Plasma cyst(e)ine in homocyst(e)inemia

Using a method developed to measure plasma protein-bound ½-cystine, we have observed that the metabolic state can affect the distribution of plasma cyst(e)ine between the bound and the free form. In five patients with homocyst(e)inemia, ½-homocystine was found to have replaced virtually all the cysteine normally present on sulfhydryl-binding sites of plasma proteins. The majority of the cyst(e)ine measured in the plasma of these patients (6.9 ± 2.4 µmol/dl, X̄ ± SD) was present in the form of a mixed disulfide with ½-homocystine (5.4 ± 0.9 µmol/dl) rather than being free in the plasma or bound to plasma proteins.