Synthesis of Cysteine from Methionine in Normal Adult Subjects: Effect of Route of Alimentation

Parenteral alimentation solutions free of cysteine, probably an essential amino acid for premature infants, were administered continuously to eight healthy men through catheters in the superior vena cava and through nasogastric tubes. When the preparation was administered parenterally, the plasma cystine concentration dropped markedly. When feeding was switched to the oral route, the concentration rose immediately, but returned to baseline only when a cystine-containing diet was fed. These studies indicate that the synthesis of cysteine from methionine is limited, even in the adult subject, when cystine-free diets are administered parenterally.