PROTEIN-LIPID INTERACTION IN BILE - EFFECTS OF BILIARY PROTEINS ON THE STABILITY OF CHOLESTEROL-LECITHIN VESICLES

The nucleation of cholesterol crystals is an obligatory precursor to c holesterol gallstone formation. Nucleation, in turn, is believed to be preceded by aggregation and fusion of cholesterol-rich vesicles. We h ave investigated the effects of two putative pro-nucleating proteins, a concanavalin A-binding protein fraction and a calcium-binding protei n, on the stability of sonicated small unilamellar cholesterol-lecithi n vesicles. Vesicle aggregation is followed by monitoring absorbance, and upon addition of the concanavalin A-binding protein fraction the a bsorbance of a vesicle dispersion increases continuously with time, Ve sicle fusion is probed by a fluorescence contents-mixing assay. Vesicl es apparently fuse slowly after the addition of the concanavalin A-bin ding protein, although inner filter effects confound the quantitative measurement of fusion rates, The rates of change of absorbance and flu orescence increase with the concentration of the protein, and the seco nd-order dimerization rate constant increases with both the protein co ncentration and the cholesterol content of the vesicles. On the other hand, the calcium-binding protein has no effect on the stability of th e vesicle dispersion. This protein may therefore affect cholesterol cr ystal formation not by promoting the nucleation process, but by enhanc ing crystal growth and packaging, Our results demonstrate that biliary proteins can destabilize lipid vesicles and that different proteins p lay different roles in the mechanism of cholesterol gallstone formatio n. (C) 1998 Elsevier Science B.V.