BIOCHEMICAL-CHARACTERIZATION OF THE 3 MAJOR SUBCLASSES OF LIPOPROTEIN-A-I PREPARATIVELY ISOLATED FROM HUMAN PLASMA

Apolipoprotein (apo) A-I is the major protein constituent of plasma hi gh-density lipoproteins (HDL). HDL consist of two major classes of apo A-I-containing lipoproteins: LpA-I and LPA-I:A-II. LpA-I includes hete rogeneous lipoprotein particles that differ in size and hydrated densi ty. LpA-I was isolated by immunoaffinity chromatography from the fasti ng plasma of 24 normal human subjects and separated by gel filtration chromatography. Three major subclasses of LpA-I were eluted: large (Lg -LpA-I), medium (Md-LpA-I), and small LpA-I (Sm-LpA-I). By nondenaturi ng gradient PAGE, Lg-LpA-I, Md-LpA-I, and Sm-LpA-I had mean Stokes dia meters of 10.8 +/- 0.5, 8.9 +/- 0.5, and 7.5 +/- 0.3 nm, respectively. The lipid/protein ratios were 1.25 +/- 0.12 for Lg-LpA-I, 0.75 +/- 0. 10 for Md-LpA-I, and 0.38 +/- 0.08 for Sm-LpA-I. Lg-LpA-I was relative ly lipid and cholesteryl ester rich compared with Md-LpA-I and Sm-LpA- I. Sm-LpA-I contained phospholipids as the major lipid component. ApoA -I was the major apolipoprotein in all LpA-1 subfractions, whereas apo E was present only in Lg-LpA-I and apoA-IV was associated with both Md -LpA-I and Sm-LpA-I. All three LpA-I subclasses exhibited predominantl y alpha mobility on agarose electrophoresis. Lg-LpA-I migrated as a di ffuse band in the fast alpha position, whereas Md-LpA-I and Sm-LpA-I m igrated to the slow alpha position. In addition, both Lg-LpA-I and Sm- LpA-I, but not Md-LpA-I, had components with pre-beta electrophoretic mobility. All three LpA-I subclasses bound specifically to Ob 1771 cel ls and promoted cholesterol efflux. Lg-LpA-I had a significantly highe r amount of LCAT and CETP activity per particle than Md-LpA-I and Sm-L pA-I. These data indicate that these LpA-I subclasses have distinctive size, electrophoretic mobility, composition, and metabolic activities and provide new insights into the molecular architecture of LpA-I and HDL.