Rats Fed a Low Protein Diet Supplemented with Sulfur Amino Acids Have Increased Cysteine Dioxygenase Activity and Increased Taurine Production in Hepatocytes

The metabolism of cysteine and cysteinesulfinate and the activities of key enzymes in cysteine catabolic pathways were investigated in hepatocytes isolated from rats fed a basal (100 g casein/kg) diet or the diet supplemented with L-methionine (3 or 10 g/kg diet) or the sulfur equivalent as L-cystine (2.4 or 8 g/kg diet). Cysteine dioxygenase activity was higher in hepatocytes from rats fed diets with the higher level of sulfur amino acid supplementation, and the higher enzyme activity was paralleled by a greater total catabolite production (taurine + sulfate) from cysteine. Taurine production as a percentage of total cysteine catabolism was significantly greater in hepatocytes from rats fed the diet with excess methionine or cystine (basal, 22%; excess methionine, 61%, excess cystine, 49%). Glutathione production was markedly lower in hepatocytes from rats fed excess sulfur amino acids such that total cysteine utilization was similar for all dietary treatments. Cysteinesulfinate decarboxylase activity and catabolism of cysteinesulfinate by hepatocytes were unaffected by the dietary supplementations. Results are in contrast to previous studies in which increased dietary protein resulted in decreased cysteinesulfinate decarboxylase activity and decreased partitioning of cysteinesulfinate to taurine vs. sulfate. Thus, sulfur amino acids may be less effective than protein in decreasing cysteinesulfinate decarboxylase activity and may result in a pattern of sulfur catabolite production from cysteine that favors taurine production.