title: Activation of rat liver adenylate cyclase by guanosine 5'-[beta,gamma-imido]triphosphate and glucagon. Existence of reversibly and irreversibly activated states of the stimulatory GTP-binding protein.
abstract: The effects of guanosine 5'-[beta-thio]diphosphate (GDP[S]) on the kinetics of activation of rat liver membrane adenylate cyclase by guanosine 5'-[beta,gamma-imido]triphosphate (p[NH]ppG) were examined. GDP[S] caused immediate inhibition of the activation by p[NH]ppG at all time points tested. Substantial inhibition by GDP[S] was observed even after the time required for the enzyme to reach its steady-state activity, but the extent of inhibition became progressively smaller as the preincubation time with p[NH]ppG increased. The rate at which adenylate cyclase became quasi-irreversibly activated was a strictly first-order process. In the presence of glucagon, the formation of the irreversibly activated state was much slower. A combination of GDP[S] and glucagon could partially reverse the quasi-irreversible activation by p[NH]ppG. Glucagon decreased the lag time required for p[NH]ppG to activate adenylate cyclase and increased the extent of activation by p[NH]ppG. This stimulatory effect of the hormone on top of guanine nucleotide decreased on preincubation with p[NH]ppG, but not with GTP. Our results suggest that the activation of adenylate cyclase by non-hydrolysable GTP analogues is a two-stage process: the formation of a reversibly activated form (G rev) is a rapid process, followed by a much slower formation of the quasi-irreversibly activated form (G irr). Glucagon can stimulate G rev but not G irr, and can partially facilitate the formation of the G rev from the G irr state.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D000242:Cyclic AMP / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005934:Glucagon / Q000494:pharmacology*; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D051381:Rats; D013873:Thionucleotides / Q000494:pharmacology
chemical_list: D013873:Thionucleotides; D006153:Guanosine Diphosphate; C032829:6-thioguanosine 5'-diphosphate; D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D005934:Glucagon; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases