Effect of cystine on the metabolism on methionine in rats
Stipanuk, M.H.; Benevenga, N.J.
Journal of Nutrition 107(8): 1455-1467
1977
ISSN/ISBN: 0022-3166 PMID: 18564 Document Number: 121135
Rats were fed on a 0.6% methionine (Met) or 0.3% Met + 0.51% cystine (Cys) diet for 10 days. Hepatic methionine adenosyltransferase activity was similar in both groups while cystathionine synthase activity was depressed by 34 to 47% in the group on the diet with cystine. Methionine metabolism was studied by using L-amino acid diets labelled with L-[1-14C]methionine. Recovery of label in expired CO2 during 8 h was 21.7 plus or minus 0.6, 15.6 plus or minus 0.4, 25.7 plus or minus 1.0, and 24.6 plus or minus 1.2 for rats on diets with 0.3% Met + 0.5% threonine (Thr), 0.3% Met + 0.51% Cys + 0.5% Thr, 0.3% Met + 0.1% Thr, and 0.3% Met + 0.51% Cys + 0.1% Thr, respectively. Cystine supplementation of the diets containing limiting threonine (0.1%) had no effect on recovery of 14C whereas supplementation of the diets containing adequate threonine (0.5%) depressed methionine oxidation by 39%. The decrease in methionine oxidation and increase in muscle protein specific activity with cystine supplementation in rats on diets first-limiting in S amino acids, and not in rats on diets first-limiting in threonine, suggests that the rate of methionine oxidation is regulated primarily by the utilization of methionine for protein synthesis. The activity of cystathionine synthase is clearly not rate-limiting for methionine oxidation under usual conditions.