Abstract
1. Mg2+ concentration dependence of adenylate cyclase activity, in a rat osteosarcoma cell line (ROS 23), 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′-(β,γ-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′-(β,γ-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.
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