title: Hormone receptor modulates the regulatory component of adenylyl cyclase by reducing its requirement for Mg2+ and enhancing its extent of activation by guanine nucleotides.
abstract: N-Ethylmaleimide treatment of rat liver plasma membranes results in an adenylyl cyclase (EC 4.6.1.1) system that shows no measurable cyclizing activity but retains both an active glucagon receptor and a receptor-sensitive regulatory component N as assessed by reconstitution into cyclase-negative (cyc-) membranes from S49 murine lymphoma. Treatment of such N-ethylmaleimide-treated membranes, termed C- liver membranes, with guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S] ) and Mg2+, followed by the removal of GTP[gamma S] by washing, yields an activated N which upon mixing with cyc- S49 membranes reconstitutes the cyc- S49 membrane adenylyl cyclase in the absence of added GTP[gamma S]. It was found that GTP[gamma S] activation of the N at saturating concentrations of GTP[gamma S] is slow at low Mg2+ concentration and accelerated by increasing Mg2+ concentrations. Addition of glucagon during the activation results in a lowering of the Mg2+ requirement for full activation from 25 mM to around 10 muM and in concomitant increases in both the rate and the extent of N activation. In contrast to its dramatic effect on Mg2+ requirement, glucagon has little (less than 2-fold) effect on the GTP[gamma S] requirement of N activation. These experiments indicate that the glucagon receptor facilitates activation of N by: (i) decreasing the apparent Km of N for Mg2+, and (ii) increasing the extent of activation that can be elicited by saturating concentrations of guanine nucleotide. It is postulated that the mechanism by which Mg2+ and receptors facilitate N activation involves dissociation of n alpha activated ADP-ribosylatable subunits (with guanine nucleotide bound to them) from n beta non-ADP-ribosylatable subunits (with receptor and Mg2+ bound to them).
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D005033:Ethylmaleimide / Q000494:pharmacology*; D005934:Glucagon / Q000378:metabolism* / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008223:Lymphoma / Q000378:metabolism*; D008274:Magnesium / Q000494:pharmacology*; D051379:Mice; D009374:Neoplasms, Experimental / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D018027:Receptors, Glucagon; D013873:Thionucleotides / Q000494:pharmacology*
chemical_list: D011956:Receptors, Cell Surface; D018027:Receptors, Glucagon; D013873:Thionucleotides; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D005934:Glucagon; D000262:Adenylyl Cyclases; D008274:Magnesium; D005033:Ethylmaleimide