GB2161159A - Improved transamination process for producing L-amino acids - Google Patents

Abstract

In a process for the transamination of an alpha keto acid to the corresponding L-amino acid using aspartic acid as the amino donor, multivalent metal ions, or salts thereof, are included in the system, to catalyze the decomposition of oxalacetic acid, a by-product of the transamination.

Description

SPECIFICATION Improved transamination process for producing amino acids This invention relates generally to an improved process for the production of L-amino acids (hereinafter "amino acids") from their a-keto acid precursors by biological transamination using aspartic acid as the amino donor. More specifically, the process disclosed herein is designed to improve the rate and yield of the transamination by increasing the rate of decomposition of oxalacetic acid, one of the reaction by-products. This results in a significant increase in the reaction rate such that substantially all of the precursor is consumed in the transamination. By eliminating the equilibrium constraint on the transamination and by allowing more precursor to be converted, amino acid yields of over 90 percent can be achieved.The reaction is catalyzed in this manner by the addition of metal ions to the transamination reaction system.

It is known that amino acid precursors may be converted enzymaticaily to their corresponding L-amino acids. For example, U.S. 3,183,170 (Kitai et al.) discloses the transamination of phenylpyruvic acid in the presence of a multi enzyme system obtained from various sources, including bacterial cells,dried cells, cell macerates or enzyme solutions. U.S. Serial No. 520,632 (Fusee), filed on August 5, 1983, discloses the microbial transamination of a-keto acids to amino acids in fed-batch fermentations using conventional precursors to convert, for example, phenylpyruvic acid to L-phenylalanine, a-ketoisocaproic acid to L-leucine, a-ketoisovaleric acid to L-valine, etc.

The a-keto acid to amino acid biotransformation is an enzymatic transamination which results in the exchange of the amino group of the amino donor and the keto group of the precursor. It generally has been recognized that enzymatic transamination is an equilibrium reaction. For example, Oishi, in Ch. 16 of The Microbial Production of Amino Acids, (Yamada et al., Ed.), "Production from Precursor Keto Acids," pp. 440-46 (1972), notes that the use of aminotransferases has necessitated a high concentration of the amino donor for a high product yield. U.S.

3,183,170 (Kitai et a.) reports that in a transamination reaction using L-glutamic acid as the amino donor, the equilibrium is shifted favorably to the right by converting the akpha-keto glutaric acid resulting from the transamination back to L-glutamic acid by reductive amination as fast as the a-keto glutaric acid is formed.

In conventional transamination processes, a number of compounds have been used as the amino donor. Oishi, at pp. 435-52 of the Yamada et al. text, states that the best amino donors are L-aspartic acid, L-leucine, L-isoleucine and L-glutamic acid and that better results are obtained when these amino acids are used in combination than when they are used singly. U.S.

Serial No. 568,300 (Walter), filed on January 5, 1984, discloses a process for driving a microbial transamination reaction towards completion by using a solution comprising approximately equimolar amounts of aspartic acid and phenylalanine precursor, pre-growing the microorganisms in the presence of the precursor, employing the biological catalyst in the form of dried cells, and/or increasing the biological catalyst loading of the system.

Oxalacetic acid (also known as oxaloacetic acid, oxosuccinic acid or keto succinic acid) is a byproduct of enzymatic transamination when aspartic acid is used as the amino donor. Oxalacetic acid has been studied in other contexts and has been found to decompose by various mechanisms. Bessman, Preparation and Assay of Oxalacetic Acid," Arch. Biochem., Vol. 26, pp. 418-21 (1950), reports spontaneous decomposition of oxalacetic acid, which is catalyzed by a number of substances. Krebs, "The Effect of Inorganic Salts on the Ketone Decomposition of Oxaloacetic Acid," J. Biochem., Vol. 36, pp. 303-05 (1942), reports that various inorganic salts increase the rate of decomposition of oxaloacetic acid into pyruvic acid and carbon dioxide.

It has been found that, under conditions as disclosed herein, the transamination of an amino acid precursor to its corresponding amino acid, where aspartic acid is selected as the primary amino donor and the precursor is an a-keto acid, can be significantly improved by catalyzing the decomposition of the oxalacetic acid byproduct of the transamination. The decomposition is catalyzed by the addition of certain multivalent metal ions or salts thereof to the reaction system.

The L-amino acid may be accumulated and collected. By decomposing oxalacetic acid in this manner, the equilibrium of the system is shifted dramatically to the right, driving the reaction substantially to completion. The result of this metal ion catalysis is to boost transamination rates to greater than about 7.0 moles per liter-hour and amino acid yields to about 90 to 100%.

One of the purposes of this invention is to improve a biological transamination reaction by increasing the reaction rate and the overall yield of amino acid based on both aspartic acid and precursor. It is, therefore, possible to decrease the proportional quantities of precursor needed to achieve the desired product yield.

By substantially increasing the rate of transamination, the time required for the system to convert a given quantity of precursor is decreased. Moreover, the increased reaction rate substantially decreases loss of product yield caused by decomposition of the precursor. In addition, this leads to decreased contamination with unreacted precursor, making the product purification and recovery less burdensome.

In the reaction system of the invention the a-keto acid precursor does not need to be pure and any inhibitory effect of the impurities which may be present is reduced or overcome.

An overall effect of the invention is to reduce significantly the costs associated with the production of amino acids by transamination.

DETAILED DESCRIPTION OF THE INVENTION The invention disclosed herein provides a unique means for driving the transamination of an amino acid precursor to its corresponding L-amino acid, using aspartic acid as the amino donor and an appropriate a-keto acid precursor, selecting a suitable intra- or extracellular transaminase, and coupling the transamination reaction with the catalyzed decomposition of the oxalacetic acid transamination by-product under conditions favorable for transaminase activity. The catalyzed decomposition is effected by the addition of multivalent metal ions, or salts thereof, to the reaction system and allowing the transamination to proceed significantly beyond equilibrium.

The transamination may be accomplished by contacting a solution comprising the a-keto acid precursor and aspartic acid with the intra- or extracellular transaminase, that is, with a cell or enzyme preparation capable of mediating the transamination. The term "transaminase" as used herein refers to an enzyme or enzyme system capable of mediating the precursor-to-amino-acid conversion just described.

Microorganisms which already have been found useful in this type of transamination process are Pseudomonas pseudoalcaligenes, Brevibacterium thiogenitalis, Pseudomonas aeriginosa, Bacillus subtillus and Escherichia coli, each of which mediate the transamination of phenylalanine precursor to L-phenylalanine. In addition, the latter four species of bacteria have been demonstrated to have activity for the production of other amino acids (L-tyrosine (Example IV), Lleucine, L-isoleucine and L-valine were demonstrated) by transamination with aspartic acid as the donor. The group of microorganisms which will be useful in this invention will, of course, be much broader. Organisms belonging to the genus Aerobacter, halophilic organisms, and yeasts such as those belonging to the genus Candida have been found to be suitable.It is expected that any microorganism which is known to demonstrate transaminase activity for the production of amino acids and which can use aspartic acid as the amino donor will be useful in this method. In addition, it is contemplated that mixed cultures of suitable strains may be used.

The growth conditions should be selected on the basis of the particular microorganism used and will be within the knowledge and skill of a person working in this area. It is preferred that the conditions favor the rapid growth of healthy cells. For example, if Pseudomonas pseudoalcaligenes is used, temperature preferably is maintained at about 35 to about 39"C and pH at about 7.5. Agitation and/or aeration is used in order to provide an aerobic environment. Suitable energy and nutient sources should be provided.

The microorganisms are grown to some predetermined cell density or growth phase. The precise growth period and cell density is not critical because density may be increased after growth, if desired, by conventional means such as centrifugation or filtration. A typical growth period of about 1 2 to about 48 hours usually will provide a workable number of cells. The cells then are harvested and may be treated to permeabilize the cell membrane. This treatment may be by drying, sonicating, incubating with toluene or nonionic surfactants, etc. It may also be desired to immobilize the cells on a suitable substrate. If viable microorganisms are used in the method of this invention, it is preferred that they be capable of excreting the amino acid product into the medium for convenient and economical recovery.In cell preparations as described above, sufficient quantities of transaminase will be present in association with the cells or cellular material to carry out the reaction.

At least about 2.0 to about 200.0 grams of cells (dry weight) per liter of substrate solution, preferably at least about 4.0 to about 80.0 grams per liter. Regardless of the form in which the cells actually used in the process, the indications of cell catalyst weights used herein are on a dry basis. Lower catalyst levels will allow significant amounts of the precursor to decompose before undergoing transamination. The extent of the precursor decomposition problem in conventional transaminations will vary, depending on the a-keto acid precursor and on the reaction conditions. For example, decomposition will be accelerated somewhat by the presence of multivalent metal salts, as well as by acidic pH conditions, ultraviolet light or the presence of oxygen.

Alternatively, a cell-free system can be used. The transamination is enzymatic and may proceed in a solution comprising the appropriate enzymes or transminases. Therefore, it will be possible to use an enzyme preparation, i.e., a crude extract or an isolated and purified enzyme, for the transamination reaction. In yet another embodiment, the enzyme may be immobilized on a suitable substrate.

The amino acid precursor is an a-keto carboxylic acid or its salt. The a-keto acid selected for use with this process will depend upon which amino acid is the desired transamination product.

For example, phenylpyruvic acid is converted to L-phenylalanine, a-ketoisocaproic acid to L leucine, a-ketoisovaleric acid to L-valine, pyruvic acid to L-alanine, ss-hydroxy-a-ketobutyric acid to L-threonine, p-hydroxyphenylpyruvic acid to L-tyrosine, indole pyruvic acid to L-tryptophan, a keto-ss-methyl valeric acid to L-isoleucine, a-ketohistidinal acid (ss-imidazolylpyruvic acid) to Lhistidine, etc. As can be seen, conventional amino acid precursors are used in the inventive process. The precursor may be purified, or may be in unpurfied form such as a sodium, calcium, potassium or ammonium hydroxide hydrolysate or a sulfuric acid precipitate.

In the preferred embodiment of this invention, a substrate solution comprising molar ratios of aspartic acid and amino acid precursor of about 1:2 to about 2:1 in a biocompatible buffer is prepared. Any biocompatible solvent or buffer which does not interfere with the reaction process and which will maintain the reaction system in the preferred pH range of about 6.5 to about 10.0 may be utilized. For example, about 0.1 to 0.5 M phosphate buffer has been found to be suitable. Alternatively, the pH can be controlled by adding a pH controller to the system to add base as needed. There will be a tendency for the pH to drop as the transamination proceeds.

The amino donor for the improved transamination of this invention must be aspartic acid or must substantially comprise aspartic acid. It has been found that the highest amino acid yield is achieved when aspartic acid is the only amino donor present in the reaction system. By using aspartic acid as the amino donor, oxalacetic acid is formed as a by-product of the transamination. The oxalacetic acid is decomposed, both spontaneously and catalytically, to yield carbon dioxide and pyruvic acid.

Of course, it will be permissible to have other amino donors present in the system and for these to be utilized by the transaminase. However, to the extent that the transamination reaction incorporates non-aspartic acid amino donors, this improved process will not operate, as there is no oxalacetic acid by-product to decompose.

The presence of the cofactor pyridoxal-5-phosphate (P-5-P) is required to facilitate the transamination. This cofactor complexes with the enzyme, is transiently converted to pyridoxamine phosphate (PMP) and is regenerated in the overall reaction. It may be desired to add small amounts of P-5-P to enhance the transamination. For example, adding P-5-P to concentrations of about 0.1 ,uM, may be desired, although small quantities of the cofactor will be present with cellular material added to the reaction system.

There are three potentially rate-limiting reactions in this improved transamination process: (1) bioconversion of the precursor to amino acid, (2) decomposition of the a-keto acid precursor to valueless products and (3) decomposition of the oxalacetic acid by-product. The first reaction, the bioconversion, easily may be driven by increasing the loading of the transamination catalyst, the catalyst being contained in the cellular material. By driving the first reaction, the extent of precursor decomposition (the second reaction) is decreased. Therefore, the critical rate-limiting step then becomes the oxalacetic acid decomposition. It can be seen that increasing the availability of transamination catalyst will be most advantageous only if the oxalacetic acid decomposition rate simultaneously is increased.

The decomposition of oxalacetic acid is a first order reaction, that is, the rate of decomposition is proportional to the concentration of oxalacetic acid in the system. At high concentrations, there is faster decomposition, but the rate slows as the decomposition lowers the concentration.

The spontaneous decomposition rate for a concentration of 0. 1 moles per liter is about 6.7 x 10-3 moles per liter per hour. Removal of oxalacetic acid from the reaction system by spontaneous decomposition will tend to drive the conversion of precursor to amino acid towards completion. However, in the intial stages of a biologically catalyzed transamination, oxalacetic acid may be produced at a rate of up to 5 to 10 times faster than its spontaneous decomposition rate, or even faster, if the transamination system allows it. Thus, even with spontaneous decomposition, there is a marked tendency for oxalacetic acid to build up in the system, especially in the early stages of the reaction, since the spontaneous decomposition rate no longer is fast enough to eliminate the effects of equilibrium on the transamination reaction.

The decomposition of oxalacetic acid may be catalyzed by the presence of inorganic multivalent metal ions, such as Al+3, Al+4, Ni+2, Mn+2, Mug+2. Pb+2, Ag+2, Fe+3, Fe+2, Zn+2, Cr+2, Cr+3, Co+2, Co+3, Pd+2, Au+3, etc., or salts thereof. For example, the metal ions may be added in soluble form, such as the sulfate, sulfide or chloride salts of these metals. Alternatively, particles of metals such as alumina, lead or iron, may be added as the catalyst. Solid pieces of metal will be advantageous from the process view-point since the catalyst will not be lost with the product stream.

The multivalent metal ions useful for catalyzing the decomposition of oxalacetic acid also are responsible for some decomposition of the a-keto acid precursor. However, the rate of catalyzed oxalacetic acid decomposition is sufficiently high that the relative quantities of precursor decomposed are unimportant in the overall reaction. That is, the transamination proceeds at a pace which allows relatively little precursor to decompose.

The choice of metal ions, or salts thereof, will be system specific. It usually will be preferred, for convenience and economy, to select those which already are present in the precursor or aspartic acid streams used in the transamination. The most preferred metal ions are those of magnesium, iron, aluminum and zinc. Iron, aluminum and zinc generally give higher rates and yields. Magnesium, although giving a somewhat lower rate and yield, is nonetheless one of the preferred metal catalysts because of its solubility and low cost.

Cost considerations may play a role,with silver and lead being more, and iron and nickel less, expensive. Copper may cause color formation, which will not be desired in certain aplications, and is believed to denature the transaminase and to chelate the amino acids. Certain metal ions, copper and iron, for example, may be cofactors or accentuators of other enzyme activity producing products which will have to be removed. Calcium ions are not desirable in this process due to their tendency to precipitate the precursor from solution. Manganese is toxic to humans and will not be a preferred choice due to problems in avoiding human exposure to process and wastestreams, although manganese salts have been demonstrated to be useful in this process.

The metal ions will increase the decomposition reaction rate by at least two orders of magnitude over the spontaneous rate of about 6.7 X 10-3 moles per liter per hour, even at very low concentrations. For example, at high oxalacetic acid concentrations, about 0.1 moles per liter, the catalyzed decomposition rate may be about 0.1 to 2.0 moles per liter per hour. At lower oxalacetic acid concentrations, about 0.02 moles per liter, the catalyzed decomposition rate may be about 0.01 to 0.2 moles per liter per hour. This rate variation is due to variations in the effectiveness and concentration of the metal catalyst used, as well as the temperature and pH of the reaction. In these ranges, the inhibition of the transamination rate by the presence of oxalacetic acid in the system is effectively eliminated.In the presence of these multivalent metal ions, volumetric transamination rates up to about 10.0 moles per liter per hour in the early stages of conversion and about .05 to 0.2 moles per liter per hour overall may be achieved with no indication of oxalacetic acid inhibition.

The concentration of metal ions used to catalyze the decomposition preferably is matched to the transamination rate, that is, to the rate at which oxalacetic acid is formed via transamination.

The presence of at least about 1.0 X 10-4 moles per liter of the ion begins to cause a detectable catalysis of oxalacetic acid decomposition. At metal ion concentrations above about 1.0 X 10-1, there will be little or no significant increase in the decomposition rate. The range typically will be about 1.0 x 10-3 to about 1.0 X 10 - ' moles per liter. It should be kept in mind when determining the quantity of salts to be added to the system, that the solubilities of the various salts will vary. For the use of insoluble metals for this catalysis, amounts up to about 20 gm/l may be used.

The cell or enzyme preparation is contacted by the aspartic acid-precursor solution in a suitable reaction vessel. The reaction conditions for the process of this invention should be selected to enhance enzyme stability and will be determined according to the overall transamination reaction system selected. The temperature range may be about 20 to about 50"C, preferably about 30 to about 40"C. The pH may be about 6.5 to about 10.0, preferably about 7.5 to about 8.5. The pH adjustment may be made with any base compatible with the enzyme system, preferably with ammonia or potassium hydroxide, either of which will help solubilize the amino acid precursor and aspartic acid.The reaction does not require aeration, unless live, growing cells are used, but there should be some agitation or movement of the cells relative to the substrate in order to maximize biocatalyst-substrate interaction.

The transamination reaction can be expected to be completed in less than about 40 hours, preferably less than 4 hours when high biocatalyst concentrations are employed. Another indicator will be the specific rate of reaction, that is, moles of amino acid produced per gram dry cell weight per hour. In the process of this invention, this indicator may range from about 0.0001 to about 0.1, preferably about 0.002 to about 0.05 moles amino acid produced per gram dry cell weight per hour. Typically, however, the specific rate will be about 0.0005 to about 0.01 moles amino acid per gram dry cell weight per hour.

The decomposition products of oxalacetic acid are carbon dioxide and pyruvic acid. The carbon dioxide will dissipate if the reaction is conducted in an open vessel and the pH is close to neutral or it may be collected either for disposal or for use in other processes. If the pH is above about 7.5, the carbon dioxide will remain dissolved in the solution. At least a minute portion of the pyruvic acid is believed to be converted to alanine by transamination or by decarboxylation with aspartic acid. The relative amount of alanine formed will vary with the biocatalyst used in the transamination. Thus, both pyuruvic acid and small amounts of alanine will be present, but both may be easily removed by conventional separation methods.

The improved biological transamination of this invention can result in extremely high amino acid yields relative to both aspartic acid and the amino acid precursor. That is, 90% or more of the aspartic acid may be transaminated in the reaction and 90% or more of the precursor may be converted to amino acid. Moreover, the transamination rate is greatly increased, which leads to both increased yields and decreased reaction times. After the reaction is complete, the amino acid product may be recovered by conventional methods. Typical methods of recovery of the desired product may include ion exchange and/or fractional crystallization processes.

The examples which follow are given for illustrative purposes and are not meant to limit the invention described herein. The following abbreviations have been used throughout in describing the invention: Ag - silver Al - aluminum Au - gold C - degrees Centigrade Co - cobalt Cr - chromium Cu - copper DS - Dveloping Solution Fe - iron gorgm - grams(s) - liter M - molar Mg - magnesium - micro ml - milliliter (s) Mn - manganese Na - sodium Ni - nickel OD - optical density P-5-P - pyridoxal-5-phosphate Pb - lead Pd - palladium % - percent PPA - phenylpyruvic acid ppm - parts per million TYR - tyrosine Zn - zinc In the Tables which illustrate the results obtained in the Examples, Specific Rate, Volumetric Rate and Yield are calculated as follows: moles produced Specific Rate = # grams cells-hour moles produced Volumetric Rate = liter-hour moles produced Yield = ~~~~~~~~~~~~~~~~~~~~~~~~~ moles theoretical limit The theoretical limit is based on the moles of precursor or aspartic acid available.

Example I (Effect of Manganese Ions on Transaminase Activity) A synthetic solution was prepared comprising 17.4 gm/l sodium phenylpyruvic acid (Na-PPA) (monohydrate) (Sigma Chemical Co.) and 11.09 gm/l aspartic acid (Sigma Chemical Co.) in a 0.1 M phosphate buffer with 0.1 yM pyridoxal-5-phosphate (P-5-P), pH adjusted to 7.5 with aqueous ammonia and sulfuric acid. Six 100 ml samples were prepared. To three samples, 1.0 X 10-3 M MnSO4 was added; the remaining samples were used as controls. The solutions were added to 150 ml jacketed stir cups, which were held at 37 C and maintained under mild agitation.

A dried cell preparation was made in the following manner: Pseudomonas pseudoalcaligenes ATCC 12815 was grown in 14 liters Trypticase Soy Broth for 24 hours at 35 C. The cells were harvested by centrifugation and dried in a vacuum oven at 37 C for 12 hours.

Dried cells were added in amounts of 10,5 and 2 grams per liter to the experimental and control solutions. The solutions were maintained at about 37 C with mild agitation. Samples were taken at 5 and 24 hours for analysis, by ferric chloride colorimetric assay for PPA (procedures described below) and by HPLC for phenylalanine and aspartic acid. The results are shown in Table I. It is apparent that for the control samples, i.e., without the addition of the metal salt, the volumetric transamination rate remains constant despite heavier catalyst loading.

The addition of MnSO4 led to a dramatic increase in the volumetric and specific transamination rates as compared with the control, in addition to higher yields of L-phenylalanine.

Table I (Addition of Mn+2) Control (no Mn+2) Time: 5 Hours 24 Hours Catalyst loading: 10 g/l 5 g/l 2 g/l 10 g/l 5 g/l 2 g/l PPA (g/l) 4.70 3.80 4.60 0.70 0.80 0.50 ASP (g/l) 3.77 2.20 2.93 2.28 4.50 1.60 PHE (g/l) 8.61 8.20 7.68 11.55 10.55 10.20 ALA (g/l) -- -- -- -- -- - Specific Rate .00104 .002 .0046 .0003 .0005 .0012 Volumetric Rate .0104 .01 .01 .003 .0027 .0025 Yield (PHE/PPA) 62% 62% 57% 84% 76% 75% Yield (PHE/ASP) 62% 62% 57% 95% 120% 89% Experimental (1.0 x 10-3 M Mn+2 added) Time: 5 Hours 24 Hours Catalyst loading: 10 g/l 5 g/l 2 g/l 10 g/l 5 g/l 2 g/l PPA (g/l) 0.30 1.25 3.89 0.05 0.40 0.30 ASP (g/l) 2.84 2.77 5.08 0.69 1.00 1.34 PHE (g/l) 12.69 10.67 7.67 13.95 12.47 12.16 ALA (g/l) 0.62 0.27 -- 0.84 0.79 0.43 Specific Rate .0015 .0026 .0046 .00035 .0006 .0015 Volumetric Rate .015 .013 .01 .0035 .0031 .003 Yield (PHE/PPA) 93% 78% 57% 101% 90% 88% Yield (PHE/ASP) 93% 78% 57% 106% 99.6% 101% 1 - Apparent analytical error.

Example II (Effect of Magnesium lons on Transaminase Activity) A synthetic solution was prepared comprising 17.71 gm/l Na-PPA (monohydrate) (Sigma Chemical Co.) and 12.35 gm/l aspartic acid (Sigma Chemical Co.) in 0.05 M phosphate buffer with 1.0 X 10-2 M MgSO4 and 0.1 yM P-5-P, pH adjusted to 7.5 with aqueous ammonia and sulfuric acid. Aliquots of 100 ml were placed in five 1 50 ml jacketed stir cups, heated to 37"C and maintained under mild agitation. Dried Pseudomonas pseudoalcaligenes ATCC 1281 5 cells were prepared as in Example I. The cells were added to the cups in the following amounts: 15, 10, 5.0, 2.0 and 1.0 grams per liter, respectively. Samples were taken at 1 and 24 hours and analyzed as in Example I.The results, shown in Table II, indicate that with the addition of MgSO4 no oxaloacetic acid repression of the transamination is observed, and the volumetric reaction rates as well as overall yields were increased.

Table II (Addition of Mg+2) Time: 1 Hour 24 Hours Catalyst loading: 15 g/l 10 g/l 5 g/l 1 g/l 15 g/l 10 g/l 5 g/l 1 g/l PPA (g/l)* 8.81 9.76 11.00 13.84 0.00 0.15 0.15 0.95 ASP (g/l) 9.44 10.29 10.13 11.75 0.92 0.93 1.18 2.08 PHE (g/l) 7.96 4.60 2.99 0.57 14.18 13.87 13.52 11.47 ALA (g/l) -- -- -- -- 0.52 0.35 0.19 0.79 Specific Rate .0032 .0028 .0036 .0035 .00024 .00035 .00068 .0029 Volumetric Rate .048 .028 .018 .0035 .0036 .0035 .0034 .0029 Yield (PHE/PPA) 56% 32% 21% 4% 100% 98% 96% 81% * as phenylpyruvic acid Example lil (Effect of Magnesium lon Concentration on Transaminase Activity) A synthetic solution was prepared comprising 17.0 gm/l Na-PPA (monohydrate) (Sigma Chemical Co.) (sodium salt) and 11.0 gm/l aspartic acid (Sigma Chemical Co.) in a 0.1 M phosphate buffer with 1.0 X 10-4 M P-5-P, pH adjusted to 7.5 with aqueous ammonia and sulfuric acid. Aliquots of 100 ml were placed in jacketed stir cups with varying amounts of magnesium sulfate to achieve the Mg+2 concentrations indicated in Table Ill. The solutions were held at 37"C and maintained under mild agitation. Dried Pseudomonas pseudoalcaligenes ATCC 12815 cells were prepared as in Example I. To each solution, 0.5 gm of dried cells were added.

Samples were analyzed as in Example I at 0, 2 and 5 hours. The results, shown in Table lil, indicate that increasing concentrations of Mg+2 ions correspond to increasing transamination rates and overall yields.

Table III (Varying Concentrations of Mg+2) Mg+2 concentration: 0 3 x 10-4 1 x 10-3 3 x 10-3 1 x 10-2 3 x 10-2 (moles/liter) 0 Hours PPA (g/l)* 17.20 17.00 16.70 16.90 17.20 17.10 ASP (g/l) 13.00 13.30 13.20 13.40 13.30 13.90 2 Hours PPA (g/l)* 9.96 7.82 7.03 6.40 8.02 9.01 ASP (g/l) 5.92 3.00 2.50 3.30 3.44 3.45 PHE (g/l) 8.93 11.73 12.80 11.52 11.07 11.60 Specific Rate .0054 .007 .0078 .007 .0067 .007 Volumetric Rate .027 .036 .039 .035 .034 .035 5 Hours PPA (g/l)* 6.50 2.40 2.21 2.63 2.12 2.70 ASP (g/l) 2.70 1.00 1.00 1.00 1.00 1.00 PHE (g/l) 12.70 15.20 15.40 15.60 16.00 15.90 Specific Rate .003 .0037 .0037 .0039 .0039 Yield (PHE/PPA) 74.0% 80.4% 92.0% 92.0% 93.0% 93.0% * as phenylpyruvic acid Example IV (Effect of Various Salts on Transaminase Activity) A synthetic solution was prepared comprising 20.6 gm/l Na-PPA (monohydrate) (Sigma Chemical Co.) and 1 3.5 gm/l aspartic acid (Sigma Chemical Co.) in a 0.1 M phosphate buffer with 0.1 !LM P-5-P. Samples of 100 ml of this solution were prepared and 1.0 X 10-3 moles per liter of pne of the following salts was added to each sample: magnesium sulfate, manganese sulfate, copper sulfate, calcium chloride, aluminum sulfate and iron sulfate. One sample was maintained as a control, with no salts added. Each solution was pH adjusted to 8.5 and mildly agitated. Dried Pseudomonas pseudoalcaligenes ATCC 1281 5 cells were prepared as in Example I. To each solution, 0.5 gm of dried cells were added. The solutions were maintained at 37"C under mild agitation. Samples were taken at 0, 5 and 24 hours and analyzed as in Example I.

The results, shown in Table IV, indicate that the Mg+2, Mn+2, Awl+3 and Fe+2 salts are effective in increasing transamination rate and yield.

Table IV (Comparison of Various Salts) MgSO4 MnSO4 CuSO4 CaCl2 Al2(SO4)3 FeSO4 Control 0 Hours PPA (g/l)* 16.30 16.04 16.30 16.20 16.55 15.90 16.40 ASP (g/l) 13.10 15.50 13.74 14.40 12.74 14.34 14.32 5 Hours PPA (g/l)* 8.33 6.33 9.00 8.10 8.30 9.30 6.20 ASP (g/l) 4.47 5.47 9.00 16.80 4.90 6.93 8.20 PHE (g/l) 8.44 8.59 3.09 2.75 8.85 7.10 6.20 Volumetric Rate .011 .011 .0037 .0033 .011 .009 .007 Specific Rate .002 .0021 .00075 .00067 .0021 .0018 .0017 24 Hours PPA (g/l)* 0.00 0.00 2.20 1.70 0.00 0.00 0.00 ASP (g/l) 0.97 1.27 8.90 5.74 1.68 3.20 4.10 PHE (g/l) 15.50 14.90 6.86 4.95 14.95 14.00 13.20 Yield (PHE/PPA) 94.9% 93.0% 45.0% 30.0% 90.0% 88.0% 80.0% Yield (PHE/ASP) 95.0% 78.0% 60.0% 36.0% 95.0% 80.0% 74.0% * as phenylpyruvic acid Example V (Comparison of Soluble and Insoluble Metals) A synthetic solution was prepared comprising 30.0 gm/l Na-PPA (monohydrate) (Sigma Chemical Co.) and 24.0 gm/l aspartic acid (Sigma Chemical Co.) in 0.1 M phosphate buffer with 0.1 ILM P-5-P and 100 ppm Barquat MB-SO (TM) (Lonza Inc.) as a disinfectant. One of the following was added to respective 100 mpl aliquots of the solution: 1.0 X 10-3 M MgSO4, 1.0 X 10-2 M MgSO4, 1.0 X 10-2 M At2(504)3, and 10 gm/l alumina pellets (approximately 40 mesh). The solutions were pH adjusted to 8.5 with aqueous ammonia and sulfuric acid. To each solution was added 5.0 gm wet Pseudomonas pseudoalcaligenes ATCC 12815 cells (72% moisture). The solutions were heated to 35"C and maintained under mild agitation. Samples were taken at 0, 1, 5 and 24 hours and analyzed as in Example I.The results, shown in Table V, indicate that the use of alumina pellets is approximately as effective as magnesium sulfate or aluminum sulfate for increasing the volumetric reaction rate and overall yield for the transamination.

Table V (Alumina pellets vs. metal salts) 10 g/l 1 x 10-3 M 1 x 10-2 M 1 x 10-2 M Alumina Metal Catalyst: MgSO4 MgSO4 Al2(SO4)3 Pellets 0 Hours PPA (g/l)* 23.54 23.07 23.82 23.83 ASP (g/l) 23.20 24.07 24.00 24.03 1 Hour PPA (g/l)* 16.52 15.31 17.40 15.17 ASP (g/l) 18.49 17.73 20.20 16.04 PHE (g/l) 7.18 9.18 6.40 8.59 Volumetric Rate .044 .053 .038 .052 Specific Rate .0031 .0038 .0028 .0037 5 Hours PPA (g/l)* 2.84 2.10 4.57 2.07 ASP (g/l) 7.73 6.22 7.73 6.02 PHE (g/l) 19.05 19.74 19.05 21.95 Volumetric Rate .025 .024 .023 .027 Specific Rate .0016 .0017 .0016 .0019 24 Hours PPA (g/l)* 0.00 0.00 0.00 0.00 ASP (g/l) 4.69 3.47 4.80 3.46 PHE (g/l) 23.45 22.79 22.20 22.75 Yield (PHE/PPA) 99.6% 98.9% 93.0% 95.5% * as phenylpyruvic acid Example VI (Production of Tyrosine) A synthetic solution was prepared by dissolving 1.25 gm p-hydroxy phenylpyruvic acid (pH PPA) (Sigma Chemical Co.) in 50 ml 0.1 M phosphate buffer along with 1.0 gm aspartic acid (Sigma Chemical Co.), 0.1,uM P-5-P, and 1.0 X 10-3 M/I zinc sulfate. The pH of the solution was adjusted to 8.0. E. coli ATCC 11303 cells were immobilized in Hypol (TM) (HFP 3000, W.

R. Grace s Co.) polyurethane foam prepared by mixing 50.0 gm of a 75% moisture cell paste with 50.0 gm of the Hypol (TM) prepolymer. The mixture was allowed to cure (about 15 minutes) and the cured foam was cut into small pieces approximately 1/4 inch in size. A total of 4.0 gm cured foam was added to the prepared pH-PPA solution. The solution was maintained at 37"C with mild agitation.

Samples were taken at 0, 3 and 18 hours and analyzed by thin layer chromatography (TLC) and HPLC for tyrosine and aspartic acid. After 3 hours, the solution became hazy with precipitate as tyrosine was produced. To recover the product, the slurry and foam were washed with an equal volume of acetone and the foam filtered out with a Buchner funnel. The solution, which comprised about 1.2 gm/l tyrosine in solution, was centrifuged at 3000 RPM. The resulting pellet of amino acid was collected and dried. A total of 1.1 gm dried product was collected and identified by TLC and HPLC as tyrosine. The product was found to have a purity of > 90% by non-aqueous titration. The results, shown in Table VI, indicate that the method described herein is suitable for the rapid production of tyrosine in high yields.

Table VI (Production of Tyrosine) Time: 0 Hours 3 Hours 18 Hours pH-PPA (g/l) 25.0 -- - ASP (g/l) 18.5 16.2 1.2 TYR 0.0 1.2 1.2 TYR 0.0 -- 1.0 Yield (TYR/pH-PPA) -- -- 85% Yield (TYR/ASP) -- -- 80% Gm/l tyrosine in solution at time of sample Gm tyrosine as total yield in dried pellet Ferric Chloride Colorimetric Assay For Phenylpyruvic Acid, Sodium Salt Principle: Phenylpyruvic acid (PPA) forms a green complex with ferric ions. The intensity of the green color is proportional to the amount of PPA present.

Reagents: 1. Developing Solution (DS) (1000 m/ > Mix the following ingredients and cool to room temperature in an ice bath. Add contents to a one liter volumetric flask and bring up to volume with deionized water. Ingredients: 0.5 gm FeCIs 6H20; 20 ml glacial acetic acid; 600 ml dimethyl sulfoxide; 200 ml deionized water.

2. Na-PPA Standard Solution (10 gm/l Add 500 mg Na-PPA H20 (Aldrich Chemical Co., Inc., 98%-100% pure) to 40 ml 0.1 M Tris/HCI buffer (pH 8.0) at 34"C. Stir until dissolved.

Place solution in a 50 ml volumetric flask and bring up to volume with Tris/HCI buffer. Store refrigerated. For 0. 1 M Tris/HCI buffer: add 900 ml deionized water to 1 2.11 gm Tris; adjust pH to 8.0 with HCI; pour into volumetric flask; add deionized water to 1000 ml.

Calibration Curve: Add Na-PPA Standard Solution and 4.95 ml of Developing Solution to glass spectrophotometer tubes. Mix on vortex. Allow to stand for exactly 10 minutes (start timing with the addition of DS to first tube). Read optical density (OD) at 640 nm. Prepare Na-PPA H20 Standard Calibration curve (vertical axis is absorbance; horizontal axis is mg Na-P PA H20/ml DS).

NA-PPA Standard mg Na-PPA H20 per ml DS 5 l 0.01 10 l 0.02 15 l 0.03 20ul 0.04 25 IL1 0.05 30,ul 0.06 Calculation: OD X 100 X dilution Na-PPA H20 (mg/ml) = slope from standard curve or ODX 100 X dilution 164.4 PPA (gm/l) = x slope from standard curve 204.16 The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims (22)

1. Process for the preparation of an L-amino acid by the transamination of an a-keto acid precursor and an amino donor, which comprises contacting the a-keto acid precursor and aspartic acid as the amino donor, with a suitable intra- or extracellular transaminase and catalyzing the decomposition of the oxalacetic acid transamination by-product by including multivalent metal ions or salts thereof in the reaction system.

2. The process of Claim 1 in which the said multivalent metal ions are selected from the group comprising Al+3, Al+4, Mg+2, Mn+2, Ni+2, Pb+2, Ag+2, Fe+2, Fe+3, Zn+2, Cr+2, Cr+3, Co+2, Co+3, Pd+2, and Au+3.

3. The process of Claim 1 in which the said multivalent metal ions are Al+3, Al+4, Mg+2, Zn+2, Fe+2 or Fe+3.

4. The process of any of Claims 1 to 3 in which the said multivalent metal ions are present in the amount of at least about 1.0 X 10-4 moles per liter of reaction system.

5. The process of Claim 4 in which the said ions are present in the amount of about 5.0 to 10-3 to about 1.0 X 10-1 moles per liter.

6. The process of any of Claims 1 to 5 in which the said multivalent metal ions are provided in the form of metal particles.

7. The process of any of Claims 1 to 5 in which the said multivalent metal ions are provided in the form of soluble salts.

8. The process of any of Claims 1 to 7 in which the oxalacetic acid, produced as a product of the transamination, is decomposed at a rate of about 0.02 to about 2.0 moles per liter per hour.

9. The process of any of Claims 1 to 8 in which the volumetric transamination rate is about 0.05 to about 10.0 moles per liter of reaction system per hour.

10. The process of any of Claims 1 to 9 in which the said a-keto acid precursor and the said L-amino acid product are: phenylpyruvic acid to produce L-phenylalanine; a-ketoisocaproic acid to produce L-leucine; a-ketoisovaleric acid to produce L-valine; pyruvic acid to produce L-alanine; ss-hydroxy-a-ketobutyric acid to produce L-threonine; p-hydroxyphenylpyruvic acid to produce Ltyrosine; indole pyruvic acid to produce L-tryptophan; a-ketohistidinal acid to produce Lhistidine; and a-keto-ss-methylvaleric acid to produce L-isoleucine.

11. The process of Claim 10 in which the said precursor is phenylpyruvic acid and the said L-amino acid is L-phenylalanine.

1 2. The process of any of Claims 1 to 11 in which the transamination is conducted at a temperature of about 20 to about 50"C and at a pH of about 6.5 to about 10.0.

1 3. The process of any of Claims 1 to 1 2 in which the molar ratio of aspartic acid to the amino acid precursor is about 1 :2 to about 2:1.

1 4. The process of any of Claims 1 to 1 3 in which the said transaminase is present in association with microorganisms selected from Pseudomonas, Brevibacterium, Bacillus, Aerobacter, Escherichia coli, Candida and halophilic microorganisms.

1 5. The process of Claim 1 4 in which the said microorganisms are present in a concentration of about 2.0 to about 80.0 grams per liter on a dry weight basis.

1 6. The process of any of Claims 1 to 1 5 in which the L-amino acid product is accumulated and collected.

1 7. The process of Claim 1 substantially as hereinbefore described.

1 8. A process for the decomposition of oxalacetic acid as it is formed by the transamination of an a-keto acid to an L-amino acid using aspartic acid as the amino donor, which comprises adding multivalent metal ions, or salts thereof, to the transamination system.

1 9. The process of Claim 1 8 in which the said metal ions are as defined in Claim 2 or 3.

20. The process of Claim 1 8 or 1 9 in which the said ions are present in a concentration of at least about 1.0 x 10-4 per liter.

21. The process of any of Claims 18 to 20 in which the said oxalacetic acid is decomposed at a rate of about 0.02 to about 2.0 moles per liter per hour.

22. L-amino acids when produced by the process of any of Claims 1 to 1 7.