Modification of the lipid-protein interaction in human low-density lipoprotein destabilizes ApoB-100 and decreases oxidizability

The interactions of the lipid and protein moiety of human low-density lipoprotein (LDL) and their influence on the oxidation behavior of LDL were modified using an amphipathic peptide, melittin, as a probe. The interaction of melittin with the LDL phospholipid surface resulted in a destabilization of apolipoprotein B-100 (apoB-100) as monitored by differential scanning calorimetry, while the characteristics of lipid core melting remained nearly unchanged. Binding of melittin caused a restriction of lipid chain mobility near the glycerol backbone, but not in the middle or near the methyl terminus of the fatty acyl chains as observed by electron paramagnetic resonance. Also, upon melittin addition, the level of copper binding to apoB-100 and the oxidizability of LDL by Cu(2+) ions were greatly reduced, as indicated by abolished tryptophan fluorescence quenching upon Cu(2+) binding and, during oxidation, prolongation of the lag phase of oxidation, attenuated consumption of alpha-tocopherol, and a lowered maximal rate of conjugated diene formation. This reduction of oxidizability could not be reversed by increasing the Cu(2+) concentration. It is deduced that interaction of Cu(2+) and alpha-tocopherol is required for reductive activation of the metal. It can be abolished by interfering with the interactions between apoB-100 and the lipid moiety of LDL which modifies the conformation of LDL and, as a consequence, hinders copper binding to apoB-100.