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Kinetics and Thermodynamics of Lipid Amphiphile Exchange between Lipoproteins and Albumin in Serum

Luís M. B. B. Estronca ^*, Maria João Moreno ^*, J. A. N. Laranjinha , L. M. Almeida  and Winchil L. C. Vaz ^*

^* Departamento de Química, Faculdade de Ciências e Tecnologia, and Laboratório de Bioquímica, Faculdade de Farmácia, Universidade de Coimbra, Coimbra, Portugal

Correspondence: Address reprint requests to Prof. Winchil L. C. Vaz, Departamento de Química, Universidade de Coimbra, 3004-535 Coimbra, Portugal. Tel.: 351-239-824861; Fax: 351-239-827703; E-Mail: wvaz{at}ci.uc.pt.

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°C, by an equilibrium binding constant of 3 x 10⁶ M^–1 and a characteristic time 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 10⁵ and 10⁶, 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 10⁵ M^–1 s^–1 for VLDL and LDL and 10⁴ 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°C. This permitted the calculation of the equilibrium thermodynamic parameters (G^o, H^o, and S^o) as well as the activation parameters (G^o, H^o, and S^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.

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