Abstract
We have examined the kinetics and thermodynamics of the exchange of a fluorescent amphiphile derived from a phospholipid, NBD-DMPE, between serum albumin and the serum lipoproteins of high density (HDL2 and HDL3), LDL, and VLDL. Binding of the fluorescent lipid amphiphile to bovine serum albumin is characterized, at 35 degrees C, by an equilibrium binding constant of approximately 3 x 10(6) M(-1) and a characteristic time < or = 0.1 s. Association of NBD-DMPE with the lipoprotein particles, if considered as a partitioning of amphiphile monomers between the aqueous phase and the lipoprotein particles, is characterized by an equilibrium partition coefficient between 10(5) and 10(6), being highest for LDL and lowest for HDL. The association of NBD-DMPE monomers with lipoprotein particles can be described by insertion rate constants on the order of 10(5) M(-1) s(-1) for VLDL and LDL and 10(4) M(-1) s(-1) for HDL. The desorption rate constants are on the order of 10(-5) s(-1) for all particles. The study was performed as a function of temperature between 15 and 35 degrees C. This permitted the calculation of the equilibrium thermodynamic parameters (deltaG(o), deltaH(o), and deltaS(o)) as well as the activation parameters (deltaG++(o), deltaH++(o), and deltaS++(o)) for the insertion and desorption processes. The association equilibrium is dominated by the entropic contribution to the free energy in all cases. The results are discussed in relation to phospholipid and amphiphile exchange phenomena involving the lipoproteins.