Studies on metabolic role of 5'-Methylthioadenosine in Ochromonas malhamensis and other microorganisms

Several compound containing a thiomethyl group were found to replace vitamin B12 in a protozoan, Ochromonas malhamensis. The order of the effectiveness was as follows: 5'-methylthioadenosine is greater than S-adenosylmethionine is greater than 5-methylthioribose is greater then L-methionine. A similar order was obtained with respect to the permeability of these compounds into the protozoan cells, except for S-adenosylmethionine. 5'-Methylthioadenosine and 5-methylthioribose as well as L-methionine markedly increased the intracellular content of L-methionine. The level of S-adenosylmethionine was also increased by them, but to lesser degree. The thiomethyl group of the compounds was established to be incorporated into S-adenosylmethionine. The metabolic fate of the thiomethyl group of 5'-methylthioadenosine cannot be distinguished from that of L-methionine. A high activity of 5'-methylthioadenosine nucleosidase was detected in the cell-free extracts of the protozoan. These results strongly suggest that 5'-methylthioadenosine would be metabolized to L-methionine would be ocnverted to S-adenosylmethionine. Like L-methionine and vitamin B12, 5'-methylthioadenosine and 5-methylthioribose may play an important role in maintenance of the C-1 pool in Ochromonas malhamensis. Neither 5'-methylthioadenosine nor 5-methylthioribose replaced vitamin B12 in some vitamin B12-requiring bacteria. This result is consistent with the fact that neither compounds was significantly taken up by these bacteria.