| Abstract: | GTP-sensitive adenylate cyclases in liver membranes achieved by glucagon and by cholera toxin pretreatment displayed similar responses to added GTP in assay with respect to magnitude and sensitivity. However, their susceptibility to GTP formed during incubation from added GDP catalyzed by membrane-associated nucleoside diphosphate kinase(mNDPK) was different. Adenylate cyclase pretreated with cholera toxin was essentially unaffected by added GDP, while further addition of glucagon produced activation. GTP-stimulated adenylate cyclase activity in toxin-treated membranes was inhibited by added GDP, whereas glucagon addition reduced the inhibitory action of GDP by two orders of magnitude. Since neither pretreatment with toxin nor glucagon addition altered GTP formation by mNDPK, these observations suggest a possible presence of a mechanism by which hormone makes adenylate cyclase susceptible to the GTP formed via mNDPK for activation. |
