid (IHOG) from indole pyruvic acid and pyruvic acid (or oxaloacetic acid), and preferable microorganisms include  Pseudomonas taetrolens  ATCC4683,  Pseudomonas coronafaciens  AJ2791,  Pseudomonas desmolytica  AJ 1582,  Erwinia  sp. AJ2917 , Xanthomonas citri  AJ2797 and  Flavobacterium rhenanum  AJ2468 is used preferably. Among these,  Pseudomonas taetrolens  ATCC4683 and  Pseudomonas coronafaciens  AJ2791 are particularly preferable. The locations where these microorganisms are deposited are indicated below. 
     (1)  Pseudomonas coronafaciens  strain AJ2791
 
(i) Deposit no.: FERM BP-8246 (transferred from FERM P-18881)
 
(ii) Deposition date: Jun. 10, 2002
 
(iii) Deposited location: National Institute of Advanced Industrial Science and Technology, International Patent Organism Depository (Chuo No. 6, 1-1-1 Higashi, Tsukuba City, Ibaraki Prefecture, Japan)
 
(2)  Pseudomonas desmolytica  strain AJ1582
 
(i) Deposit no.: FERM BP-8247 (transferred from FERM P-18882)
 
(ii) Deposition date: Jun. 10, 2002
 
(iii) Deposited location: National Institute of Advanced Industrial Science and Technology, International Patent Organism Depository (Chuo No. 6, 1-1-1 Higashi, Tsukuba City, Ibaraki Prefecture, Japan)
 
(3)  Erwinia  sp. strain AJ2917
 
(i) Deposit no.: FERM BP-8245 (transferred from FERM P-18880)
 
(ii) Deposition date: Jun. 10, 2002
 
(iii) Deposited location: National Institute of Advanced Industrial Science and Technology, International Patent Organism Depository (Chuo No. 6, 1-1-1 Higashi, Tsukuba City, Ibaraki Prefecture, Japan)
 
(4)  Flavobacterium rhenanum  strain AJ2468
 
     (i) Deposit no.: FERM BP-1862 
     (ii) Deposition date: Sep. 30, 1985
 
(iii) Deposited location: National Institute of Advanced Industrial Science and Technology, International Patent Organism Depository (Chuo No. 6, 1-1-1 Higashi, Tsukuba City, Ibaraki Prefecture, Japan)
 
(5)  Xanthomonas citri  strain AJ2797
 
(i) Deposit no.: FERM BP-8250 (transferred from FERM P-8462)
 
(ii) Deposition date: Sep. 30, 1985
 
(iii) Deposited location: National Institute of Advanced Industrial Science and Technology, International Patent Organism Depository (Chuo No. 6, 1-1-1 Higashi, Tsukuba City, Ibaraki Prefecture, Japan)
 
     Although the microorganism serving as the acquisition source of aldolase may be cultivated in any form such as liquid cultivation and solid cultivation, an industrially advantageous method is deep-aerated stir cultivation. Carbon sources, nitrogen sources, inorganic salts, and other trace nutrient elements commonly used in microbial cultivating may be used as nutrient elements of nutritive media. All nutrient sources may be used provided they may be used by the microbial strain being used. 
     Aerobic conditions are used for the aeration conditions. The cultivating temperature may be sufficient within a range in which the microorganisms grow and aldolase is produced. Thus, although the conditions are not strict, the cultivating temperature is normally 10 to 50° C. and preferably 30 to 40° C. The cultivating time varies according to other cultivating conditions. For example, the microorganisms may be cultivated until the greatest amount of aldolase is produced, and this is normally about 5 hours to 7 days, and preferably about 10 hours to 3 days. 
     Following cultivation, the microbial cells are recovered by centrifugation (e.g., 10,000×g for 10 minutes). Since the majority of the aldolase is present in the cells, the aldolase is solubilized by disrupting or lysing the microbial cells. Ultrasonic disrupting, French press disrupting or glass bead disrupting may be used to disrupt the microbial cells. In the case of lysing the cells, a method that uses egg white lysozyme, peptidase treatment, or a suitable combination thereof is adopted. 
     When aldolase derived from an aldolase-producing microorganism is purified, although the aldolase is purified by using an enzyme solubilizing solution for the starting material, if undisrupted or unlysed residue remains, re-centrifuging the solubilization solution and removing any residue that precipitates is advantageous to purification. 
     All commonly used methods for purifying ordinary enzymes may be employed to purify the aldolase, examples of which include ammonium sulfate salting-out, gel filtration chromatography, ion exchange chromatography, hydrophobic chromatography and hydroxyapatite chromatography. As a result, an aldolase-containing fraction with higher specific activity may be obtained having higher specific activity. 
     (ii) Production Process Using Recombinant DNA Technology 
     Next, an explanation is provided for a process for producing aldolase using a recombinant DNA technology. There are numerous known examples of producing useful proteins such as enzymes and physiologically active substances using a recombinant DNA technology, and the use of recombinant DNA technology enables mass production of useful proteins present only in trace amounts in nature. 
       FIG. 1  is a flowchart of a process for producing the aldolase of the present invention. 
     First, a DNA is prepared that encodes the aldolase of the present invention (Step S 1 ). 
     Next, the prepared DNA is ligated with a vector DNA to produce a recombinant DNA (Step S 2 ), and cells are transformed by the recombinant DNA to produce a transformant (Step S 3 ). Continuing, the transformant is cultivated in a medium, and the aldolase is allowed to form and accumulate in any one of the medium and cells or both (Step S 4 ). 
     Subsequently, the process proceeds to Step S 5  where purified aldolase is produced by recovering and purifying the enzyme. 
     The desired substituted α-keto acid may be produced in a large amount by using the purified aldolase produced at Step S 5  or any of the medium and cells or both in which aldolase has accumulated at Step S 4  in an aldol reaction (Step S 6 ). 
     The DNA that is ligated with the vector DNA may allow expression of the aldolase of the present invention. 
     Here, examples of aldolase genes ligated into the vector DNA include the previously described DNA indicated below: 
     (a) a DNA comprising the nucleotide sequence of SEQ ID No: 1, or the nucleotide sequence of base numbers 444 to 1118 or base numbers 456 to 1118 in the same sequence;
 
(b) a DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 1 or a nucleotide sequence of base numbers 444 to 1118 or 456 to 1118 of the same sequence, and encodes a protein having aldolase activity;
 
(c) a DNA that encodes a protein comprising the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence;
 
(d) a DNA that encodes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 2 or an amino acid sequence of residue numbers 5 to 225 of the same sequence, and has aldolase activity;
 
(e) a DNA comprising the nucleotide sequence of SEQ ID No: 15, or a nucleotide sequence of base numbers 398 to 1141 of the same sequence;
 
(f) a DNA that hybridizes under stringent conditions with a DNA having a nucleotide sequence complementary to the nucleotide sequence of SEQ ID No: 15 or a nucleotide sequence of base numbers 398 to 1141 of the same sequence, and encodes a protein having aldolase activity;
 
(g) a DNA that encodes a protein comprising the amino acid sequence of SEQ ID No: 16; and,
 
(h) a DNA that encodes a protein having an amino acid sequence that contains a substitution, deletion, insertion, addition, or inversion of one or several amino acid residues in the amino acid sequence of SEQ ID No: 16, and has aldolase activity.
 
     In the case of mass production of protein using recombinant DNA technology, it is preferable to form a inclusion body of protein by associating the protein within a transformant that produces the protein. The advantages of this expression production method include being able to protect the desired protein from digestion by proteases present within microbial cells, and being able to easily purify the desired protein by a centrifugation procedure after disrupting the microbial cells. 
     Protein inclusion bodies that have been obtained in this manner are solubilized by a protein denaturant, and after going through a renaturing procedure consisting primarily of removing the denaturant, such protein is converted to properly folded, physiologically active protein. There are numerous examples of this such as the restoration of the activity of human interleukin-2 (JP-A-S61-257931). 
     In order to obtain a active protein from a inclusion body of protein, a series of procedures including solubilization and restoration of activity are required, and these procedures are more complex than producing the active protein directly. However, in the case of large scale production in microbial cells of a protein that affects microbial growth, those effects may be inhibited by allowing the protein to accumulate within the microbial cells in the form of inactive inclusion bodies. 
     Methods for mass production of a desired protein in the form of inclusion bodies include a method in which the desired protein is expressed independently under the control of a powerful promoter, and a method in which the desired protein is expressed as a fused protein with a protein that is known to be expressed in large scale. 
     Furthermore, it is also effective to arrange the recognition sequence of a restricting protease at a suitable location in order to cut out the desired protein after having expressed in the form of a fused protein. 
     In the case of large scale protein production using recombinant DNA technology, cells such as bacterial cells,  Actinomyces  cells, yeast cells, mold cells, plant cells and animal cells may be used for the host cells that are transformed. Examples of bacterial cells for which host-vector systems have been developed include  Escherichia  species,  Pseudomonas  species,  Corynebacterium  species, and  Bacillus  species, and preferably  Escherichia coli  is used. This is because there are numerous knowledge regarding technologies for mass production of protein using  Escherichia coli . The following provides an explanation of a process for producing aldolase using transformed  E. coli.    
     A promoter normally used for heterogeneous protein production in  E. coli  may be used for the promoter that expresses a DNA encoding aldolase, examples of which include powerful promoters such as 17 promoter, trp promoter, lac promoter, tac promoter and PL promoter. 
     In order to produce aldolase in the form of a fused protein inclusion body, a gene that encodes another protein, preferably a hydrophilic peptide, is ligated either upstream or downstream of the aldolase gene with a fused protein gene. The gene that encodes another protein may be a gene that increases the amount of fused protein accumulated and enhances the solubility of the fused protein following the denaturation and regeneration steps, examples of candidates for which include T17 gene 10, β-galactosidase gene, dehydrofolate reductase gene, interferon-γ gene, interleukin-2 gene and prochymosin gene. 
     When ligating these genes with a gene that encodes aldolase, the codon reading frames are made to match. The genes may either be ligated in a suitable restriction enzyme site or using a synthetic DNA of an appropriate sequence. 
     In order to increase the amount produced, it is preferable to couple a transcription terminating sequence in the form of a terminator downstream from the fused protein gene. Examples of this terminator include T7 terminator, fd phage terminator, T4 terminator, tetracycline resistance gene terminator, and  E. coli  trpA gene terminator. 
     Multi-copy vectors are preferable for the vector used to introduce a gene that encodes aldolase or a fused protein of aldolase with another protein into  E. coli , examples of which include plasmids having a replication starting point derived from Col E1 such as pUC plasmids, pBR322 plasmids or their derivatives. A “derivative” here refers to that has undergone alteration of a plasmid by base substitution, deletion, insertion, addition or inversion. The alteration referred to here includes alteration caused by mutagenic treatment using a mutagen or UV irradiation or by spontaneous mutation. 
     It is preferable that the vector has a marker such as ampicillin resistance gene in order to select the transformant. Examples of such plasmids include commercially available expression vectors having a powerful promoter (such as pUC (Takara), pPROK (Clontech), and pKK233-2 (Clontech)). 
     A recombinant DNA is obtainable from ligating a DNA fragment, in which a promoter, a gene encoding aldolase or fused protein consisting of aldolase and another protein, and a terminator are ligated in that order, with a vector DNA. 
     When  E. coli  is transformed using the recombinant DNA and that  E. coli  is then cultivated, aldolase or a fused protein of aldolase with another protein is expressed and produced. A strain that is normally used for expression of heterogeneous genes may be used for the transformed host, and  E. coli  strain JM109(DE3) and  E. coli  strain JM109 are particularly preferable. The transformation method and method for selecting the transformant are described in, for example, Molecular Cloning, 2nd edition, Cold Spring Harbor Press (1989). 
     In the case of expressing as fused protein, the aldolase may be cut out using a restricting protease such as blood coagulation factor Xa or kallikrein that recognizes sequences not existing in aldolase as the recognition sequence. 
     A medium normally used for cultivating  E. coli  may be used for the production medium, examples of which include M9-casamino acid medium and LB medium. Furthermore, conditions of cultivating and production induction may be appropriately selected according to the type of the marker and promoter of the vector used, and type of host microorganism used. 
     The following method may be used to recover the aldolase or fused protein of aldolase with another protein. If the aldolase or its fused protein is solubilized within microbial cells, then it may be used in the form of a crude enzyme solution after recovering the microbial cells and disrupting or lysing the recovered cells. Furthermore, the aldolase or its fused protein may also be used after purifying by precipitation, filtration, column chromatography or other common technique as necessary. In this case, a purification method may also be used that uses an antibody of the aldolase or its fused protein. 
     When a protein inclusion body is formed, it is solubilized with a denaturant. Although it may be solubilized together with microbial cell protein, in consideration of the subsequent purification procedure, it is preferable to take out the inclusion body and then solubilize it. A method known in the prior art may be used to recover the inclusion body from the microbial cell. For example, the inclusion body may be recovered by disrupting the microbial cell followed by centrifugal separation. Examples of denaturants that solubilize protein inclusion bodies include guanidine hydrochloride (e.g., 6 M, pH 5-8) and urea (e.g., 8 M). 
     The protein inclusion body may be regenerated as active protein by removing these denaturants by treatment such as dialysis. Dialysis solutions such as Tris-HCl buffer or phosphate buffer may be used for dialysis, and the concentration may be from 20 mM to 0.5 M, and the pH may be from pH 5 to pH 8. 
     The protein concentration during the regeneration step is preferably held to about 500 μg/ml or less. In order to prevent the regenerated aldolase from undergoing self-crosslinking, the dialysis temperature is preferably 5° C. or lower. Furthermore, activity may also be expected to be restored by other methods used to remove denaturant such as dilution and ultrafiltration in addition to the aforementioned dialysis. 
     When the aldolase gene is derived from bacteria belonging to the genus  Pseudomonas , the aldolase may be expressed and produced by using the  Pseudomonas  species bacteria as a host in one preferable mode. Descriptions of examples of host cells in this case include a report on a recombinant expression method in  Pseudomonas syringae  by Shi-En Lu et al. (FEMS Microbiology Letters 210 (2002) p. 115-121), a report on a recombinant expression method in  Pseudomonas aeruginosa  by Olsen, R. H. et al. (Journal of Bacteriology, (1982) 150, p. 60-69) and a report on a recombinant expression method in  Pseudomonas stutzeri  by Grapner, S. et al. (Biomol. Eng., (2000), 17, p. 11-16). However, the  Pseudomonas  species bacteria used as host cells for expressing aldolase are not limited to those recited herein. 
     Next, with respect to the vector used to introduce aldolase gene into  Pseudomonas  species bacteria, a plasmid may be used that has a replication starting point which function inside  Pseudomonas  species bacterial cells. For example, plasmid pKLH4.05 has been reported by Eza Kalyaeva et al. to have a replicon TFK that function in  Pseudomonas aeruginosa . Wide-spectrum host vectors may also be used that are used to transform Gram-negative bacteria. These vectors are known to function in  Pseudomonas  species bacteria as well, examples of which include RK404 (Ditta, G. et al., Plasmid 13 (1985) p. 149-153) and RSF1010 (Frey, J. et al., Gene 24 (1982) p. 289-296). 
     In the case of using the DNA indicated in SEQ ID No: 1 for the DNA that encodes aldolase, aldolase is produced that has the amino acid sequence of SEQ ID No: 2 or 3, and in the case of using the DNA indicated in SEQ ID No: 15, aldolase is produced that has the amino acid sequence of SEQ ID No: 16. 
     [II] Method for Producing Substituted α-Keto Acid Using 
     Aldolase 
     Next, a description is provided of the method for producing substituted α-keto acid of the present invention. The method for producing substituted α-keto acid of the present invention is a method for producing the substituted α-keto acid represented by the following general formula (3): 
     
       
         
         
             
             
         
       
     
     by reacting the substituted α-keto acid represented by the following general formula (1): 
     
       
         
         
             
             
         
       
     
     with the substituted α-keto acid represented by the following general formula (2): 
     
       
         
         
             
             
         
       
     
     wherein the reaction is performed in the presence of a protein that catalyzes the reaction. 
     In general formula (1), R 1  represents an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof. R 1  may also be substituted with a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms or an amino group. R 1  is preferably a substituent having 4 or more carbon atoms and preferably 6 or more carbon atoms, while an aryl group or an aralkyl group having up to 20 carbon atoms (aryl groups such as a phenyl group, a tolyl group, a xylyl group, a biphenyl group, a naphthyl group, an anthryl group or a phenantolyl group; an aralkyl groups such as a benzyl group, a benzhydryl group, a styryl group, a phenethyl group, a trityl group or a cinnamyl group), and a heterocycle-containing hydrocarbon group having up to 11 carbon atoms (heterocyclic groups such as a furyl group, a thienyl group, a pyridyl group, a piperidyl group, a piperidino group, a morpholino group or an indolyl group; alkyl groups substituted by these heterocyclic groups) are particularly preferable. R 1  is particularly preferably a benzyl group or a 3-indolylmethyl group, and a 3-indolylmethyl group is the most preferable. Namely, phenyl pyruvic acid or indole pyruvic acid is preferable for the substituted α-keto acid of general formula (1), and indole pyruvic acid is particularly preferable. 
     In general formula (2), R 2  represents an alkyl group having 1 to 8 carbon atoms, an alkoxyl group having 1 to 8 carbon atoms, a carboxyalkyl group having 2 to 9 carbon atoms, an aryl or an aralkyl group having up to 20 carbon atoms, a heterocycle-containing hydrocarbon group having up to 11 carbon atoms, a hydroxyl group or an ester derivative thereof. When R 2  contains an aromatic ring or a hetero ring, the aromatic ring or hetero ring may be substituted with at least one type of substituent selected from the group consisting of a halogen atom, a hydroxyl group, an alkyl group having up to 3 carbon atoms, an alkoxy group having up to 3 carbon atoms and an amino group. However, when R 1  represents a hydrogen atom, a methyl group or a carboxymethyl group in general formula (1), R 2  does not represent a hydrogen atom. R 2  is preferably a hydrogen atom or a carboxyl group, and particularly preferably a hydrogen atom. Namely, the substituted α-keto acid of general formula (2) is preferably oxaloacetic acid or pyruvic acid, and particularly preferably pyruvic acid. 
     In general formula (3), R 1  and R 2  have the same meanings as R 1  and R 2  in general formulas (1) and (2). 
     In the production process of a substituted α-keto acid of the present invention, it is most preferable to synthesize IHOG shown in the following formula (4) by using indole pyruvic acid for the substituted α-keto acid of general formula (1), and using pyruvic acid for the substituted α-keto acid of general formula (2). 
     
       
         
         
             
             
         
       
     
     There are no particular limitations on the protein that catalyzes the reaction, and any protein may be used provided it is a protein that is capable of catalyzing a reaction that synthesizes the substituted α-keto acid represented by the general formula (3) by aldol condensation of the substituted α-keto acid represented by the general formula (1) and the substituted α-keto acid represented by the general formula (2). Namely, as long as the protein catalyzes the reaction, it may be a protein derived from a microorganism or a protein synthesized by a chemical synthesis process. 
     A preferable example of such a protein is the aldolase explained in section [1] describing aldolase. In this case, aldolase may be used which has been obtained by cultivating microbial cells that form protein that catalyzes the reaction (aldolase) among microorganisms belonging to the genus  Pseudomonas, Erwinia, Flavobacterium , or  Xanthomonas , or (2) aldolase may be used which has been obtained by producing a transformant that forms a protein that catalyzes the reaction using recombinant DNA technology followed by cultivating the transformant. 
     The protein that catalyzes the reaction may be added in any form to the reaction mixture provided it is able to catalyze the reaction that synthesizes the aforementioned substituted α-keto acid represented by general formula (3). Namely, the protein that catalyzes the reaction may be added to the reaction mixture by itself, or it may be added to the reaction mixture in the form of a composition having aldolase activity that contains a protein that catalyzes the reaction (aldolase). 
     A “composition having aldolase activity” here is only required to be one which contains a protein that catalyzes the reaction (aldolase), and specific examples include a culture, medium (in which microbial cells have been removed from a culture), microbial cells (including both cultivated microbial cells and washed microbial cells), a treated microbial cell product that have been disrupted or lysed, and a composition having aldolase activity obtainable from purifying any of the medium and cells or both (crude enzyme solution, purified enzyme). For example, in the case of producing a substituted α-keto acid using aldolase-producing microorganisms or cells that have been transformed by a recombinant DNA, the substrate may be added directly to the culture while cultivating, or may be used in the form of microbial cells or washed microbial cells that have been separated from the culture. Furthermore, a treated microbial cell product that have been disrupted or lysed may be used directly, or the aldolase may be recovered from the treated microbial cell product and used as a crude enzyme solution, or used after purifying the enzyme. Namely, as long as it is in the form of a fraction that has aldolase activity, it may be used in the process for producing a substituted α-keto acid of the present invention in any form. 
     In order to perform an aldol reaction using aldolase or a composition having aldolase activity, a reaction solution containing a substituted α-keto acid represented by the general formula (1), a substituted α-keto acid represented by the general formula (2) and a protein or aldolase-containing composition that catalyzes the reaction is adjusted to a suitable temperature of 20 to 50° C. and allowing to stand undisturbed, shaking or stirring for 30 minutes to 5 days while maintaining at pH 6 to 12. 
     The reaction velocity may also be improved by adding a bivalent cation such as Mg 2+ , Mn 2+ , Ni 2+ , or Co 2+  to the reaction mixture. Mg 2+  may be used preferably in terms of cost and so forth. 
     When adding these bivalent cations to the reaction solution, although any salt may be used provided it does not hinder the reaction, MgCl 2 , MgSO 4 , MnSO 4 , and so forth may be used preferably. The concentrations of these bivalent cations may be determined by simple preliminary studies conducted by a person with ordinary skill in the art. These bivalent cations may be added within the range of 0.01 mM to 10 mM, preferably 0.1 mM to 5 mM, and more preferably 0.1 mM to 1 mM. 
     As a preferable example of reaction conditions when performing the process for producing substituted α-keto acid of the present invention, an enzyme source in the form of washed cells of aldolase-expressing  E. coli  are added at 10% (w/v) to a reaction solution consisting of 100 mM buffer, 50 mM indole-3-pyruvic acid, 250 mM pyruvic acid, 1 mM MgCl 2  and 1% (v/v) toluene followed by reacting while shaking at 33° C. for 4 hours to obtain 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG). 
     The substituted α-keto acid of general formula (3) that is formed may be separated and purified according to known techniques. Examples of the method may include a method in which the substituted α-keto acid is contacted with an ion exchange resin to adsorb basic amino acids followed by elution and crystallization, and a method in which the product obtained by elution is discolored and filtrated with activated charcoal followed by crystallization to obtain substituted α-keto acid. 
     Use of the process for producing substituted α-keto acid of the present invention makes it possible to form a precursor keto acid (IHOG) of monatin from indole pyruvic acid and oxaloacetic acid (or pyruvic acid). Since IHOG may be used to derive monatin by aminating position 2, it is useful as an intermediate in monatin synthesis. 
     EXAMPLES 
     The present invention will be explained in further detail with reference to examples shown below, however, the invention is not limited thereto. In the examples, IHOG and PHOG used as substrates were synthesized according to the processes described in Reference Examples 1 and 2. 
     Example 1 
     [I] Screening of Microorganisms Having Aldolase Activity for PHOG 
     Microbial strains having aldolase activity using 4-phenylmethyl-4-hydroxy-2-oxoglutarate (PHOG) as a substrate were screened for. 
     Microorganisms (bacteria, yeast) were inoculated onto bouillon plate medium (Eiken Chemical Co., Ltd.) and cultivated at 30° C. for 24 hours. The microorganisms were then inoculated onto plates containing 0.5 g/dl glycerol, 0.5 g/dl fumaric acid, 0.3 g/dl yeast extract, 0.2 g/dl peptone, 0.3 g/dl ammonium sulfate, 0.3 g/dl K 2 HPO 4 , 0.1 g/dl KH 2 PO 4 , 0.05 g/dl MgSO 4 7H 2 O, 0.25 g/dl sodium phthalate and 2 g/dl agar powder (pH 6.5) followed by cultivating at 30° C. for 24 hours. The resulting microbial cells were inoculated into a reaction solution comprised of 100 mM Tris-HCl (pH 8.0), 50 mM PHOG, 1 mM MgCl 2 , 5 mM potassium phosphate solution (KPi) and 1% (v/v) toluene to wet cells weight of about 1% (w/v), and incuvated at 30° C. for 24 hours. The concentration of free pyruvic acid in the reaction solution was quantified by an enzymatic method using lactate dehydrogenase (LDH). 10 μl of sample was then added to 200 μl of a reaction solution comprised of 100 mM Tris-HCl (pH 8.0), 1.5 mM NADH, 5 mM MgCl 2  and 25 U/ml LDH followed by incubating at 30° C. for 10 minutes. The optical absorbance at 340 nm was measured after the reaction, and the amount of pyruvic acid in the sample was quantified from the reduction in the amount of NADH. 
     The amount of phenyl pyruvic acid formed was quantified by HPLC analysis using an “Inertsil ODS-2” column (GL Sciences Inc., 5 μm, 4.6×250 mm). The analysis conditions were as indicated below. 
     Mobile phase: 20% (v/v) acetonitrile/0.05% (v/v) aqueous trifluoroacetic acid 
     Flow rate: 1 ml/min 
     Column temperature: 40° C. 
     Detection: UV 210 nm 
     Under these conditions, PHOG was eluted at retention time of about 9.8 minutes while phenyl pyruvic acid was eluted at a retention time of about 12 minutes, and each was able to be separated and quantified. 
     The value obtained by subtracting the amount of pyruvic acid or phenyl pyruvic acid formed from PHOG in a reference (microorganism non-addition reaction) from the amount formed in a test microorganism addition reaction was used to indicate the amount formed by aldolase. As a result, aldolase activity using PHOG as substrate was found for the microbial strains listed in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Results of Screening for Microbial Strains 
               
               
                 Having Aldolase Activity for PHOG 
               
            
           
           
               
               
               
            
               
                   
                 Pyruvic 
                 Phenylpyruvic 
               
               
                 Microbial strain 
                 acid (mM) 
                 acid (mM) 
               
               
                   
               
            
           
           
               
               
               
            
               
                   Pseudomonas taetrolens  ATCC4683 
                 34.9 
                 35.0 
               
               
                   Pseudomonas coronafaciens  AJ2791 
                 33.6 
                 33.9 
               
               
                   Pseudomonas desmolytica  AJ1582 
                 1.1 
                 2.9 
               
               
                   Erwinia  sp. AJ2917 
                 0.8 
                 3.0 
               
               
                   Flavobacterium rhenanum  CCM298 AJ2468 
                 3.0 
                 6.1 
               
               
                   Xanthomonas citri  AJ2797 
                 1.0 
                 3.2 
               
               
                   
               
            
           
         
       
     
     PHOG was synthesized from phenyl pyruvic acid and oxaloacetic acid or pyruvic acid using  Pseudomonas taetrolens  ATCC4683 cells. Microbial cells of  P. taetrolens  ATCC4683 (AJ2212) were inoculated to a final concentration of about 1% (w/v) into a reaction solution comprised of 100 mM Tris-HCl (pH 8.0), 50 mM phenyl pyruvic acid, 1 mM MgCl 2 , 5 mM KPi, 100 mM oxaloacetic acid, or pyruvic acid and 1% (w/w) toluene, followed by incuvation at 30° C. for 16 hours. After the reaction, the amount of PHOG formed was quantified by HPLC. The amounts of PHOG formed from phenyl pyruvic acid and oxaloacetic acid or pyruvic acid are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Amounts of PHOG Formed from Phenyl Pyruvic Acid 
               
               
                 and Oxaloacetic Acid and/or Pyruvic Acid 
               
            
           
           
               
               
               
            
               
                   
                 Oxaloacetic 
                 Pyruvic 
               
               
                   
                 acid lot 
                 acid lot 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Microorganism addition lot 
                 14.3 (mM) 
                 9.3 (mM) 
               
               
                   
                 Control (Mg addition) lot 
                 8.6 
                 1.7 
               
               
                   
                 Control (Mg non-addition) lot 
                 trace 
                 N.D. 
               
               
                   
                   
               
            
           
         
       
     
     According to Table 2, an increase in the amount of PHOG formed was observed in the microorganism addition reaction, and PHOG was determined to be able to be formed by the action of aldolase for both the combinations of phenyl pyruvic acid+oxaloacetic acid and phenyl pyruvic acid+pyruvic acid. 
     [II] Purification of IHOG-Aldolase Derived from  Pseudomonas taetrolens  strain ATCC4683 
     IHOG-aldolase was purified as described below from a soluble fraction of  P. taetrolens  strain ATCC4683. Aldolase activity using PHOG as a substrate was measured under the following conditions. 
     Reaction conditions: 50 mM Tris-HCl (pH 8.0), 2 mM PHOG, 0.2 mM NADH, 0.2 mM KPi, 1 mM MgCl 2 , 16 U/ml of lactate dehydrogenase and 3 μL enzyme/600 μl reaction solution, optical absorbance at 340 nm measured at 30° C. 
     (1) Preparation of Soluble Fraction 
     A loopful of  P. taetrolens  ATCC4683 cells cultivated at 30° C. for 24 hours on bouillon plate medium was picked from the plate, and inoculated into a 500 ml volumetric flask containing 50 ml of enzyme production medium (0.5 g/dl glycerol, 0.5 g/dl fumaric acid, 0.5 g/dl ammonium sulfate, 0.3 g/dl K 2 HPO 4 , 0.1 g/dl KH 2 PO 4 , 0.05 g/dl MgSO 4 7H 2 O, 0.3 g/dl yeast extract, 0.2 g/dl peptone, 0.25 g/dl sodium phthalate and 0.005% Antifoam A (Sigma), adjusted to pH 6.5 with KOH) followed by shaken-culturing at 30° C. for 24 hours. 0.5 ml of the culture was inoculated into 500 ml volumetric flasks containing 50 ml of enzyme production medium followed by shake culture at 30° C. for 24 hours. The microorganisms were collected from the resulting culture (total 2 L) by centrifugation, and after washing by suspending in buffer A (20 mM Tris-HCl (pH 7.6)), were again recovered by centrifugation. The resulting washed microbial cells were suspended in 200 ml of buffer A and then ultrasonically disrupted at 4° C. for 30 minutes. The microbial cell residue was removed from the disruption solution by centrifugation (×8000 rpm, 10 min×2 times) followed by additional ultracentrifugation (×50000 rpm, 30 min). After the ultracentrifugation, the resulting supernatant was obtained as the soluble fraction. 
     (2) Anionic Exchange Chromatography: Q-Sepharose FF 
     80 ml of the aforementioned soluble fraction was applied to an anionic exchange chromatography column, Q-Sepharose FF 26/10 (Pharmacia, CV=20 ml) equilibrated with buffer A, and adsorbed onto the carrier. After washing out any unbound proteins using buffer A, the adsorbed protein was eluted using liner gradient of the KCl concentration from 0 M to 0.7 M (total: 140 ml). aldolase activity for PHOG was detected in the fraction equivalent to about 0.5 M. The same chromatography procedure was repeated twice. 
     (3) Hydrophobic Chromatography: Phenyl Sepharose HP HR 16/10 
     The fraction in which aldolase activity had been detected was dialyzed at 4° C. overnight against buffer B (50 mM Tris-HCl (pH 7.6), 1 M ammonium sulfate, pH 7.6) and then filtered with a 0.45 μm filter. The resulting filtrate was applied to a hydrophobic chromatography column, Phenyl Sepharose HP HR 16/10 (Pharmacia) equilibrated with buffer B. The aldolase was adsorbed to the carrier. 
     After washing out any unbound proteins using buffer B, aldolase was eluted using liner gradient of the ammonium sulfate concentration from 1 M to 0 M. The aldolase activity was detected in the elution fraction where the concentration of ammonium sulfate was about 0.2 M. 
     (4) Gel Filtration Chromatography: Sephadex 200 HP 16/60 
     Each fraction containing aldolase was collected, dialyzed against buffer A and filtered with a 0.45 μm filter. The resulting filtrate was concentrated using the Centriprep 10 ultrafiltration membrane. The resulting concentrate was applied to a gel filtration chromatography column, Sephadex 200 HP 16/60 (Pharmacia) equilibrated with buffer C (20 mM Tris-HCl (pH 7.6), 0.1 M KCl), and eluted at a flow rate of 1 mL/min. the aldolase was eluted in the fraction from 66 to 71 ml. The molecular weight of the aldolase as determined from the elution position of peak activity was estimated to be about 146 kDa. 
     (5) Anionic Exchange Chromatography: Mono Q HR5/5 
     The resulting fraction was filtered with a 0.45 μm filter. Here, the resulting filtrate was applied to an anionic exchange chromatography column, Mono Q HR 5/5 (Pharmacia) equilibrated with buffer A. The aldolase was adsorbed to the carrier. After washing out any unbound proteins with buffer A, protein was eluted using liner gradient of the KCl concentration from 0 mM to 700 mM (total: 24 ml). The aldolase activity was measured for each eluted fraction, and aldolase activity was observed at the elution position where the KCl concentration was about 0.4 M. 
     (6) Hydroxyapatite Chromatography: CHT-II 
     The resulting fraction was dialyzed at 4° C. overnight against buffer D (10 mM potassium phosphate buffer (pH 7.0)), and filtered with a 0.45 μm filter. Here, the resulting filtrate was applied to a hydroxyapatite chromatography column, 5 ml CHT-II (BioRad) equilibrated with buffer D. The aldolase activity was detected at the unbound fraction, aldolase was able to be separated from the adsorbed protein. 
     When the fraction purified according to the column chromatography procedures described above was applied to SDS-PAGE, a nearly single band was detected at the location corresponding to about 25 kDa. Since the estimated molecular weight as determined by gel filtration chromatography was about 146 kDa, it was suggested that the aldolase forms a hexamer. The purification table is shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Purification Table of IHOG Aldolase Derived from 
               
               
                   Pseudomonas taetrolens  Strain ATCC4683 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 specific 
                 purifi- 
                 total 
                   
               
               
                   
                 protein 
                 activity 
                 cation 
                 activity 
                 yield 
               
               
                   
                 (mg) 
                 (U/mg) 
                 fold 
                 (U) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 soluble fraction 
                 3750 
                 0.014 
                 1 
                 51 
                 100 
               
               
                 Q-sepharose 
                 510 
                 0.060 
                 4.4 
                 30.5 
                 59.8 
               
               
                 HP 26/10 
               
               
                 Phenyl sepharose 
                 21.2 
                 0.893 
                 66 
                 19.0 
                 37.2 
               
               
                 HP 16/10 
               
               
                 Sephadex200 
                 1.9 
                 4.643 
                 341 
                 8.65 
                 17.0 
               
               
                 HP 16/60 
               
               
                 monoQ HR5/5 
                 0.49 
                 10.89 
                 800 
                 5.33 
                 10.4 
               
               
                 Hydroxyapatite 
                 0.025 
                 28.70 
                 2110 
                 0.71 
                 1.4 
               
               
                 CHT-II 
               
               
                   
               
            
           
         
       
     
     [III] Determination of Internal Amino Acid Sequence of IHOG Aldolase 
     After applying an approximately 2 μg aliquot of the purified aldolase to SDS-PAGE, the sample in the SDS-PAGE gel was treated with trypsin (pH 8.5, 35° C., 20 hours) and then applied to reverse phase HPLC to separate the fragment peptides. Amino acid sequences consisting of 20 residues and 12 residues, respectively (SEQ ID Nos: 4 and 5), were determined as shown below for two of the separated fractions. 
                     TABLE 4               Determined Internal Amino Acid Sequences                                                SEQ ID No. 4   SLLDA FQNVV TPHIS DNLGR                       SEQ ID No. 5   AEIAT GALDQ SW                        
[IV] Cloning of IHOG-Aldolase Gene Derived from  P. taetrolens  Strain ATCC4683
 
     (1) Preparation of Chromosomal DNA 
       P. taetrolens  strain ATCC4683 was cultivated at 30° C. overnight with 50 ml of bouillon medium (pre-cultivation). 5 ml of this culture was inoculated into the 50 ml of fresh bouillon medium. After the cultivation until the late logarithmic growth phase, 50 ml of culture was centrifuged (12000×g, 4° C., 15 minutes) to recover the microorganisms. A chromosomal DNA was then prepared according to established methods using these microbial cells. 
     (2) Cloning of a Internal DNA of the Aldolase Gene by PCR 
     The following mixed primers (SEQ ID Nos: 6 and 7) were synthesized based on the internal amino acid sequence determined for IHOG-aldolase. 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Mixprimers Designed and Synthesized Based on 
               
               
                 Internal Amino Acid Sequence 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 SEQ ID No. 6 
                 TTY CAR AAY GTS GTS ACS CCS C 
               
               
                   
               
               
                 SEQ ID No. 7 
                 TGR TCR ATN GCN CCS GTN GCR ATY TCN GC 
               
               
                   
               
            
           
         
       
     
     Amplification was performed by PCR with the synthesized mixed primers and the chromosomal DNA of  P. taetrolens  ATCC4683 as a template. The PCR reaction was performed with the PCR Thermal PERSONEL (Takara) for 30 cycles under the conditions indicated below. 
     94° C., 30 seconds 
     55° C., 30 seconds 
     72° C., 1 minute 
     The PCR product was applied to agarose gel electrophoresis, and an approximately 500 bp fragment was amplified. This DNA fragment was cloned to pUC18, followed by determination of the nucleotide sequence. The amino acid sequence estimated from the obtained DNA fragment matched with the internal amino acid sequence of IHOG aldolase, thereby confirming that the desired aldolase gene had been obtained. 
     (3) Cloning of the Full Length Gene by Colony Hybridization 
     The full-length gene was cloned by Southern hybridization analysis and colony hybridization with the DNA fragment amplified by PCR. The DNA probe was produced by using DIG High Prime (Roche Diagnostics K.K.), incubating overnight (i.e., O/N) at 37° C. according to the instruction manual to label the probe. Southern hybridization analysis was performed according to the manual by digesting 1 μg of chromosomal DNA with each of restriction enzyme, electrophoresing in 0.8% agarose gel and blotting onto a nylon membrane. Hybridization was performed using DIG Easy Hyb (Roche Diagnostics K.K.) by pre-hybridizing at 50° C. for 1 hour, adding the probe and then hybridizing with O/N. Bands were detected using the DIG Nucleotide Detection Kit. As a result, an approximately 4 kbp PstI fragment was detected that strongly hybridized by using the PCR fragment as a probe. Next, this PstI fragment was obtained by colony hybridization. 20 μg of chromosomal DNA was treated with PstI followed by applying to agarose gel electrophoresis and recovering a fragment of which size was about 4 kbp. This was then ligated into pUC118 to produce a library in  E. coli  JM109. The colonies were then transferred to a nylon membrane fitter (Hybond-N, Amersham) followed by alkaline denaturation, neutralization and immobilization treatment. Hybridization was performed using DIG Easy Hyb. The filter was immersed in a buffer and pre-hybridized at 42° C. for 1 hour. Subsequently, the previously prepared labeled probe was added, followed by hybridization at 42° C. for 16 hours. After washing with SSC, colonies that hybridized with the probe were detected using the DIG Nucleotide Detection Kit (Roche Diagnostics K.K.). As a result, a clone was obtained that strongly hybridized with the probe. 
     The nucleotide sequence of plasmid DNA recovered from the obtained clone was determined, and it was clearly demonstrated to have the nucleotide sequence of SEQ ID No: 1. An orf of 678 bp that contained the nucleotide sequence corresponding to the determined amino acid sequence (numbers 507 to 566 and numbers 1046 to 1082 in SEQ ID No: 1) was found and the entire length of the desired aldolase gene was obtained. 
     (4) Expression of IHOG-Aldolase in  E. coli  (Part 1) 
     The fragment amplified using the primers shown in Table 6 (SEQ ID Nos: 8 and 9) from a chromosomal DNA of  P. taetrolens  ATCC4683 was digested with BamHI/HindIII, inserted into the BamHI/HindIII site of pUC18, to construct plasmid pUCALD.  E. coli  JM109 was transformed with thus obtained pUCALD, and the transformant was cultured at 37° C. overnight in LB medium containing 50 μg/ml of ampicillin (pre-cultivation). The pre-culture was inoculated at 1% in 50 ml of LB medium followed by cultivating at 37° C. IPTG was added to a final concentration of 1 mM at about 2 hours after the start of cultivating, followed by additionally cultivating for 3 hours. After cultivating, the microorganisms were recovered, washed, and then suspended in 1 ml of 20 mM Tris-HCl (pH 7.6) followed by disrupting the microbial cells using a Multi-Beads Shocker (Yasui Kikai Corporation). The crude extract was then centrifuged at 15000 rpm for 10 minutes, and the resulting supernatant was used as a crude enzyme solution. 
     
       
         
           
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Primer 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 SEQ ID No. 8 
                 ALD-5′ Bam(5′-GCC GGA TCC ACA AGG GTT 
               
               
                   
                 CAG TCA TTC ATG G-3′) 
               
               
                   
               
               
                 SEQ ID No. 9 
                 ALD-3′ Hind(5′-CCG AAG CTT TCA GTT CGC 
               
               
                   
                 CAG GCC AGC C-3′) 
               
               
                   
               
            
           
         
       
     
     When aldolase activity was measured using the crude enzyme solution and PHOG as a substrate, in contrast to aldolase activity for PHOG not being detected in the  E. coli  harboring pUC18 (control), aldolase activity for PHOG of 0.81 U/mg protein was detected in the strain harboring pUCADL. As a result, the gene was demonstrated to encode the desired aldolase. 
     [V] Synthesis of 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG) from Indole-3-pyruvic Acid and Pyruvic Acid Using an Aldolase-Expressing Strain 
     4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG) was synthesized from indole-3-pyruvic acid and pyruvic acid with washed cells of the aldolase-expressing  E. coli  produced in [IV] as the enzyme source. The amount of IHOG was determined by HPLC analysis using the “Inertsil ODS-2” (GL Sciences Inc., 5 μm, 4.6×250 mm). The analysis conditions are as indicated below. 
     Mobile phase: 40% (v/v) acetonitrile/5 mM tetrabutyl 
     ammonium dihydrogen phosphate solution 
     Flow rate: 1 mL/min 
     Column temperature: 40° C. 
     Detection: UV 210 nm 
     Washed cells of aldolase-expressing  E. coli  were added at 10% (w/v) to a reaction solution comprised of 100 mM buffer (Tris-HCl 8.0, 9.0, or glycine-NaOH 10.0) 50 mM indole-3-pyruvic acid, 250 mM pyruvic acid, 1 mM MgCl 2  and 1% (v/v) toluene, and allowed to react by shaking at 33° C. for 4 hours. The enzyme reaction solution was suitably diluted and the amount of IHOG formed was quantified. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Amount of IHOG Formed by Aldolase 
               
            
           
           
               
               
               
            
               
                 pH 
                 aldolase 
                 IHOG (mM) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 8 
                 + 
                 9.2 
               
               
                 8 
                 − 
                 0.42 
               
               
                 9 
                 + 
                 12.1 
               
               
                 9 
                 − 
                 1.6 
               
               
                 10 
                 + 
                 10.7 
               
               
                 10 
                 − 
                 5.4 
               
               
                   
               
            
           
         
       
     
     As a result, the amount of IHOG produced increased in the aldolase-expressing  E. coli  addition group, and IHOG was formed by the aldolase. 
     Example 2 
     High-Level Expression of IHOG-Aldolase in  E. coil  (Part 2) 
     (1) Construction of Plasmid pTrp4 Containing trp Promoter and rmB Terminator 
     The promoter region of the trp operon in a chromosomal DNA of  E. coli  W3310 by PCR with the oligonucleotides shown in Table 8 as primers (combination of SEQ ID No: 10 and 11), and the resulting DNA fragment was ligated into a pGEM-Teasy vector (Promega).  E. coli  JM109 was then transformed with this ligation mixture, and a strain was selected among ampicillin-resistant strains that had the desired plasmid in which the trp promoter was inserted in the opposite orientation of the lac promoter. Next, the DNA fragment containing trp promoter obtained by treating this plasmid with Eco01091/EcoRI was ligated with the product of treating pUC19 (Takara) with Eco01091/EcoRI.  E. coil  JM109 was then transformed with this ligation mixture, a strain was selected among ampicillin-resistant strains that had the desired plasmid, and that plasmid was named pTrp1. Next, pKK223-3 (Amersham-Pharmacia) was treated with HindIII/HincII, and the resulting DNA fragment containing rmB terminator was ligated with the product of treating pTrp1 with HindIII/PvuII.  E. coli  JM109 was then transformed with this ligation mixture, a strain was selected among ampicillin-resistant strains that had the desired plasmid, and that plasmid was named pTrp2. Next, the trp promoter region was amplified by PCR with pTrp2 as a template and the oligonucleotides shown in the table as primers (combination of SEQ ID Nos: 10 and 12). This DNA fragment was treated with Eco0109I/NdeI and ligated with the product of treating pTrp2 with Eco0109I/NdeI.  E. coli  was then transformed with this ligation mixture, a strain was selected among ampicillin-resistant strains that had the desired plasmid, and that plasmid was named pTrp4. 
                     TABLE 8               Primer                                            SEQ ID No. 10   5′-side   GTATCACG AGGCCCT AGCTGTGGTGTCATGGTCGGTGATC                       Eco0109               SEQ ID No. 11   3′-side   TTCGGGGATTC CATATG ATACCCTTTTTACGTGAACTTGC                           Ndel               SEQ ID No. 12   3′-side   GGGGGGGG CATATG CGACCTCCTTATTACGTGAACTTG                        Ndel                    
(2) Construction of Aldolase Gene-Expressing Plasmids ptrpALD1 and ptrpALD2, and Expression in  E. coli  
 
     A fragment amplified from a chromosomal DNA of  P. taetrolens  ATCC4683 using the primers shown in Table 9 (SEQ ID Nos: 9 and 13) was digested with NdeI/HindIII to construct plasmid ptrpALD1 inserted into the NdeI/HindIII site of pTrp4. This plasmid expresses aldolase gene composing the amino acid sequence of SEQ ID No: 3 translated from the 444th ATG in the nucleotide sequence of SEQ ID No: 1 as the initiation codon. Furthermore, a fragment amplified from a chromosomal DNA of  P. taetrolens  ATCC4683 with primers (SEQ ID Nos: 9 and 14) was digested with NdeI/HindIII to construct plasmid ptrpALD2 inserted into the NdeI/HindIII site of pTrp4. This plasmid expresses aldolase gene composing the amino acid sequence of SEQ ID No: 2 translated from the 456th ATG in the nucleotide sequence of SEQ ID No: 1 as the initiation codon. Each of the constructed expression plasmids was introduced into  E. coli  JM109, and the transformants were cultured at 37° C. overnight in LB medium containing 50 μg/ml of ampicillin (pre-cultivation). The pre-culture was inoculated at 1% in 50 ml of LB medium followed by cultivation at 37° C. The microbial cells were harvested, washed and suspended in 1 ml of 20 mM Tris-HCl (pH 7.6) followed by disrupting the microbial cells using a Multi-Beads Shocker (Yasui Kikai Corporation). The crude extract was then centrifuged at 15000 rpm for 10 minutes and the resulting supernatant was used as a crude enzyme solution. 
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Primer 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 SEQ ID No. 9 
                 ALD-3′ Hind(5′-CCG AAG CTT TCA GTT CGC CAG GCC AGC C-3′) 
               
               
                 SEQ ID No. 13 
                 ALD-5′ Nde-1(5′-GGT TCA GTC ACA TAT GGA GGT CGC TAT GTC-3′) 
               
               
                 SEQ ID No. 14 
                 ALD-5′ Nde-2(5′-ATG GAG GTC CAT TAG TCA TTG CCC GGT TCA CGC-3′) 
               
               
                   
               
            
           
         
       
     
     Measuring aldolase activity using the crude enzyme and PHOG for the substrate, in contrast to aldolase activity for PHOG not being detected in the  E. coli  harboring pTrp4 (control), PHOG-aldolase activity of 16.1 U/mg protein was detected in the strain harboring ptrpALD1, while PHOG-aldolase activity of 36.0 U/mg protein was detected in the strain harboring ptrpALD2. As a result, regardless of which aldolase comprising the amino acid sequence of SEQ ID No: 2 or 3 was used, both aldolases were demonstrated to have aldolase activity. 
     Example 3 
     Cloning of IHOG-Aldolase Derived from  Pseudomonas coronafaciens  strain AJ2791 
     (1) Preparation of Chromosomal DNA 
       P. coronafaciens  strain AJ2791 was cultivated at 30° C. overnight using 50 ml of bouillon medium (pre-cultivation). 5 ml of this culture was inoculated into 50 ml of bouillon medium. After the cultivation until the late logarithmic growth phase, 50 ml of culture was centrifuged (12000×g, 4° C., 15 minutes) to recover the microorganisms. A chromosomal DNA was then prepared according to established methods using these microbial cells. 
     (2) Cloning of Full Length Gene by Southern Analysis and Colony Hybridization 
     Cloning of IHOG-aldolase gene derived from  P. coronafaciens  strain AJ2791 (hereinafter, “PcALD”) was performed by Southern hybridization analysis and colony hybridization with IHOG-aldolase gene derived from  P. taetrolens  strain ATCC4683 as a probe. The full length of IHOG-aldolase gene was amplified by PCR from a chromosomal DNA of  P. taetrolens  ATCC4683 using the primers ALD-5′ Nde-1 (SEQ ID No: 13) and ALD-3′ Hind (SEQ ID No: 9) shown in Table 9. The full length gene was then obtained by Southern analysis and colony hybridization with the amplified DNA fragment as a probe. The DNA probe was produced by using DIG High Prime (Roche Diagnostics K.K.), O/N incubating at 37° C. according to the instruction manual to label the probe. Southern hybridization analysis was performed according to the manual by completely digesting 1 μg of chromosomal DNA of  P. coronafaciens  strain AJ2791 with each type of restriction enzyme, electrophoresing in 0.8% agarose gel and blotting onto a nylon membrane. Hybridization was performed using DIG Easy Hyb (Roche Diagnostics K.K.) by pre-hybridizing at 37° C. for 1 hour, adding the probe and then hybridizing with O/N at 37° C. The membrane was washed twice at room temperature for 5 minutes using 2×SSC, and then washed twice at 37° C. for 15 minutes using 0.1×SSC. Bands were detected using the DIG Nucleotide Detection Kit. As a result, an approximately 2.2 kbp PstI/HindIII fragment was detected that hybridized with the probe. Next, this PstI/HindIII fragment was obtained by colony hybridization. 20 μg of chromosomal DNA was treated with PstI/HindIII followed by applying to agarose gel electrophoresis and recovering a fragment of which size was about 2.2 kbp. This was then ligated into pUC118 treated with PstI/HindIII to produce a library in  E. coil  JM109. The colonies were then transferred to a Nylon membrane filter (Hybond-N, Amersham) followed by alkaline denaturation, neutralization and immobilization treatment. Hybridization was performed using DIG Easy Hyb. The Nylon membrane filter was immersed in a buffer and pre-hybridized at 37° C. for 1 hour. Subsequently, the previously prepared labeled probe was added and hybridized at 37° C. for 16 hours. The Nylon membrane filter was washed twice at room temperature for 5 minute using 2×SSC and then washed twice at 37° C. for 15 minutes using 0.1×SSC. Colonies that hybridized with the probe were detected using the DIG Nucleotide Detection Kit (Roche Diagnostics K.K.). As a result, a clone was obtained that hybridized with the probe. 
     When the nucleotide sequence of plasmid DNA recovered from the positive clone was determined, a 744 bp orf was found, and the full length of the desired aldolase was obtained. The constructed plasmid inserted with the PcALD gene was named pUCPcALD. 
       E. coli  JM109 was transformed with pUCPcALD, and the transformant was cultured at 37° C. for overnight in LB medium containing 50 μg/ml of ampicillin (LB-amp medium) (pre-cultivation). Thus obtained preculture was inoculated at 1% in 50 ml of fresh LBmedium, followed by cultivation at 37° C. IPTG was added to a final concentration of 1 mM at about 2 hours after the start of cultivating followed by additionally cultivating for 3 hours. After cultivating, the microorganisms were recovered, washed, and then suspended in 1 ml of 20 mM Tris-HCl (pH 7.6) followed by disrupting the microbial cells with a Multi-Beads Shocker (Yasui Kikai Corporation). The crude extract was then centrifuged at 15000 rpm for 10 minutes, and the resulting supernatant was used as a crude enzyme. 
     When aldolase activity was measured using the crude enzyme and PHOG as a substrate, in contrast to aldolase activity for PHOG not being detected in the  E. coli  harboring pUC18 (control), PHOG-aldolase activity of 39.3 U/mg protein was detected in the strain harboring pUCADL. As a result, the gene was demonstrated to encode the desired aldolase. 
     Example 4 
     Purification of Recombinant Enzyme of Aldolase Derived from  Pseudomonas taetrolens  Strain ATCC4683 
     Recombinant aldolase derived from  P. taetrolens  ATCC4683 (PtALD) was purified as described below from the soluble fraction of  E. coli  that highly expresses PtALD. Aldolase activity was determined by measuring aldolase activity when using PHOG as a substrate under the following conditions. 
     Reaction conditions: 50 mM Tris-HCl (pH 8.0), 2 mM PHOG, 0.2 mM NAD, 0.2 mM KPi, 1 mM MgCl 2 , 16 U/ml of lactate dehydrogenase and 3 μL enzyme/600 μl reaction mixture, optical absorbance at 340 nm measured at 30° C. 
     (1) Preparation of Soluble Fraction 
     A loopful of  E. coli  JM109/ptrpALD2 cultured at 37° C. for 16 hours on LB-amp plate medium were picked from the plate, inoculated into a test tube containing 3 ml of LB-amp medium and shake-culture was performed at 37° C. for 16 hours. 0.5 ml of this culture was then inoculated into ten 500 ml volumetric flasks containing 50 ml of LB-amp medium followed by culturing at 37° C. for 16 hours. The microorganisms were recovered from the resulting culture by centrifugation, and after washing by suspending in buffer A (20 mM Hepes-KOH (pH 7.6)), were again recovered by centrifugation. The resulting washed microbial cells were suspended in 25 ml of buffer A and then ultrasonically disrupted at 4° C. for 30 minutes. The microbial cell residue was removed from the crude extract by centrifugation (×8000 rpm, 10 min×2 times) after which the resulting supernatant was used as the crude fraction. 
     (2) Anionic Exchange Chromatography: Q-Sepharose FF 
     23 ml of the crude fraction was applied to a Q-Sepharose FF 26/10 anionic exchange chromatography column (Pharmacia, CV=20 ml) equilibrated with buffer A and adsorbed onto the carrier. After washing out any unbound proteins using buffer A, the PtALD was eluted while linearly changing the KCl concentration from 0 M to 0.7 M (total: 140 ml). The aldolase activity for PHOG was observed in the fraction equivalent to about 0.5 M. 
     (3) Hydrophobic Chromatography: Phenyl Sepharose HP HR 16/10 
     The fraction detected aldolase activity was dialyzed at 4° C. overnight against buffer B (20 mM Hepes-KOH (pH 7.6), 1 M ammonium sulfate, pH 7.6) and then filtering the supernatant obtained with a 0.45 μm filter. The resulting filtrate was applied to a hydrophobic chromatography column, Phenyl Sepharose HP HR 16/10 (Pharmacia) equilibrated with buffer B. This procedure resulted in the aldolase being adsorbed to the carrier. 
     After washing out any unbound proteins using buffer B, aldolase was eluted using liner gradient of the ammonium sulfate concentration from 1 M to 0 M. The aldolase activity was detected at the elution position where the concentration of ammonium sulfate was about 0.2 M. 
     When the fraction purified according to the chromatography procedures described above was applied to SDS-PAGE, a single band was observed at the location corresponding to about 25 kDa with CBB staining. The resulting recombinant PtALD solution was dialyzed at 4° C. overnight against buffer A. As a result of the procedure, 17 ml of 350 U/ml PtALD solution was obtained. 
     Example 5 
     Purification of Recombinant Enzyme of Aldolase Derived from  Pseudomonas coronafaciens  strain AJ2791 
     Recombinant aldolase derived from  P. coronafaciens  AJ2791 (PcALD) was purified from the soluble fraction of  E. coli  that highly expresses PcALD. 
     A loopful of  E. coli  JM109/pUCPcALD cells cultivated at 37° C. for 16 hours on LB-amp plate medium was picked from the plate, inoculated into a test tube containing 3 ml of LB-amp medium and shake-culture was performed at 37° C. for 16 hours. 0.5 ml of this culture was then inoculated into seven pieces of 500 ml volumetric flasks containing 50 ml of LB-amp medium (+0.1 mM IPTG) followed by shake-culturing at 37° C. for 16 hours. The microorganisms were recovered from the resulting culture by centrifugation, and after washing by suspending in buffer A (20 mM Hepes-KOH (pH 7.6)), were again recovered by centrifugation. The resulting washed microbial cells were suspended in 30 ml of buffer A and then ultrasonically disrupted at 4° C. for 30 minutes. The microbial cell residue was removed from the crude extract by centrifugation (×8000 rpm, 10 min×2 times) after which the resulting supernatant was used as the crude extraction fraction. 
     The resulting crude extraction fraction was purified by chromatography using the same procedure as the purification of recombinant PtALD described in Example 4. When a fraction purified by a two-stage chromatography procedure using Q-Sepharose and Phenyl Superose was applied to SDS-PAGE, a single band was observed at the location corresponding to about 25 kDa with CBB staining. The resulting recombinant PcALD solution was dialyzed at 4° C. overnight against buffer A. As a result of the aforementioned procedure, 10 ml of 108 U/ml PcALD solution was obtained. 
     Example 6 
     Synthesis of 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate from Indole-3-pyruvic Acid and Pyruvic Acid Using PtALD and PcALD 
     4-(Indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG) was synthesized from indole-3-pyruvic acid and pyruvic acid using the PtALD and PcALD produced in Examples 4 and 5 as enzyme sources. The amount of IHOG was determined by HPLC analysis using the “Inertsil ODS-2” (GL Sciences Inc., 5 μm, 4.6×250 mm). The analysis conditions are as indicated below. 
     Mobile phase: 40% (v/v) acetonitrile/5 mM tetrabutyl ammonium dihydrogen phosphate solution 
     Flow rate: 1 ml/min 
     Column temperature: 40° C. 
     Detection: UV 210 nm 
     PtALD and PcALD were added at 1.8 U/ml and 0.8 U/ml, respectively, to a reaction solution comprised of 100 mM Hepes-KOH (pH 8.5), 50 mM indole-3-pyruvic acid, 250 mM pyruvic acid and 1 mM to 5 mM MgCl 2 , and allowed to react at 33° C. for 4 hours. The enzyme reaction solution was suitably diluted and the amount of IHOG formed was quantified. As a result, IHOG was able to be formed from the aldolases. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                 MgCl 2  (mM) 
                 IHOG (mM) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 PcALD 
                 1 
                 4.9 
               
               
                   
                   
                 5 
                 5.4 
               
               
                   
                 PtALD 
                 1 
                 11.4 
               
               
                   
                   
                 5 
                 10.5 
               
               
                   
                   
               
            
           
         
       
     
     Example 7 
     Enzymatic Synthesis of Various Substituted α-Keto Acids Using PtALD 
     The aldol condensation activity of pyruvic acid was measured for the various substrates shown in Table 11 using the PtALD produced in Example 4. A reaction solution was prepared composed of 100 mM glycine-NaOH (pH 9), 50 mM substrate, 250 mM sodium pyruvate, 1 mM MgCl 2  and 0.1 mg/ml PtALD, and incubated at 33° C. for 2 hours. The resulting reaction solution was subjected to ultrafiltration with an ultrafiltration membrane, Microcon 10 (Amicon) for a fraction molecular weight of 10000. 2 μl of the filtrate was diluted in 200 μl of 50% acetonitrile/0.1% aqueous ammonia solution followed by ESI-MS analysis. 
     As a result, peaks in the corresponding m/z values were detected for the substrates shown in Table 12, thus indicating that the desired pyruvic acid aldol condensates were formed. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Aldol Condensates of Pyruvic Acid for Various Substrates Using PtALD 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Exact MS 
                 [M − H] −   
               
               
                 Substrate 
                 Substrate 
                 Aldol Condensate 
                 (calculated) 
                 (measured) 
               
               
                   
               
               
                 indol-3-pyruvic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 291.1  
                 290.2  
               
               
                   
               
               
                 β-hydroxypyruvic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 192.03 
                 191.19 
               
               
                   
               
               
                 2-ketobutyric acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 190.05 
                 189.19 
               
               
                   
               
               
                 3-methyl-2-oxo butanoic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 204.06 
                 203.08 
               
               
                   
               
               
                 α-ketogrutarate 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 234.04 
                 233.45 
               
               
                   
               
               
                 pyruvic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 176.03 
                 175.2  
               
               
                   
               
               
                 D-arabinose 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 238.07 
                 237.15 
               
               
                   
               
               
                 L-mannose 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 268.08 
                 267.72 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Aldol Condensates of Pyruvic Acid for Various Substrates Using PtALD 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Exact MS 
                 [M − H] −   
               
               
                 Substrate 
                 Substrate 
                 Aldol Condensate 
                 (calculated) 
                 (measured) 
               
               
                   
               
               
                 L-ribose 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 238.07 
                 237.05 
               
               
                   
               
               
                 N-acetyl-L- mannosamine 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 309.27 
                 308.48 
               
               
                   
               
            
           
         
       
     
     Example 8 
     Enzymatic Properties of PtALD 
     A study was conducted on the following parameters regarding the PtALD prepared in Example 4. 
     Basic reaction conditions: 50 mM Hepes-KOH (pH 8.0), 2 mM PHOG, 5 mM MgCl 2 , 3 mM KPB (pH 8), 16 U/ml of lactate dehydrogenase, measure a optical absorbance at 340 nm measured at 30° C. 
     (1) Kinetics Constants When Using PHOG as Substrate 
     Vmax (for PHOG)=91.7 μmol/min/mg, Km (for PHOG)=0.10 mM, Km (for MgCl 2 )=0.019 mM, and Ka (for KPi)=0.95 mM were respectively determined from the results shown in  FIGS. 2 to 4 . The aldolase activity increased by 2.7 times as compared with a non-addition lot following addition of 1 mM MgCl 2 , and increased 2 times as compared with a non-addition lot following addition of 5 mM KPB. 
     (2) pH Stability 
     pH stability was measured over a range of pH 3 to pH 11. The buffers used for measurement were as follows: sodium acetate buffer (pH 3, 4, 5, 6), potassium phosphate buffer (pH 6, 6.5, 7, 7.5), Tris-HCl buffer (pH 7, 7.5, 8, 8.5), glycine-NaOH buffer (pH 9, 9.5, 10), CAPS-NaOH buffer (pH 10, 10.5, 11). The residual activity after incubating PtALD at 37° C. for 30 minutes in each of the buffer solutions at a concentration of 100 mM was measured under the basic reaction conditions. Those results are shown in  FIG. 5 . 
     (3) Temperature Stability 
     Residual activity was measured under the basic measurement conditions after incubating PtALD at 25 to 70° C. for 30 minutes in 100 mM potassium phosphate buffer (pH 7.0) and 100 mM glycine-NaOH buffer (pH 9.0). Those results are shown in  FIG. 6 . 
     Example 9 
     Synthesis of Substituted α-Keto Acid Using PtALD 
     The aldol condensation activity of the PtALD prepared in Example 4 was measured using acetaldehyde and α-ketobutyrate. A reaction solution composed of 100 mM glycine-NaOH (pH 9), 50 mM acetoaldehyde, 250 mM α-ketobutyrate, 1 mM MgCl 2  and 0.1 mg/ml PtALD was prepared, and it was incubated at 33° C. for 16 hours. The resulting reaction solution was subjected to ultrafiltration with an ultrafiltration membrane, Microcon 10 (Amicon) for a fraction molecular weight of 10000. 10 μl of the filtrate was diluted in 200 μl of 50% acetonitrile/0.1% aqueous ammonia solution followed by ESI-MS analysis. 
     As a result, in contrast to the exact mass (calculated value) of the predicted product being 146.06, a peak (m/z=+146.8) of the m/z value corresponding to [M+H] was detected, thereby confirming formation of the desired aldol condensate according to the reaction shown below. 
     
       
         
         
             
             
         
       
     
     Reference Example 1 
     Synthesis of 4-hydroxy-4-(3-Indolylmethyl)-2-ketoglutarate (IHOG) 
     7.50 g (35.8 mmol, content: 97.0% by weight) of indole-3-pyruvic acid and 14.18 g (107.4 mmol) of oxaloacetic acid were added to 64.45 ml of aqueous solution including 18.91 g (286.5 mmol, content: 85% by weight) of potassium hydroxide and dissolved. This mixed solution was then stirred at 35° C. for 24 hours. 
     Furthermore, the mixed solution was neutralized (pH 7.0) by addition of 40.0 ml of 3 N hydrochloric acid to obtain 153.5 g of a neutralized reaction solution. This neutralized reaction solution contained 5.55 g of 4-hydroxy-4-(3-indolylmethyl)-2-ketoglutarate, and the yield was 53.3% (with respect to indole pyruvic acid). 
     Water was added to this neutralized reaction solution to bring to a volume of 168 ml and then passed through a resin column (diameter. 4.8 cm) filled with 840 ml of a synthetic adsorbent (DIAION-SP207, Mitsubishi Chemical Corporation). Furthermore, water was passed through the column at a flow rate of 23.5 ml/min and by recovering the product from 1.73 to 2.55 (UL-R), an aqueous solution containing 3.04 g of highly pure 4-hydroxy-4-(3-indolylmethyl)-2-ketoglutarate was obtained at a yield of 54.7% (with respect to the amount charged into the resin). 
     (NMR Measurement) 
       1 H-NMR (400 MHz, D 2 O): 3.03 (d, 1H, J=14.6 Hz), 3.11 (d, 1H, J=14.6 Hz), 3.21 (d, 1H, J=18.1 Hz), 3.40 (d, 1H, J=18.1 Hz), 7.06-7.15 (m, 3H), 7.39 (d, 1H, J=7.8 Hz), 7.66 (d, 1H, J=7.8 Hz) 
       13 C-NMR (100 MHz, D 2 O): 35.43, 47.91, 77.28, 109.49, 112.05, 119.44, 119.67, 121.91, 125.42, 128.41, 136.21, 169.78, 181.43, 203.58 
     Reference Example 2 
     Synthesis of 4-phenylmethyl-4-hydroxy-2-ketoglutarate (PHOG) 
     5.0 g (30.5 mmol) of phenylpyruvic acid and 12.1 g (91.4 mmol) of oxaloacetic acid were added to 25 ml of water in which 13.8 g of potassium hydroxide (purity: 85%) was dissolved and the resulting mixture was allowed to react at room temperature for 72 hours. The pH value of this reaction solution was then adjusted to 2.2 using concentrated hydrochloric acid followed by extracting with ethyl acetate. After washing the organic layer with a saturated aqueous NaCl solution and drying with anhydrous magnesium sulfate and then concentrating, the residue was obtained. The residue was recrystallized from ethyl acetate and toluene to obtain 2.8 g (11.3 mmol) of 4-phenylmethyl-4-hydroxy-2-ketoglutarate in the form of crystals. 
     (NMR Measurement) 
       1 H-NMR (D 2 O) δ: 2.48 (d, J=14.4 Hz, 0.18H), 2.60 (d, J=14.4 Hz, 0.18H), 2.85-3.30 (m, 3.64H), 7.17-7.36 (m, 5H) 
     (Measurement of Molecular Weight) 
     ESI-MS Calculated Value for C 12 H 12 O 6 =252.23, analytical value=251.22 (MH − ) 
     INDUSTRIAL APPLICABILITY 
     The aldolase of the present invention is a novel aldolase that catalyzes an aldol condensation reaction of indole pyruvic acid and pyruvic acid (or oxaloacetic acid), and may be preferably used to synthesize 4-(indol-3-ylmethyl)-4-hydroxy-2-oxoglutarate (IHOG), which is useful as an intermediate in monatin synthesis.