Importance of sarcosine formation in methionine methyl carbon oxidation in the rat

Experiments in vitro using rat liver slices indicated that the incorporation of the methionine methyl carbon into sarcosine and serine was dependent upon available glycine and most probably involves glycine methyltransferase. Although the sarcosine methyl carbon was rapidly oxidized to CO2, its formation accounted for only a small proportion of the oxidation of the methionine methyl carbon to CO2 under these conditions. In vivo experiments using a sarcosine trapping pool with 0.3% to 3.0% L-[methyl-14C]methionine in the diet indicated that from 5% to 14% of the absorbed methionine methyl carbon was metabolized via sarcosine, and that this accounted for only 10% to 20% of the observed oxidation of the methyl carbon to CO2. The adaptive response of the rat to high levels of dietary methionine, as indicated by greater oxidation of the methyl carbon to CO2, is in part due to increased sarcosine synthesis. The failure of supplemental glycine to stimulate oxidation of the methionine methyl carbon to CO2 in rats receiving 3% methionine plus 10% sarcosine may be due to sufficient glycine being produced from sarcosine metabolism.