The mechanism of apolipoprotein B-100 thiol depletion during oxidative modification of low-density lipoprotein

Oxidation of low-density lipoprotein (LDL) is recognized to be a key step in atherogenesis. Previous studies show that LDL contains low-molecular- weight antioxidants such as vitamin E, β-carotene, and ubiquinol, which can retard oxidative modification. In this report, we have evaluated the antioxidant potential of apolipoprotein B-100 (apo-B) thiols during LDL oxidation. Bath apo-B thiols and vitamin E were depleted concomitantly during the lag phase of Cu²⁺ -mediated LDL oxidation. The rate of thiol depletion was significantly inhibited by the lipophilic spin trap N-tert-butyl-α- phenylnitrone (PBN) but not by the water-soluble spin trap α-(4-pyridyl-1- oxide)-N-tert-butylnitrone (POBN). Blocking apo-B thiols with sulfhydryl modifying agents increased the oxidizability of LDL. As with Cu²⁺, peroxynitrite also caused depletion of apoB thiols, and again thiol depletion was inhibited by PBN but not by POBN. A PBN/lipid-derived radical adduct was observed by the electron spin resonance technique during oxidation of LDL with peroxynitrite. We conclude that apo-B thiol depletion is mediated by lipid peroxidation, prior to the onset of the propagation phase of LDL oxidation. The implications of apoB thiols as intrinsic antioxidants of LDL are discussed.