Effect of short-term administration of nitrous oxide on plasma concentrations of methionine, tryptophan, phenylalanine and S-adenosyl methionine in man.

Plasma concentrations of methionine, tryptophan, phenylalanine and S-adenosyl methionine (SAM) were measured at the beginning and end of general anaesthesia (for routine surgery) lasting between 25 and 217 min which included 60-70% nitrous oxide. Plasma methionine, tryptophan and phenylalanine concentrations in patients prepared for surgery averaged 68 (SD 11.3)% of values in a group of normal control subjects. At the end of administration of nitrous oxide (mean duration 88 min) there were no significant changes in the mean methionine and phenylalanine concentrations, but the mean concentration of tryptophan was reduced by 15% (P = 0.0075). SAM concentrations averaged 40% greater in males than females in all groups, but there were no significant differences between the concentrations in the control subjects and the patients before or after the administration of nitrous oxide.