EFFECTS OF MAGNESIUM ON CYCLIC-GMP HYDROLYSIS BY THE BOVINE RETINAL ROD CYCLIC-GMP PHOSPHODIESTERASE

Knowledge of the kinetics of the rod cyclic GMP phosphodiesterase is e ssential for understanding the kinetics and gain of the light response . Therefore, the interactions between Mg2+, cyclic GMP, and purified, trypsin-activated bovine rod cyclic GMP phosphodiesterase (EC3.1.4.17) were examined. The effects of Mg2+ and of cyclic GMP on the rod phosp hodiesterase activity were mutually concentration-dependent. Formation of a free Mg-cyclic GMP complex is unlikely due to its high dissociat ion constant (K-d = 19 mM). Plots of 1/velocity versus 1/[cyclic GMP] as a function of [Mg2+] and 1/velocity versus 1/[Mg2+] as a function o f [cyclic GMP] intersected to the left of the 1/velocity axis. This is consistent with the formation of a ternary complex between the phosph odiesterase, Mg2+, and cyclic GMP. A competitive inhibitor of the phos phodiesterase relative to cyclic GMP, 3-isobutyl-1-methylxanthine, non -competitively inhibited the enzyme relative to Mg2+. Pb2+, a competit ive inhibitor of the phosphodiesterase relative to Mg2+ [D. Srivastava , R. L. Hurwitz and D. A. Fox (1995) Toxicol. Appl. Pharmacol, in the press] non-competitively inhibited the enzyme relative to cyclic GMP. Collectively these results are suggestive of a rapid equilibrium rando m binding order of Mg2+ and cyclic GMP to the rod phosphodiesterase.