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. Althoug h the phenomenon is well established, the kinetics and molecular mechanism of passive apo exchange/transfer have not been defined in detail. In this s tudy, the kinetic parameters governing the movement of radiolabeled apoC mo lecules from human very low density lipoprotein (VLDL) to high density lipo protein (HDL3) particles were measured using a manganese phosphate precipit ation assay to rapidly separate the two types of lipoprotein particles. In the case of VLDL labeled with human [C-14]apoCIII(1), a large fraction of t he apoCIII1 transfers to HDL3 within 1 minute of mixing the two lipoprotein s at either 4 degrees or 37 degrees C. As the diameter of the VLDL donor pa rticles is decreased from 42-59 to 23-25 nm, the size of this rapidly trans ferring apoCIII(1) pool increases from about 50% to 85%. There is also a po ol of apoCIII(1) existing on the donor VLDL particles that transfers more s lowly. This slow transfer follows a monoexponential rate equation; for 35-4 0 nm donor VLDL particles the pool size is similar to 20% and the t(1/2) is similar to 3 h. The flux of apoCIII molecules between VLDL and HDL3 is bid irectional and all of the apoCIII seems to be available for exchange so tha t equilibrium is attained. It is likely that the two kinetic pools of apoCI II are related to conformational variations of individual apo molecules on the surface of VLDL particles. The rate of slow transfer of apoCIII1 from d onor VLDL (35-40 nm) to acceptor HDL3 is unaffected by an increase in the a cceptor to donor ratio, indicating that the transfer is not dependent on co llisions between donor and acceptor particles. Consistent with this, apoCII I1 molecules can transfer from donor VLDL to acceptor HDL3 particles across a 50 kDa molecular mass cutoff semipermeable membrane separating the lipop rotein particles. These results indicate that apoC molecules transfer betwe en VLDL and HDL3 particles by an aqueous diffusion mechanism. (C) 1999 Publ ished by Elsevier Science B.V. All rights reserved.