CONFORMATIONAL-CHANGES IN OXIDIZED PHOSPHOLIPIDS AND THEIR PREFERENTIAL HYDROLYSIS BY PHOSPHOLIPASE-A(2) - A MONOLAYER STUDY

Cleavage of oxidized fatty acids by phospholipase A2 has been implicat ed as the first step in the repair mechanism for oxidative damage to m embrane phospholipids. However, the mechanism by which this enzyme pre ferentially hydrolyzes oxidized fatty acyl chains is poorly understood . Using a lipid monolayer technique, we found that the molecular surfa ce areas of -2-(9/13-hydroperoxylinoleoyl)-phosphatidylcholine (PLPC-O OH) and itoyl-2-(9/13-hydroxylinoleoyl)phosphatidylcholine (PLPC-OH) w ere increased by as much as 50% relative to the parent nonoxidized 1-p almitoyl-2-linoleoylphosphatidylcholine (PLPC). These experimental dat a directly indicate a drastically changed molecular conformation of ox idized 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 P LPC-OOH and PLPC-OH monolayers much faster than PLPC monolayers. In al l cases, the presence of serum albumin in the subphase enhanced monola yer breakdown by extracting hydrolysis products from the monolayer, bu t monolayer breakdown was always much faster for oxidized than for non oxidized PLPC. This did not appear to be due to change in the extent o f monolayer penetration by phospholipase A2, since enzyme-monolayer in teraction studies revealed essentially identical penetration behavior of bee venom phospholipase A2 with PLPC, PLPC-OOH, and PLPC-OH monolay ers. We propose that the altered molecular conformation of oxidized ph ospholipids facilitates access to the sn-2 ester bond, thereby ensurin g their preferential hydrolysis in the presence of a phospholipase A2.