The phosphatase activity of the plasma membrane Ca2+ pump. Activation by acidic lipids in the absence of Ca2+ increases the apparent affinity for Mg2+

The purified PMCA supplemented with phosphatidylcholine was able to hydrolyze pNPP in a reaction media containing only Mg2+ and K+. Micromolar concentrations of Ca2+ inhibited about 75% of the pNPPase activity while the inhibition of the remainder 25% required higher Ca2+ concentrations. Acidic lipids increased 5-10 fold the pNPPase activity either in the presence or in the absence of Ca2+. The activation by acidic lipids took place without a significant change in the apparent affinities for pNPP or K+ but the apparent affinity of the enzyme for Mg2+ increased about 10 fold. Thus, the stimulation of the pNPPase activity of the PMCA by acidic lipids was maximal at low concentrations of Mg2+. Although with differing apparent affinities vanadate, phosphate, ATP and ADP were all inhibitors of the pNPPase activity and their effects were not significantly affected by acidic lipids. These results indicate that (a) the phosphatase function of the PMCA is optimal when the enzyme is in its activated Ca2+ free conformation (E2) and (b) the PMCA can be activated by acidic lipids in the absence of Ca2+ and the activation improves the interaction of the enzyme with Mg2+.