The effect of nitrous oxide-induced inactivation of cobalamin on plasma amino acid levels in the rat.

Rats were maintained in an atmosphere of equal volumes of oxygen/nitrous oxide (1/1) for up to 7 d and plasma levels of methionine, glycine, serine, histidine, homocysteine and S-methylcysteine were measured. There was a fall in plasma methionine and a rise in plasma serine levels. There were no significant changes in glycine and histidine levels. Homocysteine and S-methylcysteine were not detected in rat plasmas. The fall in plasma methionine was due to loss of cobalamin-dependent methionine synthetase activity. The rise in plasma serine may be due to decline in its metabolism via methenyltetrahydrofolate cyclohydrolase which is concerned in oxidizing the methenyl-carbon ( =CH-), initially derived as a methylene-carbon (-CH2-)from serine, to formate (-CHO).