Abstract/Summary

Oxidised forms of low-densitylipoprotein (LDL) are widely belived to beinvolved in the pathogenesis of atherogenesis,but large clinical trials have not shown protectionof cardiovascular diseaseby antioxidants. Recently, it has been shownthat LDL can be oxidised by iron in the lysosomes of macrophages. We hypothesised that antioxidants would protect LDL against oxidation less well at lysosomal pH than at pH 7.4.LDL was enriched with α-tocopherol by incubating plasma with α-tocopherol and isolating the LDL. This enrichment inhibited LDL oxidation by copper ions (Cu2+) at pH 7.4,but not at pH4.5, as shown by spectrophotometry at 234 nm to measure conjugated dienes and by HPLC to measure individual oxidised lipids. α-Tocopherol enrichment did not inhibit LDL oxidation by Fe3+ (2, 5 or 20 μM) at pH4.5 ,but inhibiteitby 5 or 20 μM Fe2+,but not 2 μM Fe2+. This might help to explain whyα-tocopherol did not inhibit cardiovascular diseases inthe large clinical trials. The antioxidant tempol and probucol inhibited the late phase of LDL oxidation by Fe2+and Cu2+at pH 4.5 more than the early phase, possibly because they were located mainly in the phospholipid monolayer of LDL, rather than in the cholesteryl ester of the LDL particle. There is a suggestion that lysosomal dysfunction plays an important role in atherosclerosis. Thelysosomal oxidation of LDL aggregated by sphingomelinaseresulted in the production of the advanced lipid peroxidation product (ceroid).α-Tocopherol enrichment of macrophagesdid notprotectthem against apoptosis induced by H2O2.The work presented here also demonstrated LDL oxidisedFe2+at pH 4.5, decreasedendothelium-dependent vasodilatationof rat aortic rings. This suggests that VIlysosomallyoxidised LDL released from dead cells in atherosclerotic lesionsmight damage the endothelium.Takentogather,these results suggest that inhibiting the oxidation of LDLin lysosomes might be a therapy of atherosclerosis.