INTERFACIAL PROPERTIES OF RECOMBINANT HUMAN CHOLESTEROL ESTER TRANSFER PROTEIN

We investigated the interfacial behavior of recombinant human choleste rol ester transfer protein (rCETP) using monolayer and surface balance techniques. rCETP bound to egg phosphatidylcholine monolayers spread at the air/water interface with a maximum surface pressure of 23 milli newtons (mN)/m at subphase concentrations between 3 and 5 x 10(-5) g/d l; the estimated dissociation constant was 7.5 x 10(-6) g/dl or 1 nM?. The binding of rCETP to the lipid interface decreased linearly with i ncreasing initial surface pressure; rCETP was excluded at pressures gr eater than 31 mN/m. rCETP catalyzed the desorption of [C-14]cholestero l oleate from mixed lipid monolayers in a concentration dependent fash ion. Similar studies with apolipoproteins A-I and A-IV established tha t cholesterol ester desorption was not caused by changes in surface pr essure or cholesterol ester solubility. The desorption rate was propor tional to subphase rCETP concentration, but at all concentrations surf ace radioactivity remained constant until surface pressure reached a p lateau. The calculated binding stochiometry was one molecule of choles terol ester desorbed for every 1000 molecules of rCETP in the subphase . We conclude that rCETP is surface active, binds to phospholipid mono layers with an affinity equivalent to that of the plasma apolipoprotei ns, and effects the desorption of cholesterol ester molecules from pho spholipid monolayers by a carrier mechanism. Moreover, the relatively low equilibrium surface pressure of rCETP suggests that when bound to lipid the entire rCETP molecule may not penetrate the interface.