Oxidised low density lipoprotein (LDL) was considered to be important in the pathogenesis of atherosclerosis, but the large clinical trials of antioxidants, including the first one using probucol (the PQRST Trial), failed to show benefit and have cast doubt on the importance of oxidised LDL. We have shown previously that LDL oxidation can be catalysed by iron in the lysosomes of macrophages. The aim of this study was therefore to investigate the effectiveness of antioxidants in preventing LDL oxidation at lysosomal pH and also establish the possible mechanism of oxidation. Probucol did not effectively inhibit the oxidation of LDL at lysosomal pH, as measured by conjugated dienes or oxidised cholesteryl esters or tryptophan residues in isolated LDL or by ceroid formation in the lysosomes of macrophage-like cells, in marked contrast to its highly effective inhibition of LDL oxidation at pH 7.4. LDL oxidation at lysosomal pH was inhibited very effectively for long periods by N,N'-diphenyl-1,4-phenylenediamine, which is more hydrophobic than probucol and has been shown by others to inhibit atherosclerosis in rabbits, and by cysteamine, which is a hydrophilic antioxidant that accumulates in lysosomes. Iron-induced LDL oxidation might be due to the formation of the superoxide radical, which protonates at lysosomal pH to form the much more reactive, hydrophobic hydroperoxyl radical, which can enter LDL and reach its core. Probucol resides mainly in the surface monolayer of LDL and would not effectively scavenge hydroperoxyl radicals in the core of LDL. This might explain why probucol failed to protect against atherosclerosis in various clinical trials. The oxidised LDL hypothesis of atherosclerosis now needs to be re-evaluated using different and more effective antioxidants that protect against the lysosomal oxidation of LDL.

氧化的低密度脂蛋白（LDL）被认为在动脉粥样硬化的发病机理中很重要，但是抗氧化剂的大规模临床试验，包括第一个使用普罗布考的试验（PQRST试验），未能显示出益处，并且对该药物的重要性产生了怀疑。氧化的低密度脂蛋白。先前我们已经表明，LDL氧化可以被巨噬细胞溶酶体中的铁催化。因此，本研究的目的是研究抗氧化剂在溶酶体pH值下预防LDL氧化的有效性，并建立可能的氧化机理。如通过共轭二烯或氧化的胆固醇酯或色氨酸残基在分离的LDL中或在巨噬细胞样细胞的溶酶体中形成类固醇，普罗布考不能有效抑制溶酶体pH下LDL的氧化，与它在pH 7.4下高效抑制LDL氧化形成鲜明对比。溶酶体pH值对LDL的氧化作用在很长一段时间内非常有效地被抑制N，N'-二苯基-1,4-苯二胺，比普罗布考疏水性更强，已被其他人抑制兔的动脉粥样硬化，而半胱胺则是一种在溶酶体中蓄积的亲水性抗氧化剂。铁诱导的LDL氧化可能是由于形成了超氧化物自由基，该超氧化物自由基在溶酶体的pH值下质子化，形成了更具反应性的疏水性氢过氧自由基，后者可以进入LDL并到达其核心。普罗布考主要存在于LDL的表面单层，不会有效清除LDL核心中的氢过氧自由基。这可以解释为什么普罗布考在各种临床试验中未能预防动脉粥样硬化。