In Vivo Metabolism of 5′-Methylthioadenosine in Lemna

Abstract

Evidence is presented that Lemna converts 5′-methylthioadenosine (MTA) to methionine. The methylthio moiety and four of the ribose carbons of the nucleoside contribute the methylthio and the four-carbon moieties of methionine. Plants grown in the presence of inhibitors which block methionine biosynthesis convert MTA to methionine at a rate sufficient to sustain normal growth (at least 4.4 nanomoles per colony per doubling with a molar yield of at least 65%). The pathway for conversion is shown to be constitutive in plants grown in standard medium and to function at a rate sufficient to dispose of MTA arising as a result of polyamine synthesis, and to explain the observed rate (1.4 nanomoles per colony per doubling) of preferential recycling of methionine sulfur (Giovanelli, Mudd, Datko 1981 Biochem Biophys Res Commun 100: 831-839). Rapid entry of methionine methyl into S-adenosylmethionine and phosphorylcholine was observed for plants grown in standard medium. Adenine generated during this cycle is efficiently salvaged into ADP and ATP.