ROLE OF APOLIPOPROTEIN B-DERIVED RADICAL AND ALPHA-TOCOPHEROXYL RADICAL IN PEROXIDASE-DEPENDENT OXIDATION OF LOW-DENSITY-LIPOPROTEIN

The peroxidation of low density lipoprotein (LDL) may play an importan t role in the modification of the lipoprotein to an atherogenic form. The oxidation of LDL by peroxidases has recently been suggested as a m odel for in vivo transition metal ion-independent oxidation of LDL (Wi eland, E., S. Pathasarathy, and D. Steinberg. 1993. Proc. Nail. Acad. Sci USA. 90: 5929-5933). It is possible that in vivo the peroxidase ac tivities of proteins, such as prostaglandin synthase and myeloperoxida se, promote LDL oxidation. We have used horseradish peroxidase (HRP) a nd H2O2 as a model of peroxidase-dependent oxidation of LDL and we obs erved the following during HRP/H2O2-initiated LDL oxidation, i) The ox idation of alpha-tocopherol occurred with the concomitant formation of alpha-tocopheroxyl radical. This was followed by the production of an apolipoprotein B (apoB)-derived radical. The apoB radical and the alp ha-tocopheroxyl radical were formed under both aerobic and anaerobic c onditions. ii) Inclusion of N-t-butyl-alpha-phenylnitrone (PEN) did no t inhibit alpha-tocopheroxyl radical formation. The ESR spectrum of a PBN/LDL-lipid derived adduct was observed after prolonged incubation. iii) There was formation of conjugated dienes, lipid hydroperoxides an d thiobarbituric acid reactive substances. Our data indicate that HRP/ H2O2 oxidizes both a-tocopherol and apoB to the corresponding radicals and concomitantly initiates lipid peroxidation.