1. The mixed disulphide of cysteine and homocysteine is known always to be present in the plasma of patients with homocystinuria, an abnormality of methionine metabolism. Recently we have shown that it is also detectable in low concentration in the plasma of normal fasting man. In the present study we measured mixed disulphide concentrations after an overnight fast in 24 normal men and compared the findings with those obtained in 24 normal premenopausal women of similar age.
2. The mean value for men (± sd) of 3·3 ± 0·8 μmol/l was significantly higher than that for women (2·4 ± 0·7 μmol/l; P < 0·001). Of the other neutral and acidic amino acids measured mean values for leucine, isoleucine and valine (P < 0·001) and cystine (P < 0·01) were also higher in the men but methionine concentrations were not significantly different.
3. The higher branched-chain amino acid concentrations in men could be related to larger muscle bulk and protein intake, but the higher cysteine—homocysteine mixed disulphide concentrations are consistent with differences in methionine metabolism between men and women under the age of 50 years.