CHOLESTEROL INTERACTION WITH RECOMBINANT HUMAN STEROL CARRIER PROTEIN-2

The interaction of human recombinant sterol carrier protein-2 (SCP-2) with sterols was examined. Two independent ligand binding methods, Lipidex 1000 binding of [H-3]cholesterol and a fluorescent dehydroergosterol binding assay, were used to determine the affinity of SCP-2 for sterols. Binding analysis indicated SCP-2 bound [H-3]cholesterol and dehydroergosterol with a K-d of 0.3 and 1.7 mu M, respectively, and suggested the presence of a single binding site. Phase fluorometry and circular dichroism were used to characterize the SCP-2 sterol binding site, Alterations in dehydroergosterol lifetime, SCP-2 tryptophan lifetime, and SCP-2 tryptophan quenching by acrylamide upon cholesterol binding demonstrated a shielding of the SCP-2 tryptophan from the aqueous solvent by bound sterol. Differential polarized phase fluorometry revealed decreased SCP-2 tryptophan rotational correlation rime upon cholesterol binding. Circular dichroism of SCP-2 indicated that cholesterol elicited a small decrease in SCP-2 alpha helical content. The data suggest that SCP-2 binds sterols with affinity consistent with a lipid trans fer protein that may act either as an aqueous carrier or at a membrane surface to enhance sterol desorption.