On the Kinetics and Temperature Dependence of Adrenaline-Adenylate Cyclase Interactions

Adrenaline and beta adrenergic blocking drugs were shown to interact rapidly and reversibly with catecholamine-sensitive adenylate cyclases from Ehrlich ascites cells, rat liver, and rat fat cell ghosts within the temperature range of 15-37°. Rates of binding and dissociation of both adrenaline and beta -blocking drugs are faster than the time resolution of the methods (1-2 min). Arrhenius plots for the enzyme from Ehrlich cells showed inflection temperatures (about 24°) for basal and adrenaline-stimulated activities, energies of activation ( E a ) being about 10 kcal/mole lower above this temperature. Differences in E a between basal and adrenaline-stimulated activities were not apparent. Fluoride-stimulated activity did not show a clear inflection point. Unstimulated enzyme from rat brain cortex showed an inflection temperature at about 24°, similar to that for Ehrlich cell cyclase, and no inflection was seen for fluoride-stimulated activity. The activation constant of adrenaline for Ehrlich cell adenylate cyclase was 4-6-fold lower at 15° compared to 37°. This would not explain the inflection in Arrhenius plots, however, since the latter were obtained at maximally stimulating concentrations of adrenaline.