Effects of Mg2+, Mn2+ and Ca2+ on adenylcyclase activity. Evidence for a metallic site

In membrane preparations from immature erythrocytes from rats the effects of the divalent cations Mg2+, Mn2+ and Ca2+ on basal activity of adenylcyclase as well as on enzyme activity stimulated by isoprenaline (Ipn) or guanylyl-imidodiphosphate [Gpp(NH)p] were investigated.--Mn2+ is a ten-fold stronger activator of the enzyme than Mg2+ irrespective of the stimulant used. At suboptimal concentrations of the cations at all concentrations of Gpp(NH)p used (10(-6) to 10(-3) M) reaction velocities increase progressively over an incubation period of 40 min. Optimal cation concentrations, however, i.e. 3 x 10(-3) M Mn2+ and 3 x 10(-2)M Mg2+ elicit a constant and at 10(-4) M Gpp(NH)p maximal reaction velocity. In contrast, the Ipn-stimulated cAMP synthesis proceeds linearly at all Ipn concentrations used; a change of cation concentrations elicits only a change in reaction velocity, which is maximal at 10(-3) M Mn2+ and 10(-2) M Mg2+ respectively.--Ca2+ inhibits adenylcyclase activity in a non-competitive manner, irrespective of the stimulant and ion concentration used. The Mg2+-activated enzyme, however, is more susceptible to the inhibiting effect of Ca2+ than the Mn2+-activated enzyme.--It is concluded that Mn2+ and Mg2+ are allosteric effectors of the enzyme adenylcyclase, acting at a Me2+-site of the catalytic unit of adenylcyclase.