The resolution of agonist alpha 2-adrenergic receptor complexes from unoccupied receptors or antagonist-alpha 2 receptor complexes using DEAE chromatography.

Alpha 2-adrenergic receptors linked to inhibition of adenylate cyclase activity in human platelet membranes can be isolated using DEAE chromatography following solubilization into digitonin-containing buffers. This procedure permits resolution of agonist-occupied receptors from antagonist-occupied receptors. Antagonist-occupied receptors co-elute with unoccupied receptors. Adenylate cyclase activity elutes independently of all alpha 2-receptor activities. A greater resolution of agonist-receptor complexes from antagonist-receptor complexes is obtained using DEAE chromatography than reported earlier using approaches that rely entirely on agonist-stabilized increases in apparent receptor size. Consequently, DEAE chromatography may be of considerable value in isolating these agonist-receptor complexes to permit identification of the membrane component(s) which are more stably associated with the receptor subsequent to agonist occupancy and thus might be involved in receptor-cyclase coupling.

Mn2+-uncoupling of the catecholamine-sensitive adenylate cyclase system of rat reticulocytes. Parallel effects on cholera toxin-catalyzed ADP-ribosylation of the system.

High concentrations of Mn2+ interfere with functional interactions between the GTP-binding regulatory protein (G) and the catalytic moiety (C) of adenylate cyclase without perturbing interactions between receptor (R) and component G in rat reticulocyte membranes. The ability of cholera toxin to ADP-ribosylate component G and to enhance GTP-stimulated adenylate cyclase activity also appears to be correlated with the efficacy of the communication of component G with the adenylate cyclase system. Thus, increasing the concentration of Mn2+ in rat reticulocyte membrane during in vitro incubations causes a parallel loss of Gpp(NH)p-stimulated adenylate cyclase activity, cholera toxin-catalyzed [32P]ADP-ribosylation of the 42 000 Mr subunit of component G and cholera toxin-catalyzed enhancement of GTP-sensitive adenylate cyclase activity. Removal of Mn2+ by washing the membranes completely restores the sensitivity of adenylate cyclase activity. Removal of Mn2+ by washing the membranes completely restores the sensitivity of adenylate cyclase to all effectors, including cholera toxin. The data suggest that exposure of membranes to Mn2+ provides a useful tool for reversibly uncoupling catecholamine-sensitive adenylate cyclase systems. The data also suggest that the extent of cholera toxin-catalyzed ADP.-ribosylation of membrane substrates, i.e., the G component may rely on functional communication among the various components of the adenylate cyclase system. A corollary of the latter is that the amount of [32P]ADP-ribose-product detected in a membrane may reflect both the quantity and coupling efficiency of component G.