title: Activation of adenylate cyclase in human platelet membranes by guanosine 5'-[beta gamma-imido]triphosphate is inhibited by cyclic-AMP-dependent phosphorylation. Slow activation occurs in the absence of ATP.
abstract: Incubation of platelet membranes with guanosine 5'-[beta gamma-imido]triphosphate causes a slow increase in GS (stimulatory GTP-binding protein) activation of adenylate cyclase. Mg2+ is necessary for this slow activation. This process is inhibited in the presence of ATP, and inhibition is greater if cyclic AMP is also included in the incubation. Adenosine 5'-[beta gamma-imido]triphosphate instead of ATP in the incubation facilitates the slow activation in the presence of cyclic AMP, and incubation of membranes with cyclic-AMP-dependent protein kinase inhibitor decreased inhibition of the slow activation of adenylate cyclase by ATP and cyclic AMP. A protein of 45 kDa in platelet membranes is phosphorylated in a cyclic-AMP-dependent manner. The transition from a reversibly activated form of GS to an irreversibly activated form is substantially slower in the presence of ATP and cyclic AMP. We propose that guanosine 5'-[beta gamma-imido]triphosphate-activated GS may exist in phosphorylated or non-phosphorylated forms, and that the non-phosphorylated form is the more active of the two species. The non-phosphorylated form of GS may correspond to the irreversibly activated state.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000097:blood*; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology; D000242:Cyclic AMP / Q000494:pharmacology*; D004789:Enzyme Activation / Q000187:drug effects; D004837:Epinephrine / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology*; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D010766:Phosphorylation
chemical_list: D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D000262:Adenylyl Cyclases; D008274:Magnesium; D004837:Epinephrine