Mechanism of high-density lipoprotein subfractions inhibiting copper-catalyzed oxidation of low-density lipoprotein

Objective: To investigate the role of HDL subfractions, HDL2 and HDL3, on the oxidation of LDL catalyzed by 5 μM Cu2+ ion, and to illustrate the mechanism of the generation of conjugated diene and thiobarbituric acid reactive substances (TBARS) during LDL oxidation. Methods: LDL was incubated for 8 h with 5 μM Cu2+ ion in phosphate-buffered saline (PBS) alone, or in the presence of HDL2, HDL3, HSA, BSA, or transferrin. Meantime, LDL was incubated for 24 h with 10 μM Ni2+ ions in PBS. The amount of conjugated diene and TBARS in each sample of LDL were measured. Results: (a) HDL2 and HDL3 could inhibit the generation of conjugated diene, but could not inhibit the generation of TBARS; (b) the transferrin containing HDL3 shows the ability of inhibiting the generation of both conjugated diene and TBARS; (c) the transferrin presented in blood exhibits the inhibitory effect on the generation of conjugated diene and TBARS, however, when the transferrin is saturated with Fe3+ ion, it could not inhibit the generation of TBARS; (d) HSA and BSA could prevent the generation of conjugated diene and TBARS; (e) Ni2+ ion could induce the generation of conjugated diene, but the amount of TBARS was much smaller than that induced by Cu2+ ion. Conclusion: HDL2 and HDL3 play important role in the copper-catalyzed oxidation of LDL; it is absolutely necessary to require chelation of Cu2+ ion for inhibiting generation of TBARS; whereas, inhibition of conjugated diene can be fulfilled either by chelating Cu2+ ion, or the free radicals scavenger.