Synthesis of l-cystathionine by the γ-replacement reaction of cystathionine γ-lyase from Streptomyces phaeochromogenes

Cystathionine γ-lyase (EC 4.4.1.1) occurring in Streptomyces phaeochromogenes (IFO 3105) can catalyze not only the α,γ-elimination reaction of l-homoserine to form α-ketobutyrate and ammonia but also the γ-replacement reaction of l-homoserine in the presence of l-cysteine to form l-cystathionine. Optimal reaction conditions for the enzymatic synthesis of l-cystathionine using the γ-replacement reaction were defined. Since concentrations of greater than 10 mM l-cysteine inhibit the γ-replacement reaction, it is necessary to keep the concentration of l-cysteine around 10 mM for a high yield of l-cystathionine. Under optimal reaction conditions, defined as the conditions producing the highest yield, 1.39 mmol of lcystathionine (0.3 g) was synthesized enzymatically from 20 mmol of l-homoserine and 5.8 mmol of l-cysteine in 100 ml of reaction mixture.