The Administration of Radioactive L-Cystathionine to a Human Cystinuric

The present communication is concerned with experimental work carried out some years ago, in which cystathionine labeled with radioactive sulfur was administered to a human cystinuric (4). In 1936, Brand, Block, Kassell, and Cahill (3) advanced the hypothesis that carboxyaminopropyl-S-cysteine [later called cystathionine (2)] might be an intermediate in the formation of cysteine from methionine in the body. The synthesis of L-cystathionine (5) made it possible to subject this interesting hypothesis to experimental test. It was first demonstrated that cystathionine may be used in lieu of dietary cystine for the growth of rats (5) and subsequently that slices and extracts of rat liver are capable of cleaving cystathionine to cysteine (5). Tracer experiments then demonstrated that the sulfur of cystathionine may be utilized for cystine synthesis by the rat (10). In the present report direct evidence is offered that L-cystathionine serves as a source of sulfur for cystine synthesis