The possible involvement of lecithin-cholesterol acyltransferase (LCAT) inthe metabolism of oxidized phosphatidylcholine (PC) in plasma was investigated. A variety of oxidized products are formed from PC following oxidation of low density lipoproteins (LDL). A significant increase in LDL oxidation levels in patients with familial LCAT deficiency (FLD) has been previously demonstrated by a sensitive sandwich ELISA for oxidized LDL using the monoclonal antibody DLH3 which recognizes oxidized products of PC. In the present study, we found that LCAT produces various metabolites from oxidized PC and that oxidized PC molecules in LDL particles serve as substrates. When the neutral lipid fraction was separated by TLC after the incubation of oxidized 1-palmitoy1-2-[1-¹⁴C]linoleoyl PC with human plasma, a number of radioactive bands were formed in addition to cholesteryl ester. These products were not formed from native 1-palmitoy1-2[1-¹⁴C]linoleoyl PC. Plasma from FLD patients also failed to form the additional products from oxidized PC. The addition of dithio-bis(nitrobenzoate) (DTNB), an LCAT inhibitor, or the inactivation of LCAT activity by treating the plasma at 56°C for 30min abolished the generation of these products from oxidized PC. The activity was recovered in the high density lipoprotein (HDL) fraction but not in the LDL fraction separated from normal plasma. When Ipalmitoy1-2-[1-¹⁴C](9-oxononanoyl) PC and 1-stearoy1-2-[1-₁₄C] (5-oxovaleroyl)PC, PC oxidation products that contain short chain aldehydes, were incubated with human plasma, radioactive products in the neutral lipid fraction were observed on TLC. LDL containing oxidized PC was measured by sandwich ELISA using an anti-apolipoprotein B antibody and DLH3. The reconstituted oxidized PC-LDL particles were found to have lost their ability to bind DLH3 upon incubation with HDL, while the reactivity of the reconstituted oxidized PC-LDL remained unchanged in the presence of DTNB. These results suggest that LCAT is capable of metabolizing a variety of oxidized products of PC and preventing the accumulation of oxidized PC in circulating LDL particles.