CA2+-DEPENDENT PHOSPHOLIPID AND ARACHIDONIC-ACID BINDING BY THE PLACENTAL ANNEXIN-VI AND ANNEXIN-IV

Using an assay system in which phospholipids were immobilised on phenyl-Sepharose, we examined the affinities of the placental annexins VI and IV for binding to specific phospholipids over a range of Ca2+ concentrations. Purified annexin VI showed half-maximal binding to phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol at Ca2+ concentrations of 0.6, 0.4 and 3.5-mu-M, respectively, compared to values of 4.5, 4.5 and 20-mu-M Ca2+, respectively for purified annexin IV. These values did not change significantly in the presence of other proteins from the family. Neither annexin VI or IV bound to phosphatidylinositol bisphosphate and phosphatidylcholine, even at millimolar concentrations of Ca2+. However, both proteins bound to arachidonic acid, oleic acid and palmitic acid in a Ca2+-dependent manner, using the same assay system. The level of binding for both proteins was significantly increased when mixtures of phosphatidylcholine and arachidonic acid were examined. A dose-dependent inhibition of phospholipase A2 by both annexins VI and IV, at millimolar concentrations of Ca2+ was observed when phosphatidylcholine liposomes were used as a substrate. These results raise questions about the interpretation of experiments in which the release of arachidonic acid is used as a measure of lipase activity, and of the validity of the substrate-depletion model for the inhibition of phospholipases by the annexins.