Effects of Mg2+, Mn2+ and Ca2+ on adenylcyclase activity

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Abstract
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 10−3M) reaction velocities increase progressively over an incubation period of 40 min. Optimal cation concentrations, however, i.e. 3×10−3M Mn2+ and 3×10−2M Mg2+ elicit a constant and at 10−4M 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 10−3M Mn2+ and 10−2M 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.

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