Novel hexokinase

Abstract: The present invention relates to a hexokinase of excellent stability in solution whether in the presence or the absence of glucose, and the present enzyme is a novel substance derived from the cultured of thermophilic yeast, Kluyveromyces , and it can be advantageously used for determination of ATP and glucose, and for determination of a vital component which produces ATP or glucose (e.g. creatine kinase).

Description: Field of the invention The present invention relates to a novel hexokinase of excellent stability in solution, its preparation and its use. Prior art A hexokinase (EC 2.7.1.1; hereinafter sometimes referred to as HK) is an enzyme catalyzing the reaction that a hexose such as glucose is phosphoesterified into a hexose-6-phosphate ("Microbiological Dictionary", p978, August 23, 1989, published by Gihodo). However, the well-known hexokinase at present from any origin is extremely unstable in solution, for instance, when it is stored at 37° C, the residual rate falls to approximately 50% within only 5 days in a buffer of a glucose determination reagent. Problems to be solved by the invention Since a hexokinase of the prior art is extremely unstable in solution, it cannot be stored for a long term, therefore it must be stored in a refrigerator below 4° C or prepared in use. That is, the hexokinase in solution cannot be stored at room temperature, therefore a determination reagent or a kit for determination using it cannot be prepared freely. Means for solving the problems The present invention was made in order to meet the present requirements to the conventional technique described above, that is, in order to provide a HK which can be used with previously dissolved in a buffer etc. by the development of a novel HK which can be stored from room temperature to high temperatures without lowering its activity for a long term even in solution to perform determination easily and rapidly, and in order to reduce the cost by extending a shelf life. As a result of intensive study from every point of view, it has been successful in collecting a hexokinase of excellent stability in solution from a bacterial body of thermophilic yeast belonging to Kluyveromyces, and it has been confirmed that the obtained hexokinase was a novel enzyme which has been previously unknown at that time to achieve the present invention by further study. The present invention as defined by the claims will be described in detail below. The HK according to the present invention has not only physicochemical properties described below, but stability and reactivity especially as hereunder, therefore it is a novel and excellent one. 1) The HK exhibits excellent stability in storage tests in solution at 37° C whether in the presence or the absence of glucose. 2) The HK has high reactivity to glucose, therefore the amount of glucose can be rapidly determined even at low temperatures by a small enzyme amount of it. In order to prepare the HK according to the present invention, it is needed first to breed and select thermophilic yeast which is able to grow at 35° C or above, preferably 40° C or above. For the present invention Kluyveromyces fragilis IFO 1777 is used. Following the description in Applied Biochemistry and Biotechnology Vol.39/40. pp201-211 by I.Ballesteros et al. (1993) etc., these type culture strains are cultured at high temperatures about 45° C to obtain colonies, the desired strain is selected from the colonies and cultured, and the hexokinase is extracted from the culture in the conventional method. For instance, the bacterial body is collected, crushed, treated with phosphate buffer, and centrifugalized to obtain the supernatant. As the HK extract, the supernatant is purified by a well-known method of purification such as chromatography, treatment with ammonium sulfate, and ion exchange. BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 shows residual rates of the HK of this invention in a buffer of a CPK activity determining reagent. Figure 2 shows residual rates of the HK in a buffer of a glucose determining reagent. Figure 3 shows enzyme following performance of the HK in determination of glucose. Figure 4 shows enzyme following performance of the HK in determination of creatine kinase activity. Figure 5 shows relationship between dilution linearity of the sample and enzymatic amounts of the HK in determination of creatine kinase activity. PREPARATION EXAMPLE: Preparation of Hexokinase A strain of Kluyveromyces fragilis IFO 1777 was cultured, and from the obtained bacterial body of the thermophilic yeast a hexokinase was extracted and purified as follows. (1) Culture of Thermophilic Yeast, Kluyveromyces Strain A culture medium comprising the yeast extract (0.3%), malt extract (0.3%), peptone (0.5%), glucose (1%), and agar (1.5%) was adjusted to pH 6.0 and sterilized in an autoclave, and a suspension of the type culture bacterial body of Kluyveromyces was smeared on the agar medium. Then, the medium was allowed to stand at 45° C for one night to perform colony formation. And then single colonies on the agar medium were picked up with a loop, suspended in the liquid medium described above without containing agar (10 mL) in a test tube, and cultured with shaking at 45° C for 1 night. The results of determination of hexokinase (HK) content by the use of the varieties of type culture strains belonging to Kluyveromyces will be summarized in Table 1 below. Strain activity Result of culture Amount of HK (at 37° C) pH of medium OD 600 IFO10005 5.28 1.27 0.34U IFO 1777 4.62 5.35 5.80 IFO 1735 4.67 0.25 0.25 Preparation of a extract solution from the bacterial body was performed in the following manner. That is, the bacterial body obtained by the culture with shaking in the test tube having 10 mL of the medium was collected by centrifugation, suspended in 5 mL of 20 mM K-PO 4 buffer (containing 1 mM 2ME, 0.5 mM EDTA, 0.02% NaN 3 , and 0.1 mM PMSP; pH 7.5), and treated with an ultrasonic crusher (UD-200; trade name, Tomy Co.) with ice-water cooling for 5 minutes. The solution of crushed bacterial body obtained above was centrifugalized, and the supernatant is separated as the HK extract solution. Determination of the HK enzymatic activity was performed in the following conditions. Temperature: 37° C; Instrument: Hitachi U-2,000 spectrophotometer; Wavelength: 340 nm; Composition of the substrate reagent: 0.1 M TEA-HCl (pH 7.5), 50 mg/mL of glucose, 5 mM MgCl 2 , 1 mM ATP, 1 mM NAD + , and 2 U/mL of G6FD (LM) As concerns the enzymatic activity, 1 unit means the quantity producing 1 µmole of NADH per a minute under the reaction conditions given above. In separating and purifying hexokinase from the bacterial body of the thermophilic yeast, Kluyveromyces fragilis (IFO1777) was selected which has the highest content of the enzyme per test tube cultured among the three type culture strains described above. Needless to say, strains belonging to Kluyveromyces are not restricted to this type culture strain. Kluyveromyces fragilis (IFO1777) was deposited under Budapest Treaty in National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry of 1-3, Higashi 1 chome, Tsukuba-shi, Ibaraki-ken, 305, Japan on February 6, 1997 and assigned deposted No. FERM BP-5812. (2) Purification of Hexokinase In obtaining HK from the bacterial body of a thermophilic yeast, Kluyveromyces , IFO 1777 strain having high enzymatic content per a cultured bacterial body was selected based on the results of the culture with shaking in test tubes mentioned above, and culture with shaking of 3.5 L in flasks was performed using the incubated in test tubes as the seed of culture. After culturing at 45° C for 9 hours, 30 g wet of the bacterial body of the thermophilic yeast was collected from the cultured suspension by centrifugation. The obtained bacterial body was suspended in 5 times amount of the extract buffer described above, and content of the HK activity per a bacterial body was determined by ultrasonic crushing treatment to give 59 U/g wet bacterial body. Separation and purification of HK were performed using the bacterial body extract solution prepared by the ultrasonic crushing treatment as a crude enzyme solution, by ion-exchange chromatography using DEAE-cephalose CL6B gel, the chromatography again, and gel filtration chromatography of HPLC with TSK gel G3,000 SW column. The sample finally obtained had the specific activity of 169 U/mg protein (calculated on E 280 nm (concentration 1%) =10.0), and the yield based on 100% of the total activity of the bacterial body extract solution which was the starting material in the enzyme purification was 18.2%. (3) Property of Hexokinase As the result of the investigation of enzymological properties of a HK derived from the bacterial body of Kluyveromyces (IFOl777) based on the HK sample obtained here, the HK had the properties given below and was identified as a novel enzyme that has been unknown so far. The obtained hexokinase exhibits the physicochemical properties below. (A) Stability: The hexokinase is stably stored in solution at 37° C whether in the presence or the absence of glucose. The stability of it provides at least 80% of the residual activity after 7 days at 37° C. (B) Reactivity: The hexokinase has high reactivity to glucose, therefore the amount of glucose can be rapidly determined even at low temperatures by a small enzymatic amount of it. (C) Substrate specificity: The hexokinase has high reactivity especially to glucose and fructose. The degrees of reactivity based on 100% to glucose were as follows (the substrate concentration of varieties of monosaccharide was 0.1 M in the determination of the substrate specificity). glucose 100% fructose 189.4% mannose 68.1% galactose 62.2% xylose 24.3% sorbitol 6.8% mannitol 0.1% (D) Molecular weight: 60,000 (gel filtration chromatography) (E) Isoelectric point: PI=4.70 (chromatofocusing) The hexokinase according to the present invention includes all the enzyme below; an enzyme preparation prepared by purification of the strain cultured mentioned above, a recombinase prepared by transduction of a hexokinase gene derived form thermophilic yeast into a host such as E.coli, and a modified enzyme prepared by protein cross-linking reaction etc. The novel hexokinase of excellent stability in solution according to the present invention has the excellent properties described above, therefore the hexokinase (HK) can be used to advantage in the place of the HK that has been well-known so far in a variety of conventional methods of determining glucose and/or ATP using a hexonase such as a determination method of glucose using hexokinase with linking glucose-6-phosphate dehydrogenase (G6PD) (HK-G6PD method) etc., and the present invention can advantageously provide an excellent novel determination reagent and a determination kit using it. As described above, the novel HK according to the present invention can be used to advantage for determination of ATP and/or glucose accurately, rapidly, and inexpensively without skill, therefore it can be used naturally for a system of direct determination of ATP and/or glucose, and also used to advantage for a system of determination of vital components which produce ATP or glucose, and the present invention can advantageously provide a method of determining said vital components, a determination reagent, and a determination kit characterized by using the HK. An example of a process for the determination of vital components which produce ATP or glucose is that of creatine kinase activity described below: "A process for the determination of creatine kinase activity comprising bringing a sample into contact with a reaction system containing at least creatine phosphate, ADP, hexokinase or glucokinase, glucose, glucose-6-phosphate dehydrogenase, and thionicotinamide adenine dinucleotide (hereinafter referred to as thio-NAD) or thionicotinamide adenine dinucleotide phosphate (hereinafter referred to as thio-NADP), and detecting a detectable change." As said vital components, in addition to creatine kinase described above, all vital components which produce ATP or glucose in the reaction system can be applied to the present invention. Hereinafter, description is made of examples according to the present invention. Example 1: Storage Stability of Hexokinase in Solution The storage stability of a hexokinase derived from thermophilic yeast, Kluyveromyces , in a variety of test reagents was determined under the conditions given below. In addition, a variety of hexokinase which were used as the control reagents and their origin will be described. HK Enzyme Reagent: 1) Hexokinase derived from Kluyveromyces (this invention) 2) Hexokinase derived from Saccharomyces cerevisiae, PI (Oriental Yeast Co., Ltd.) 3) Hexokinase derived from Saccharomyces cerevisiae , PII (Boehringer) 4) Hexokinase derived from Bacillus (Asahi Chemical Industry Co., Ltd.) Conditions: (1) Temperature in storage: 4° C and 37° C (2) Days in storage: sampling after 0, 5, 10, and 14 days (3) Initial amount of HK: 0.5 U/ml (determination at 37° C) (4) Buffers for storage stability tests: (a) Buffer for CPK activity determination reagent 100 mM imidazole acetic acid buffer (pH 6.7) containing 2 mM EDTA, 10 mM magnesium acetate, and 25 mM N-acetylcysteine (b) Buffer for glucose determination reagent 100 mM tris hydrochloric acid buffer (pH 7.5) containing 5 mM magnesium acetate In addition, 0.02% sodium azide was added as an antiseptic to the solution for storage tests at 4° C and 37° C given above for investigation of storage stability in solution. (5) Calculation of the degree of storage stability in solution: The proportion of the residual activity to the initial HK activity was plotted against the storage days to estimate half-life of the varieties of HK based on the rate of residual activity. The results are shown in Figure 1 (buffer (a)) and Figure 2 (buffer (b)), respectively. Example 2: Enzyme Following Performance in Glucose Determination In order to confirm the enzyme following performance of a hexokinase derived from thermophilic yeast ( Kluyveromyces ) in glucose determination, a reagent for glucose determination was prepared on the basis of the method of determining glucose and the composition of the reaction reagent in glucose determination described in Clinical Chemistry Vol.20 No.4, p247-254 (1991). Conditions: (1) Sample and blank The samples were a standard glucose solution of 200 mg/dL (Denka Seiken Co.) and the diluted solution of it to 60 mg/dL with 0.9% aqueous solution of NaCl. As the blank 0.9% aqueous solution of NaCl was used. (2) Conditions in determination Instrument: Cobasfara (trade name, Baxter Co.); Temperature: 37°C; Wavelength: 340 nm (3) Ratio of the sample and the reagents Sample: First reagent: Second reagent = 4:400:100 (4) Composition of the reagents First reagent Second reagent Concentration of reaction solution ATP --- 6 mM 1.2 mM NADP+ 1.5 mM --- 1.2 mM G6PD 2.5 U/ML --- 2 U/mL Both the First reagent and Second reagent described above were dissolved with the solution containing 100 mM tris hydrochloric acid buffer (pH 7.5) and 5 mM magnesium acetate. (5) Determination of HK following performance(*11) in glucose determination HK following performance was determined as the relationship between the apparent glucose value after addition of HK of a variety of concentration into the First reagent described above and the HK amount added into the First reagent. (6) Calculation of apparent glucose value based on ΔA 340 variation ΔA 340 was obtained by subtracting the initial absorption at mixing the First reagent and the sample from A 340 at 3 minutes after starting the reaction by addition of the Second reagent, and the apparent glucose concentration of the sample was calculated. The results are shown in Figure 3. Example 3: Enzyme following performance in determination of creatine kinase activity The enzyme following performance of a hexokinase derived from thermophilic yeast (Kluyveromyces ) in determination of creatine kinase activity was confirmed as follows. That is, to the composition of the reagent prepared for determination of CK (creatine kinase) activity, the composition of the final reaction solution described in Clinical Chemistry Vol.19 No.2, p185-208 (1990) was applied. The conditions were as follows. Conditions : (1) Sample and blank Control serum on the market (Consera and Consera A, Nissui Seiyaku Co.) was used. As the blank, 0.9% aqueous solution of NaCl was used. (2) Conditions in determination Instrument: Cobasfara; Temperature: 37° C (3) Ratio of the sample and the reagents Sample: Purified water: First reagent: Second reagent = 6:4:240:60 (µL) (4) Calculation of CK activity ΔA 340 /min was obtained by the rate assay for 1.5 minute ranging from 2.5 to 4 minutes after mixing the sample and the determination reagents. (5) Relationship between CK activity and HK following performance The apparent value of CK activity obtained from the time course from the beginning of the reaction by addition of HK of a variety of concentration into the First reagent was plotted against the amount of HK added into the First reagent. The composition of the reagent used above will be shown in Table 2 given below. Provided that the buffer dissolving the CK activity determination reagent contained 100 mM imidazole acetic acid buffer (pH 6.7), 2 mM EDTA, 10 mM magnesium acetate, and 20 mM glucose. 1st reagent 2nd reagent Final concentration ADP 2.5 mM --- 2 mM NAC 25 mM --- 20 mM AMP 6.25 mM --- 5 mM AP5A 12.5 µM --- 10 µM NADP+ 2.5 mM --- 2 mM G6PD --- 15 U/mL 3 U/mL CP --- 150 mM 30 mM NAC: N-acetylcysteine, CP: creatine phosphate, The results are shown in Figure 4. In order to investigate the range which is linearly measurable in determination of creatine kinase activity, the relationship between the dilution linearity of the sample in determination of creatine kinase and the HK amount added into the First reagent was determined under the conditions given below, and the results are shown in Figure 5. Conditions : (1) Sample The sample was 3,000 U/L of creatine kinase derived from skeletal muscle of rabbit (determination at 25° C) diluted to 6 grades. (2) Creatine kinase dilution solution Creatine kinase was diluted with 50 mM tris-HCl buffer (pH 8.5) containing 1% BSA, 10 mM NAC, and 0.05% NaN 3 (in each example, the recommended method described in the transactions by Japanese Society of Clinical Chemistry was applied to the buffer for the storage tests and the varieties of the determination reagents used for investigation of the properties of the hexokinase). Effect of the invention A novel hexokinase according to the present invention has high stability in solution, consequently it exerts a remarkable effect that the activity does not fall even in solution at high temperatures. Therefore, the present enzyme can be stored for a long term even in a buffer and is quite advantageous to preparing a determination reagent and a kit for it, furthermore, it can accurately measure and extends shelf life of the merchandise to highly exert an economical effect.

Claims: A purified hexokinase obtainable by isolation from the thermophilic yeast Kluyveromyces fragilis IFO 1777, wherein said hexokinase retains at least 80% of its initial enzymatic activity in creatine phosphokinase activity determination in a 100 mM imidazole acetic acid buffer (pH 6.7) containing 2 mM EDTA, 10 mM magnesium acetate and 25 mM N-acetylcysteine at 37ºC for 7 days, and said hexokinase has a greater substrate specificity for glucose and fructose compared to mannose, galactose, xylose, sorbitol and mannitol. The hexokinase of claim 1, wherein said hexokinase has a molecular weight of about 60 kilodaltons based on gel filtration. The hexokinase of claim 1, wherein said hexokinase has an isoelectric point at a pH value of 4.70. A method of preparing the hexokinase of claim 1, comprising the steps of: culturing the thermophilic yeast Kluyveromyces fragilis IFO 1777, to produce said hexokinase; and collecting said hexokinase from said culture.

Inventors: FUJITA TUYOSI, MATSUKAWA HIROKAZU, FUJITA, TUYOSI, MATSUKAWA, HIROKAZU
Applicants: ORIENTAL YEAST CO LTD, ORIENTAL YEAST CO., LTD.
Application year: 1997
Main classification: C12N9
IPC codes: C12N1, C12Q1, C12N9, C12R1