PNNL

Abstract

Human high-density lipoproteins (HDL) are heterogenous in composition and recent evidence indicates HDL is a mixture of structurally-related particles that perform distinct physiological functions. However, our molecular understanding of specific subspecies is incomplete, largely because they are difficult to isolate due to similar physico-chemical properties. In previous studies investigating human HDL that contain the major scaffold protein apolipoprotein A-I (APOA1) but lack a second scaffold protein (APOA2), designated LpA-I, we observed this subfraction is composed primarily of two distinctly-sized populations human plasma. Using high-resolution size exclusion chromatography, we isolated these “large” and “small” populations and performed a deep molecular characterization of each using mass spectrometry. The LpA-I large population was spherical with a diameter of 109 Å and contained 3 molecules of APOA1 while the small population was 91 Å and contained between 3-4 molecules of APOA1. The large population contained the highest proteomic diversity. Overall lipid contents were relatively similar between the subfractions, though the large particles tended to be enriched in ceramides and depleted in free cholesterol. The data indicate that the major driver of LpA-I HDL particle size is the scaffold formed by APOA1, with non-scaffold minor proteins contributing little to particle size. We propose that the presence of APOA2 disrupts APOA1-mediated particle size quantization, driving most of the particle size heterogeneity observed in HDL as a whole. This understanding of the determinants of HDL particle size serves as a basis for understanding the functional heterogeneity of HDL in a variety of disease states.