Properties of Human Plasma Lipid Transfer Protein in Aqueous Solution and at Interfaces

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Abstract
Human plasma lipid transfer protein (cholesteryl ester transfer protein) has been characterized for its solution and surface properties. The protein is monomeric in aqueous solution up to 0.62 g/L (11.7 microM) as demonstrated by sedimentation equilibrium. It binds to the surface of a lipid microemulsion having an average diameter of 26 nm made from triolein and egg yolk phosphatidylcholine, with an estimated dissociation constant 1.2 x 10(-8) M, and the maximum saturation binding level is 8 protein molecules per particle regardless of the presence of apolipoprotein A-I. Circular dichroism measurement indicated that the protein in solution is predominantly in the beta-sheet/beta-turn conformation with some alpha-helix, and this profile does not undergo drastic change by its binding to the lipid surface. The analysis of the behavior of the protein in its monomolecular layer at the air-buffer interface indicated that it is also monomeric at the interface. LTP molecules occupied the same area per amino acid as other apolipoproteins in the monolayer but had a higher collapse pressure of its monolayer (18 dyn/cm), and the protein stayed at the interface even after the overcompressing monolayer far beyond the collapsing pressure up to 40 dyn/cm.