title: Kinetics of irreversible activation of adenylate cyclase of fat cell membranes by phosphonium and phosphoramidate analogs of gtp1.
abstract: The ability of guanylylimidodiphosphate (GMP=P(NH)P) and guanylylmethylenediphosphonate (GMP-P(CH2)P) to activate adenylate cyclase activity has been studied by incubating these analogs with fat cell membranes followed by thorough washing of the membranes before assay of enzyme activity. GMP-P(NH)P is hydrolyzed by membrane preparations from several tissues. A pyruvate kinase regenerating system maintains the concentration of GMP-P(NH)P and thereby augments the ability of suboptimal concentrations of GMP-P(NH)P to activate adenylate cyclase. GTP inhibits activation of fat cell membrane adenylate cyclase by GMP-P(NH)P but this inhibition is overcome by time. This is consistent with the virtually irreversible nature of the GMP-P(NH)P activation, and with the inability of GTP to reverse the stimulated state of the enzyme. Although the initial rate of enzyme activation is highly dependent on the concentration of GMP-P(NH)P, with increasing times of incubation nearly the same maximal extent of activation is seen over a wide range of concentrations. Thus, it is not possible to estimate true affinity constants (at equilibrium) for GMP-P(NH)P, as anticipated from the virtually irreversible character of the activation process.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000273:Adipose Tissue / Q000187:drug effects / Q000201:enzymology*; D000324:Adrenocorticotropic Hormone / Q000494:pharmacology; D000577:Amides / Q000494:pharmacology*; D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology; D004789:Enzyme Activation / Q000187:drug effects; D004912:Erythrocytes / Q000201:enzymology; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D063065:Organophosphonates / Q000494:pharmacology*; D051381:Rats; D014422:Turkeys
chemical_list: D000577:Amides; D063065:Organophosphonates; D006160:Guanosine Triphosphate; D000324:Adrenocorticotropic Hormone; D000262:Adenylyl Cyclases; D008274:Magnesium; D007545:Isoproterenol