PARAOXONASE INHIBITS HIGH-DENSITY-LIPOPROTEIN OXIDATION AND PRESERVESITS FUNCTIONS - A POSSIBLE PEROXIDATIVE ROLE FOR PARAOXONASE

HDL levels are inversely related to the risk of developing atheroscler osis. In serum, paraoxonase (PON) is associated with HDL, and was show n to inhibit LDL oxidation, Whether PON also protects HDL from oxidati on is unknown, and was determined in the present study. In humans, we found serum HDL PON activity and HDL susceptibility to oxidation to be inversely correlated (r(2) = 0.77, n = 15). Supplementing human HDL w ith purified PON inhibited copper-induced HDL oxidation in a concentra tion-dependent manner. Adding PON to HDL prolonged the oxidation lag p hase and reduced HDL peroxide and aldehyde formation by up to 95%. Thi s inhibitory effect was most pronounced when PON was added before oxid ation initiation. When purified PON was added to whole serum, essentia lly all of it became HDL-associated. The PON-enriched HDL was more res istant to copper ion-induced oxidation than was control HDL. Compared with control HDL, HDL from PON-treated serum showed a 66% prolongation in the lag phase of its oxidation, and up to a 40% reduction in perox ide and aldehyde content. In contrast, in the presence of various PON inhibitors, HDL oxidation induced by either copper ions or by a free r adical generating system was markedly enhanced. As PON inhibited HDL o xidation, two major functions of HDL were assessed: macrophage cholest erol efflux, and LDL protection from oxidation. Compared with oxidized untreated HDL, oxidized PON-treated HDL caused a 45% increase in cell ular cholesterol efflux from J-774 A.1 macrophages. Both HDL-associate d PON and purified PON were potent inhibitors of LDL oxidation. Search ing for a possible mechanism for PON-induced inhibition of HDL oxidati on revealed PON (2 paraoxonase U/ml)-mediated hydrolysis of lipid pero xides (by 19%) and of cholesteryl linoleate hydroperoxides (by 90%) in oxidized HDL. HDL-associated PON, as well as purified PON, were also able to substantially hydrolyze (up to 25%) hydrogen peroxide (H2O2), a major reactive oxygen species produced under oxidative stress during atherogenesis. Finally, we analyzed serum PON activity in the atheros clerotic apolipoprotein E-deficient mice during aging and development of atherosclerotic lesions. With age, serum lipid peroxidation and les ion size increased, whereas serum PON activity decreased. We thus conc lude that HDL-associated PON possesses peroxidase-like activity that c an contribute to the protective effect of PON against lipoprotein oxid ation, The presence of PON in HDL may thus be a major contributor to t he antiatherogenicity of this lipoprotein.