RATS FED A LOW-PROTEIN DIET SUPPLEMENTED WITH SULFUR AMINO-ACIDS HAVEINCREASED 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 hepat ocytes isolated from rats fed a basal (100 g casein/kg) diet or the di et supplemented with L-methionine (3 or 10 g/kg diet) or the sulfur eq uivalent as L-cystine (2.4 or 8 g/kg diet). Cysteine dioxygenase activ ity was higher in hepatocytes from rats fed diets with the higher leve l of sulfur amino acid supplementation, and the higher enzyme activity was paralleled by a greater total catabolite production (taurine + su lfate) from cysteine. Taurine production as a percentage of total cyst eine catabolism was significantly greater in hepatocytes from rats fed the diet with excess methionine or cystine (basal, 22%; excess methio nine, 61%, excess cystine, 49%). Glutathione production was markedly l ower in hepatocytes from rats fed excess sulfur amino acids such that total cysteine utilization was similar for all dietary treatments. Cys teinesulfinate decarboxylase activity and catabolism of cysteinesulfin ate by hepatocytes were unaffected by the dietary supplementations. Re sults are in contrast to previous studies in which increased dietary p rotein 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 decre asing cysteinesulfinate decarboxylase activity and may result in a pat tern of sulfur catabolite production from cysteine that favors taurine production.