ROLE OF LIPOPROTEIN-COPPER COMPLEX IN COPPER CATALYZED-PEROXIDATION OF LOW-DENSITY-LIPOPROTEIN

The oxidative modification of low-density lipoprotein (LDL) is suggested to play an important role in the pathogenesis of atherosclerosis. The present study examined the role of the formation of LDL-copper (Cu) complex in the peroxidation of LDL. The amount of copper bound to LDL increased during incubation performed with increasing concentrations of CuSO4. More than 80% of the copper bound to the LDL particle was observed in the protein phase of LDL, suggesting that most of the copper ions formed complexes with the ligand-binding sites of apoprotein. The addition of histidine (1 mM), known to form a high affinity complex with copper, and EDTA (1 mM), a metal chelator, during the incubation of LDL with CuSO4 prevented the formation of both thiobarbituric acid-reactive substances (TBARS) and LDL-Cu complexes. EDTA inhibited the copper-catalyzed ascorbate oxidation whereas histidine had no effect, suggesting that the copper within the complex with histidine is available to catalyze the reaction, in contrast to EDTA. These observations indicate that the preventive effect of histidine on the copper-catalyzed peroxidation of LDL is not simply mediated by chelating free copper ions in aqueous phase. Evidence that copper bound to LDL particle still has a redox potential was provided by the observed increase in TBARS content during incubation of LDL-Cu complexes in the absence of free copper ions. The addition of either histidine or EDTA to LDL-Cu complexes inhibited the formation of TBARS by removing copper ions from the LDL forming the corresponding complexes. However, there still remained small amounts of copper in the LDL particles following the treatment of LDL-Cu complexes with histidine or EDTA. The copper ions remaining in the LDL particle lacked the ability to catalyze LDL peroxidation, suggesting that there may be two types of copper binding sites in LDL: tight-binding sites, from which the copper ions are not removed by chelation, and weak-binding sites, from which copper ions are easily removed by chelators. The formation of TBARS in the LDL preparation during incubation with CuSO4 was comparable to the incubation with FeSO4. In contrast, the formation of TBARS in the LDL-lipid micelles by CuSO4 was nearly eliminated even in the presence of ascorbate to promote metal-catalyzed lipid peroxidation, although a marked increase in TBARS content was observed in the LDL-lipid micelles with FeSO4, and with FeCl3 in the presence of ascorbate. These findings suggest that the protein moiety of LDL, apoprotein, is necessary for the copper-catalyzed peroxidation of LDL. From these results, we propose that the formation of LDL-Cu complexes is essential for the copper-catalyzed peroxidation of LDL. In addition, our observations suggest the possibility of the existence of two types of copper binding sites in LDL, of which the weak-binding sites would be involved in the peroxidation of LDL.