Studies On (K+ + H+)-Atpase .4. Effects Of Phospholipase-C Treatment

(1) The total phospholipid content of a gradient purified (K+ + H+)-ATPase preparation from pig gastric mucosa is 105 mumol per 100 mg protein, and consists of 29% sphingomyelin, 29% phosphatidylcholine, 28% phosphatidylethanolamine, 10% phosphatidylserine and 4% phosphatidylinositol. The cholesterol content corresponds to 50 mumol per 100 mg protein. (2) Treatment with phospholipase C (from Clostridium welchii and Bacillus cereus) results in an immediate decrease of the phosphate content. Up to 50% of the phospholipids are hydrolyzed by each phospholipase C preparation alone, without further hydrolysis by increased phospholipase concentration or prolonged incubation time. Combined treatment with the two phospholipase C preparations, sequentially or simultaneously, hydrolyzes up to 65% of the phospholipids. (3) The (K+ + H+)-ATPase and K+ stimulated p-nitrophenylphosphatase activities are decreased proportionally with the total phospholipid content, indicating that these enzyme activities are dependent on phospholipids. (4) Phospholipase C treatment does not change optimal pH, Km value for ATP and temperature dependence of the gastric (K+ + H+)-ATPase, but slightly decreases the Ka value for K+. (5) Phospholipase C treatment lowers the AdoPP[NH]P binding and phosphorylation capacities, suggesting that inactivation occurs primarily on the substrate binding level. (6) Most of the results can be understood by assuming that hydrolysis of the phospholipids by phospholipase C leads to aggregation of the membrane protein molecules and complete inactivation of the aggregated ATPase molecules.