Formation of apolipoprotein-specific high-density lipoprotein particles from lipid-free apolipoproteins A-I and A-II

We have shown previously that apolipoprotein A (apoA)-I-containing high-den sity lipoprotein (HDL) particles are formed by the conjugation of lipid-fre e apoA-I with lipids derived from other lipoprotein fractions in a process dependent on nonesterified fatty acids, generated by the lipolysis of very- low-density lipoprotein (VLDL) or provided exogenously. In the present stud y, we show that this process is also able to generate HDL particles contain ing apoA-II (A-II HDL) and both apoAI and apoA-II (A-I/A-II HDL). When lipi d-free apoA-II was incubated with either VLDLs and lipoprotein lipase or LD Ls and sodium oleate, a significant proportion of the apoA-II was recovered in the HDL density fraction. This was associated with the formation of sev eral populations of HDL-sized particles with pre-beta 2 electrophoretic mob ility, which contained phospholipids and unesterified cholesterol as their main lipid constituents. When both lipid-free apoA-I and lipid-free apoA-II were incubated with LDL and sodium oleate, both apolipoproteins were recov ered in HDLs that contained phospholipids and unesterified cholesterol as t heir main lipids. Two populations of particles had diameters of 7.4 and 10. 8 nm and pre-beta 2-migration; there was also a population of pre-beta 1-mi grating particles of diameter 4.7 nm. ApoA-I and apoA-II were both present in the larger HDLs, whereas only apoA-I was present in the smaller particle s. Immune affinity chromatography on an anti-(apoA-I)-Sepharose column reve aled that 10-20 % of the apoA-II resided in particles that also contained a poA-I. The majority of the A-I/A-II HDL were present in a population of pre -beta 2 particles of 10.8 nm diameter. These results in vitro illustrate a potential mechanism for the formation of HDLs containing both apoA-I and ap oA-II.