Redox status and protein binding of plasma homocysteine and other aminothiols in patients with hyperhomocysteinemia due to cobalamin deficiency

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Abstract
We determined reduced, oxidized, and protein-bound homocysteine, cysteine, and cysteinylglycine in plasma from 13 patients with hyperhomocysteinemia (total homocysteine in the range 30.6–159.8 µmol/L) due to cobalamin deficiency. Reduced homocysteine (x̄ ± SD: 1.87 ± 2.06 µmol/L) was markedly above normal (0.24 ± 0.12 µmol/L) in most patients, and the reduced fraction increased as an exponential function of the total homocysteine concentration. The ratio of reduced homocysteine to total homocysteine was positively correlated with the reduced-total ratio for cysteine and cysteinylglycine, suggesting redox equilibrium between different aminothiol species. The free oxidized and the protein-bound forms of homocysteine account for most of the homocysteine in plasma of these patients. The amount of protein-bound homocysteine was negatively correlated with the concentrations of both protein-bound cysteine and cysteinylglycine, indicating displacement of these aminothiols by homocysteine.