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.