The detection of cysteine‐homocysteine mixed disulphide in plasma of normal fasting man

Abstract

Cysteine-homocysteine mixed disulphide, formed in the degradation of methionine, is detected routinely in the plasma of fasting patients homozygous for homocystinuria and in some obligate heterozygotes. It has not hitherto been identified in the plasma of normal fasting man. Using a highly cross-linked resin with Li citrate buffers on a JEOL Amino Acid Analyser, the mixed disulphide was detected in every one of the plasma samples from 20 normal fasting subjects. The mean concentration was 3.25 .mu.mol/1 (SD 0.85, N = 20), with a range of 1.68-4.85 .mu.mol/1. The other neutral and acidic amino acids were within the accepted range. Circulating homocysteine is apparently normally not immediately transformed to cystathionine or remethylated to methionine; some combines with cysteine to form measurable amounts of mixed disulphide. Since homocysteine may produce endothelial damage, the present findings could be relevant to understanding the pathogenesis of vascular disease.