THE EFFECT OF HISTIDINE MODIFICATION ON COPPER-DEPENDENT LIPID-PEROXIDATION IN HUMAN LOW-DENSITY-LIPOPROTEIN

Lipid peroxidation and subsequent oxidative modification of low-densit y lipoprotein (LDL) have been implicated as causal events in atheroscl erosis. Cu2+ may play an important role in LDL oxidation by binding to histidine residues of apolipoprotein B-100 (apo B) and initiating and propagating lipid peroxidation. To investigate the role of histidine residues, we used diethylpyrocarbonate (DEPC), a lipid-soluble histidi ne-specific modifying reagent. When LDL (0.1 mg protein/ml, or 0.2 mu M) was incubated with DEPC (1 mM), at least 76 +/- 7% of the histidine residues in apo B were modified. Treatment of LDL with DEPC led to an increase in the rate of Cu2+-induced initiation of lipid peroxidation (R-i), but a significant decrease in the rate of propagation. These c hanges resulted in an overall increased resistance of IDL to oxidation , with a significantly increased lag phase preceding the propagation p hase of lipid peroxidation. In contrast to DEPC, ascorbate completely prevented the initiation of LDL oxidation (R-i = 0). Our data indicate that there are two types of copper/histidine binding sites on apo B: those facing the lipid core of the LDL particle, which mediate the pro pagation of lipid peroxidation and are modified by DEPC; and those fou nd on the surface of the LDL particle exposed to the aqueous environme nt, which are responsible for mediating the initiation of lipid peroxi dation and are modifiable by ascorbate in the presence of Cu2+.