title: The apparent preferential interaction of human plasma high density apolipoprotein A-I with gel-state phospholipids.
abstract: The enthalpy, entropy and free energy of activation was measured for the transfer of the tryptophan residues of apolipoprotein A-I from a more hydrophobic environment of a lipoprotein particle containing dimyristoylphosphatidylcholine (with or without 12% cholesterol) to an aqueous solvent in the presence of varying concentrations of guanidinium chloride. The free energy of activation was approximately 25 kcal/mol at 50 degrees C for all the conditions studied. The enthalpy of activation was greatest under conditions where a large degree of unfolding occurs when the protein dissociated from lipid. However, under these conditions the unfavourable activation enthalpy was compensated for by a favourable activation entropy resulting in the insensitivity of the free energy of activation to the condition of measurement. Apolipoprotein A-I has an apparent affinity for gel-state lipid which results from the very slow rate of dissociation of the lipoprotein particle below 40 degrees C. It is unlikely that the association of apolipoprotein A-I with dimyristoylphosphatidylcholine is thermodynamically stable only in the temperature region of the phase transition but that the association exhibits a large kinetic stability, especially at lower temperatures or in the absence of guanidinium chloride.
mesh_terms: D016632:Apolipoprotein A-I; D001053:Apolipoproteins / Q000097:blood*; D005782:Gels; D006801:Humans; D007700:Kinetics; D010743:Phospholipids / Q000378:metabolism*; D011485:Protein Binding; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D016632:Apolipoprotein A-I; D001053:Apolipoproteins; D005782:Gels; D010743:Phospholipids