VARIATIONS IN DIETARY-PROTEIN BUT NOT IN DIETARY-FAT PLUS CELLULOSE OR CARBOHYDRATE-LEVELS AFFECT CYSTEINE METABOLISM IN RAT ISOLATED HEPATOCYTES

To determine If previously observed effects of dietary protein on hepa tic cysteine metabolism were due specifically to increases in dietary protein or to the accompanying decreases in dietary carbohydrate, two experiments were conducted. In one experiment, rats were fed diets tha t contained different levels of protein vs. an isocaloric mixture of f at + cellulose and a constant amount of carbohydrate. In the other, ra ts were fed diets that contained a constant amount of protein but diff erent levels of carbohydrate vs. an isocaioric mixture of fat + cellul ose. Diets were fed for 2-3 wk and hepatocytes were then isolated. Hep atic cysteine dioxygenase activity increased and cysteinesulfinate dec arboxylase and gamma-glutamylcysteine synthetase activities decreased in a stepwise manner when protein was added to the diet at the expense of fat + cellulose. Changes in cysteine dioxygenase, cysteinesulfinat e decarboxylase and gamma-glutamylcysteine synthetase activities were consistent with changes in rates of cysteine catabolism, taurine produ ction and glutathione synthesis, respectively, by intact hepatocytes i ncubated with 0.2 mmol/L cysteine. When the carbohydrate to fat + cell ulose ratio was varied, but the protein level was held constant, littl e or no change in enzyme activities or levels of metabolite production was observed. Regulation of the activities of enzymes involved in cys teine metabolism is predominantly due to changes in dietary protein in take and not to the associated changes in intake of other dietary macr onutrients.