Predominance of esterified hydroperoxy-linoleic acid in human monocyte-oxidized LDL

Low density lipoprotein that was oxidized by activated human monocytes was analyzed to determine the identity of oxidized fatty acids present and the conditions required for their formation. The oxidized lipids were also analyzed under conditions allowing preservation of their oxidation state. Using reversed-phase high performance liquid chromatography (HPLC) analysis of native and saponified lipid extracts of oxidized low density lipoprotein (LDL), we found that the major fatty acid oxidation product was esterified hydroperoxyoctadecadienoic acid (HPODE), the oxidized product of the most abundant polyunsaturated fatty acid in human LDL, linoleic acid. Although some esterified hydroxyoctadecadienoic acid (HODE) was also detected, the reduction of HPODE to HODE did not appear to be monocyte-dependent. Essentially all of the HPODE was found to be esterified with the majority being esterified to cholesterol followed by phospholipids and generally following the abundance of esterified linoleic acid within the lipid classes. The percent of cholesteryl linoleate converted to cholesteryl HPODE and cholesteryl HODE at the end of the 24-h incubation was determined to be approximately 13.5%. The formation of oxidized esterified linoleic acid in the LDL was shown to require immunological activation of the human monocytes, a previously observed requirement for general LDL oxidation in this culture system. The oxidized esterified linoleic acid was present in the supernatant with the LDL and was not cell-associated. HPODE formation on LDL was prevented by including superoxide dismutase (SOD) or eicosatetraynoic acid (ETYA) during the 24-h coincubation of activated monocytes with LDL whereas indomethacin was without effect. The analysis of the lipid oxidation products in oxidized LDL can provide insight into the mechanisms involved in oxidation of LDL by activated human monocytes.