Effect of Substrate Analogues on the Kinetics of the Reaction Catalyzed by Adenosine Triphosphate: Arginine Phosphotransferase

A study has been made of the specificity of arginine kinase for both the nucleotide and guanidino substrates.The results have indicated that the magnesium complexes of dADP, UDP, GDP, and CDP, as well as MgADP-, can function as acceptors of the phosphoryl group of phosphoarginine.The initial velocity data obtained with each of the magnesium-nucleoside diphosphate complexes have been analyzed on the basis that the reaction mechanism is rapid equilibrium random, irrespective of the structure of the nucleotide and values obtained for the various kinetic parameters.The values indicate that while the maximum velocities of the reactions differ markedly according to the identity of the nucleotide, all these substrates combine more readily with free enzyme than with the enzyme-phosphoarginine complex.MgCDP-, MgGDP-, and MgUDPhave also been used as inhibitors of the reaction in the presence of MgADP-.The resulting inhibition patterns and values determined for the inhibition constants lend support to the assumption that all reactions have a rapid equilibrium, random mechanism.AMP and magnesiuminorganic pyrophosphate were found to act as inhibitors.It appears that the enzyme has an absolute specificity for L-arginine since no reaction was obtained with D-arginine, canavanine, or creatine.However, both creatine and phosphocreatine can react with arginine kinase as judged by the fact that both these guanidino compounds are capable of inhibiting the reaction. Afethods Aleasurement oj InitialVelocity-Initial velocities of the reaction in the reverse direction were determined at 30" by estimating 7764