The effect of dietary cysteine level on cysteine metabolism in rats.

Male, Sprague-Dawley rats were fed L-amino acid diets that contained 0.4% L methionine and either 0, 0.2% (control), or 2.6% L-cysteine (free base) for 5 or 20 days. Hepatic cysteine dioxygenase activity in rats fed 2.6% cysteine was 5 and 3-times as great as in pair-fed control rats at 5 and 20 days, respectively. Cysteine sulfinate decarboxylase activity in liver of rats fed 2.6% cysteine for 20 days was about 40% of the pair-fed control level. The activity of cysteine sulfinate aminotransferase and the rate of cysteine desulfhydration were not influenced by dietary cysteine content. Rats that had been fed these diets for 5 days were intubated with 5 g of diet that contained either 0.2% or 2.6% L-[35S] cysteine. The 24-hour urinary excretion of total 35S, 35SO4, and [35S] taurine by rats that had been fed 2.6% cysteine for 5 days and intubated with 0.2% L[35S] cysteine was 1.4-, 1.8-, and 2.4-fold, respectively, that of pair-fed control rats. About 2.1, 1.8, and 9.1 times as much 35S, 35SO4, and [35S] taurine, respectively, were excreted by rats fed 2.6% cysteine for 5 days and intubated with 2.6% L-[35S] cysteine as was excreted by pair-fed control rats. Urinary excretion of these metabolites was not different between rats fed 0 and 0.2% cysteine. These results indicate that the cysteine sulfinate pathway plays a major regulatory role in the metabolism of excess cysteine.