The use of cyst(e)ine in the removal of protein-bound homocysteine.

Competition between homocyst(e)ine and cyst(e)ine for binding sites on plasma proteins was examined both in vivo and in vitro. Plasma-free cyst(e)ine concentrations were elevated in rats fed diets adequate or deficient in vitamin B6 containing 2.4% L-cystine; however, plasma protein-bound cysteine was not increased. Feeding high cystine diets did not slow the accumulation or decrease the concentration of plasma protein-bound or free homocyst(e)ine in vitamin B6-deficient rats. An in vitro experiment demonstrated that plasma protein-bound cysteine increased when plasma was incubated with increasing concentrations of cysteine, and decreased with increasing homocysteine concentrations. Plasma protein-bound homocysteine concentration was increased by increasing the concentration of homocysteine in the incubation medium; however, increasing the cysteine concentration failed to decrease bound homocysteine. The affinity of homocysteine for binding sites on plasma proteins appeared to be high because cysteine did not displace the bound homocysteine. Therefore, feeding a high cystine diet is unlikely to cause a decrease in bound homocysteine in homocystinuric patients due to competition for binding sites, but may still be beneficial because plasma cystine concentrations are below normal in individuals with homocystinuria.