Studies on Adaptation of Urea Cycle Enzymes in the Rat

Previous papers in this Symposium have been devoted to a biochemical and genetic analysis of control mechanisms involved in the biosynthesis of arginine in Escherichia coli. I should like to report on some studies involving a group of enzymes catalyzing many of the same chemical reactions in rat liver, namely those concerned with the synthesis of urea: carbamyl phosphate synthetase, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinate cleavage enzyme, and arginase (for discussions of the enzymatic steps of urea synthesis and general problems of urea synthesis see Ratner [1955] and Cohen and Brown [1960]). This sequence of enzymes serves both biosynthetic and degradative functions in the rat, i.e., the synthesis of arginine, and the formation of urea, in which form amino nitrogen is excreted in mammals. It is the latter function with which I shall deal.