Adenyl Cyclase

The effects of several divalent cations, ATP, F-, and epinephrine, on adenyl cyclase from cardiac tissue were examined. Mn++ was more effective than Mg++ in activating the enzyme and the apparent Ka for Mn++ (0.7 mm) was less than that for Mg++ (2 to 3 mm). Co++ produced weaker stimulation than Mg++. ATP concentrations in excess of divalent metal ion inhibited the cardiac enzyme. F- increased the velocity of the reaction at all Mg++ concentrations, even those above saturation. It was concluded that the stimulatory effect of F- was due to an increase in Vmax, there being no significant effect on the Km for substrate (MgATP) or on the apparent Ka for Mg++. Epinephrine, like F-, stimulated the cardiac enzyme at all Mg++ concentrations and did not prominently affect the apparent Ka for Mg++ or the Km for the MgATP substrate. The actions of F- and epinephrine on the enzyme in washed particles of brain and of F- and glucagon on particulate preparations of liver were also examined. In each case the prominent effect of F- and hormone was to increase reaction velocity. It is suggested that a common feature of hormonal (and F-) stimulation of adenyl cyclase may be an increase in Vmax of the enzyme, reflecting increased reactivity of the catalytic site with substrate.