Low MWHigh MW

Water treatment agent

Absorbable compound

Since Quastel and Woolf (1) observed the reversible formation of L-aspartic acid from fumaric acid and ammonium ion by a cell suspension of Escherichia coli, many reports have been published on aspartase (E.C., which catalyzes this reaction. This enzyme degrades L-aspartic acid into fumaric acid and ammonium ion under physiological conditions, but the reverse reaction proceeds in high concentrations of ammonia (Fig. 2).

High-yield conversion of L-aspartic acid from fumaric acid and ammonia was first reported by Sumiki et al. (2) in 1928. This method employed resting yeast cells to produce L-aspartic acid. Fermentation methods using medium containing fumaric acid, and enzymatic methods using bacterial aspartase activity have also been reported (3-5).

A process for industrial production of L-aspartic acid was first established in 1973; bacterial cells having aspar-tase activity were immobilized in a gel matrix to stabilize the enzyme activity. A continuous operation was also reported (6-9). In the immobilized-enzyme process, the enzyme cost was reduced compared with that of a batch process.

However, in the immobilized-cell process, cells had to be immobilized and packed into reactor columns under aseptic conditions. Contamination-free operations were needed, which resulted in additional costs. Also, it did not seem to be possible to use the reactor columns for conventional purposes because different specifications were required according to each individual reaction mode or the chemical properties of substrate and product.

Figure 1. Uses of aspartic acid.

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