Distribution of high density lipoprotein particles with different apoprotein composition: particles with A-I and A-II and particles with A-I but no A-II.

High density lipoproteins (HDL) were subfractionated by equilibrium CsCl gradient centrifugation of the d 1.063-1.21 g/ml HDL fraction isolated from two men and two women. The various HDL subfractions were analyzed for their apoproteins (apo) A-I, A-II, B, D, and E and the major lipid contents. ApoA-I and A-II were found throughout the density gradient with the maximum concentration between the d 1.105 and 1.120 g/ml fractions. ApoE was found in all HDL fractions with the higher concentration in the lower density fractions. Conversely, the concentration of apoD increased as the density of the HDL fraction increased. Each density subfraction underwent quantitative precipitation with anti-A-I and anti-A-II immunoglobulin. Essentially all A-II in all density subfractions was precipitated with either immunoglobulin. Particles from each density subfraction precipitated with anti-A-II immunoglobulin had an A-I/A-II molar ratio of approximately 2.0 (range 1.9-2.3). However, particles precipitated with anti-A-I immunoglobulin had A-I/A-II molar ratios identical to the A-I/A-II ratio of the subfraction (range 2.1-7.1). The subfractions (d 1.105-1.149 g/ml fractions) with A-I/A-II molar ratios of about 2 had the least proportion of A-I in particles containing A-I but not A-II. Conversely, the subfractions (d 1.063-1.075 g/ml fractions) with the highest A-I/A-II molar ratio had the greatest proportion of apoA-I in particles containing A-I but not A-II. These data indicate that HDL contains at least two types of particles: particles with both A-I and A-II in a 2:1 molar ratio, and particles containing A-I but no A-II. The variation in A-I/A-II ratio observed in different HDL density subfractions was due to the different proportions of these two types of particles.