Activation of adenylate cyclase from purified platelet membranes by prostaglandin E₁ and its inhibition by L-epinephrine: Mechanistic effects

Activation and inhibition of adenylate cyclase in purified human platelet membranes by hormones and guanyl nucleotides was studied. The rate constant of enzyme activation (k(on) was measured using the GTP analog guanylimidodiphosphate (Gpp(NH)p), and the rate constant of enzyme deactivation (k(off) was determined using the guanosine diphosphate analog guanosine 5'-O-(2-thio)-diphosphate (GDPβS). PGE₁ which was found previously to accelerate k(on) has been found to accelerate also k(off), sec^-1 (6 min^-1) to 0.2 sec^-1 (12 min^-1). The α₂-adrenergic inhibitory hormone 1-epinephrine did not alter k(on), whether measured in the absence or presence of GTP, and also did not alter k(off), whether measured under basal or PGE₁-dependent GTP-ase activity at any concentration of GTP tested. These results suggest that the α₂-adrenergic receptor exerts its inhibitory effect on adenylate cyclase by a mechanism which does not involve its direct interaction with the GTP regulatory protein that is associated with the stimulatory hormone. These results support the view that the inhibitor involves the interaction of the inhibitory receptor with a GTP binding unit separate from the one mediating hormonal stimulation. This regulatory unit attenuates the adenylate cyclase activity either by interacting directly with the catalytic moiety or by modulating the interaction of the stimulatory GTP regulatory protein with the catalytic moiety.