Increased membrane acyl chain ordering activates adenylate cyclase.

The thermodynamic parameters suggest that PGE1, NaF, and increased acyl chain ordering all activate adenylate cyclase by stabilizing the enzyme in a more reactive conformation, similar to those seen with activation by PGEl and NaF.

Abstract

Adenylate cyclase activity was studied in a CHO-Kl somatic cell mutant that is defective in the regulation of plasma membrane cholesterol content. Changes in cholesterol content lead to parallel changes in membrane acyl chain ordering as assessed by electron spin resonance techniques. Both prostaglandin El (PGE1) and sodium fluoride (NaF) increase adenylate cyclase activity in membrane preparations from these cells. Increasing membrane order by increasing the amount of cholesterol in the membrane also led to an increase in basal adenylate cyclase activity. However, the relative stimulation by PGEl and NaF was significantly decreased in cells with increased membrane ordering. This was presumably due to the higher basal activity. The potency of these compounds in activating adenylate cyclase was unchanged with increased membrane ordering. Cholera toxin also activated adenylate cyclase when added to intact cells. Although the lag period for activation was unchanged by increased membrane ordering, the relative stimulation was again decreased. Thermodynamic analysis of the activation of adenylate cyclase by PGEl and NaF revealed increases in both enthalpy and entropy of activation. The activation caused by increased membrane ordering resulted in similar thermodynamic parameters, again resulting in an increased enthalpy and entropy of activation. The thermodynamic parameters suggest that PGE1, NaF, and increased acyl chain ordering all activate adenylate cyclase by stabilizing the enzyme in a more reactive conformation. Since the thermodynamic parameters of the activation by increased membrane acyl chain ordering are similar to those seen with activation by PGEl and NaF, it is possible that the effects of these substances are mediated through increases in acyl chain ordering.