title: The role of Mg2+ in hormone stimulation of rat osteosarcoma adenylate cyclase.
abstract: 1. Mg2+ concentration dependence of adenylate cyclase activity, in a rat osteosarcoma cell line (ROS 2/3), exhibits two apparent affinities with Km values of approx. 2 mM and 10 mM. 2. Only one Mg2+ affinity with a Km value of around 1 mM was apparent at saturating concentrations of: (i) guanosine-5'-(beta, gamma-imido)triphosphate; (ii) parathyroid hormone and GTP; and (iii) (-)-isoproterenol and GTP. 3. Conversely, at saturating concentrations of Mg2+ (40 mM) only high hormone concentrations, acting on low affinity sites, stimulated adenylate cyclase. 4. At saturating concentrations of guanosine-5'-(beta, gamma-imido)triphosphate, hormone stimulation decreased with increasing Mg2+ concentrations and none was seen at 40 mM Mg2+. The findings suggest that hormone stimulation of adenylate cyclase is associated with Mg2+ activation of a 'high hormone affinity' responsive state dependent on triphosphoguanine nucleotide. The hormone effect on Mg2+ affinity fully accounts for hormone stimulation of adenylate cyclase at physiologically relevant concentrations.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology*; D007545:Isoproterenol / Q000494:pharmacology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D012516:Osteosarcoma / Q000201:enzymology*; D010281:Parathyroid Hormone / Q000494:pharmacology*; D051381:Rats
chemical_list: D010281:Parathyroid Hormone; D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D000262:Adenylyl Cyclases; D008274:Magnesium; D007545:Isoproterenol