Abstract:
This invention relates to a method for producing (R)-malic acid comprising contacting, in a reaction medium, maleic acid with (A) a microbial maleate hydratase capable of hydrating maleic acid to form (R)-malic acid or (B) a microorganism containing the maleate hydratase, microbial maleate hydratase and a method for producing same. In accordance with the present invention (R)-malic acid with high optical purity is supplied efficiently and economically.

Description:
This is a continuation-in-part application of International Application Ser. No PCT/JP91/00025 filed Jan. 14, 1991. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for producing (R)-malic acid [D-(+)-malic acid] by hydrating maleic acid. The present invention also relates to an enzyme catalyzing a reaction in which maleic acid is hydrated into (R)-malic acid and a method for producing the enzyme. 
     BACKGROUND OF THE INVENTION 
     (R)-malic acid [D-(+)-malic acid] is an unnatural malic acid and is useful as a precursor for the synthesis of various chiral compounds such as an optically active isoserine, an optically active pantolactone, an optically active β-lactam intermediates and an optically active pheromones. 
     Although L-(-)-malic aid occurs naturally and now being produced from fumaric acid by enzymic method, no efficient process for (R)-malic acid production was developed until now. Chemical hydration of fumaric acid or maleic acid anhydride produce DL-malic acid (racemic malic acid). Several syntheses of (R)-malic acid or its derivatives have been known. Wynberg, Staring et al. [Wynberg, H., Staring, E. G. J., J. Amer. Chem. Soc., Vol. 104, pp. 166 to 168 (1982)] synthesized (R)-malic acid in 79% overall yield by application of asymmetric cycloaddition catalyzed by quinidine. Henrot et al. [Henrot, S., Larcheveque, M., Petit, Y., J. Chem. Soc., Perkin Trans. 1. Synth. Commun., Vol. 16, p. 183 (1986)]described the synthesis of (R)-malic acid from (R)-aspartic acid in three steps and 68% overall yield. Seebach et al. [Hungerbuhler, E., Seebach, D., Wasmuth, D., Helv. Chim. Acta., Vol. 64, p. 1467 (1981)] and also Alpegiani et al. [Alpegiani, M. and Hanessian, S., J. Org. Chem., Vol. 52, pp. 278 to 279 (1987)] described the synthesis of (R)-dimethyl malate from (R,R)-dimethyl tartarate. 
     (R)-malic acid was also prepared by resolution of DL-malic acid as described in German Patent Application (OLS) No. 2,933,895, JP-A-57-56439, JP-A-60-204741 (the term &#34;JP-A&#34; as used herein refers to a &#34;published unexamined Japanese patent application&#34;), EP-A-149,885 (the term &#34;EP-A&#34; as used herein refers to a &#34;published unexamined European patent application&#34;), Synthesis, p. 214 (1985), and U.S. Pat. No. 4,912,042. High performance liquid chromatography resolved DL-malic acid [Benecke, I , J. Chromatogr., Vol. 291, p. 155 (1984)]. Rom 67,279 (1979) [Chem. Abstr., Vol. 93, 238846a (1980)] described asymmetric synthesis using polarized electrets. 
     However, these methods are disadvantageous from an industrial viewpoint since the starting materials or resolving agents are expensive. The formation of (S)-malic acid by enzymic method is known as described in Kitahara, K., Fukui, S. and Misawa, M., J. Gen. Appl. Microbiol., Vol. 6, p. 108 (1960). (S)-malic acid was also produced from maleic acid via fumaric acid by a bacterium which has cis-trans-isomerase catalyzing the formation of fumaric acid from maleate [Otsuka, K., Agric. Biol. Chem., Vol. 25, p. 726 (1961)]. Sacks, W. and Jensen, C. O. [Sacks, W., Jensen, C. O., J. Biol. Chem., Vol. 192, p. 231 (1951)] obtained malic acid from maleic acid using a hydratase from corn kernels but enantiomorph of the malic acid formed was not specified. D-malate formation from maleic acid was noted with the extract of mammalian kidney [Taggart, J. V., Angielski, S., Morell, H., Biochem. Biophys. Acta., Vol. 185, p. 220 (1969)]. (R)-malic acid formation from maleic acid using cell-extract of a Pseudomonas strain isolated from soil is described in Rahatekar, H. L., Maskatl, F. S., Subramanian, S. S., Raghavendra Rao, M. R., Indian J. Biochem., Vol. 5, p. 143 (1968) and Hopper, D. J., Chapman, P. J., Sagney, S., Biochem. J., Vol. 110, p. 798 (1968). However, the properties of the enzyme has never been specified and no industrial method enabling the supply of (R)-malic acid has never been developed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome the above-described disadvantage of the prior art and provide a method for producing (R)-malic acid biochemically and efficiently. 
     Another object of the present invention is to provide a novel enzyme useful in the biochemical production of (R)-malic acid and a method for efficiently producing the enzyme. 
     In view of the usefulness of (R)-malic acid, intensive investigation has been made on a biochemical method for producing (R)-malic acid from maleic acid available inexpensively. Not only microorganisms from culture collection have been investigated but also newly isolated microorganisms have been investigated for an appropriate enzyme. As a result, a novel enzyme, microbial maleate hydratase, has been found in the present invention to be useful in the biochemical hydration of maleic acid to obtain (R)-malic acid and a method for producing this enzyme efficiently has been also found in the present invention. 
     Therefore, in one embodiment, the present invention provides a method for producing (R)-malic acid comprising hydrating maleic acid by contacting, in a reaction medium, maleic acid with (A) a microbial maleate hydratase capable of hydrating maleic acid to form (R)-malic acid or (B) a microorganism containing the maleate hydratase. In another embodiment, the present invention provides a method for producing microbial maleate hydratase by cultivating microorganism capable of producing the maleate hydratase in the presence of a certain organic substance. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The microorganisms which can be used in the present invention contain maleate hydratase which hydrates maleic acid to form (R)-malic acid. The enzyme used in the method of the present invention is an enzyme capable of hydrating maleic acid to produce (R)-malic acid and is named maleate hydratase. The enzyme is classified as class 4.2.1.31 according to the nomenclature of the international enzyme classification. 
     Microorganisms which produce an enzyme capable of converting maleic acid into (R)-malic acid can be selectively isolated based on the capability of producing (R)-malic acid from maleic acid and distributed ubiquitously in microorganisms including bacteria, fungi and yeast. Examples of such microorganisms include bacteria belonging to the genera, Arthrobacter, Brevibacterium, Corynebacterium, Bacillus, Acinetobacter, Pseudomonas, Microbacterium, Aeromonas, Escherichia, Alcaligenes, Proteus, Providencia, Paracoccus, Protaminobacter, Serratia, Xanthomonas, Amycoplatopsis, Streptomyces, Rhodococcus, Cellulomonas, Hafnia, Cytophaga, Flavobacterium, Klebsiella, Micrococcus, Ancylobacter, Morganella, Planococcus, Kluyvera, Kurthia, Achromobacter, and Citrobacter. Examples of the yeast to be used include Saccharomyces, Saccharomycopsis, Yarrowia, Candida, Debaryomyces, Hansenula, Kloeckera, Kluyveromyces, Lipomyces, Rhodotorula, Schizosaccharomyces, Torulopsis, Trichosporon, and Trigonopsis. Examples of the fungi to be used include the microorganisms of the genera, Aspergillus, Penicillium, Rhizopus, and Trichoderma. Particular examples of the microbial strains to be used are those shown in examples. These microbial strains are available from type culture collections (ATCC: American Type Culture Collection, Rockville, Md., U.S.A., IFO: Institute For Fermentation, Osaka, Japan). These microorganisms can be wild strain or mutants. Strains containing genes for maleate hydratase obtainable by DNA recombination technique can also be used in the present invention. 
     The microbial maleate hydratase can be those extracted from the above described microorganisms cells or the cultured broth. Immobilized enzymes and immobilized microorganisms containing the enzymes can also be employed as long as they exhibit maleate hydratase activity. 
     In order to obtain a culture containing a maleate hydratase activity by cultivating microorganism capable of producing this enzyme, usual cultivation methods can be used; namely cultivating in a nutrient medium containing organic compounds as a carbon source, organic and/or inorganic compounds as a nitrogen source, and mineral salts, at pH 4 to 10, at 10° to 40° C. It is preferred to add at least one compound selected from the group consisting of maleic acid and citraconic acid since a culture having high maleate hydratase activity can be obtained by the addition of such compound. The concentration of these compounds to be added in a medium is generally from about 0.1 to 3% by weight, preferably from about 0.5 to 2% by weight. The effect of these compounds is shown in Table 1 hereinbelow and Examples 1 and 2. From these data, it can be seen that the medium containing at least one such compound increases the yield of the enzyme remarkably. 
     
                       TABLE 1______________________________________Induction of maleate hydratase by citraconic acid andmaleic acid in Brevibacterium helvolum ATCC 11822Addition to      Growth at 68 Hours                    Specific   InductionBasal Medium*      O.D. (660 nm) Activity** Fold______________________________________None       1.5           0.65       1.0Citraconic 4.1           3.13       4.8Acid (1%)Maleic Acid      4.5           2.56       3.9(1%)______________________________________ *Basal Medium: NH.sub.4 Cl 0.1%, KH.sub.2 PO.sub.4 0.14%, Na.sub.2 HPO.sub.4.12H.sub.2 O 0.31%, MgSO.sub.4.7H.sub.2 O 0.025%, meat extract 5.0% (%: w/v). **Determined by the amount of (R)malic acid formed when reaction mixture (total volume 2,000 μl, pH 7.2) containing 50 mM TrisHCl buffer (1,700 μl), 100 mM maleate (200 μl), and crude enzyme preparation (cells sonicate supernatant, 100 μl) was reacted at 30° C. for 60 minutes. 
    
     Furthermore, both solid medium and liquid medium can be used in the present invention. Other conditions of cultivation of the enzyme producing microorganisms can be selected appropriately such that the strains can grow well according to the knowledge of those skilled in the art. 
     Although upon prolonged cultivation or addition of a releasing agent the enzyme is usually released from the cells and is obtainable by centrifugation of the broth, the malate hydratase contained in the cells can be released as a crude enzyme solution by destructing the cells by grinding or ultrasonication and extracting the enzyme therefrom. Of course, the cells as they are can be used as an enzyme preparation. 
     Purified maleate hydratase can be obtained from the crude enzyme solution by conventional enzyme purification methods such as an organic solvent fractionation method, an ammonium sulfate differential precipitation method, dialysis, isoelectric point precipitation method, and column chromatography alone or in combination. When a solid medium is used, water is added to the solid medium containing microbial cells, and the mixture as it is or after collecting the cells is subjected to the above-described ultrasonication or the like treatment to obtain a crude enzyme solution. 
     Of course, the maleic acid as a substrate can be used in the form of a physiolgically acceptable salt thereof, e.g., sodium salt or potassium salt, and the resulting (R)-malic acid can be also obtained in the form of salt thereof, e.g., sodium salt or potassium salt. 
     The microorganism cells having maleate hydratase activity or enzyme preparation derived therefrom thus obtained can be contacted with the substrate by adding the enzyme preparation in a solution containing the substrate and incubating the reaction mixture until the reaction proceeds or by adding the substrate in a culture broth of the microorganism followed by incubation for reaction. Alternatively, the enzyme can be contacted with the substrate in the form of enzyme preparations or cells separated from a culture broth of the microorganism of the present invention, physicochemically or biochemically treated cells such as washed cells, lyophilized cells and acetone-dried cells, extract solution, purified preparations, immobilized preparations, etc. 
     The concentration of the substrate varies depending on whether a batch system or a continuous system is used. In the batch system, it ranges generally from about 0.1 to 30%, preferably from about 0.5 to 10% by weight based on the weight of the reaction medium. In the continuous system, slightly lower ranges of the concentration, namely 0.05 to 20% by weight, is preferred. The reaction can be carried out usually at about 5° to 50° C., preferably at about 20° to 45° C., at a pH of about 4 to 10, preferably at a pH of about 6 to 9. The reaction time varies depending on the means of standing, stirring, flowing down through the column containing the immobilized enzyme, etc., or the form or activity of the enzyme but usually it ranges from about 1 to 100 hours. 
     The process of the reaction can be monitored by monitoring the generation of malic acid using thin layer chromatography or high performance liquid chromatography. The malate concentration in a reaction mixture can be determined also by colorimetrically after reaction with the mixture of sulfuric acid and 2,7-naphthalenediol according to the method of Goodban, A. E. and Stark, J. B., Anal. Chem., Vol. 29, p. 283 (1957). (R)-malic acid in the reaction mixture is monitored by the method of Krebs, H. A. and Egglestone, L. V., Biochem. J., Vol. 37, p. 334 (1943)] Based on these values (malic acid and (R)-malic acid), optical purity of the (R)-malic acid in the reaction mixture can be estimated. The value of the optical purity is not exactly correct but the method is conveniently applicable for tracing the reaction. Exact optical purity of the reaction mixture and isolated (R)-malic acid is determined by high performance liquid chromatography which enables the resolution of DL-malic acid into (R)-malic acid and (S)-malic acid. The conditions of the chromatography are as follows: column, MCI GEL CRS 10W (4.6×50 mm) (made by Mitsubishi Kasei, Japan); eluent, 0.5 mM CuSO 4  /10% (v/v) acetonitrile; velocity, 1.3 ml/min.; temperature, 25° to 26° C.; detection, at 258 nm. Specific rotation of the product is also determined for the determination of the optical purity of the isolated product. 
     Hereinafter, the enzymological characteristics of microbial maleate hydratase of the present invention will be explained. 
     (1) Action and specificity 
     The enzyme catalyzes a reaction in which maleic acid is hydrated to form (R)-malic acid. It does not act on fumaric acid. 
     (2) Optimum pH 
     The enzyme is active at pH 6.0 to 9.0 and most active at 7.0 to 8.0. 
     (3) pH stability 
     The enzyme is stable generally at pH 6.0 to 9.0 and particularly at pH 7.0 to 8.0. 
     (4) Optimum temperature 
     The enzyme acts well at 20° to 50° C. and its optimum temperature is about 40° C. 
     (5) Temperature stability 
     The enzyme is stable below 35° C. At 45° C. or more it inactivates rapidly. 
     (6) Molecular weight 
     The molecular weight of the enzyme estimated by the gel filtration method (using Sepharose 4B) is about 13.5×10 4 . Using Sephacryl S-200, the value is about 6×10 4 . 
     (7) Inhibitors 
     In a concentration of 1 mM, PCMB (p-chloromercuribenzoic acid) inhibits 100% activity, but EDTA and PMSF (phenylmethylsulfonylfluoride) shows no inhibition. IAA (iodoacetic acid) and NaIO 4  (sodium metaperiodate) inhibits 12% and 17%, respectively. NEM (N-ethylmaleimide) shows 33% inhibition at 10 mM. Thus, the enzyme seems to contain SH group in its active site. 
     For the recovery of (R)-malic acid from the reaction mixture, known method for (S)-malic acid can be applied. Thus, ion-exchange treatment, concentration and crystallization process are applied after removal of solids such as cells from the reaction mixture by centrifugation or filtration. 
     Hereinafter, the present invention will be described in greater detail with reference to examples which should by no means construed as limiting the present invention thereto. In the following examples all percentages are by weight unless otherwise indicated. 
     EXAMPLE 1 
     A large test tube (2.4×19.5 mm) containing 5 ml of a seed culture medium was sterilized and Arthrobacter globiformis IFO 12137 was inoculated and shake cultured at 26° C. for 24 hours. One and a half ml of the seed culture prepared as described above was inoculated into an Erlenmeyer flask containing 30 ml of a growth medium. The inoculated flask was shake cultured at 26° C., 220 r.p.m., for 24 hour. Cells were collected from 120 ml of the cultured broth by centrifugation and after washing 2 times with 50 mM phosphate buffer (pH 7.0), resuspended into 20 ml of 100 mM phosphate buffer (pH 7.0) containing 1.0% maleic acid, and 0.1% NaCl. The reaction mixture thus prepared was incubated in a large test tube with shaking (195 r.p.m.) at 26° C. 
     The composition of the seed medium was: glucose 1%, peptone 0.5%, meat extract 0.3%, yeast extract 0.3%, NaCl 0.25%, pH 7.0. The composition of the growth medium was: glucose 1%, NH 4  Cl 0.1%, KH 2  PO 4  0.14%, Na 2  HPO 4 .12H 2  O 0.31%, MgSO 0.025%, meat extract 0.5%, citraconic acid as indicated in Table 2, pH 7.0. 
     After incubation of the reaction mixture for 19.5 to 48 hours, malic acid was produced and its optical purity (expressed as enantiomer excess, e.e.%) was determined. As shown in Table 2, after 48 hour incubation 4.7 g/liter of (R)-malic acid (e.e.73%) was produced even with the cells grown in a medium containing no citraconic acid. The cells grown in a medium containing citraconic acid produced (R)-malic acid more rapidly. 
     
                       TABLE 2______________________________________Malic acid production (g/liter) and opticalpurity (e.e. %) of the produced (R)-malic acidIncubation Time      Citraconic Acid Added into Growth Medium(hours)    0%        0.5%     1.0%    2.0%______________________________________19.5       0.3 g/l   5.2 g/l  5.2 g/l 5.0 g/l                 (73)*   (71)    (81)24.0       0.3 g/l   7.2 g/l  6.4 g/l 3.9 g/l                (80)     (79)    (78)28.0       0.4 g/l   4.6 g/l  6.2 g/l 3.2 g/l                (79)     (81)    (61)48.0       4.7 g/l   0.5 g/l  0.2 g/l 3.0 g/l      (73)                       (58)______________________________________ *The values in parentheses are e.e. %. 
    
     EXAMPLE 2 
     The same procedures as in Examples 1 were repeated except that Pseudomonas fragi IFO 3458, Pseudomonas putida IFO 3738 and Arthrobacter oxydans IFO 12138 were used as the microorganism. (R)-malic acid was produced as shown in Table 3. The result shown in Table 3 also shows the effect of citraconic acid in the growth medium. 
     
                       TABLE 3______________________________________Malic acid production (g/liter) and opticalpurity (e.e. %) of the produced (R)-malic acid    Strain    IFO 3458  IFO 3738    IFO 12138Incubation Time      Citraconic Acid Added (g/liter)(hours)    0       10      0     10    0______________________________________19.5       0.2 g/l 2.3 g/l 0.2 g/l                            0.7 g/l                                  0.2 g/l              (100)*28         0.3 g/l 3.4 g/l 0.3 g/l                            0.3 g/l                                  2.2 g/l              (88)                (57)48         0.3 g/l 0.9 g/l 0.2 g/l                            0.2 g/l                                  0.4 g/l______________________________________ *The values in parentheses are e.e. %. 
    
     EXAMPLE 3 
     The same procedures as in Example 1 were repeated except that Brevibacterium helvolum ATCC 11822 was used as the microorganism and a medium supplemented with 1% citraconic acid was used as the growth medium. In this case, 4.4 g/liter of (R)-malic acid was produced after 48 hours incubation and the optical purity (e.e.) of the (R)-malic acid was 100%. After centrifugation of the reaction mixture, the clear supernatant was passed through a column of strongly acidic cation exchange resin (Dowex 50, H +  form) and then loaded on a column of strongly basic anion exchange resin (Dowex-1X8, formate form). The column was washed with water and then the acids were eluted stepwise with increasing strength of aqueous formic acid. Fractions of the effluent containing malic acid (eluted between 0.5 N and 1 N formic acid) was collected and concentrated to syrup. The syrup was cooled and crystals appeared was washed with small amount of cold acetonitrile and dried. The yield of the (R)-malic acid isolated was 2.5 g from 1 liter of the reaction mixture and its optical purity (e.e.) was 100%. 
     EXAMPLE 4 
     The same procedures as in Example 3 were repeated except that the microorganisms as shown in Table 4 were used. The concentration of the malic acid produced and the optical purity (e.e.) of the (R)-malic acid produced were shown in Table 4. 
     
                       TABLE 4______________________________________          Malic Acid                    Optical Purity          Produced  [(R)-Malic          (g/liter) acid, e.e. %]            24      48      24    48Microorganism    Hours   Hours   Hours Hours______________________________________Pseudomonas fluorescence 2.8           89IFO 3081Pseudomonas pseudoalcaligenes            3.2     4.8     83    81ATCC 12815Corynebacterium          2.3           93acetoacidophilum ATCC 13870Corynebacterium callunae 2.7           90ATCC 15991Corynebacterium glutamicum                    2.0           83ATCC 31808Corynebacterium vitarumen            5.9             85ATCC 10234Arthrobacter nicotianae            1.6             52ATCC 21279Arthrobacter citreus     1.2           100AICC 17775Arthrobacter ureafaciens 3.2           85ATCC 7562Bacillus alvei ATCC 6344 2.7           88Bacillus brevis ATCC 8185                    5.7           88Acinetobacter calcoaceticus                    4.6           81ATCC 14987______________________________________ 
    
     EXAMPLE 5 
     The same procedures as in Example 1 were repeated except that Arthrobacter globiformis IFO 12137 was used as microorganism and growth medium was supplemented with 0.5% citraconic acid and the reaction mixture was incubated statically. After reaction for 24 hours, 6.3 g/liter of (R)-malic acid (e.e. 99%) was produced, and after 48 hour reaction, 9.1 g/liter of (R)- malic acid (e.e. 91%) was produced. 
     EXAMPLE 6 
     Cell-free extract was prepared by sonic treatment of the cells of the microorganisms used in Example 3 and Example 4. Except that the cell-free extract as above prepared was used in place of the cells in reaction mixture, the same procedures as in Example 3 and Example 4 were repeated. About 1.0 g/liter of (R)-malic acid was produced with each microorganism. 
     EXAMPLE 7 
     Brevibacterium helvolum ATCC 11822 was shake cultured in a 300 ml Erlenmeyer flask containing 30 ml of a sterilized medium consisting of 3% glucose, 0.5% K 2  HPO 4 , 0.5% KH 2  PO 4 , 0.025% MgSO 4 .7H 2  O, 0.001% FeSO 4 .4H 2  O, 0.001% MnSO 4 .4H 2  O, 0.5% meat extract, 0.3% yeast extract, 0.5% peptone, 0.5% citraconic acid (pH 7.0) at 26° C. for 24 hours. Cells were collected from the culture by centrifugation and suspended into 30 ml of 4% maleic acid solution (pH of the solution was adjusted to 7.0 with NaOH). The reaction mixture thus prepared was incubated statically at 35° C. for 72 hours. (R)-malic acid was produced at a concentration of 4.44% in the reaction mixture and the optical purity (e.e.) of the (R)-malic acid was 98.0%. 
     EXAMPLE 8 
     Brevibacterium helvolum ATCC 11822 was shake cultured in a 300 ml Erlenmeyer flask containing 3u ml of a sterilized medium consisting of 2% glucose, 0.5% NH 4  Cl, 0.5% K 2  HPO 4 , 0.5% KH 2  PO 4 , 0.025% MgSO 4 .7H 2  O, 0.001% FeSO 4 .7H 2  O, 0.001% MnSO 4 .4H 2  O, 0.5% meat extract, 0.3% yeast extract, 0.5% peptone and the balance water (pH 7.0) at 26° C. for 24 hours. Cells were collected by centrifugation and treated with cold acetone. Thus prepared acetone-dried cells were suspended in 4% maleic acid solution (pH 7.0 with NaOH) to produce 30 ml of reaction mixture. The reaction mixture was incubated statically at 35° C. for 72 hours. (R)-malic acid was produced at a concentration of 3.03% in the reaction mixture and the optical purity (e.e.) of the (R)-malic acid was 87.1%. 
     EXAMPLE 9 
     Brevibacterium ketoglutamicum ATCC 15587 was shake cultured in a 300 ml Erlenmeyer flask containing 30 ml of a sterilized medium consisting of 1% glucose, 0.1% NH 4  Cl, 0.14% KH 2  PO 4 , 0.3% Na 2  HPO 4 .12H 2  O, 0.025% MgSO 4 .7H20, 0.5% meat extract, 1% citraconic acid and the balance water (pH 7.0) at 26° C. for 24 hours. Cells were collected by centrifugation and suspended into 5 ml of 100 mM phosphate buffer (pH 7.0) containing 1% maleic acid in a large test tube and shake cultured at 26° C. for 48 hours. (R)-malic acid was produced at a concentration of 1.09 g/liter in the reaction mixture and the optical purity (e.e.) of the (R)-malic acid was 85%. 
     EXAMPLE 10 
     The same procedures as in Example 5 were repeated except that Arthrobacter globiformis IFO 12137 was used as the microorganism. After 20 hour reaction, (R)-malic acid was produced at a concentration of 4.3 g/liter and the optical purity (e.e.) of the (R)-malic acid was 79.8%. The reaction mixture was centrifuged to obtain supernatant. The supernatant (255 ml) was adjusted to pH 4.5 and boiled 10 minutes at 100° C. After the boiling, the formed precipitate was removed and the clear supernatant obtained was passed through a column of strongly acidic cation exchange resin (Diaion SK-1B, 20 to 50 mesh, H +  form) 50 ml and then loaded on a column of strongly basic anion exchange resin (Dowex 1X8, 50 to 100 mesh, formate form) 50 ml. The latter column was washed twice with 100 ml distilled water and the malic acid was eluted with 1 N formic acid. The fraction containing malic acid was collected and concentrated in vaccuo to syrup. The crystal appeared after cooling the syrup was washed with small amount of cold acetonitrile and dried to obtain 0.276 g (R)-malic acid. The chemical purity and optical purity (e.e.(of the crystal was 92.2% and 98.5%, respectively. Mother liquor of the crystal and the acetonitrile-washed crystal solution were mixed and concentrated under reduced pressure, and 0.266 g of (R)-malic acid was obtained by repeating crystallization and washing. The chemical purity and optical purity of the second crystal were 89.3% and 100%, respectively. 
     EXAMPLE 11 
     Cells of Brevibacterium helvolum ATCC 11822 grown on a medium containing 2% citraconic acid, 5% meat extract, 0.1% NH 4  Cl, 0.14% KH , 0.31% Na 2  HPO 4 .12H 2  O, 0.025% MgSO 4 .7H 2  O (pH 7.0) was disrupted by sonication. Proteins precipitated with ammonium sulfate (43 to 60% saturation) was further purified by chromatography on Sepharose 4B gel. Thus, the activity of the maleate hydratase increased 16-fold compared with that of cell-free extract by the procedures described above. 
     EXAMPLE 12 
     Microorganisms shown in Table 5 were shake cultured in a 300 ml Erlenmeyer flask which contains a sterilized medium consisting of 1% glucose, 0.5% NH 4  Cl, 0.5% K 2  HPO 4 , 0.5% KH 2  PO 4 , 0.025% MgSO 4 .7H 2  O, 0.001% MnSO 4 .4H 2  O, 0.5% meat extract, 0.5% peptone, 0.3% yeast extract, 0.5% citraconic acid and the balance water (pH 7.0) at 26° C., 220 r.p.m. for 24 hours. Cells were collected from the culture by centrifugation and after 2-times washing with 50 mM phosphate buffer (pH 7.0) suspended into 5 ml of 50 mM phosphate buffer (pH 7.0) containing 1% maleic acid and 0.1% NaCl. The reaction mixture thus prepared was incubated statically at 26° C. for 72 hours. Malic acid concentration in the reaction mixture at 24 hour incubation, 48 hour incubation and 72 hour incubation were shown in Table 5. Optical purity of the malic acid produced as the percentage of (R)-malic acid was also shown in Table 5. 
     
                       TABLE 5______________________________________          Malic Acid Produced and          Optical Purity [% of (R)-form]Microorganism    24 Hours 48 Hours 72 Hours______________________________________Aeromonas punctata            2.2 g/l  7.0 g/l  8.2 g/lATCC 11163                (&gt;95%)   (&gt;95%)Aeromonas sp.    3.7 g/l  8.7 g/l  3.2 g/lATCC 21763       (&gt;95%)   (&gt;95%)   (&gt;95%)Alcaligenes faecalis            7.4 g/l  7.6 g/l  8.5 g/lIFO 3160         (&gt;95%)   (&gt;95%)   (&gt;95%)Escherichia coli 2.1 g/l  5.7 g/l  8.9 g/lATCC 4157                 (&gt;95%)   (&gt;95%)Escherichia coli 1.7 g/l  6.1 g/l  9.3 g/lATCC 10798                (&gt;95%)   (&gt;95%)Microbacterium ammoniaphilum            4.4 g/l  4.6 g/l  6.5 g/lATCC 15354                         (&gt;95%)Proteus mirabilis            4.9 g/l  7.9 g/l  8.7 g/lATCC 15290                (&gt;95%)   (&gt;95%)Providencia stuarti            3.7 g/l  7.8 g/l  9.1 g/lATCC 25825                (&gt;95%)   (&gt;95%)______________________________________ 
    
     EXAMPLE 13 
     Microorganisms shown in Table 6 were shake cultured in a 300 ml Erlenmeyer flask which contains a sterilized medium at 26° C., 220 r.p.m. for 24 hours (bacteria) or 48 hours (yeast and fungi). Cells were collected from the culture by centrifugation and after 2-times washing with 50 mM phosphate buffer (pH 7.0) suspended into 5 ml of 50 mM phosphate buffer (ph 7.0) containing 1% maleic acid and 0.1% NaCl. The reaction mixture thus prepared was incubated statically at 26° C. for 96 hours. 
     The composition of the medium for bacteria consisted of 1% glucose, 0.5% NH 4  Cl, 0.5% K 2  HPO 4 , 0.5% KH 2  PO 4 , 0.025% MgSO 4 .7H 2  O, 0.001% MnSO 4 .4H 2  O, 0.5% meat extract, 0.5% citraconic acid and the balance water (pH 7.0). For the growth of yeast, yeast extract (0.2%) was supplemented to the above-described medium for bacteria and the concentration of meat extract in the medium was changed to 0.3%. The pH of the medium for yeast was adjusted to 6.0. For the growth of fungi, yeast extract (0.2%) and meat extract (0.2%) were supplemented to the medium for bacteria and the concentration of meat extract in the medium was changed to 0.1%. The pH of the medium for fungi was adjusted to pH 5.5. 
     Malic acid concentration in the reaction mixture at 24 hour incubation, 48 hour incubation and 96 hour incubation were shown in Table 6. Optical purity of the malic acid produced as the percentage of (R)-malic acid was also shown in Table 6. 
     
                       TABLE 6______________________________________          Malic Acid Produced and          Optical Purity [% of (R)-form]Microorganism    24 Hours 48 Hours  96 Hours______________________________________Paracoccus denitrificans            6.0 g/l  9.4 g/l   8.7 g/lATCC 19367                          (99.9%)Protaminobacter ruber            0.5 g/l  0.8 g/l   1.4 g/lIFO 3708                            (99.9%)Serratia rubidae 2.3 g/l  6.0 g/l   7.0 g/lATCC 11634                          (99.8%)Xanthomonas translucens            6.0 g/l  9.5 g/l   7.7 g/lIFO 13558                           (99.9%)Amycoplatopsis orientalis            6.2 g/l  7.1 g/l   7.2 g/lATCC 19795                          (99.9%)Streptomyces coelicolor            4.3 g/l  9.6 g/l   10.4 g/lATCC 10147                          (99.9%)Rhodococcus erythropolis            4.4 g/l  6.5 g/l   9.4 g/lIFO 12320                           (99.9%)Cellulomonas cellasea            4.7 g/l  7.7 g/l   10.3 g/lATCC 487                            (99.9%)Hafnia alvei ATCC 9760            3.8 g/l  9.4 g/l   7.6 g/l                               (99.9%)Cytophaga sp. ATCC 9760            0.4 g/l  0.6 g/l   1.3 g/l                               (99.9%)Flavobacterium aquatile            2.3 g/l  4.0 g/l   5.7 g/lATCC 8375                           (99.8%)Klebsiella pneumoniae            2.5 g/l  4.1 g/l   6.0 g/lATCC 8308                           (99.9%)Micrococcus aurantiacus            1.4 g/l  2.5 g/l   4.4 g/lATCC 11731                          (99.6%)Ancylobacter sp. 1.7 g/l  3.8 g/l   6.2 g/lATCC 21373                          (99.6%)Morganella morganii            0.3 g/l  0.6 g/l   1.3 g/lATCC 25830                          (99.9%)Planococcus citreus            0.7 g/l  1.2 g/l   1.6 g/lATCC 14404                          (99.9%)Kluyvera cryocrescens            2.1 g/l  4.9 g/l   7.8 g/lATCC 14238                          (100.0%)Kurthia zopfii   0.3 g/l  0.5 g/l   1.3 g/lATCC l0538                          (99.9%)Achromobacter cycloclastes          10.4 g/lATCC 2I921                          (99.9%)Citrobacter freundii                6.6 g/lATCC 6750                           (99.8%)Saccharomyces cerevisiae            3.7 g/lATCC 18824                          (99.9%)Saccharomycopsis lipolytica         0.88 g/lIFO 0746                            (98.6%)Yarrowia lipolytica                 1.1 g/lATCC 16617                          (&gt;99.9%)Candida utilis ATCC 9950            0.43 g/l                               (81.8%)Candida utilis IFO 1086             0.40 g/l                               (79.0%)Debaryomyces polymorphus            0.71 g/lIFO 1189                            (95.5%)Hansenula subpelliculosa            0.67 g/lIFO 0808                            (97.0%)Kloeckera apiculata IFO 0175        1.95 g/l                               (98.6%)Hansenula wickerhamii               0.71 g/lATCC 16767                          (96.5%)Kloeckera javanica IFO 1094         3.83 g/l                               (&gt;99.9%)Kluyveromyces wickerhamii           0.32 g/lIFO 1675                            (75.5%)Lipomyces starkeyi                  0.87 g/lATCC 20147                          (96.0%)Rhodotorula glutinis                3.64 g/lATCC 20147                          (99.9%)Schizosaccharomyces pombe           0.68 g/lATCC 2476                           (93.4%)Torulopsis pinus ATCC 22996         4.31 g/l                               (99.6%)Torulopsis spherica                 0.26 g/lATCC 8549                           (80.3%)Trichosporon cutaneum               0.54 g/lIFO 0173                            (87.3%)Trigonopsis variabilis              0.52 g/lIFO 0755                            (93.4%)Aspergillus niger ATCC 6275         8.11 g/l                               (99.9%)Aspergillus oryzae var.             5.85 g/lviridis ATCC 22788                  (98.5%)Penicillium chrysogenum             5.98 g/lATCC 9480                           (99.1%)Penicillium citrinum                4.15 g/lATCC 14994                          (91.4%)Rhizopus chinensis var.             1.96 g/lliquefaciens IFO 4737               (96.7%)Trichoderma reesei                  5.03 g/lATCC 13631                          (98.8%)Trichoderma longibrachiatum         5.02 g/lIFO 4847                            (99.3%)______________________________________ 
    
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.