Prostaglandin-stimulated GTP hydrolysis associated with activation of adenylate cyclase in human platelet membranes

In membranes purified from human blood platelets, basal guanosine triphosphate (GTP) hydrolysis is reduced by a factor of ≃6 by exposure to N-ethylmaleimide (10 mM). This decreased background enables the detection of an additional GTP hydrolysis in the presence of prostaglandin E₁ (PGE₁). The PGE₁-stimulated GTPase has several properties correlated with PGE₁-stimulated adenylate cyclase in this preparation. The two enzymes have similar dose-response relationships (half-maximal stimulation at 0.1 μM PGE₁). Exposure to cholera toxin blocks the PGE₁-stimulated GTPase and activates adenylate cyclase. Both enzymes are activated by submicromolar concentrations of GTP, although the K(m) for the GTPase is about 10 times greater than that for the adenylate cyclase. The data are discussed in relation to the hypothesis that hormone-stimulated adenylate cyclase (i) is activated as a regulatory component binds a molecule of GTP and (ii) is deactivated as this molecule is hydrolyzed.