In the stimulation of rat hepatic adenylyl cyclase by gluca5on or epinephrine we observe an abrupt change in the energy of activation at 32 C (seen as an increase in the slope of the Arrhenius plot). The energy of activation for the cyclase reaction above 32OC is about 1.7 times that found below this temperature. Cyclase activity stimulated by fluoride, prostaglandin E l , or 1 -propanol, or activity in the absence of added stimulators does not show this change. The structural differences between the hormones suggest that they interact with the cyclase system at different loci. But the mechanism by which they stimulate cyclase activity appears to involve a common, temperature-dependent step. 2. In the presence of 1-propanol the change at 32OC in the energy of activation of the hormone-stimulated activity is not observed. 3. In view of the relatively large mole fraction of cholesterol present in the rat liver plasma membrane (which appears to inhibit phase transitions in bulk membrane lipids), it is suggested that this thermal sensitivity resides in protein rather than lipid components or that the cyclase is restricted to cholesterol-poor membrane regions.
The occurrence of anomalous Arrhenius plots of enzyme activities (with abrupt changes of slope) for both membrane-bound and soluble enzymes is reviewed.