Rates of Oxidation of the Methionine and S-Adenosylmethionine Methyl Carbons in Isolated Rat Hepatocytes

The methyl group of methionine (Met) is metabolized via activation to S-adenosylmethionine (AdoMet) and subsequent transmethylation and also by a transamination-decarboxylation route. To estimate the contribution of pathways independent of AdoMet formation in the oxidation of physiological levels of Met to CO2, the oxidation rates of L-[methyl-14C]Met and L-[methyl-14C]AdoMet were compared in hepatocyte suspensions. Hepatocytes took up AdoMet from the medium and oxidized its methyl group to CO2 at a rate of 280 ± 22 nmol/g wet cells per hour, which was constant at extracellular concentrations between 0.1 and 1.0 mM AdoMet and Met. Hepatocytes took up L-[methyl-14C]Met and reached an intracellular specific activity within 10 min that was