A method for enzymatically producing L-aspartic acid from ammonium fumarate by using a microorganism with aspartase activity, Escherichia coli, is known. Japanese Patent Publication No. 55-35110 (i.e., JP-B-55-35110) describes a method in which E.coli aspartase is immobilized on an ion-exchange resin Duolite A-7. Applied Biochemistry and Biotechnology vol. 13, pp.231-240 (1986) describes a method for continuously producing L-aspartic acid wherein a reactor is filled with E.coli cells immobilized in .kappa.-carrageenan by gel entrapment. These methods, however, are unsatisfactory in terms of industrial productivity since the immobilized aspartase has a low activity or since the gel entrapment immobilization causes diffusion barrier or rate-limiting diffusion.
Since conventional immobilized aspartases do not have a sufficiently high activity, the reaction has to be carried out at 30.degree. C. or higher, in which case cooling is required because aspartase loses its activity by heat of reaction. Particularly, the activity is remarkably decreased at a temperature exceeding 40.degree. C. (Applied Microbiology vol. 27, No. 5, pp.878-885 (1974); Applied Microbiology vol. 27, No. 5, pp. 886-889 (1974)). To prevent a rise in temperature in a reactor caused by heat of reaction, the reactor needs to be equipped with an internal cooling tube or a cooling device such as jacket, which renders the reactor complicate. Further, prior art immobilized aspartases cannot be used to carry out enzymatic reactions at a high LHSV due to their low activities. In methods that use aspartases immobilized by gel entrapment, it is also impossible to carry out an enzymatic reaction at a high LHSV (Liquid Hour Space Velocity) since diffusion barrier becomes greater.
The reaction at high LHSV is required to achieve the improved productivity of L-aspartic acid by the enzymatic reaction using immobilized aspartases. Such a reaction can be realized with an immobilized aspartase which has a sufficiently high activity to react at a low temperature and which leads to a low diffusion barrier and a low pressure loss. Unfortunately, such an immobilized aspartase has not yet been prepared in the art. In this situation, the present inventors have now prepared an immobilized aspartase with the above preferable properties and have now found that the reaction at high LHSV is achieved substantially without removing heat by introducing a substrate solution into a reactor while controlling a temperature of the substrate solution to be lower than a temperature of impairing the stability of aspartase by at least a rise in a temperature caused by heat of reaction, thereby ensuring the stable activity of the immobilized aspartase.