Dl. Bella et al., VARIATIONS IN DIETARY-PROTEIN BUT NOT IN DIETARY-FAT PLUS CELLULOSE OR CARBOHYDRATE-LEVELS AFFECT CYSTEINE METABOLISM IN RAT ISOLATED HEPATOCYTES, The Journal of nutrition, 126(9), 1996, pp. 2179-2187
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.