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 (Ea) being about 10 kcal/mole lower above this temperature. Differences in Ea 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.
ACKNOWLEDGMENTS Extensive technical assistance by Mrs. S. Kulshrestha is gratefully acknowledged. The author is indebted to Dr. David Cook for making available an APL computer program of the procedure of Bogartz (7) and a further APL program analyzing significance of regressions of one-line and two-line fits to points in an Arrhenius plot (from a manuscript in preparation).
- Copyright ©, 1974, by Academic Press, Inc.