Determination of the turn-off reaction for the hormone-activated adenylate cyclase

Previous work suggested that hormonal activation of adenylate cyclase involves the introduction of GTP to the regulatory site, and subsequent hydrolysis of the bound GTP terminates the activation. In many tissues the turn-off GTPase reaction cannot be readily measured because of a high background of nonspecific GTP hydrolysis. To circumvent this problem a general assay for the turn-off reaction has now been developed. The adenylate cyclase is first activated by hormone and GTP and the introduction of GTP is then stopped either by addition of an excess of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) or by addition of a receptor blocking agent. The decay of adenylate cyclase activity brought on by these inhibitors is used to calculate the rate constant of the turn-off reaction. In turkey erythrocyte and rat parotid membranes the rate constant of the decay process as determined with GDP beta S is similar to that determined with the beta-adrenergic blocker propranolol. The rate constants (min-1 at 30 degrees C) for various adenylate cyclase preparations are 10 for turkey erythrocyte, 7.5 for rat parotid, and 6.2 for the rat liver enzyme. The finding of similar rate constants in the various preparations indicates that GTP hydrolysis at the regulatory site is a general mechanism for terminating the activation of adenylate cyclase.