The alkaline phospholipase A1 of rat liver cytosol.

1. Rat liver cytosol contains a heat-sensitive phospholipase A1 active against phosphatidylethanolamine, 1-acylglycerophosphoethanolamine and, to a very much lesser extent, phosphatidylcholine and phosphatidylinositol. 2. Activity towards a pure phosphatidylethanolamine substrate is invoked by the presence of water-soluble cations that do not precipitate at the pH optimum of the enzyme (9.5). In this activation bivalent cations, e.g. Mg2+, Ca2+, Mn2+, Sr2+ and Ba2+, are effective at much lower concentrations (2.5-5 mM) than univalent cations K+, Na+ and NH4+ (100 mM). 3. In the absence of such cations the enzyme can be activated by cationic amphiphiles containing quaternary nitrogen or by basic proteins. 4. It is concluded that these agents activate the enzyme by reducing the negative zeta potential on the substrate at the high pH optimum (9.5) and allow interaction with the enzyme whose isoelectric point is at 7.15. 5. The activated enzyme is markedly inhibited by mixing the phosphatidylethanolamine substrate with many other phospholipids that exist in cell membranes, e.g. phosphatidylcholine, phosphatidylinositol. On the other hand, both phosphatidylcholine and phosphatidylinositol can be hydrolysed much more readily if they are mixed with an excess of phosphatidylethanolamine. 6. Such results on the inhibition and substrate specificity of the enzyme, coupled with birefringence measurements, allow the tentative conclusion that phospholipid substrates are only attacked when they exist in a hexagonal or non-bilayer structure and not in the bilayer (lamellar) form.