Regulation of Enzymic Activity by Metabolites

A kinase which catalyzes the phosphorylation of uridine and cytidine to uridine S-phosphate and cytidine 5'-phosphate, respectively, has been purified 97-fold from extracts of the Novikoff ascites rat tumor.Several nucleoside triphosphates may be used as phosphate donors; among them ATP, dUTP, dGTP, and dATP are the most effective.In contrast, UTP and CTP are potent inhibitors of the kinase.Studies on kinetics showed that the inhibition by CTP was competitive with respect to concentration of ATP and not competitive with respect to concentration of phosphate acceptor.Upon aging of the enzyme, the inhibitory effect of CTP was partially lost and CTP became effective as a phosphate donor; the catalytic activity of the enzyme was retained.From the evidence presented, it has been proposed that there exist at least three distinct binding sites on the surface of the enzyme molecule; one for the phosphate donor, one for the phosphate acceptor, and a third one for the regulatory ligand.These findings may have physiological implications in that the anabolism of pyrimidine ribonucleosides is regulated by the terminal products of the pathway, UTP and CTP.Some enzymic proteins are subject to modulation of their catalytic activities by interaction with compounds which are not substrates or products of the reaction catalyzed.As Monod, Changeux, and Jacob (1) pointed out, such a property of the protein molecule, when the regulatory compound is a terminal product of the metabolic sequence involved, may provide an expla.nation of the fine metabolic coordination displayed by living cells.The terminal product of the pyrimidine nucleotide biosynthetic pathway, i.e.