Methionine metabolism in rainbow trout fed diets of differing methionine and cystine content.

The metabolism of methionine was studied in rainbow trout fed diets containing different levels of methionine and cystine. Growth data indicated that methionine requirement was between 0.5 and 1% dry diet in the absence of dietary cystine, but 0.5% was adequate when dietary cystine was 2%. In fish fed diets deficient in sulfur-containing amino acids, elevated hepatic activities of glutathione reductase were found, whereas glutathione peroxidase and glutathione levels were unaffected. Plasma and liver concentrations (18 hours after feeding) of methionine and cystine were affected by dietary methionine, but dietary cystine had little effect. Cystine appeared to be converted to taurine in the liver. In fish injected intraperitoneally with [14COOH]- and [14CH3]methionine, over 24 hours the carboxyl group was oxidized more than the methyl group and more was incorporated into protein. However, much more of the methyl group was incorporated into the lipid fraction. The results suggest the operation of the transsulfuration pathway of methionine catabolism. Oxidation of methionine was related to its concentration in the tissues, and little affected by dietary cystine. A 28-day experiment on the metabolism of injected [14COOH]methionine showed that its turnover was slow, and much of the radioactivity was associated with protein.