Tissue nonprotein sulfhydryl content and weight gain of rats as affected by dietary methionine level

The effect of dietary methionine level on tissue nonprotein sulfhydryl content (NPSH) and weight gain was systematically evaluated in young adult rats (approximately 360 g) fed amino acid diets. In 28-day feeding experiments, weight gain and liver and skeletal muscle NPSH increased, but blood NPSH decreased as dietary methionine rose from 0 to 0.8%. The requirement for weight maintenance (0.2% methionine) did not sustain maximum liver and skeletal muscle NPSH, whereas the requirement for maximum weight gain (0.6% methionine) did. Maximum skeletal muscle NPSH was attained by 0.4% methionine and maximum liver NPSH by 0.5% methionine. In another experiment, diets containing 0.4, 0.5, and 0.6% methionine were fed for 1, 8, 29, and 50 days. Liver and skeletal muscle NPSH were lower, whereas blood NPSH was higher with the 0.4% methionine diet. These differences in NPSH were significant at all times for liver, at 8 days for skeletal muscle, and at 29 days for blood. Weight gain did not differ significantly among the groups at any time. In all experiments, weight gain was similar with 0.4 and 0.5% methionine even though liver NPSH was 40-50% higher with 0.5% methionine. The data suggest that tissue NPSH may serve as a cysteine reservoir and spare dietary sulfur-containing amino acids during marginal intake. Also, weight gain may be an unreliable measure of sulfur-containing amino acid needs under some circumstances.