CHARACTERIZATION OF LIPID EXCHANGE PROTEINS ISOLATED FROM SMALL-INTESTINAL BRUSH-BORDER MEMBRANE

Subjecting rabbit small intestinal brush border membrane vesicles (BBM V) to freeze-thaw cycles releases water-soluble lipid exchange (transf er) proteins into the supernatant. They differ widely in apparent mole cular weight and catalyze cholesterol, phosphatidylcholine, and phosph atidylinositol exchange between two populations of small unilamellar l ipid vesicles. In order to determine their interrelations, the smalles t water-soluble lipid exchange protein was purified to homogeneity by gel filtration on Sephadex G-75 and cation exchange chromatography on Mono S. It is a basic protein of apparent molecular mass of 13 +/- 0.5 kDa. The purified protein was used to raise polyclonal antibodies, Po lyclonal antibodies were also produced against a lipid exchange protei n of apparent molecular mass of 100-120 kDa. By comparing lipid exchan ge, lipid binding, and immunological properties of the water soluble l ipid exchange proteins it can be shown that the 13-kDa (peak 3) protei n is related to the 100-120 kDa (peak 1) protein; the properties of th ese two proteins are different from those of the peak 2 lipid exchange protein of apparent molecular mass of 22 kDa. Based on the immunologi cal cross-reactivity observed between the 13 and 100-120 kDa and the l ipid binding properties of these two proteins, a working hypothesis is proposed: both proteins are probably part of an intergral membrane pr otein of the brush border membrane that facilitates cholesterol and ph osphatidylcholine absorption in this membrane. Evidence derived from i mmunogold labeling of BBMV supports the notion that this protein is lo cated on the external (luminal) side of the brush border membrane. The analogous behavior of rabbit and human small intestinal brush border membrane in terms of lipid absorption and the release of water-soluble lipid exchange proteins is discussed.