Kinetics and mechanism of exchange of apolipoprotein C-III molecules from very low density lipoprotein particles

Transfer of apolipoprotein (apo) molecules between lipoprotein particles is an important factor in modulating the metabolism of the particles. Although the phenomenon is well established, the kinetics and molecular mechanism of passive apo exchange/transfer have not been defined in detail. In this study, the kinetic parameters governing the movement of radiolabeled apoC molecules from human very low density lipoprotein (VLDL) to high density lipoprotein (HDL3) particles were measured using a manganese phosphate precipitation assay to rapidly separate the two types of lipoprotein particles. In the case of VLDL labeled with human 14C]apoCIII1, a large fraction of the apoCIII1 transfers to HDL3 within 1 minute of mixing the two lipoproteins at either 4 degrees or 37 degrees C. As the diameter of the VLDL donor particles is decreased from 42-59 to 23-25 nm, the size of this rapidly transferring apoCIII1 pool increases from about 50% to 85%. There is also a pool of apoCIII1 existing on the donor VLDL particles that transfers more slowly. This slow transfer follows a monoexponential rate equation; for 35-40 nm donor VLDL particles the pool size is approximately 20% and the t1/2 is approximately 3 h. The flux of apoCIII molecules between VLDL and HDL3 is bidirectional and all of the apoCIII seems to be available for exchange so that equilibrium is attained. It is likely that the two kinetic pools of apoCIII are related to conformational variations of individual apo molecules on the surface of VLDL particles. The rate of slow transfer of apoCIII1 from donor VLDL (35-40 nm) to acceptor HDL3 is unaffected by an increase in the acceptor to donor ratio, indicating that the transfer is not dependent on collisions between donor and acceptor particles. Consistent with this, apoCIII1 molecules can transfer from donor VLDL to acceptor HDL3 particles across a 50 kDa molecular mass cutoff semipermeable membrane separating the lipoprotein particles. These results indicate that apoC molecules transfer between VLDL and HDL3 particles by an aqueous diffusion mechanism.