Temperature dependence of adenylate cyclase activity from rat white adipocytes

The effects of temperature on adenylate cyclase (AC) activity from rat white adipocytes were studied. Arrhenius plots of the data were found to be biphasic for basal AC activity, with a break near 27 degrees C. Noradrenaline and corticotropin induced a shift in the break with a rise in energy of activation (Ea) on both sides of the break. Aabove break point only, Ea increased with respect to hormone does as a hyperbolic function. The maximum value was in the range 17-21 Kcal x mol-1. Temperature was shown to have only a slight effect on the binding capacity of corticotropin to its receptor sites. The possibility of the existence of multiple thermodynamic states for the enzyme is envisaged. Basal AC activity is thermodynamically different from the hormone-stimulated enzyme. Hormones induce changes in the basal conformation of the enzyme, and this is reflected in modifications of Arrhenius plots. The maximal state of activation reached with high doses of hormones could be interpreted as a 'desensitization' of enzyme and/or enzyme systems to membrane lipid interactions.