Fourier transform infrared spectroscopy evaluation of low density lipoprotein oxidation in the presence of quercetin, catechin, and alpha-tocopherol.

We investigated the changes in human LDL primary and secondary lipid oxidation products and modification of the apolipoprotein B-100 (apoB-100) secondary structures during Cu2+-mediated oxidation by FTIR spectroscopy in the presence of catechin, quercetin, and alpha-tocopherol at physiological concentrations. Catechin- and quercetin-containing samples had slower rates and longer lag phases for conjugated diene hydroperoxide (CD) formation than alpha-tocopherol-containing samples; however, all antioxidant-treated LDL samples generated similar CD levels (P< 0.05). A lower maximum (98.4 nmol/mg LDL protein) of carbonyl compounds was produced in the quercetin- and catechin-treated samples than in alpha-tocopherol samples. Modification of the apoB-100 secondary structures corresponded closely to the formation of carbonyls and was hampered by the presence of antioxidants. Physiological concentrations of catechin and quercetin offered similar levels of protection against modification by carbonyls of the apoB-100 at advanced stages (carbonyls approximately 96.0 nmol/mg LDL protein) but not at the intermediate stages (carbonyls approximately 58.0 nmol/mg LDL protein) of LDL oxidation probably owing to differences in the protein-binding mechanisms of catechin and quercetin. Relationships between peroxide formation, carbonyl products, and LDL protein denaturation were shown by the FTIR approach. The FTIR technique provided a simple new tool for a comprehensive evaluation of antioxidant performance in protecting LDL during in vitro oxidation.