IMMOBILIZED MICROBIAL CELLS AND PROCESS FOR PREPARING AND USING SAME

Abstract: A composition comprising immobilized cells having enzyme activity, the cells being immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable prepolymer material selected from the group con­ sisting of polyazetidine prepolymers, carboxymethyl cellu­ lose, polyurethane hydrogel prepolymers and polymethy­ lene isocyanates, said polymer being cured at a temperature below the temperature at which the enzyme activity of the microbial cells is significantly reduced, the cell crosslinked polymer composition constituting a coating on a solid inert carrier. The composition may be used in various ways, for example, to prepare L-aspartic acid.

Description: The present invention is concerned with the immobilization of microbial cells and processes for preparing and using the same. The invention is particularly concerned with an improved process for manufacturing L-aspartic acid using immobilized microbial cells, notably Escherichia coli ( E. coli ) cells, which contain L-aspartase activity. However, the immobilization and use of other cells are also contemplated. Background to The Invention There is a considerable amount of prior art regarding the immobilization of E. coli or other microbial cells for use in the preparation of L-aspartic acid. For example, U.S. patent 3,791,926 (Chibata et al) describes a process for the production of L-aspartic acid which involves polymerizing a monomer selected from acrylamide, N,N'-lower alkylene-bis (acrylamide) and bis(acrylamidomethyl) ether in an aqueous suspension containing an aspartase-producing microorganism such as E. coli ATCC no. 11303. The resultant immobilized aspartase-producing microorganism is treated with ammonium fumarate or a mixture of fumaric acid or its salt and an inorganic ammonium salt which by enzymatic reaction gives L-aspartic acid. The immobilization of E. coli cells containing aspartase activity and use of the resulting immobilized cells for the production of L-aspartic acid are also described by Fusee et al, Applied and Environmental Microbiology , Vol. 42, No. 4, pages 672-676 (October 1981). According to Fusee et al, the cells are immobilized by mixing a suspension of the cells with a liquid isocyanate-capped polyurethane prepolymer (HYPOL ® ) so as to form a "foam" containing the immobilized cells. Sato et al ( Biochimica et Biophysica Acta , 570(1979) pages 179-186) have disclosed the immobilization of E. coli cells containing aspartase activity with K-carrageenan, and use of the immobilized preparation for the production of L-aspartic acid. Additional literature disclosures describing the immobilization of microbial cells in urethane prepolymers or polyurethanes or the like include the following: (a) Immobilization of Microbial Cells in Polyurethane Matrices, by Klein et al, Biotechnology Letters , Vol. 3, No. 2, pages 65-70 (1981); (b) Hyrophilic Urethane Prepolymers: Convenient Materials for Enzyme Entrapment, Biotechnology and Bioengineering , Vol. XX, pages 1465-1469 (1978); (c) Transformation of Steroids by Gel-Entrapped Cells in Organic Solvent, by Omata et al, European J. Applied Microbiology and Biotechnology , 8, pages 143-155 (1979); and (d) Entrapment of Microbial Cells and Organelles With Hydrophilic Urethane Prepolymers, by Tanaka et al, European J. Applied Microbiology and Biotechnology , 7, 351-354 (1979). The above-noted processes for preparing L-aspartic acid using immobilized microbial cells suffer from various dis advantages. For example, K-carrageenan gum and polyurethane "foam" as disclosed by Fusee et al and Sato et al are relatively soft and compressible. Hence when these immobilized cell compositions are used, in a column through which ammonium fumarate is passed for conversion to ammonium aspartase, they tend to be compressed and plug up, particularly where high flow rates and/or relatively tall column heights are involved. Objects of the Invention The principal object of the present invention is to provide an improved process for preparing L-aspartic acid using L-aspartase active microbial cells, preferably E. coli cells, which have been immobilized in a special way whereby the resulting composition is highly effective for the preparation of L-aspartic acid batchwise or in continuous fashion. A more specific object includes the provision of an improved process for preparing L-aspartic acid from ammonium fumarate using immobilized E. coli cells which maintain optimum L-aspartase activity for relatively long periods of time. Another specific object of the invention is to provide novel immobilized microbial systems suitable for use in making L-aspartic acid which obviate problems encountered in prior procedures involving the use of immobilized cells. Other objects will also be hereinafter apparent. Summary of The Invention According to one aspect of the invention, there is provided a process for preparing L-aspartic acid by contacting ammonium fumarate or its equivalent with an immobilized microbial cell composition comprising E . coli ATCC 11303 cells or equivalent cells containing L-aspartase activity, the cells being immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable polyazetidine prepolymer material at a temperature below the temperature at which the L-aspartase activity of the microbial cells is significantly reduced, the cell/cross-linked polymer composition constituting a coating on a solid inert carrier. The application of the immobilized cell/crosslinked polymer coating on the carrier provides a highly advantageous form of the cell/polymer composition for use in preparing L-aspartic acid. It is a particularly advantageous feature that the cell/polymer systems of the invention can be provided as a dry coating on the carrier without significant loss of the L-aspartase activity. An especially preferred aspect of the invention contemplates the provision of immobilized E . coli cells having L-aspartase activity, the cells being immobilized by means of a crosslinked, water-insoluble polymer obtained by curing a polyazetidine prepolymer. The indicated prepolymer systems can be crosslinked at temperature below 40°C and in the presence of relatively large volumes of water containing E. coli cells without significantly deteriorating the L-aspartase activity of the cells. It is a surprising aspect of the invention that such aqueous crosslinking conditions leave the E. coli cells with their L-aspartase activity even though the cells are immobilized in insoluble, crosslinked polymer networks. Another unique aspect of the invention is that the wet dispersion of the E. coli cells in the aqueous polymer solutions can be taken to dryness while the immobilized cells still surprisingly retain most of their original L-aspartase activity. The drying process has the advantage of providing strong, well-crosslinked, insoluble compositions in the form of coatings, membranes, particles, etc. having high concentrations of cells retaining L-aspartase activity. The novel immobilized L-aspartase cell compositions disclosed herein have been found to outperform the immobilized L-aspartase active E. coli cell compositions previously reported by, for example, Fusee et al and Sato et al in the references noted above. Prepolymer materials suitable for use to provide the crosslinked polymer network for immobilizing the E. coli cells according to the invention include: (1) polyazetidine prepolymers which may be crosslinked in aqueous solution by reaction with =NH, -SH, -OH, -COOH; or other polyazetidines which may be crosslinked by H₂O removal, heat, or by changing to a more basic pH. The following is an idealized structure of a representative polyazetidine such as Polycup ® 172 (Hercules, Inc.) which is useful for present purposes: where R is typically (CH₂)₄ The above-noted polymer system may be used to immobilize L-aspartase active E. coli cells to provide cell/polymer compositions having a variety of different forms and shapes. For example, the immobilized cell/polymer composition may be prepared in the form of membranes, filaments, fibers, tubes, beads or the like. A particularly important embodiment of the invention involves providing the immobilized cell/polymer composition as a coating on an appropriately shaped support which advantageously, although not necessarily, is in the form of solid or reticulated beads or particles composed of an inert solid organic or inorganic, porous or non-porous material. As noted earlier, the composition of the invention can be cured and dried without substantively affecting the L-aspartase activity of the cells and, as a consequence, the composition can be made up in the form of dried coated beads or other particulate material and stored until needed for use. Typically, useful supports or carriers for the immobilized cell/crosslinked polymer compositions of the invention include the following in bead or particle form: Molecular sieves Ion exchange resins Alumina Silica and silica gel Foraminifera skeletons Polymer latexes Metals It is a useful property of the polymer systems used in the present invention that they can be contacted or mixed in water with E. coli cells containing L-aspartase and then crosslinked (or cured) to form insoluble and relatively fixed polymer matrices which hold or otherwise immobilize the E. coli cells. It is particularly useful that the crosslinking conditions which need to be used are mild enough so that the L-aspartase activity of the E. coli mutant cells is maintained. As indicated, a particularly desirable composition according to the invention is obtained by applying the E. coli cells containing L-aspartase activity and the immobilizing polymer as a coating on hard inorganic or organic polymer beads. Using these relatively noncompressible compositions an L-aspartase active catalyst bed capable of high throughput in a fixed bed or fluidized bed is possible. As noted earlier, the L-aspartase active immobilized cell compositions previously described in the literature involving the use of relatively soft, compressible k-carrageenan gum or polyurethane foam are compressible and therefore tend to plug under high flow rates, and/or tall column (bed) heights. Various methods may be used to immobilize the E. coli ATCC 11303 cells, while retaining their L-aspartase activity, by combination with cured or crosslinked forms of prepolymer according to the invention. The preferred method for immobilizing E . coli ATCC 11303 cells is as follows: E . Coli ATCC 11303 cells are advantageously mixed with an aqueous solution of the azetidine prepolymer so as to obtain a homogeneous mixture after which the polyazetidine may be cured or crosslinked to give an insoluble L-aspartase active composition by any of the following means: Removal of part or nearly all of the water at temperatures below 60°C (usually between 40° and 0°C) and pressures of 760 to 1.0 Torr; raising the pH above 7.5; exposing the E. coli /polyazetidine mixture to polyamines (using, for example, 100 parts of the composition for each 1-2 parts/wt. of polyethylene imine, polyamino ion exchange resins, diethylene triamine, ethylene diamine, etc.). The immobilized cell/polymer compositions of the invention may be used to make aspartic acid by either a batch or continuous process. However, it is noted that these compositions, particularly when coated onto a bead or other particulate support, are especially effective for the continuous conversion of aqueous solutions of ammonium fumarate to ammonium L-aspartate according to the following equation: As illustrative of processes which may be used to prepare L-aspartic acid or ammonium L-aspartate according to the invention are the following: (i) A batch type process wherein the catalyst compositions are stirred in from 0.1 to 5.0 molar (preferably 0.5 to 2.0 molar) solutions of ammonium fumarate in water at 5.0 to 10.0 pH (preferably 7.5 to 9.5 pH) for periods of 1.0 to 100 hours (preferably 8 to 48 hours) at temperatures below 50°C (preferably 20° to 40°C). Broadly from 0.05 to 50 g of immobilized cells, preferably from 1.0 to 15 g, are used per 1.0 mole of starting ammonium fumarate. After the conversion, the catalyst compositions may be removed by filtration or the equivalent for reuse in converting fresh batches of fumarate solutions. The product solutions are obtained in a form suitable for conventional processing to isolate the L-aspartic acid (acidification, precipitation, filtration, washing, recrystallization, drying). (ii) A continuous process wherein the catalyst compositions, e.g. coated beads, are placed in columns and the solutions of ammonium fumarate (concentrations, pH's, and temperatures are the same as described above for the batch processes) are passed through the catalyst beds either from above or from below (fluidized bed mode). The rates of passage of these fumarate solutions may range from 0.1 to 1000 space velocities/hour. For example, 5.0 liters of solution per hour may be passed through 1.0 liter of catalyst bed representing 5.0 space velocities (S.V.) per hour. Preferably the fumarate solution flow rates which yield essentially 100% conversion of the fumarate to the L-aspartate fall in the range of 0.5 to 20.0 S.V./hour. The effluent from these columns of catalyst beds is suitable for conventional processing to isolate L-aspartic acid (as outlined above in the batch processes). The invention is illustrated by the following Examples: Example 1 Preparation of E. coli Containing L-Aspartase Immobilized With Polyazetidine Gels. a) A mixture of 2.0 g of an aqueous polyazetidine solution (Hercules Polycup ® 172, as received 12% solids in H₂O) and 2.0 g of E. coli ATCC 11303 paste was stirred rapidly with a magnetic stirring bar for five minutes at 25°C, to insure a homogeneous dispersion. This mixture was then poured out onto a polystyrene surface to dry at 16 hours at 25°C. The resultant 2" X 2" X 1/16" thick flexible membrane was stripped from the surface and found to weight 0.7 g. This film was used in L-aspartase measurements (see Table I) and was estimated to contain 2.0 g of E. coli cells (hydrated state). b) A homogeneous mixture of 2.0 g Polycup ® 172 and 2.0 g of E. coli ATCC 11303 was distributed onto 30 g of hydrated 5Å, 8-12 mesh molecular sieve beads by means of tumbling at 25°C and 5 to 20 Torr for 45 minutes. After 16 more hours drying at 25°C in open air the resultant coated beads weighed 30.4 g. A 5.0 ml aliquot of these coated beads weighed 3.9 g. This sample was used for L-aspartase activity (see Table I) and was estimated to have 0.26 g of E. coli cells c) A homogeneous dispersion of 2.0 g E. coli ATCC 11303 in 2.0 g of Polycup ® 172 at 25°C was dispersed onto 30.0 g of Amberlite R IRA 45 ion exchange beads (as received, 42% H₂O, =N-H form) by hand mixing with a wooden stick for 30 minutes at 25°C. A thin layer of the somewhat sticky mixture was allowed to stand in air at 25°C for 16 hours. The resultant free flowing beads weighed 25.8 g. A 3.0 g sample of these dry coated beads had a volume of 5.0 ml. Upon soaking in 1.5 M ammonium fumarate solution the volume expanded to 5.5 ml. A 5.0 ml aliquot of these wet coated beads was taken for L-aspartase measurements (see Table I) and the estimated E. coli content was 0.21 g d) A homogeneous dispersion of 2.0 g E. coli ATCC 11303 in 4.0 g deionized water and 2.0 g Polycup ® 172 was added over a period of 3.5 hours in three equal portions to 10.0 g of Amberlite ® IRA 45 (as received, 42% H₂O, free =N-H form) containing sixty 0.25 diameter Teflon ® balls. This mixture was tumbled at 25°C at 5 to 20 Torr. After a total of five hours of drying at reduced pressure the resultant dry, coated beads (Teflon ® balls removed) weighed 7.8 g. A 5.0 aliquot of these dry, free flowing coated beads weighed 2.9 g. After saturating with 1.5 M ammonium fumarate solution the volume swelled to 6.25 ml. A 5.0 ml aliquot of these wet, coated beads was taken for L-aspartase measurements (see Table I). The estimated E. coli content of this aliquot was e) A homogeneous dispersion of 3.0 g E. coli ATCC 11303 in 6.0 ml deionized water plus 3.0 g of Polycup ® 172 was spread onto 10.0 g of Amberlite ® IRA 45 (as received, 42% H₂O, free =N-H form) in the same manner of drying at reduced pressure described above. A total of 10.7g dry, coated, free flowng beads were obtained. A 5.0 ml aliquot of dry coated beads weighed 3.0 g. After soaking in 1.5 M ammonium fumarate solution this volume swelled to 5.75 ml. A 5.0 ml aliquot of the wet, coated beads was taken for L-aspartase measurement (see Table 1). The estimated E. coli content of this 5.0 ml aliquot was 0.73 g f) A homogeneous dispersion of 4.0 g E. coli ATCC 11303 in 4.0 g of Polycup ® 172 and 8.0 g deionized water was added in approximately six equal portions over a 5.5 hour period at 25°C and 5 to 20 Torr to 10.0 g of Amberlite ® IRA 938 beads (Rohm and Haas, 73% H₂O, 0.38 mm diameter, tert. amine chloride salt form) containing eight 0.5 inch diameter Teflon ® balls. After 6 hours total of tumbling at 25°C and 5 to 20 Torr the resultant dry, coated free flowing beads weighed 10.3 g and had a volume of 16.6 ml. Soaking these beads in excess 1.5 M ammonium fumarate solution (pH 8.5) For 16 hours at 25°C gave 17.5 ml wet beads. A 5.0 ml aliquot of the wet beads, estimated to contain 1.14 g E. coli ATCC 11303 was taken for L-aspartase activity measurements (see Table I). Example 2 (comparative) Coating Molecular Sieve Beads and Ion Exchange Resin Beads with E. coli Cells Containing L-Aspartase Using No Polymer Binders. To demonstrate the utility of the above described polymer systems used to immobilize the E. coli cells containing L-aspartase, the following experiments were conducted to coat materials with the cells containing L-aspartase alone, i.e., with no polymer binders. a) A homogeneous slurry of 1.0 g E. coli cells, ATCC 11303, containing L-aspartase in 5.0 ml of Na phosphate buffer (0.1 M, pH 7.5) was added in four equal portions over a period of 3.5 hours to 10.0 g of hydrated 5A molecular sieve beads (8-12 mesh) also containing ten 0.5 inch diameter Teflon ® balls. This mixture was tumbled at 25°C and 5 to 10 Torr. After a total of about 4.0 hours the resultant dry, coated, free flowing beads weighed 9.9 g. A 5.0 ml aliquot of these dry beads weighed 4.2 g. The volume did not change upon soaking in 1.5 M ammonium fumarate solution. The estimated E. coli cell content of this aliquot of coated beads was 0.42 g A 5.0 ml aliquot of the coated beads was gently stirred in 50 ml 1.5 M ammonium fumarate solution at 25°C for 24 hours. The solution became very turbid indicating considerable amounts of material were sloughing off the beads. The beads were isolated by decantation and again gently stirred in 50.0 ml fresh 1.5 M ammonium fumarate at 25°C for L-aspartase activity measurement (see Table I). However, even after one hour the solution became very turbid again. On several more occasions the beads were isolated and gently stirred in fresh 1.5 M ammonium fumarate solution. The beads continued to give off considerable amounts of material to make the solutions very turbid. This behavior was not observed in any of the preparations of L-aspartase active cell coatings of beads where the polymer binders were also added to the formulation. In all the polymer examples the ammonium fumarate solutions over the coated beads remained clear. b) In a similar way, a slurry of 10.0 g E. coli ATCC 11303 in 30 g of aqueous phosphate buffer solution (0.1 M, pH 7.5), but no polymer binders, was coated onto 50.0 g of Amberlite ® IRA 45 (as received from Rohm and Haas, 42% H₂O, =N-H form) over a period of 4.5 hours at 25°C and 5 to 20 Torr. The resultant 47.6 g of free flowing dry, coated beads had a volume of 83.5 ml. Upon soaking these beads in excess 1.5 M aqueous ammonium fumarate the solution became thick and opaque from the massive amount of material detached from the beads. All attempts to filter (cotton wadding, fiberglass mat, 43 mesh nylon screen, medium porosity sintered glass) off the beads from this opaque mother liquid failed because the thick gelatinous particles quickly plugged the filters. This behavior was not observed in any of the examples where the polymer binders were incorporated into the formulation. In these cases the aqueous solutions over the beads remained clear at all times, and flowed quickly through filters. Example 3 L-Aspartase Activity Measurements Shown in Table I are the L-aspartase activities of the immobilized E. coli ATCC 11303 compositions described in the preceding examples. In each case the samples were preconditioned by gently shaking in 50 ml of a 1.5 M ammonium fumarate solution in deionized water (pH 8.5) containing 0.002 M MgSO₄ for a period of 8 to 16 hours at 25°C. The solution was then drained from each sample and 50 ml fresh fumarate solution added prior to gentle shaking again for 8 to 16 hours at 25°C. At this point all the samples prepared with the polymer binders discussed in the previous examples showed no evidence of material sloughing off the solid supports; i.e., their supernatant solutions were all clear. Samples prepared using no polymer binders (Example 5) continued to give very turbid supernatant solutions even after several washings. This indicates the cell material was not securely bound to the solid supports. The next step in each example was to take a 5.0 ml aliquot (or in the case of the membrane, a 2 inch X 2 inch X 0.125 inch section) of the washed and well drained composites and gently shake in a fresh 50.0 ml of the above mentioned ammonium fumarate solutin at 25°C. Samples were removed after 60 minutes and diluted 1/1000 with deionized water. The decrease in optical density at 280 nm from the starting 1.5 M ammonium fumarate solution (observed to be 0.43 at 1/1000 dilution) indicated the L-aspartase activity. The optical density of a 1 5 M L-aspartic acid ammonium salt (pH 8.5) at 280 nm is essentially zero (<0.005). The observed conversion of the fumarate to the L-aspartate is shown in Table I as moles of aspartic acid/hr/liter of catalyst bed and as moles of aspartic acid/hr/kg wet E. coli . For example in experiment #4d using polyazetidines, cell paste and ion exchange beads, the optical density at 280 nm dropped from 0.43 to 0.32 after 60 minutes at 25°C. The calculations are: Also shown in Table I are values calculated from data in the literature showing the L-aspartase performance at 37°C of a polyurethane foam (HYPOL ®) containing E. coli ATCC 11303. Here a 0.777 liter bed having 126 g of cells required 32 minutes to convert 2.0 liters of 1.0 M fumarate 100% to L-aspartate at 37°C. These workers also report the average rate of conversion of 1.5 M fumarate to be 8% lower than 1.0 M fumarate solution. They also observed rates at 37°C to be 1.67 times rates at 25°C. Thus a fair estimate of their L-aspartic acid production rate for comparison to our data (25°C, 1.5 M fumarate) is: Comparing these values to the data obtained representative of the present invention shows the superiority of the present process over the polyurethane foam system. Example 4 Continuous Production of L-Aspartic Acid (as ammonium L-aspartate). A 5.0 ml aliquot of the wet beads from Example #4d were placed in a 0.7 cm diameter X 15 cm jacketed glass column. The column temperature was maintained at 37°C. A fresh 1.5 M ammonium fumarate solution in deionized water containing 0.002 m MgSO₄ (all at pH 8.5) was continuously passed up thorugh the bed of beads at a rate of 0.5 ml/min. Samples of the effluent product solution were taken periodically and analyzed for L-aspartate content as 1/1000 dilutions in deionized water as described in Example 6. The bulk of the effluent was collected but not returned to the column. The following data shows that over a period of 50 days of continuous running the L-aspartase activity of the coated beads remained essentially unchanged. At the 62.5% conversion rate for a flow of 6.0 bed volumes/hr (0.5 ml/min through 5.0 ml of catalyst bed) the production rate of L-aspartic acid is: For a period of one hour at 37°C the same column was run at the rate of 4.0 bed volumes/hr (0.33 ml/min through 5.0 ml of catalyst bed) and the average rate of fumarate (1.5 M) conversion was 80.0%. This corresponds to: Based on these conversion rates of 62.5% and 80.0% at 6.0 and 4.0 bed vol/hr respectively, a conservative estimate by graphical methods, of the flow rate required to achieve 100% conversion is At 1.5 bed volumes/hr the L-ASP production rate is: These values (100% conversion at 1.5 bed vol/hr, and are significantly better than values of 1.1 bed vol/hr for 100% conversion and reported for the continuous operation of a column of E. coli ATCC 11303 cells immobilized on k-carrageenan gel. In a further variation of the invention, it is contemplated that the aqueous ammonium L-aspartate solution obtained in Example 4 may be used directly, i.e. without recovery or separation of dry, solid aspartic acid, for reaction with benzyl chloroformate to form the benzyloxy carbamate of the amine of aspartic acid, this carbamate being an important intermediate in the preparation of the commercially important sweetener, L-aspartyl-L-phenylalanine methyl ester. Presently this benzyloxy carbamate is made by starting with dry, crystalline solid L-aspartic acid. The acid is dissolved in aqueous sodium hydroxide prior to reaction with benzyl chloroformate according to the following reactions: The immobilized whole cell enzymatic process for converting ammonium fumarate to ammonium L-aspartate according to the invention provides directly a basic aqueous product stream of, for example, 1.5 molar 98.5% ammonium L-aspartate solution which, after removal of ammonia, is suitable for direct use in the reaction of enzyl chloroformate (or other chloroformate) to make the desired aqueous solution of benzyl (or other) oxycarbamates. This should make it possible to avoid the need to isolate dry aspartic acid crystals and the time and expense incident thereto. For example, the isolation of dry aspartic acid crystals from the ammonium L-aspartate solution of Example 4 would generally require the following: (1) acidification of the 1.5 molar ammonium L-aspartate product (L-ASP) stream using approximately 0.7 to 0.8 moles of H₂SO₄ to precipitate the L-ASP at pH of 2.8. This offers a prospective corrosion problem and the expense involved in disposing of the ammonium sulfate by-product; (2) although the L-ASP product stream may contain 98.5% L-ASP (the rest is ammonium fumarate), the precipitate of L-ASP and washing with water gives only an 89-93% yield of solid L-ASP. The retention of 5-9% of the L-ASP in the mother liquor is an appreciable loss, considering the value of the L-ASP. (3) subsequent drying of the precipitated L-ASP crystals is also costly (estimated at about 5-10 cents per pound). Use of the aqueous ammonium L-ASP product directly, according to the invention, eliminates the disadvantages inherent in steps (1) - (3). Adjustment of the ammonium L-ASP product stream is appropriate to obtain optimum results. For example, the addition of sodium hydroxide (in the order of 0.5 to 2.0 moles/mole ammonium L-ASP) results in displacement of the NH₄⁺ as NH₃ which can be boiled off and used to make additonal ammonium fumarate. Most of the NH₄⁺ ion should be removed to facilitate the chloroformate reaction. The invention is not limited to the immobilization of E. coli cells or the use of such immobilized cells for the production of L-aspartic acid. Thus, other cells may be immobilized in generally similar fashion using the indicated curable polymeric materials. Such further immobilized cell variations, and uses thereof, are exemplified in the following examples: Example 5 Production of L-Alanine Via Immobilization of a Pseudomonas dacunhae Immobilized with Polyazetidine Gels. Pseudomonas dacunhae ATCC 21192 was cultured on a medium of: peptone (Bacto-Difco), 9.0 g/liter; casein hydrolysate (Casamino acids, Difco), 2.0 g/liter; KH₂PO₄, 0.5 g/liter; MgSO₄·7H₂O, 0.1 g/liter; and L-glutamic acid sodium salt, 15.0 g/liter, pH adjusted to 7.2 with NH₄OH. The fermentation was carried out under aerobic conditions for 23 hours at 30°C, 300 rotations/min. A mixture of 3.62 grams (wet weight) of cell paste and 3.6 grams of aqueous polyazetidine solution (Hercules Polycup ® 172) was stirred to homogenity at 25°C by hand mixing with a wooden stick. This mixture was dispersed on 3.6 g of Amberlite ® IRA 938 ion exchange bead (Rohm & Haas) which had been previously air dried at 25°-30°C for 60 hours to remove greater than 85% of the moisture associated with the beads. The thin film of the mixture on the beads was allowed to air dry at 25°C for 24 hours. The resulting free flowing beads occupied a volume of 20 ml. These beads were washed with 5 volumes of normal saline and an 18 ml. aliquot was placed in a flask containing .5 liters of 1.5 M ammonium aspartate solution, pH 5.5 (adjusted with H₂SO₄) which was held at 37°C. The entire mixture was stirred for a period of 48 hours while the pH was maintained by the addition of sulfuric acid as needed. Analysis of the liquid at the end of this period on HPLC (uNH₂ Bondapak column, Waters Assoc.; eluant CH₃CN, 25%, 5 mM KHPO₄ pH 4.4, 75%) showed that L-aspartic acid had been decarboxylated to form L-alanine in 98% yield. Example 6 Preparation of Penicillin-G Acylase Containing Bacillus megaterium Immobilized With Polyazetidine Gels. Bacillus megaterium ATCC 14945 was cultured under aerobic conditions as previously described (U.S. Patent 3,446,705). A mixture of 10.5 g (wet weight) of the cell material and 10.5 g of an aqueous polyazetidine solution (Hercules Polycup ® 172) was stirred to homogeneity at 25°C by hand mixing with a wooden stick. This mixture was dispersed onto 9.8 g of Amberlite ® IRA 938 ion exchange beads (Rohm and Haas) which had been previously air-dried at 25°-30°C for 60 hours which removed>85% of the moisture from the beads. A thin layer of the mixture gas allowed to air dry at 25°C for 24 hours. The resultant free flowing beads weighed 13.1 g and occupied a volume of 53 ml. A 1247 mg aliquot of these beads corresponding to 1 gram of free cells was assayed for penicillin acylase activity. 6-Aminopenicillanic acid production was quantitated by high pressure liquid chromatography (HPLC). The activity of the immobilized preparation was 19.7 µ moles hr⁻¹ g cell paste⁻¹. This value was 67% of the free cell activity. Example 7 Preparation of Penicillin-G Acylase Containing Escherichia coli Immobilized With Polyazetidine Gels. Escherichia coli ATCC 9637 was cultured aerobically as described by Sato et al, Eur. J. Appl. Microbiol. 2:153-160 (1976). A mixture of 20.7 g (wet weight) of cell paste and 20.7 g of aqueous polyazetidine solution (Hercules Polycup ® 172) was stirred to homogeneity at 25°C by hand mixing with a wooden stick. This mixture was dispersed onto 19.3 g of Amberlite ® IRA 938 ion exchange beads (Rohm and Haas) which had been previously air-dried at 25°-30°C for 60 hours to remove>85% of the moisture associated with the beads. A thin film of the mixture was allowed to air dry at 25°C for 24 hours. The resultant free flowing beads weighed 27.4 g and occupied a volume of 95 ml. A 1325 mg aliquot of these beads corresponding to 1 gram of free cells was assayed for penicillin acylase activity. 6-Amino penicillanic acid production was quantitated by high-pressure liquid chromatography (HPLC). The activity of the immobilized preparation was 53.6 µ moles hr⁻¹ gram cell paste⁻¹ which was 99% of the free cell activity. Example 8 Immobilization of Arthrobacter species Containing Glucose Isomerase Activity for Production of High Fructose Corn Syrup. Arthrobacter species ATCC 21748 was grown under submerged aerobic conditions and the cells were harvested. The cells had a glucose isomerase activity of 420 µ moles hr⁻¹·gm⁻¹ cell paste (wet weight) at 60°C. To 47 gm of Polycup ® 172 was added 47 gm of Arthrobacter species ATCC 21748 (wet weight) and the mixture was stirred vigorously for 5-10 minutes. The Polycup ® /cell mixture was then added slowly to 46.4 gm of air dried IRA ® 938 beads with mixing and the resultant beads were dried at room temperature for 24 hours. To a flask containing 5 ml of the Arthrobacter species ATCC 21748 immobilized beads was added 50 ml of 0.2 M phosphate buffer, pH 7.5, 10 mM MgSO₄, 1 mM CoCl₂ and the solution was heated to 60°C with stirring. 450 mg of α-D-glucose was added to the flask and the flask was placed in an incubated shaker at 60°C for 1 hour at 150 rpm. The glucose isomerase activity of the immobilized Arthrobacter species ATCC 21748 was 530 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 60°C as determined by the cysteine/H₂SO₄ reaction. [(Dische, Biochim. Biophys. Acta , 39, 140 (1960)]. 10 Ml of the Arthrobacter species ATCC 21748 immobilized beads were added to a 0.9 x 30 cm jacketed glass column which was maintained at 60°C by a circulating waterbath. Prewarmed substrate solution containing 250 mM glucose, 5 mM MgSO₄ and 0.5 mM CoCl₂ in 50 mM Tris-HCl buffer, pH 7.5, was passed through the column. After 6 days of continuous operation the glucose isomerase activity of the immobilized Arthrobacter species ATCC 21748 was 280 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 60°C and after 37 days the glucose isomerase activity was 18 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 60°C. Example 9 Immobilization of Arthrobacter simplex Having Steroid Dehydrogenase Activity for The Production of Prednisolone or Related Steroids. Arthrobacter simplex ATCC 6946 was grown under submerged aerobic conditions and the cells were harvested. The cells have a Δ-1-dehydrogenase activity of 980 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 34°C. To 20 gm of Polycup ® 172 was added 20 gm of A. simplex ATCC 6946 and the mixture was stirred vigorously for 5-10 minutes. The Polycup ® /cell mixture was then added slowly to 18.6 gm of air dried IRA ® 938 beads with mixing and the resultant beads were dried at room temperature for 24 hours. To a flask containing 5 Ml of the A. simplex ATCC 6946 immobilized beads was added 36 Ml of 20 mM phosphate buffer (pH 7.0) followed by 4 Ml of ethanol containing 60 mg of hydrocortisone. The flask was placed in an incubated shaker at 34°C for 1 hr at 150 rpm. The-Δ-1-dehydrogenase activity of the immobilied A. simplex ATCC 6946 was 440 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 34°C as determined by absorbance at 285 nm. Example 10 Immobilization of a Streptomyces species Having Glucose Isomerase Activity for the Production of High Fructose Corn Syrup. Streptomyces phaeochromogenes NRRL B3559 was grown under submerged aerobic conditions and the cells were harvested. The cells had a glucose isomerase activity of 7.7 µ moles·min⁻¹·gm⁻¹ cell paste (wet weight) at 60°C. To 64.5 gm of Polycup ® 172 was added 64.5 gm of S. phaeochromogenes NRRL B3559 and the mixture was vigorously stirred 5-10 minutes. The Polycup ® /cell mixture was then slowly added to 60.0 gm of air dried IRA ® 938 beads with mixing and the resultant beads were dried at room temperature for 24 hours. 10 Ml of the S. phaeochromogenes NRRL B3559 immobilized beads were added to a 0.9 x 30 cm jacketed glass column which was maintained at 60°C by a circulating waterbath. Prewarmed substrate solution containing 250 mM glucose, 100 mM Na₂SO₃, 10 mM MgSO₄ and 1 mM CoCl₂ adjusted to pH 7.0 with HCl was passed through the column at a flowrate of 28 ml/hr. The glucose isomerase activity of the immobilized S. phaeochromogenes NRRL B3559 was 7.7 µ moles·min⁻¹·gm⁻¹ cell paste (wet weight) at 60° C as determined by the cysteine/H₂SO₄ reaction (Dische et al, supra). Example 11 Immobiliztion of a Rhodosporidium Species High In Phenylalanine Ammonia Lyase for the Production of Phenylalanine. Rhodosporidium toruloides ATCC 10788 was grown under submerged aerobic conditions and the cells were harvested. The cells had an L-phenylalanine ammonia-lyase activity of 112 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 30°C. To 14.9 gm of Polycup ® 172 was added 14.9 gm of R. toruloides ATCC 10788 and the cells mixture was vigorously stirred for 5-10 minutes. The Polycup ® /cell mixture was then added slowly to 13.8 gm of air dried IRA ® 938 beads with mixing and the resultant beads were dried at room temperature for 24 hours. To a flask containing 1 Ml of the R. toruloides ATCC 10788 immobilized beads was added 5 Ml of 25 mM Tris-HCl buffer (pH 8.8), 25 mM L-phenylalanine, 0.005% cetyl pyridinium chloride. The flask was placed in a Dubnoff shaking incubator at 30°C for 1 hour at 100 rpm. The phenylalanine ammonia-lyase activity of the immobilized R. toruloides ATCC 10788 was 10 µ moles·hr⁻¹·gm⁻¹ cell paste (wet weight) at 30°C as determined by absorbance at 278 nm. Those in the art will appreciate from the prior literature how the immobilized cell/polymer compositions disclosed in Examples 5-11 can be used to make the products indicated. Thus, with respect to Example 5, it is known that the production of L-alanine via decarboxylation of aspartic acid is mediated by the enzyme, L-aspartic-β-decarboxylase. The presence of this enzyme is well known in many genera of microorganisms, as reported by Chibata et al, Applied Microbiology , Vol. 13, No. 5, pages 638-645 (1965). It is produced by Clostridium perfringens , Desulfovibrio desulfurican s, Nocardia globerula , Pseudomonas reptilivora , Acetobacter species, Achromobacter species, and Alcaligenes faecalis . Chibata et al determined that alanine forming strains were fairly common in the genera Acetobacter , Achromobacter , Pseudomonas , Torula , Torulopsis , Absidia , Aspergillus , Mucor , and Oospora . Chibata et al increased the knowledge previously obtained by Meister and coworkers and determined that L-aspartic acid was converted stoichiometrically to alanine. Isolated yields of L-alanine over 90% from L-aspartic acid were easily obtained. This process is described with respect to the organisms, Pseudomonas dacunhae and Achromobacter pestifer in U.S. Patent No. 3,458,400, which claims a process for production of L-alanine in an aqueous nutrient medium, by dried living cells, or by cell free extracts. A modification of this process was patented by Chibata et al, 1975, U.S. Patent No. 3,898,128, by immobilization of the microorganism in acrylamide polymers. A further patent on the use of the enzyme from Pseudomonas dacunhae was obtained in 1969, U.S. Patent No. 3,463,704. Continued research by Shibatani et al, Applied & Environmental Microbiology , Vol. 38, No. 3, pages 359-364 (1979) has shown that additional species have been found by other workers to possess L-aspartic-β-decarboxylase and shows that production of this enzyme can be stimulated by the addition of certain amino acids, such as glutamate. An improved continuous method of production was published by Yamamoto et al, Biotechnology & Bioengineering, Vol. XXII, pages 2045-2054 (1980), when the whole organism was immobilized in carrageenan gel. Additionally, others have worked on the production of alanine from Pseudomonas dacunhae and Alcaligenes faecalis immobilized in HYPOL ® foam [Fusee et al, American Society for Microbiology Abstracts , Dallas (March 1980)]. The present invention contemplates using the exemplified cell/polymer composition to produce L-alanine from aspartic acid in lieu of the previously used cell compositions. A great deal of work has also previously been done with respect to the immobilization of the enzyme penicillin acylase. See, for example, Biochimie , 62 : 317-321 (1980). Marconi et al have immobilized penicillin acylase in cellulose triacetate fibers [Biotechnology and Bioengineerin g 22 :735-756 (1980); Biotechnology and Bioengineerin g 21 :1057-1073 (1979)], and a number of U.S. patents have issued on the enzyme immobilization: U.S. patents 3,278,391; 3,116,218; 3,190,586; 3,446,705; 3,622,462; 3,736,230; 3,766,009; 3,801,962; 3,883,394; 3,900,488; 3,499,909; 3,736,230; 4,001,264; 4,113,566; and 4,230,804. It appears that relatively few papers on whole cell immobilization are available. Tanabe Seiyaku has published a report of cells entrapped in polyacrylimide gel. Chibata et al (1974), German Patent 2,414,128, and Mandel et al, published on entrapment in polyacrylamide gel [ Prikl. Blkhim. Mikrobiol . 11 :219-225 (1975)]. Immobilized whole cells of Bacillus megaterium or Achromobacter adsorbed on DEAE cellulose have been used by Toyo Jozo, Fujii et al, Japanese patent 73 99393 and Sato et al, 1976, Eur. J. Applied Microbiology 2:153-160 (1976), have reported on the production of 6-APA from pencillin-G by using immobilized E. coli having high penicillin acylase activity. It will be appreciated that various other modifications may also be made without deviating from the invention, the scope of which is defined in the following claims wherein:

Claims: A composition comprising immobilized cells having enzyme activity, the cells being immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable polyazetidine prepolymer said polymer being cured at a temperature below the temperature at which the enzyme activity of the microbial cells is significantly reduced, the cell/crosslinked polymer composition constituting a coating on a solid inert carrier. A composition according to claim 1 wherein the cells are whole cells of E. coli which have L-aspartase activity. A composition according to claim 1 wherein the cells are whole cells of Pseudomonas dacunhae , Bacillus megaterium , Arthrobacter species , Arthrobacter simplex , Streptomyces phaeochromogenes or Rhodosporidium toruloides . A composition according to any preceding claim wherein the carrier is a particulate support. A process for preparing an immobilized microbial cell composition according to claim 1 which comprises mixing together an aqueous dispersion of the cells and a polyazetidine prepolymer which is capable of being cured into a crosslinked polymer and curing the prepolymer in said mixture. A process according to claim 5 wherein the mixture is cured after application as a coating to a carrier. A process according to claim 5 or claim 6 wherein the coating is dried. A process for preparing L-aspartic acid which comprises contacting ammonium furmarate with an immobilized microbial cell composition comprising cells containing L-aspartase activity immoblized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable polyazetidine prepolymer material said polymer being cured at a temperature below the temperature at which the L-aspartase activity of the microbial cells is significantly reduced, the cell/crosslinked polymer composition constituting a coating on a solid inert carrier. A process according to claim 8 wherein the cells are whole cells of E. coli 11303. A process for preparing L-alanine which comprises contacting L-aspartic acid solution with an immobilized cell/polymer composition comprising cells containing L-aspartate decarboxylase activity immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable prepolymer of polyazetidine, said composition comprising a coating on a solid inert carrier. A proces according to claim 10 wherein the cell/polymer composition comprises immobilized Pseudomonas dacunhae cells. A process for preparing 6-Amino-penicillanic acid which comprises using an immobilized cell/polymer composition comprising cells containing penicillin-G acylase activity immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable prepolymer of polyazetidine, said composition comprising a coating on a solid inert carrier. A process for producing high fructose corn syrup which comprises using an immobilized cell/polymer composition comprising cells containing glucose isomerase activity immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable prepolymer of polyazetidine, said composition comprising a coating on a solid inert carrier. A process for producing prednisolone or related steroids which comprises using an immobilized cell/polymer composition comprising cells having steroid dehydrogenase activity immobilized by means of a fixed insoluble, crosslinked polymer obtained by curing a curable prepolymer of polyazetidine, said composition comprising a coating on a solid inert carrier. A process for producing phenylalanine which comprises using an immobilized cell/polymer compositon comprising cells containing phenylalanine ammonia-lyase activity immobilized by means of a fixed, insoluble, crosslinked polymer obtained by curing a curable prepolymer of polyazetidine, said composition comprising a coating on a solid inert carrier. A process according to any of claims 8 to 15, wherein the carrier comprises beads or other particulate material.

Inventors: CALTON, GARY J., WOOD, LOUIS L.
Applicants: RHONE-POULENC CHIMIE S.A.
Application year: 1983
Main classification: C12N11
IPC codes: C12P37, C12N11, C12P13, C12P33, C12R1, A23L1, C12P19