The Allosteric Inhibition by Calcium of Soluble and Partially Purified Adenylate Cyclase from Turkey Erythrocytes

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Abstract
Adenylate cyclase from turkey erythrocyte membranes was solubilized in Lubrol‐PX and partially purified (22‐fold) by molecular sieve chromatography on Biogel A5M. The molecular weight of the enzyme was found to be 316000. The partially purified solubilized enzyme was found to retain all the kinetic and regulatory properties of the native membrane‐bound enzyme except its sensitivity to β‐agonists. The enzyme responds to Mg2+ in a positively cooperative fashion, with a Hill coefficient of nH= 2.0. The enzyme is inhibited by Ca2+ in a positively cooperative fashion with a Hill coefficient of nH= 2.0. The calcium effect is only on the Kcat of the reaction and not on the binding and kinetic parameters of the enzyme towards the other ligands such as MgATP and Mg2+. The Mn2+‐supported adenylate cyclase is not inhibited by Ca2+ as was found for the native membrane‐bound enzyme.