PENETRATION OF AN EMULSION SURFACE BY CHOLESTERYL ESTER TRANSFER PROTEIN

Quenching of the intrinsic fluorescence of cholesteryl ester transfer protein (CETP) by spin labelled fatty acids (5-NS and 16-NS) was inves tigated to determine the degree to which the protein penetrated the ph ospholipid monolayer surface of a lipid emulsion. When bound to the ph ospholipid surface approximately 50% of the fluorophores of the transf er protein were accessible to quenching by 5-NS whose nitroxy group lo cates near the monolayer surface. On the other hand, only 22% of the f luorophores of CETP were accessible to quenching by 16-NS whose nitrox y group locates deeper in the surface monolayer. Quenching of the CETP fluorescence by an aqueous phase quencher (acrylamide) shows that the protein undergoes a conformational change on binding which increases the proportion of the tryptophan residues exposed to the aqueous phase . The results indicate that CETP does not penetrate the lipid surface to a significant degree.