ROLE OF ALPHA-TOCOPHEROXYL RADICAL IN THE INITIATION OF LIPID-PEROXIDATION IN HUMAN LOW-DENSITY-LIPOPROTEIN EXPOSED TO HORSE RADISH PEROXIDASE

Heme-containing (per)oxidases including horse radish peroxidase (HRP)/ H2O2 have been shown to oxidatively modify isolated low-density lipopr otein (LDL) in vitro and oxidized LDL is implicated in the early event s leading to atherosclerosis. The role of alpha-tocopherol (alpha-TOH) in the oxidation of LDL by HRP/H2O2 is unclear, although alpha-tocoph eroxyl radical (alpha-TO.), which is formed during this process, can a ct as a chain transfer agent of lipid peroxidation in LDL. By combinin g HPLC and EPR spectroscopy, we hereby show that during HRP/H2O2-induc ed oxidation of human LDL: (i) the accumulation of cholesteryl linolea te hydroperoxides and hydroxides (CE-O(O)H) occurs concomitantly with the formation of alpha-TO. and consumption of alpha-TOH in the absence of other detectable organic (g approximate to 2) radicals; (ii) the r ates of alpha-TH. formation and subsequent decay reflect the rates of both alpha-TOH consumption and CEO(O)H accumulation; (iii) CE-O(O)H ac cumulation is directly dependent on the level of endogenous alpha-TOH, and vitamin E supplementation results in increased lipid oxidizabilit y; (iv) the inhibition of HRP activity by catalase plus urate results in a persistent alpha-TO. signal, the decay (t(1/2) approximate to 20 min) of which is accompanied by continued accumulation of CE-O(O)H, wi th complete cessation of lipid peroxidation upon loss of the chromanox yl signal. These results demonstrate a direct correlation between alph a-TOH/alpha-TO. and the extent of HRP/H2O2-induced LDL lipid peroxidat ion, and that this type of oxidative modification can occur in the abs ence of g approximate to 2 radicals other than alpha-TO.. Together, th e results support a role for tocopherol-mediated peroxidation but not the involvement of a protein radical in the initiation of LDL lipid pe roxidation induced by HRP/H2O2.