Effect of excess dietary methionine on the catabolism of cysteine in rats.

The effect of excess methionine (MET) on cysteine (CYS) catabolism was investigated in rats prefed either a control (10% casein + 0.3% L-MET) or high-MET (10% casein + 3.0% L-MET) diet for 50 or 20 days. The activities of cysteine dioxygenase, cysteine desulfhydrase and cystathionase were increased in high-MET rats to levels 5.9, 2.7 and 2.7 times, respectively, those of control rats after 5 days and to 2.9, 2.0 and 2.7 times control levels after 20 days. Cysteine aminotransferase and cystathionine synthase activities were increased to 1.5 and 1.7 times control values after 5 days but were not significantly different from control values at 20 days. Following gastric intubation of 5 g of an L-amino acid diet containing 0.2% L-[35S]CYS, the 24-hour urinary exretion of 35SO4, [35S]taurine and total 35S (% of administered dose) and the [35S]taurine:35S and [35S]taurine:35SO4 ratios were increased in rats prefed excess MET for 5 or 20 days. When 2.6% L-[35S]CYS was administered similarly, no significant differences between high-MET and control rats were observed. However, the [35S]taurine:35S and [35S]taurine:35SO4 ratios were elevated in both high-MET and control rats given 2.6% L-[35S]CYS over those for control rats fed 0.2% L-[35S]CYS. The increase in cysteine dioxygenase activity and the increase in [35S]taurine:35SO4 ratio in rats fed excess MET or given a load dose of CYS suggest that the cysteine sulfinic acid pathway plays a major role in the regulation of CYS degradation.