Mechanism of action of des-His1-[Glu9]glucagon amide, a peptide antagonist of the glucagon receptor system.
We have investigated the mechanisms through which des-His1-[Glu9]glucagon amide functions as a peptide antagonist of the glucagon receptor/adenylyl cyclase system. Studies with radiolabeled peptides identified that (i) the antagonist bound to intact hepatocytes according to a single first-order process, whereas the rate of association of glucagon with the same preparation could be described only by the sum of two first-order processes; (ii) the interaction of the antagonist with saponin-permeabilized hepatocytes was not affected by the addition of GTP to the incubation medium or by the elimination of Mg2+, whereas the interaction of glucagon with the same cell preparation was modified significantly by the presence of the nucleotide or by the absence of the divalent metal ion; (iii) the dissociation of antagonist from intact hepatocytes incubated in buffer was complete, whereas that of agonist was not; and (iv) the antagonist bound to intact hepatocytes at steady state according to a single binding isotherm (as did both agonist and antagonist in permeabilized hepatocytes), whereas glucagon bound to the intact cell system with two clearly defined apparent dissociation constants. A model is presented for the mechanism of action of the glucagon antagonist in which the analog binds to glucagon receptors in a Mg(2+)- and GTP-independent fashion and in which resulting ligand-receptor complexes fail to undergo sequential adjustments necessary for the stimulation of adenylyl cyclase.
Post
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SR
Department of Biochemistry and Molecular Biology, University of Chicago, IL 60637.
Rubinstein
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PG
Tager
H S
HS
D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D000242:Cyclic AMP / Q000502:physiology; D004285:Dogs; D005934:Glucagon / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D066298:In Vitro Techniques; D008099:Liver / Q000502:physiology; D011964:Receptors, Gastrointestinal Hormone / Q000037:antagonists & inhibitors*; D018027:Receptors, Glucagon; D015398:Signal Transduction; D013329:Structure-Activity Relationship
D011964:Receptors, Gastrointestinal Hormone; D018027:Receptors, Glucagon; C080071:glucagonamide, desHis(1)-Glu(9)-; D005934:Glucagon; D000242:Cyclic AMP; D000262:Adenylyl Cyclases