Effects of Various Natural Antioxidants on the Cu²⁺-Mediated Oxidative Modification of Low Density Lipoprotein

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We have reported in our previous paper that several flavan-3-ol derivatives (tea polyphenols) inhibited the Cu²⁺-mediated low density lipoprotein (LDL) oxidation in vitro. (-)-Epigallocatechin gallate (EGCG), in particular, exhibited strong inhibition. In this study, we have compared the antioxidative effects of EGCG with those of other natural antioxidants, such as flavonols, sesaminol, curcuminoid derivatives, tocopherol analogues, and theaflavins. The antioxidative effects were monitored by conjugated diene formation in LDL which was carried out at 37°C with 5 μM CuSO₄ with or without antioxidants. Dibutyl hydroxytoluene (BHT) was used as a reference compound. The lag-time before the onset of conjugated diene formation was more than 100 min in the presence of 0.5 μM EGCG, theaflavin, myricetin, quercetin, and sesaminol. The ability to prolong the lag-time was in the order of sesaminol>quercetin>EGCG>theaflavin≥myricetin>BHT>α-tocopherol. Among the 4 tocopherol analogues used, α-tocopherol showed the strongest antioxidative activity. We have also studied the effects of EGCG, BHT, and α-tocopherol on cholesteryl ester (CE) degradation and apolipoprotein B 100 (apo B 100) fragmentation in the Cu²⁺-mediated oxidative modification of LDL. EGCG was the most effective inhibitor of CE degradation and apo B 100 fragmentation.