Occurrence of S ‐adenosylhomocysteine hydrolase in prokaryote cells

1 The occurrence of S ‐adenosylhomocysteine hydrolase (EC 3.3.1.1) was found in a variety of prokaryotes. These prokaryotes did not exhibit any activities of S ‐adenosylhomocysteine nucleosidase (EC 3.2.2.9) and S ‐ribosyl‐homocysteine hydrolase (EC 3.3.1.3), which had been the generally accepted prokaryote enzymes for the regeneration of free homocysteine from S ‐adenosylhomocysteine in the activated methyl cycle. In these prokaryotes S ‐adenosylhomocysteine hydrolase was suggested to be the only enzyme functioning for the regeneration of free homocysteine by enzymological and immunochemical studies. 2 S‐Adenosylhomocysteine hydrolase was purified and crystallized from cells of a prokaryote, Alcaligenes faecalis. The purified enzyme was found to be homogenous on ultracentrifugation and gel electrophoresis. Its relative molecular mass is approximately 280000 and it is composed of six identical subunits with a M r of approximately 48000. The NH 2 ‐terminal and COOH‐terminal amino acids are lysine and tyrosine respectively. The enzyme contains 6 ma1 NAD/mol. Some nucleosides, such as formycin A, nebularine, adenosine N 1 oxide and so on, are able to substitute for adenosine yielding the corresponding S ‐nucleosidylhomocysteine congeners. Modification of the 5′‐hydroxymethyl group in adenosine leads to the most potent inhibition of the thioether formation of homocysteine with adenosine. 3 The enzyme from A. faecalis has some immunological similarities to other prokaryote S ‐adenosylhomocysteine hydrolases, but is different from the enzymes of animal sources.