The Metabolism of Sulfur Xxiii. the Influence of the Ingestion of Cystine, Cys- Teine, and Methionine on the Excretion of Cystine in Cystinuria* by Howard

The recognition of methionine as an important building stone of the protein molecule is one of the most significant advances in our knowledge of the chemistry of the biologically important sulfur compounds (1). Methionine has been shown to function similarly to cystine both for purposes of growth in rats fed a diet inadequate in its cystine content (2) and for the detoxication of monobromobensene (3). Thus a close relationship, in function, at least, between these two amino acids of the protein molecule may be assumed. The recent studies of Brand and associates, which are concerned with the fate of methionine, cystine and cysteine (4), and homocystine and homocysteine (5) in the cystinuric individual, have furnished still further evidence of the interrelationship of the various sulfur-containing amino acids and have indicated a possible origin of the cystine excreted in this perplexing error of metabolism. It has been difficult to correlate the failure of cystine, when fed to a cystinuric patient, to increase the urinary cystine and its ready oxidation to and excretion as sulfate with the increased elimination of urinary cystine which has been observed repeatedly to occur when a diet high in its content of protein (and presumably cystine) has been fed (6). Brand (4)