Adenylate cyclase system of bovine adrenal plasma membranes

The adenylate cyclase system present in a preparation enriched in plasma membranes derived from bovine adrenal cortex was investigated in considerable detail. This system is stimulated by adrenocorticotropic hormone (ACTH), by biologically active analogs of this hormone, and by fluoride ion. The preparation contains sodium-potassium- and magnesium-dependent ATPases that are markedly inhibited by 50 mM sodium fluoride. Incorporation of a pyruvate phosphokinase ATP generating system into the adenylate cyclase assay medium provided constant substrate levels. In the presence of the ATP generating system, the rate of cyclic AMP formation (basal, fluoride, and ACTH-activated) was proportional to enzyme concentration and was linear with time. Proportionality with respect to enzyme concentration as concerned the hormone-activated adenylate cyclase was achieved only when the ratio of hormone to enzyme protein was kept constant. The temperature optimum of the adenylate cyclase, basal or activated, was approximately 30 degrees. Michaelis-Menten kinetics were observed when the ratio of Mg2+ to ATP was approximately 6:1. Both calcium and ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid completely inhibited the adenylate cyclase system at concentrations of 5 and 0.5 mM, respectively. GTP was inhibitory at concentrations of 10-2 M but had little effect at lower concentrations. Freezing in liquid nitrogen and storage at -60 degrees exerted little effect on the fluoride-stimulated enzyme but lowered hormone stimulated activity. Preincubation in the presence of ACTH afforded a high degree of stabilization of the enzyme system while preincubation with a biologically inactive analog afforded no protection.