Abstract
The effect of phospholipids was tested on the p-nitrophenylphosphatase activity of the Ca 2+ pump. Acidic phospholipids like phosphatidylserine and phosphatidylinositol inhibited the phosphatase activity, while neutral phospholipids like phosphatidylcholine did not. This result contrasts sharply with the known activating effect of acidic phospholipids on the Ca 2+-ATPase activity of the pump. It is known that the phosphatase activity of the Ca 2+ pump can be elicited either by calmodulin and Ca 2+ or by ATP and Ca 2+. Unlike calmodulin, acidic phospholipids failed to stimulate the phosphatase activity. Furthermore, calmodulin-activated phosphatase was completely inhibited by acidic phospholipids. Maximal inhibition of the ATP-activated phosphatase was only 70%. Inhibition by acidic phospholipids was non-competitive regarding to calmodulin, suggesting that acidic phospholipids and calmodulin do not bind to the same domain of the pump. The presence of Ca 2+ was essential for the inhibition, and the apparent affinity for Ca 2+ for this effect was increased by acidic phospholipids. Results are consistent with the idea that acidic phospholipids stabilize an enzyme-Ca complex lacking phosphatase activity.
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