Isolation and characterization of human apolipoprotein M-containing lipoproteins

Apolipoprotein M (apoM) is a novel apolipoprotein with unknown function. In this study, we established a method for isolating apoM-containing lipoproteins and studied their composition and the effect of apoM on HDL function. ApoM-containing lipoproteins were isolated from human plasma with immunoaffinity chromatography and compared with lipoproteins lacking apoM. The apoM-containing lipoproteins were predominantly of HDL size; similar to 5% of the total HDL population contained apoM. Mass spectrometry showed that the apoM-containing lipoproteins also contained apoJ, apoA-I, apoA II, apoC-I, apoC-II, apoC-III, paraoxonase 1, and apoB. ApoM-containing HDL (HDLapoM+) contained significantly more free cholesterol than HDL lacking apoM (HDLapoM-) (5.9 +/- 0.7% vs. 3.2 +/- 0.5%; P < 0.005) and was heterogeneous in size with both small and large particles. HDLapoM+ inhibited Cu2+-induced oxidation of LDL and stimulated cholesterol efflux from THP-1 foam cells more efficiently than HDLapoM-. In conclusion, our results suggest that apoM is associated with a small heterogeneous subpopulation of HDL particles. Nevertheless, apoM designates a subpopulation of HDL that protects LDL against oxidation and stimulates cholesterol efflux more efficiently than HDL lacking apoM.