Studies on the Regulation of One-Carbon Metabolism

The regulation of serine hydroxymethyltransferase has been investigated in Escherichia coli 113-3 (ATCC 11105). It was found that extracts from bacteria grown in the presence of low levels of methionine (or cyanocobalamin) showed a 2- to 18-fold increase in specific activity over extracts of organisms grown at high concentrations of these compounds. The addition of methionine or S-adenosylmethionine to the serine hydroxymethyltransferase assay mixture had no effect on the specific activity. This provides evidence against regulation via feedback inhibition. It appears that the biosynthesis of this enzyme is controlled by the level of methionine in the growth medium. Dihydrofolate reductase, thymidylate synthetase, and N5,N10-methylenetetrahydrofolate dehydrogenase were not affected by the methionine or cyanocobalamin level. Serine hydroxymethyltransferase was purified 49- to 70-fold from E. coli 113-3 grown in medium containing either low or high levels of methionine. The pH optimum, kinetic constants, and molecular weights were identical for the enzymes from both preparations. The molecular weight of serine hydroxymethyltransferase was found to be 170,000 which differs from that of the enzyme purified from rabbit liver.