Brief Report: Lack of Effect of L-Methionine Ingestion on ¹⁴CO₂ Excretion from L-Histidine (Imidazole-2-¹⁴C) in Folic Acid and Vitamin B₁₂ Deficient Humans

Three human subjects with folic acid and one with vitamin B₁₂ deficiency failed to show any effect of ingestion of L-methionine (400-800 µg./kg. body weight) on the production of ¹⁴CO₂ from parenterally administered high specific activity L-histidine (imidazole-2-¹⁴C). It is suggested that the inhibitory effect of methionine on intestinal absorption and cell membrane transport of histidine can explain previously published effects of methionine on the oxidation of the imidazole-2 carbon atom of L-histidine and that a direct effect of methionine on histidine metabolism need not be invoked.