Hepatic cysteine sulfinic acid decarboxylase activity in rats fed various levels of dietary casein.

A series of experiments was conducted to examine the effects of dietary protein intake on hepatic cysteine sulfinic acid decarboxylase (EC 4.1.1.29) activity and urinary taurine excretion. When rats were fed diets containing 18, 30, 45 or 60% casein for 1 wk, hepatic cysteine sulfinic acid decarboxylase activity (CSAD) decreased in a progressive and significant manner. Enzyme activity in rats fed a 60% casein diet was 25% of the activity measured in rats fed 18% casein. The time course of the change in CSAD activity was examined in rats fed a 60% casein diet. Within 24 h of switching rats from a moderate (18% casein) to high (60% casein) protein diet, enzyme activity decreased by 50% and continued to decline in rats fed the high protein diet for 7 d. The observed decrease in enzyme activity was reversed when rats were refed the 18% casein diet. The half-life of CSAD was calculated to be 2 and 7 d during the diet switch from 18 to 60% casein and from 60 to 18% casein, respectively. The change in enzyme activity was evident after a single high protein meal. In contrast to CSAD activity, urinary taurine excretion increased 140-fold within 2 d of switching rats from an 18 to 60% casein diet. Upon refeeding of the 18% casein diet taurine excretion rapidly decreased. These findings indicate that CSAD responds in a rapid and reversible manner to dietary protein.