title: Effect of hormone and guanyl nucleotide pretreatment on the activation energy of pancreatic adenylate cyclase.
abstract: The activation energy of adenylate cyclase by p[NH]ppG in rat pancreatic plasma membranes was estimated to be 141-189 kj/mol. When a high concentration of secretin or CCK-8 (C-terminal octopeptide of cholecystokinin-pancreozimin) was added to the assay medium, the activation energy was reduced to 73 kj/mol. This hormone effect was exerted on the activation energy of the activation process of adenylate cyclase by p[NH]ppG. Indeed, when plasma membranes were preactivated with p[NH]ppG alone or with p[NH]ppG and CCK-8 and then washed, there resulted a persistent activation with low activation energy (65 and 48 kj/mol, respectively). A similar low activation energy was observed in membranes preincubated with GMP and CCK-8. The latter treatment could not induce persistent activation but facilitated the activation by p[NH]ppG, suggesting that the step of p[NP]ppG activation requiring a high activation energy in the absence of hormone had developed during preincubation with GMP and CCK-8, and had not been reversed by membrane washing. By contrast, EDTA pretreatment did not influence p[NH]ppG activation while provoking a reversible deactivation of persistently activated adenylate cyclase.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D002766:Cholecystokinin / Q000494:pharmacology*; D004492:Edetic Acid / Q000494:pharmacology; D004789:Enzyme Activation; D006150:Guanine Nucleotides / Q000494:pharmacology*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D010179:Pancreas / Q000201:enzymology*; D051381:Rats; D012633:Secretin / Q000494:pharmacology*
chemical_list: D006150:Guanine Nucleotides; D012633:Secretin; D006165:Guanylyl Imidodiphosphate; D002766:Cholecystokinin; D004492:Edetic Acid; D000262:Adenylyl Cyclases