SECRETORY PHOSPHOLIPASE A(2) AND LIPOPROTEIN-LIPASE ENHANCE 15-LIPOXYGENASE-INDUCED ENZYMATIC AND NONENZYMIC LIPID-PEROXIDATION IN LOW-DENSITY LIPOPROTEINS

The oxidation of low-density lipoprotein (LDL) is thought to contribut e to atherogenesis. 15-Lipoxygenase (15LO) induces LDL oxidation, and phospholipase A(2) enhances this process [Sparrow, C. P., Parthasarath y, S., and Steinberg, D. (1988) J. Lipid Res. 29, 745-753]. As the und erlying mechanism of the enhancing effect has not been investigated pr eviously, we here show that in the presence of soybean 15LO (SLO) or h uman 15LO (rhLO), the addition of lipoprotein lipase, porcine pancreat ic, or human type IIa secretory phospholipase A(2) (sPLA(2)) greatly e nhanced the accumulation of hydro(pero)xides of all major classes of L DL's lipids. Hydroperoxides of free fatty acids accumulated exclusivel y as enzymic products with kinetics reflecting both the formation of f ree fatty acids and the initial 'build-up' of alpha-tocopheroxyl radic al. In contrast, hydroperoxides of cholesteryl esters and phosphatidyl choline accumulated linearly over comparatively longer periods of time and, in the case of rhLO, well beyond inactivation of the oxygenase. With SLO, formation of oxidized esterified lipids occurred nonenzymica lly, independent of the presence of lipase and despite the oxygenase r emaining active until the end of the incubation. Enhancement of rhLO-i nduced LDL lipid peroxidation by sPLA(2) was eliminated by a neutraliz ing anti-sPLA(2) antibody, indicating that lipolytic activity was requ ired for this effect. LDL depleted of alpha-tocopherol was resistant t o oxidation by 15LO alone, whereas lipase overcame this resistance, de monstrating that lipases enhance 15LO-induced enzymic and nonenzymic p eroxidation of LDL lipids. This is likely due to provision of free fat ty acid substrate, resulting in an enhanced rate of free radical forma tion which itself causes nonenzymic peroxidation of esterified lipids. As lipases and 15LO are present in atherosclerotic lesions, our findi ngs could be of pathophysiological significance.