The mechanism of oxidation of model compounds of apo B protein in LDL by Cu(II) complexes

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Although it is assumed that the bound copper ion is in redox-active state and capable of producing free radicals initiating the chain reaction, the mechanism of copper-induced peroxidation of LDL is still unclear.Thus, the mechanism of peroxidation of linoleic acid, which is the most abundant polyunsaturated fatty acid in LDL, by some Cu (II) complexes as model compounds of copper bound to apo B protein of LDL was examined by absorption spectroscopy and HPLC. The absorbance at 234 nm due to the formation of conjugated dienes in the reaction of Cu (II) complexes with linoleic acid acid increased with incubation time, reaching a maximum level, and decreased gradually thereafter. The time to reach the maximal value of absorbance at 234 nm was depended on the redox potential of Cu (II) complexes, suggesting that Cu (II) complexes with a high redox potential could oxidize easily linoleic acid to generate linoleic acid hydroperoxide (LOOH). In addition, the addition of hydrogen peroxide (HィイD22ィエD2OィイD22ィエD2) enhanced the oxidation with incubation time more rapidly than in the absence of HィイD22ィエD2OィイD22ィエD2.The addition of ascorbic acid to Cu (II) (HGG) suppressed the increase in absorbance at 234 nm, depending on the increase in the concentration of ascorbic acid. Fe (III) EDTA-mediated oxidation of linoleic acid was accelerated by the addition of ascorbic acid. On the other hand, the addition of Trolox (a water soluble derivative of α-tocopherol) to Cu (II) (HGG) suppressed the increase in absorbance at 234 nm. On contrary, Fe (III) EDTA-mediated oxidation of linoleic acid was completely inhibited by the addition of Trolox.In the presence of acetyl-L-lysine, the reaction of Cu (II) complexes with LOOH led to the formation of fluorescence with excitation maximum at 360 nm and emission maximum at 433 nm.