ETHANOL STIMULATES THE PLASMA-MEMBRANE CALCIUM-PUMP FROM HUMAN ERYTHROCYTES

The plasma membrane Ca2+-ATPase from human erythrocytes can be stimulated by different treatments such as addition of calmodulin or acidic phospholipids and controlled proteolysis. In this report we show that short chain alkyl alcohols also stimulated this enzyme. At 5% (v/v) ethanol, the maximal velocity of the enzyme was about 2.4-fold higher than in the control, and thus, was also higher than the maximal velocity obtained in the presence of calmodulin (about 2-fold). When ethanol and calmodulin were present simultaneously, the stimulatory effect was additive (3.4-fold stimulation). On the other hand, the stimulatory effect of ethanol was preserved after treatment of the enzyme with trypsin to stimulate the Ca2+-ATPase and render it independent of calmodulin, thus suggesting that the interaction of ethanol and calmodulin with the Ca2+-ATPase occurred through a different mechanism. Other short chain alkyl alcohols (methanol, n-propanol and n-butanol) stimulated the Ca2+-ATPase activity to the same extent than ethanol but with different efficacy. Thus, the larger the carbon number, the lower the concentration needed to get the same maximal stimulation. Ethanol also increased the affinity of the enzyme for ATP to a larger extent and additively, when compared to calmodulin. All the effects of ethanol mentioned above were identically observed on the membrane-bound enzyme (i.e., erythrocyte ghosts) ruling out any effect of the alcohols attributable to the solubilized purified enzyme. Furthermore, Ca2+ transport by inside-out vesicles was also stimulated by ethanol, showing both the same concentration-dependence as the Ca2+-ATPase activity and the additive effect observed when calmodulin was also present. The stimulatory effect of ethanol was significant at pharmacological concentrations, thus suggesting potential implications of toxicological relevance.