On the origin of the carbon chain of cysteine in the rat.

Evidence has been presented to show that in rat liver preparations serine is probably condensed with homocysteine to yield cystathionine which is then cleaved to cysteine (1). Employing N16-labeled serine, Stetten found the isotopic nitrogen in the cystine isolated from rat tissues (2). These data served as the basis for the suggestion that the carbons of cystine originate from those of serine. Sakami demonstrated the synthesis of serine from glycine and ‘(formate” (3) in rat liver, and, later, the same worker showed that serine carbons are synthesized in the rat from glycine and the carbon of the methyl group of choline (4). Prior to that direct evidence was presented to show that the methyl group of sarcosine is oxidized in rat liver preparations to yield formaldehyde (5). In the present report, data will be described to demonstrate the synthesis of the carbons of cysteine from glycine and the methyl group carbon of methionine in the intact rat.