The detection of both the activities of β-cystathionase and O-acetylhomoserine (OAH) sulfhydrylase showed the presence of two pathways for the synthesis of L-homocysteine in a facultative methylotroph, Pseudomonas FM518. L-Methionine, the following conversion product of L-homocysteine, inhibited the activities of both the enzymes competitively, and repressed β-cystathionase but not OAH sulfhydrylase. Activities of serine-O-transacetylase and O-acetylserine (OAS) sulfhydrylase which are involved in the biosynthesis of L-cysteine, the precursor of L-cystathionine, were also inhibited by L-methionine. The Ki value of OAS sulfhydrylase for L-methionine (1.6mM)was small enough to consider that the L-cysteine biosynthesis was regulated by L-methionine in vivo. These results and regulatory features showed that the transsulfration pathway involving cystathionine might function for the biosynthesis of homocysteine in Pseudomonas FM518.

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