Binding of a Glucagon Photoaffinity Label to Rat Liver Plasma Membranes and Its Effect on Adenylate Cyclase Activity before and after Photolysis

The concentration-dependent stimulation of adenylate cyclase by the photoaffinity reagent 2-[(2-nitro-4-azidophenyl)sulfenyl]-Trp²⁵-glucagon (glucagon-NAPS) and also its binding characteristics were compared with those of the native hormone. The derivative was found to be slightly more potent in stimulating adenylate cyclase than glucagon, in the presence of guanosine 5′-triphosphate (GTP). ¹²⁵I-Labeled glucagon-NAPS or ¹²⁵I-labeled glucagon bound rapidly to receptors and was competitively displaced by unlabeled glucagon or glucagon-NAPS. Glucagon-NAPS displaced bound radiolabeled hormone at a lower concentration than did glucagon in the absence of GTP. Scatchard analysis of the binding data obtained from displacement of bound radiolabeled ligand with unlabeled peptide demonstrated a heterogeneous population of saturable glucagon binding sites. Glucagon-NAPS displayed a higher affinity (0.7 nM) for the high-capacity sites (80–90% of total binding sites) than glucagon (4.0 nM) in the absence of GTP. In the presence of the nucleotide, both ligands had approximately the same affinity (0.5–0.6 nM). Hill plot analysis of the binding data suggested noncooperative interactions. Photoaffinity labeling of plasma membranes with glucagon-NAPS resulted in an irreversible activation of adenylate cyclase with a reduced response to further stimulation by glucagon, glucagon-NAPS, and NaF.