Sodium-Potassium-activated Adenosine Triphosphatase

The (Na+ + K+)-ATPase from Electrophorus electroplax was shown to require phospholipids. Treatment of the enzyme with phospholipase A from Naja naja venom removed almost all phosphatides with concomitant loss of enzymatic activity and of the ability to form phosphoryl enzyme. On the other hand digestion with phospholipase C from Clostridium perfringens removed nearly all of the lecithin, two-thirds of the phosphatidylethanolamine, and none of the phosphatidylserine. ATPase activity was partly inhibited by phospholipase C whereas the steady state level of the phosphoryl enzyme and the rate of the Na+-dependent nucleotide transphosphorylation were not impaired. Phospholipase C treatment produced an increased proportion of the E2 conformation at a given level of Mg++; inhibition of the ATPase and p-nitrophenylphosphatase by high Mg++ were more pronounced with the phospholipase C-treated enzyme. The reduced turnover of the phospholipase C-treated ATPase probably arises from this same effect. It was concluded that phosphatidylserine is a requirement for the formation of phosphoryl enzyme as well as its dephosphorylation, and phosphatidylethanolamine acts as a modifier which influences the affinity of the enzyme for Mg++.