N-homocysteinylation of apolipoprotein A-I impairs the protein's antioxidant ability but not its cholesterol efflux capacity.

ABSTRACT

A high homocysteine (Hcy) level is a risk factor for atherosclerosis. Hcy can be added to proteins through a process known as N-homocysteinylation. This is thought to be a potential cause of atherosclerosis induction. We previously reported that N-homocysteinylated apolipoprotein A-I (N-Hcy-apoA-I) was identified in normal human plasma. In this study, the effect of N-homocysteinylation on the functions of apoA-I was examined. A kinetic study using dimyristoyl phosphatidylcholine (DMPC) liposomes indicated that N-Hcy-apoA-I showed increased lipid-binding activity compared to wild-type apoA-I. Two reconstituted high-density lipoprotein (rHDL) particles of different sizes (approximately 8.2 nm and 7.6 nm in diameter) were produced by mixing apoA-I and 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC). However, an increased ratio of large to small particles was found in rHDL prepared with N-Hcy-apoA-I. The normal apoA-I antioxidant ability, estimated by the suppression of conjugated diene formation in low-density lipoprotein (LDL) induced by copper sulfate oxidation, was considerably impaired when using N-Hcy-apoA-I. Although N-Hcy-apoA-I functioned as an oxidant, no significant difference was observed in the cholesterol efflux capacity from THP-1 macrophages between wild-type apoA-I and N-Hcy-apoA-I. These results suggest that N-Hcy-apoA-I might be proatherogenic due to its oxidative behavior but not an attenuation of cholesterol efflux capacity.