Activation energy and phospholipid requirements of membrane-bound adenosine triphosphatases

In order to evaluate the role of lipids in the function of membrane ATPase reactions, the apparent activation energies of membrane-bound (Na⁺ + K⁺)-ATPase and membrane-bound Mg²⁺-ATPase have been measured under conditions frequently supposed to alter the membrane lipids in vitro.In the case of (Na⁺ + K⁺)-ATPase, the untreated enzyme was shown to have two different activation energies as shown by an Arrhenius plot comprising two straight lines which intersect at the “critical temperature.” Treatment of the preparation with detergents or with phospholipase C causes some alteration in the spécifie activity of the enzyme but did not significantly alter the activation energies or the critical temperature. After treatment with phospholipase A, however, the Arrhenius plot appeared linear over the entire temperature range studied. Subsequent treatment of phospholipase A-treated preparations with phosphatidylserine restored the control response.Conversely, untreated preparations of Mg²⁺-ATPase give an Arrhenius plot which is neither linear nor composed of two intersecting straight lines. This plot, which we regard as curvilinear, does not permit a unique value of the activation energy to be determined. The shape of this plot is unaltered by detergent or by treatment with phospholipase C. In contrast to (Na⁺ + K⁺)-ATPase, it is also unaffected by treat-with phospholipase A or phospholipase A followed by phosphatidylserine.We conclude that although (Na⁺ + K⁺)-ATPase and Mg²⁺-ATPase are frequently closely associated in many membranes, their functions involve the presence of different membrane lipids.