Oxidation of Low-Density Lipoproteins Produces Levuglandin-Protein Adducts

Abstract

Free-radical oxidation of human low-density lipoprotein (LDL) produces levuglandin (LG)-protein adducts that were detected with an enzyme-linked immunosorbent assay using LGE2-KLH antibodies which recognize LGE2-derived pyrroles. The level of immunoreactivity increases with time of oxidation and reaches a maximum by 8 h. The yield of pyrrole varies nonlinearly with the level of LG adduction to LDL. At low LG:LDL ratios, such as those detected in oxidized LDL, the reaction of primary amino groups with LGE2 produces mostly non-pyrrole adducts that are not immunoreactive. Concomitant phospholipolysis must occur if the generation of immunoreactive epitopes in LDL involves oxidation of arachidonyl phospholipids. Thus, since a protein adduct prepared from synthetic LGE2-2-lysophosphatidylcholine ester showed, at most, only 0.5% cross-reactivity with the LGE2-KLH antibodies, the epitopes detected in oxidized LDL are almost certainly not protein adducts of LG-phospholipid esters. As expected, hydrolysis of the carboxylic ester in the protein adduct of LGE2-2-lysophosphatidylcholine ester by treatment with phospholipase A2 produced a fully immunoreactive LGE2-protein adduct.