Evidence for the presence of a glycosphingolipid-transfer protein in rat brain cytosol

Proteins in the postmicrosomal supernatant fraction of rat brain catalyzed the transfer of bovine brain galactocerebroside, sulfatide, and ganglioside GM1 from unilamellar liposomes to the rat erythrocytes or ghosts. The vesicles were made with egg yolk lecithin, cholesterol, 3H-labelled glycolipid, and a trace of [14C]triolein as a nonexchangeable marker. The routine assay of the glycosphingolipid transfer consisted of incubation of the donor liposomes with erythrocytes in the presence or absence of supernatant protein in physiological buffer at 37 °C for various time intervals. After the incubation, the erythrocytes were separated from the vesicles by centrifugation and the extent of protein-catalyzed transfer of labelled glycolipid in the membrane-bound total lipid fraction was determined by scintillation spectrometry. The fraction of [3H]glycosphingolipid transferred is represented by a change in the 3H/14C ratios at initial and subsequent time intervals. The glycosphingolipid transfer catalyzed by the supernatant protein was found to be logarithmic, whereas the protein-independent transfer was linear over a period of 3–4 h. The rate constant (K) and half time (t1/2) of the protein-catalyzed transfer reaction of cerebrosides and sulfatides were almost the same, while the transfer of ganglioside GM1 occurred at a slightly faster rate, probably owing to the greater aqueous solubility of this lipid. The transfer activity was also increased in a manner dependent on the amount of supernatant protein added up to 10 mg. The catalytic activity of the protein was lost when heated at 70 °C for 5 min. The pH optimum of the activity was around 7.4. Divalent metal ions Ca2+, Mg2+, and Mn2+ at a concentration of 0.1–2.0 mM had no appreciable effect on the transfer of cerebroside. However, Ca2+ at the concentration tested notably inhibited sulfatide transfer. Approximately, 3–5 pmol of the glycosphingolipids was transferred from the vesicles to the erythrocytes per milligram of supernatant protein per hour. The transferred radioactivity can be exclusively recovered in the red cell membrane bound glycolipid fraction, as analyzed by high performance liquid chromatography. The active material was partially purified (over 30-fold) by ammonium sulfate precipitation and gel permeation chromatography on Sephadex G-75, with an indicated molecular weight of about 21 000.