Effect of dietary proteins and amino acid deficiencies on urinary excretion of nitrogen and the urea synthesizing system in rats.

A mechanism of regulation in urea biosynthesis under conditions of varying dietary protein quality was investigated in rats adapted to 10% protein diets. A reverse correlation was observed between activities of urea cycle enzymes and urinary excretion of urea under these conditions. The concentrations of several non-essential amino acids in livers of rats fed a 10% gluten diet significantly increased as compared with those of rats fed a 10% whole egg protein diet without change in ornithine concentration. The contents of threonine, serine and ornithine in livers of rats fed a methionine-free diet were higher than those of rats fed a basal diet. The capacity of urea synthesis in vivo of rats fed gluten or a methionine-free amino acid mixture was the same as that of rats fed whole egg protein or basal amino acid mixture. The results suggest that under conditions of varying dietary protein quality, the liver level of free amino acids may regulate mainly the rate of urea synthesis without changes in activities of urea cycle enzymes and that the concentrations of ornithine in addition to free amino acids in liver may play an important role in an increase of urea synthesis in the methionine-deficiency.