Conformational changes in oxidized phospholipids and their preferential hydrolysis by phospholipase A2: A monolayer study

Cleavage of oxidized fatty acids by phospholipase A2 has been implicated as the first step in the repair mechanism for oxidative damage to membrane phospholipids. However, the mechanism by which this enzyme preferentially hydrolyzes oxidized fatty acyl chains is poorly understood. Using a lipid monolayer technique, we found that the molecular surface areas of 1-palmitoyl-2-(9/13-hydroperoxylinoleoyl)-phosphatidylcholine (PLPC-OOH) and 1-palmitoyl-2-(9/13-hydroxylinoleoyl)phosphatidylcholine (PLPC-OH) were increased by as much as 50% relative to the parent nonoxidized 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC). These experimental data directly indicate a drastically changed molecular conformation of oxidized phospholipids in which the hydroperoxy or hydroxy group in the sn-2 fatty acid is close to the lipid-water interface. Phospholipases A2 from porcine pancreas and from bee venom were shown to break down PLPC-OOH and PLPC-OH monolayers much faster than PLPC monolayers. In all cases, the presence of serum albumin in the subphase enhanced monolayer breakdown by extracting hydrolysis products from the monolayer, but monolayer breakdown was always much faster for oxidized than for nonoxidized PLPC. This did not appear to be due to change in the extent of monolayer penetration by phospholipase A2, since enzyme-monolayer interaction studies revealed essentially identical penetration behavior of bee venom phospholipase A2 with PLPC, PLPC-OOH, and PLPC-OH monolayers. We propose that the altered molecular conformation of oxidized phospholipids facilitates access to the sn-2 ester bond, thereby ensuring their preferential hydrolysis in the presence of a phospholipase A2.