A rat brain cytosol protein which accelerates the translocation of galactosylceramide, lactosylceramide and glucosylceramide between membranes

Abstract

The cytosol fraction of rat brains accelerated the transfer of [ 3H]galactosylceramide, [ 3H]lactosylceramide, and [ 3H]glucosylceramide from donor to acceptor liposomes, which contained one of these glycosphingolipids as a constituent. All the three glycosphingolipid transfer activities were eluted from a Sephadex G-75 column at an identical position; from a K av value of 0.377 a molecular weight of 18000 was estimated for the transfer protein. The glycosphingolipid transfer reactions were characterized by the use of the active protein fraction obtained by the Sephadex chromatography. The translocation of each of the three [ 3H]glycosphingolipids from the donor liposomes to the acceptor liposomes was shown by thin-layer chromatography of the lipids extracted from the acceptor liposomes, which were separated from the donor liposomes after the incubation. In all three glycosphingolipid transfers, it was found that the transfer rates were highest when liposomes lacking in glycosphingolipids were used as an acceptor. Addition of one of the three glycosphingolipids to the acceptor liposomes as a constituent at 1 mol% resulted in a decrease in the measured rates of all three glycosphingolipid transfers: the effect was strongest with galactosylceramide, intermediate with glucosylceramide, and weak with lactosylceramide. In all three glycosphingolipid transfer assays, larger amounts of glycosphingolipids were exchanged between liposomes when the assay was performed using liposomes containing 10 mol% of a glycosphingolipid instead of using liposomes containing 1 mol% of the glycosphingolipid; the results suggest that phospholipids in the liposomes participate in some unknown way in the lipid transfer mediated by the glycosphingolipid transfer protein. The Sephadex G-75 fraction of the glycosphingolipid transfer protein did not facilitate the transfer of [ 3H]phosphatidylethanolamine from the donor to acceptor liposomes.