METABOLISM OF S-ADENOSYLMETHIONINE IN GERMINATING PEA SEEDS - TURNOVER AND POSSIBLE RELATIONSHIPS BETWEEN RECYCLING OF SULFUR AND TRANSMETHYLATION REACTIONS

Tissue slices of pea cotyledons incubated with methionine labeled in the methyl group with 3H and 14C and with methionine 35S, rapidly incorporated radioactivity into S-adenosylmethionine. Such slices transferred to unlabeled methionine solutions showed a rapid turnover of labeled S-adenosylmethionine. Kinetic studies showed that sulfur-labeled and methyl-labeled S-adenosylmethionine had quite different rates of turnover. Such differences were related to the possible recycling of sulfur. Attempts to demonstrate direct methylation of S-adenosylhomocysteine in vitro were not successful. Experiments with S-adenosyl-8-14C-l-homocysteine demonstrated conversion to adenosine-14C and cell-free extracts catalyzed the synthesis of methionine from homocysteine, in the presence of S-adenosylmethionine, S-methylmethionine, or 5-methyltetrahydrofolate. It is concluded that in the metabolic turnover of S-adenosylmethionine the resulting S-adenosylhomocysteine is rapidly cleaved to give rise to adenosine and homocysteine. Homocysteine is then methylated by 5-methyltetrahydrofolate and the resulting methionine is utilized for regeneration of S-adenosylmethionine. © 1969.