Dietary and hormonal regulation of urea cycle enzymes in rat liver.

When 6-week-old rats fed normal diet (22% protein) were transferred to 10 and 75% protein diet, the levels of urea cycle enzymes showed decreases and increases respectively. The activities expressed as units per gram wet weight of liver had not stabilized after 7 days on the new diet; the corresponding specific activities were closer to leveling off. Four daily injections of cortisol raised CPS, ASS, and arginase. The percentage increases were greater on a 10 than on a 75% protein diet. Adrenalectomy of rats fed 10% protein decreased all urea cycle enzymes; on 75% protein, only arginase decreased. All enzymes could be raised to control levels within 24 h by three injections of cortisol. Thyroxine produced only slight increases in urea cycle enzymes. On a 10% protein diet, all five enzymes were raised by thyroidectomy, and further raised by injection of thyroxine.