title: Kinetic and thermodynamic analyses of spontaneous exchange between high-density lipoprotein-bound and lipid-free apolipoprotein A-I.
abstract: It is thought that apolipoprotein A-I (apoA-I) spontaneously exchanges between high-density lipoprotein (HDL)-bound and lipid-free states, which is relevant to the occurrence of preβ-HDL particles in plasma. To improve our understanding of the mechanistic basis for this phenomenon, we performed kinetic and thermodynamic analyses for apoA-I exchange between discoidal HDL-bound and lipid-free forms using fluorescence-labeled apoA-I variants. Gel filtration experiments demonstrated that addition of excess lipid-free apoA-I to discoidal HDL particles promotes exchange of apoA-I between HDL-associated and lipid-free pools without alteration of the steady-state HDL particle size. Kinetic analysis of time-dependent changes in NBD fluorescence upon the transition of NBD-labeled apoA-I from HDL-bound to lipid-free state indicates that the exchange kinetics are independent of the collision frequency between HDL-bound and lipid-free apoA-I, in which the lipid binding ability of apoA-I affects the rate of association of lipid-free apoA-I with the HDL particles and not the rate of dissociation of HDL-bound apoA-I. Thus, C-terminal truncations or mutations that reduce the lipid binding affinity of apoA-I strongly impair the transition of lipid-free apoA-I to the HDL-bound state. Thermodynamic analysis of the exchange kinetics demonstrated that the apoA-I exchange process is enthalpically unfavorable but entropically favorable. These results explain the thermodynamic basis of the spontaneous exchange reaction of apoA-I associated with HDL particles. The altered exchangeability of dysfunctional apoA-I would affect HDL particle rearrangement, leading to perturbed HDL metabolism.
authors.affiliation: Institute of Health Biosciences, Graduate School of Pharmaceutical Sciences, Tokushima University , 1-78-1 Shomachi, Tokushima 770-8505, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Handa
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Kimura
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Oka
authors.forename: Yuki
authors.initials: Y
authors.lastname: Takechi
authors.forename: Keiichiro
authors.initials: K
authors.lastname: Okuhira
authors.forename: Michael C
authors.initials: MC
authors.lastname: Phillips
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Saito
mesh_terms: D016632:Apolipoprotein A-I / Q000378:metabolism* / Q000493:pharmacokinetics; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000378:metabolism* / Q000493:pharmacokinetics; D011485:Protein Binding / Q000502:physiology; D015202:Protein Engineering / Q000379:methods; D013816:Thermodynamics
chemical_list: D016632:Apolipoprotein A-I; D008075:Lipoproteins, HDL