Synergistic activation of adenylate cyclase by guanylyl imidophosphate and epinephrine

Abstract

A kinetic analysis of the synergistic activation of turkey erythrocyte adenylate cyclase by 1-catecholamines and guanylyl imidodiphosphate (Gpp(NH)p) is described. The role of the catecholamine hormone is to facilitate the activation of the enzyme by the guanyl nucleotide according to the following mechanism: .**GRAPHIC**. where R is the receptor, E the enzyme, G the guanyl nucleotide effector and H the hormone. The binding steps are fast and reversible, but the conversion of the inactive enzyme E to its active stable form (E") occurs with a rate constant of k = 0.7 min-1. This step is essentially irreversible in the presence of high Gpp(NH)p concentrations. In the absence of .beta.-agonist (1-catecholamine) and at low free Mg2+ concentrations, the activation of the enzyme is insignificant. At high Mg2+ concentration the coversion of E to E" occurs slowly in the absence of hormone, probably by another pathway. Thus, the presence of guanyl nucleotide at the allosteric site is obligatory but not sufficient to induce the conversion of the inactive enzyme to its active form. The process of enzyme activation requires both Gpp(NH)p and hormone and under these conditions is essentially irreversible. The permanently active enzyme is stable in the absence of hormone and Gpp(NH)p, and its high catalytic activity is stable for many hours. However, hormone and ATP induce a conversion of the high activity to the low activity form. The process of enzyme activation by Gpp(NH)p and its reversal are probably hormone dependent. Both processes are blocked by the .beta.-blocker propranolol.