Biosynthesis of Homocysteine in a Facultative Methylotroph, Pseudomonas FM518

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 (1.6 m m) 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.