Cystathionine γ-Lyase from Streptomyces phaeochromoǵenes

This chapter discusses the determination of cystathionine γ- Lyase from streptomyces phaeochromogenes. Cystathionine γ-lyase was first purified and crystallized from rat liver, but is also widely distributed among fungi. The microbial enzyme is purified and crystallized from Streptomyces phaeochromogenes (IFO 3105). Cystathionine β-synthase and cystathionine γ-synthase activities are also detected in crude extracts of S. phaeochromogenes, but cystathionine β-lyase is not. The enzyme is assayed by measuring the rate of formation of either α-ketobutyric acid or cysteine from L-cystathionine, or the formation of α-ketobutyric acid from L-homoserine. Gaitonde's acid/ninhydrin assay is highly specific, there being essentially no reaction with homocysteine. The lack of color development with homocysteine means that the assay is specific for γ-lyase activity and that β-lyase activity does not interfere. The assay is suitable, therefore, for crude extracts with cystathionine as substrate. The purified enzyme catalyzes the α,γ-elimination reaction of L-homoserine and L-cystathionine at 2.60 and 1.90 μmol/min/ mg of protein, respectively.