Activation of methionine for transmethylation.

Enzymatic activation of methionine has been clearly established as a prerequisite for the transfer of the methyl group of this compound to a variety of methyl acceptors (1). The reaction involves an ATP’-dependent conversion of methionine to AMe. Enzyme systems capable of catalyzing this activation have been found in the tissues of many mammalian species (2, 3) and in extracts of Escherz&a coli (4, 5). A more general role for the product, AMe, has been revealed recently with the report that, after decarboxylation, it can function also as a donor of its aliphatic side chain to putrescine in the biosynthesis of spermidine (5). AMe is not found in appreciable amounts in liver extracts; in yeast, however, a nucleoside closely related to AMe accumulates in large quantities under certain nutritional conditions (6). This nucleoside, thiomethyladenosine, was isolated from yeast as long ago as 1912 (7), but only recently Schlenk and his coworkers established that it is formed as a chemical breakdown product of AMe during extraction of the yeast (8). An additional possibility is that some thiomethyladenosine may arise enzymatically in yeast, as it presumably does in E. coli, as the moiety remaining after the transfer of the aliphatic side chain of AMe to a suitable acceptor. In either case, however, these facts indicated that yeast contains the enzymatic capacity for the synthesis of AMe. The present paper describes the isolation and some of the properties of the yeast MAE which were studied in the hope of further elucidating the intimate mechanism of the activation process. Furthermore, a comparison of the yeast enzyme with the analogous enzyme from mammalian liver seemed of interest from the viewpoint of comparative biochemistry.