Catecholamine and divalent cation effects on frog liver adenylate cyclase

Frog liver adenylate cyclase was characterized with respect to divalent cation interaction and hormonally stimulated activities. The enzyme catalyzed the synthesis of cyclic [³²P]3′,5′-AMP from α-³²P-labeled ATP. The activity of the enzyme was linear with time and00 protein concentration. The K_m for ATP was 0.5 mM, in the presence or absence of stimulators. The temperature optimum was 25°. GTP (10^?4M) increased the stimulation of adenylate cyclase by epinephrine. Similar activities were obtained using 5 mM Mg²⁺ or Mn²⁺. At higher concentrations, both ions inhibited epinephrine-stimulated, but not basal or fluoride-stimulated activities. Approximately equivalent hormonal stimulation was obtained with maximal stimulating concentrations of epinephrine, isoproterenol, glucagon, and prostaglandin E₁. Norepinephrine was less stimulatory. Only catecholamine-stimulated activities were inhibited by propranolol (10^?5M). The data suggest that catecholamines stimulate frog liver adenylate cyclase through interactions with β adrenergic receptors. The adenylate cyclase in frog liver differs from its mammalian counterpart in its response to temperature and maximally stimulatory concentrations of hormones.