ACTIVATION OF ADENYLATE-CYCLASE BY ALCOHOLS REQUIRES THE NUCLEOTIDE-BINDING PROTEIN

Abstract

Ethanol activated adenylate cyclase in mouse striatal membranes, but significant activation of adenylate cyclase by ethanol concentrations below 500 mM was found only in the presence of 5''-guanylylimidodiphosphate [Gpp(NH)p] or other guanine nucleotides. Ethanol did not alter the amount of guanine nucleotide required for half-maximal activation of adenylate cyclase, but further increased adenylate cyclase activity under conditions wherein the nucleotide binding protein was preloaded with Gpp(NH)p or when hydrolysis of added GTP was blocked using cholera toxin. The stimulation of adenylate cyclase activity by NaF was also accentuated by ethanol. Ethanol, propanol and butanol all increased adenylate cyclase activity in the presence of Gpp(NH)p, and their effects on adenylate cyclase activity were linearly correlated with their respective C chain lengths. Equivalent membrane concentrations of ethanol and chloroform produced similar increases in adenylate cyclase activity under conditions where hydrolysis of added GTP was inhibited. Chloroform and ethanol had opposite effects on adenylate cyclase activity in assays containing GTP and membranes not treated with cholera toxin. The apparent Km of adenylate cyclase for Mg-ATP and the Arrhenius activation energy for the enzyme in membranes incubated with Gpp(NH)p were similar in the presence and absence of ethanol. Ethanol. in concentrations up to 750 mM, did not alter the pattern of stimulation of adenylate cyclase by Ca and calmodulin. Ethanol apparently modifies the equilibrium for the interaction of the nucleotide-loaded G-protein with the catalytic unit of adenylate cyclase to favor formation of the active nucleotide-G-protein-catalytic unit complex. Chloroform appears to prevent normal nucleotide-G-protein interaction but seems to act similarly to ethanol if the binding of nucleotide to the G-protein occurs before the addition of chloroform to the enzyme assay.