Phospholipid requirements for (Na + + K +)-ATPase activity: Head-group specificity and fatty acid fluidity

Abstract

We have re-examined the question of phospholipid activation of latent (Na + + K +)-ATPase activity in delipidized preparations. The influence of both polar head-groups and fatty acyl chain fluidity have been examined. The results indicate the following: 1. 1. The enzyme can be reactivated not only by phosphatidylserine, but also equally well by phosphatidylglycerol. A large variety of other purified phospholipids were found to have little or no effect. 2. 2. This specificity for (Na + + K +)-ATPase reactivation correlates with the observation that only phosphatidylserine and phosphatidylglycerol vesicles show substantial discrimination for K + over Na + in terms of permeability. 3. 3. Maximal reactivation is obtained only when the fatty acyl chains are fluid. Activation by dipalmitoyl phosphatidylglycerol is inhibited below its transition temperature, or in the presence of cholesterol. Arrhenius plots for the activation by dipalmitoyl phosphatidylglycerol show a break for (Na + + K +-ATPase activity corresponding to the presumed transition temperature for the gel to liquid-crystalline transition for this phospholipid. We suggest the term “viscotropic” to describe such effects of membrane fluidity on enzyme activity. 4. 4. The ATPase preparations used in this paper were obtained by deoxycholate treatment of microsomal fractions of rabbit kidney and beef brain. This treatment results in the removal of more than 90% of the phospholipid and inhibition of (Na + + K +-ATPase activity. It was noted that exclusive specificity for reactivation by phosphatidylserine and phosphatidylglycerol was obtained only with those preparations where the inhibition of (Na + + K +)-ATPase activity was essentially complete. 5. 5. These results are discussed in relation to previous work on the phospholipid requirement for (Na + + K +)-ATPase activity, the ability of phospholipids to exhibit K + Na + discrimination and current concepts of protein-lipid interactions in biological membranes.