Apolipophorin III from Hyphantria cunea shows different anti-oxidant ability against LDL oxidation in the lipid-free and lipid-bound state.

Insect apolipophorin III (apoLp-III) and human apolipoprotein A-I (apoA-I) are major protein constituents of the lipoprotein system that share various properties. In order to compare the anti-oxidant ability of apoLp-III and apoA-I in the lipid-free and lipid-bound state, both proteins were purified and synthesized individually as a palmitoyloleoyl phosphatidylcholine (POPC)-reconstituted high-density lipoprotein (rHDL) using the same molar ratio. In the lipid-bound state, apoLp-III and apoA-I showed good anti-oxidant activities against copper-mediated LDL oxidation. Furthermore, apoLp-III and apoA-I, in the lipid-bound state, exhibited potent activities in ferric ion-reducing ability (FRA). However, lipid-free apoLp-III lost the anti-oxidant activity and FRA ability in contrast to lipid-free apoA-I. Lipid-free apoA-I treatment prevented the cellular uptake of oxLDL in macrophages, as visualized by oil-red O staining and detection assays for malondialdehyde (MDA) and lipid hydroperoxide (LPO) in the culture media. However, lipid-free apoLp-III did not prevent the uptake of oxLDL. These results indicate that the putative conformational change of apoLp-III during lipid association is critical for the maintenance of anti-oxidant activity and that the physiologic role of apoLp-III may differ when it is in the lipid-free state and the lipid-bound state.