Oxidation of LDL by rabbit and human 15-lipoxygenase: prevalence of nonenzymatic reactions

15-Lipoxygenase (15-LO)-induced oxidation of lipids in human LDL may be pro -atherogenic. However, the extent to which 15-LO promotes enzymatic oxidati on of esterified (i.e., major) lipids in LDL may depend on various factors. Here, we show that overall, LDL lipid oxidation was favored with high acti vity of human 15-LO, that phospholipids were the preferred esterified subst rate, and that low temperature maintained a higher proportion of enzymatic product. However, under all conditions, 15-LO induced alpha -tocopherol con sumption and the accumulation of nonenzymatic products that predominated wi th increasing time of incubation and inactivation of the enzyme. Lysates pr epared from cells overexpressing human 15-LO oxidized linoleic acid readily and in an almost exclusive enzymatic manner. In sharp contrast, such lysat es failed to oxidize LDL lipids unless linoleic acid was added, in which ca se nonenzymatic oxidation of LDL lipids occurred.jlr We conclude that altho ugh purified 15-LO can oxidize isolated LDL lipids in vitro, such oxygenati on always includes non-enzymatic reactions that likely play a major role in the more extensive oxidation of LDL by cell-derived 15-LO.