The Metabolism of Methionine by Single Comb White Leghorn and Black Australorp Chicks

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Abstract
The utilization of methionine for synthesis of cystine by Single Comb White Leghorn and Black Australorp chicks was studied, using a low-cystine content, semipurified diet. A growth response was obtained from methionine supplementation of the basal diet. With either breed of chick no additional growth response was obtained from cystine supplementation of a diet containing adequate methionine. Experiments using L-methionine S35 to study the utilization of methionine for cystine synthesis showed a breed difference in the amount of methionine converted to cystine. More of the radioactive methionine was converted to cystine S35 by Single Comb White Leghorn chicks than by the Black Australorp chicks. The cystine S35 isolated from the carcass hydrolysate of Single Comb White Leghorn chicks had a higher specific activity than that isolated from the carcass hydrolysate of the Black Australorp chicks. Because of the good growth rate obtained with methionine supplementation of a low-cystine content basal diet, the conversion of methionine to cystine appears to be adequate to meet the physiological needs for growth in both breeds studied.