Hypercysteinemia and delayed sulfur excretion in cirrhotics after oral cysteine loads.

Biosynthesis of cysteine from methionine via the hepatic transsulfuration pathway is impaired in some cirrhotic patients, who therefore might require cysteine in the diet. However, because further metabolism of cysteine also occurs primarily in the liver, the metabolic clearance of this amino acid could be impaired in cirrhosis. We administered oral loads of L-cysteine to cirrhotic patients and healthy volunteers. Plasma cyst(e)ine (free and protein-bound cysteine, and 1/2 cystine) and urinary sulfur-containing constituents were measured at various times postload. Cirrhotic subjects exhibited a greater maximal plasma cyst(e)ine concentration and plasma elimination half-life (t1/2) and a delayed excretion of metabolic end products after an oral L-cysteine load. The postload increase in total plasma cyst(e)ine was accounted for primarily by an increase in the disulfide form (cystine). These studies show that cirrhotics have an impaired ability to clear cyst(e)ine from the plasma.