Effects of cadmium on canine renal cortical adenylate cyclase

The present studies examine the effects of cadmium (Cd2+) on adenylate cyclase activity in basolateral renal cortical membranes from normal dogs. Cd2+, in the dose range of 1 to 200 microM caused a dose-dependent inhibition of adenylate cyclase activity due to competitive inhibition with respect to the allosteric activator Mg2+ (increase in the Kact for Mg2+ from 13.8 to 37.2 mM). In addition, increasing Cd2+ concentrations from 0 to 25 microM resulted in a purely competitive inhibition with respect to ATP. The Km for ATP increased from 61 to 128 microM. When Mn2+ at a concentration of 20 mM was substituted for Mg2+, Cd2+ maintained its inhibitory effect at concentrations lower than 25 microM. At higher concentrations it stimulated enzyme activity in a dose-dependent fashion. In the absence of other divalent cations Cd2+ was a potent stimulator of basal adenylate cyclase activity, far more potent than the physiological activator of the system Mg2+. Kact for Cd2+ was 1.0 mM (Kact for Mg2+ 20 mM), whereas the Vmax showed a fourfold increase compared with the Mg2+-stimulated membranes. However, under these conditions, synthetic bovine parathyroid hormone (1-34) or 5'-guanylylimidodiphosphate did not stimulate adenylate cyclase. It is concluded that Cd2+ behaves as a partial agonist in this system, due to its ability to form a new enzymatic substrate complex: Cd-ATP, which competes with the physiological substrate Mg-ATP at the catalytic site of the enzyme. In addition, Cd2+ in the absence of other divalent cation stimulates basal enzyme activity, presumably through interaction at an additional site, closely related to the allosteric metal regulatory site of this enzyme system.