Temperature Dependence of β Receptor, Adenosine Receptor, and Sodium Fluoride Stimulated Adenylate Cyclase from Turkey Erythrocytes

The individual temperature dependencies of the processes which control the activity of turkey erythrocyte adenylate cyclase have been determined. The temperature dependence of the fraction of activable cyclase units experiences a thermal transition at 24 °C for all three modes of enzyme activation: I-epinephrine, adenosine, and NaF. This thermal transition probably reflects the phase transition in the inner monolayer of the membrane which influences the behavior of the GTP regulatory unit which is involved in all three modes of enzyme activation. The “rate constant” of enzyme activation by adenosine reflects two thermal transitions, at 24 and at 35 °C; the apparent rate constant of cyclase activation by NaF activation experiences a transition only at 24 °C whereas the rate constant of the β-receptor-bound agonist decreases monotonously with no “breaks” on the Arrhenius plot. Following the temperature dependence of the fluorescence intensity of dansylphosphatidylethanolamine embedded in both sides of the membrane and exclusively in the outer monolayer, one can assign the thermal transition of 24 °C to the inner monolayer and the other two transitions to the outer monolayer (10 and 35 °C). We interpret these results as follows, (a) The monomolecular rate constant characterizing the activation of cyclase by the precoupled adenosine receptor experiences both the transition at 24 and 35 °C, indicating that the latter may span the bilayer. (b) The receptor activates the cyclase units only in fluid areas since it can diffuse exclusively in the fluid areas of the membrane and is unable to interact with cyclase units in “frozen” areas. The linear dependence of the logarithm of the rate constant on 1/T for the β receptor reflects the change of membrane fluidity as a function of temperature.