Apolipoproteins C-I, C-II, and C-III: Kinetics of Association with Model Membranes and Intermembrane Transfer

The apoproteins (apo) C-I, C-II, and C-III are low molecular weight amphiphilic proteins that are associated with the lipid surface of the plasma chylomicron, very low density lipoprotein (VLDL), and high-density lipoprotein (HDL) subfractions. Purified apoC-I spontaneously reassociates with VLDL, HDL, and single-bilayer vesicles (SBV) of l-palmitoyl-2-oleoyl-.sn-glycero-3-phosphocholine. ApoC-I also transfers reversibly from VLDL to HDL and from VLDL and HDL to SBV. The kinetics of association of the individual apoC proteins with SBV are second order overall and first order with respect to lipid and protein concentrations. At 37 °C, the rates of association were 2.5 × 10¹⁰, 4.0 × 10¹⁰ and 3.8 × 10¹⁰ M^-1 s^-1 for apoC-I, apoC-II, and apoC-III, respectively. Arrhenius plots of association rate vs temperature were linear and yielded activation energies of 11.0 (apoC-I), 9.0 (apoC-II), and 10.6 kcal/mol (apoC-III). The kinetics of vesicle to vesicle apoprotein transfer are biexponential for intermembrane transfer, indicating two concurrent transfer processes. Rate constants at 37 °C for the fast component of dissociation were 11.7, 9.5, and 9.9 s^-1 while rate constants for the slow component were 1.3, 0.6, and 0.9 s^-1 for apoC-I, apoC-II, and apoC-III, respectively. The dissociation constants, K_d, of apoC-I, apoC-II, and apoC-III bound to the surface monolayer of phospholipid-coated latex beads were 0.5, 1.4, and 0.5 μM, respectively. These studies show that the apoC proteins are in dynamic equilibrium among phospholipid surfaces on a time scale that is rapid compared to lipolysis, lipid transfer, and lipoprotein turnover.