Studies on metabolic role of 5?-methylthioadenosine in Ochromonas malhamensis and other microorganisms

Abstract

Several compounds 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 > S-adenosylmethionine > 5-methylthioribose > 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 a 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 via 5-methylthioribose and then the l-methionine would be converted 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.