Sterol and squalene carrier protein interactions with fluorescent delta 5,7,9(11)-cholestatrien-3 beta-ol

The fluorescent sterol delta 5,7,9(11)-cholestatrien-3 beta-ol (cholestatrienol) was used as an analogue of cholesterol to determine the properties of the sterol in aqueous buffer and the interaction of cholesterol with sterol and squalene carrier protein (SCP). Cholestatrienol was synthesized and purified to a stable product by reverse phase high performance liquid chromatography. The critical micelle concentration of cholestatrienol in aqueous buffer was 1 nM while its maximum solubility was 1.15 microM as ascertained from fluorescence polarization and light scattering properties, respectively. Several lines of evidence indicated a close molecular interaction of cholestatrienol with purified rat liver SCP. The fluorescence emission spectrum of monomeric cholestatrienol in aqueous buffer was blue shifted upon addition of SCP. The fluorescence lifetime of monomeric cholestatrienol in aqueous buffer was increased by SCP from 5 to 12 ns. The SCP increased the fluorescence polarization of monomeric cholestatrienol from 0.002 to 0.38 in aqueous buffer. The close molecular interaction of cholestatrienol with SCP was also demonstrated by energy transfer experiments. Fluorescence energy transfer from tyrosine residues of SCP to the conjugated triene fluorophore in cholestatrienol had a transfer efficiency of 59%. R, the apparent distance between the tyrosine energy donor and the cholestatrienol energy acceptor, was 16.3 A. Binding analysis indicated that cholestatrienol interacted with SCP with an apparent KD = 0.5 microM and a Bmax = 3.54 microM. One mol of cholestatrienol was bound per mol of SCP. These results demonstrate the utility of cholestatrienol not only as a membrane sterol probe molecule but also as a probe for sterol-protein interactions.