The Specificity of S-Adenosylmethionine Derivatives in Methyl Transfer Reactions

Abstract

Abstract The preparation, purification, and analysis of a variety of new sulfonium derivatives of methionine is reported. Deamination of S-adenosyl-l-methionine with nitrous acid yielded, depending on experimental conditions, S-adenosyl l-(2-hydroxy-4-methylthio)butyric acid or S-inosyl-l-(2-hydroxy-4-methylthio)butyric acid. S-Inosyl-l-methionine was obtained by enzymatic deamination of S-adenosyl-l-homocysteine to S-inosyl-l-homocysteine and methylation of the latter by methyl iodide. 4-Dimethylsulfonium-l-(2hydroxy)butyric acid has been prepared from S-methyl-l-methionine sulfonium salt. The sulfonium derivatives were purified by ion exchange chromatography and were characterized by ultraviolet spectrophotometry, paper and thin layer chromatography, electrophoresis, and hydrolysis to known fragments. The new methionine sulfonium derivatives and S-adenosyl-(5')-3-methylthiopropylamine were investigated as methyl donors and as inhibitors with three purified methyltransferases: histamine methyltransferase, acetylserotonin methyltransferase, and homocysteine methyltransferase. The replacement of the 2-amino group of the methionine moiety by the hydroxyl group resulted in complete loss of activity in all systems investigated; these deaminated derivatives were also inactive as inhibitors of S-adenosyl-l-methionine in the transmethylations. The methyl donor capacity of inosine derivatives and of decarboxylated S-adenosylmethionine ranged between inactivity and an effect equal to that of the biological methyl donor. Homocysteine methyltransferase was the least specific of the three enzymes. Experiments with S-inosylmethionine, racemic with respect to the sulfonium pole, established that both sulfonium diastereoisomers were utilized in the methylation of homocysteine. S-Adenosyl-l-homocysteine was found to be a very effective inhibitor of the systems investigated; the enzymatic removal of its amino group in the adenine part led to Sinosylhomocysteine, which was without effect as an inhibitor. 5'-Methylthioadenosine, a catabolite of S-adenosylmethionine, inhibited the methylation of histamine and acetylserotonin. On the basis of these results, three binding sites for S-adenosylmethionine on the methyl transfer enzymes may be postulated.