title: Magnesium regulation of the beta-receptor-adenylate cyclase complex. II. Sc3+ as a Mg2 antagonist.
abstract: Sc3+ bears the same relationship to Mg2+ as La3+ to Ca2+, a similar ionic radius but increased charge. Therefore, the possibility was investigated that Sc3+ would be a Mg2+ antagonist at Mg2+ sites on the beta-adrenergic receptor-adenylate cyclase complex of the murine S49 lymphoma cell. Sc3+ is consistently much more potent than La3+ in inhibiting adenylate cyclase regardless of the mode of activation. IC50 values for Sc3+ of 10-30 microM were observed, whereas those for La3+ were about 300 microM. However, Sc3+ does not block the ability of Mg2+ to increase beta-receptor affinity for agonist nor alter agonist affinity by itself. Furthermore, Sc3+ is a weak inhibitor of the beta-receptor-mediated inhibition of Mg2+ influx. In cyc- S49 membranes, in which the catalytic subunit of cyclase cannot interact with the nucleotide-coupling protein(s), Sc3+ is as potent as in wild-type S49 membranes and again more potent than La3+. Substrate kinetics show that Sc3+, like Mg2+, modulates adenylate cyclase activity by affecting the Vmax without altering the Km for substrate. The data suggest that Sc3+ is a specific antagonist of Mg2+ at the Mg2+ site on the catalytic subunit and support the suggestion that there are two distinct sites for Mg2+ with different functions, one site on the coupling protein(s) and one on the catalytic subunit. It was also found that an apparent complex of Sc3+ and F-, ScF4-, is a potent inhibitor of adenylate cyclase, with an IC50 of 3 microM.
mesh_terms: D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D001665:Binding Sites; D005459:Fluorides / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D007811:Lanthanum / Q000494:pharmacology; D008274:Magnesium / Q000037:antagonists & inhibitors*; D051379:Mice; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*; D012538:Scandium / Q000494:pharmacology*
chemical_list: D000067956:Adenylyl Cyclase Inhibitors; D011941:Receptors, Adrenergic; D011943:Receptors, Adrenergic, beta; D007811:Lanthanum; D006160:Guanosine Triphosphate; D008274:Magnesium; D005459:Fluorides; D012538:Scandium