In vivo oxidation of the methyl group of hepatic 5-methyltetrahydrofolate.

Methionine given parenterally to rats caused rapid disappearance of methyltetrahydrofolate from the liver and a corresponding rise in tetrahydrofolate and formyl-tetrahydrofolate concentrations. When [14C]H3--H4folate was given, methionine caused an increased [14C]0(2) excretion, indicating that oxidation of the methyl group had occurred. Methionine was more effective than S-adenosylmethionine at causing oxidation, but serine was ineffective. The lowest dose of methionine to produce an effect was 0.5 mumol, which is less than the daily dietary intake in a rat. The data suggest that the concentration of methylfolate in rat livers is controlled by the concentrations of methionine.