Photoaffinity labeling of the glucagon receptor with a new glucagon analog

Stabilization of soluble active rat liver glucagon receptor

Active glucagon receptor was solubilized with 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate (Chaps) from rat liver plasma membranes but rapidly (less than 8 h) lost activity. Either inclusion of 1X Hanks' balanced salt solution in the 3 mM Chaps solubilization buffer or its addition after solubilization increased the percentage of total binding attributable to specific glucagon binding from approximately 10 to greater than 80%; of great importance, it increased the stability from near zero binding at 8 h to 50% binding at 48 h (4 degrees C). Of the Hanks' solution components, either NaCl (137 mM) or CaCl2 (1.26 mM) was effective in increasing specific binding to approximately 70 and 60% respectively: Mg salts were ineffective. Soluble receptor binding activity was assayed by dextran-coated charcoal adsorption of free hormone. The assay is rapid, simple, and reproducible. It is suitable for monitoring receptor activity during purification and molecular characterization. Competition binding studies gave an IC50 value of 10-20 nM (slope factor approximately 1), with or without GTP. Dissociation assays revealed GTP sensitivity when receptors were solubilized either as glucagon-receptor complexes or free receptor. Active glucagon-receptor complexes could be eluted from wheat germ lectin-agarose: neither concanavalin A-agarose nor soybean agglutinin-agarose bind receptor. A glucagon degrading activity which co-solubilized with the receptor but did not require detergent for extraction was distinguishable from the soluble receptor not only by solubility but also by its heat stability (30 degrees C), its inhibition by bacitracin, its affinity for glucagon, its retention of activity for at least 1 week at 4 degrees C, and its size.

Guanine nucleotide regulation of the interconversion of the two-state hepatic glucagon receptor system of rat

To investigate whether guanine nucleotides regulate interconversion of the two-state hepatic glucagon receptor we have utilized kinetic assays of glucagon binding to partially purified rat liver plasma membranes. Dissociation of glucagon at 30 degrees C exhibited biexponential character in either the absence or presence of GTP, indicating that the system previously seen in intact hepatocytes is independent of intracellular modulators. In each case the receptors underwent a time-dependent conversion from a low affinity to a high affinity state. However, GTP decreased the fraction of receptors in the high affinity state. The rank order for stabilizing the low affinity state was Gpp(NH)p greater than GTP greater than GDP much greater than GMP = no nucleotides. Data from competition binding assays with increasing concentrations of GTP allow calculation of equilibrium constants which are 3.32 nM for glucagon and receptor in the absence of GTP, 18.6 nM for glucagon and receptor in the presence of GTP, 1.55 microM for the association of receptor and GTP presumably linked to an N protein, and 8.86 microM for the association of the glucagon-receptor complex and GTP again presumably linked to an N protein, Glucagon binding to receptor is noncooperative in both the absence and presence of GTP, distinguishing this system from the beta-adrenergic system. With GTP, binding to the low affinity state is favored because of the relative affinities reported. Therefore, GTP regulates the activation by slowing the conversion of the receptor from a low affinity to high affinity form.