Covalent attachment of immunoglobulins to liposomes via glycosphingolipids.

We describe a method for covalent binding of proteins to large unilamellar liposomes which involves the periodate oxidation of glycosphingolipids in the vesicle membrane. Proteins such as IgG and F(ab')2 may then be attached to the aldehyde groups on the glycolipid by Schiff-base formation at pH 9.5 and reduction with NaBH4, or by reductive amination with NaBH3CN at pH 8.4. Exposure of the vesicles to periodate, protein coupling and separation from unbound protein by a novel method of flotation is discontinuous dextran gradients does not release the vesicle contents when performed at pH 8.4. Studies on the oxidation of neutral glycolipid-containing vesicles, and on the oxidation of encapsulated glycerol 1-phosphate show that periodate influx into neutral vesicles during a 4 h exposure is appreciable at pH 5.5 but not at pH 8.4. Under optimal conditions, approx. 20% of the protein may be coupled to vesicles, and a ratio of 100--200 microgram of protein/mumol of lipid is readily achieved. This method will be of great importance for the antibody-mediated targeting of vesicles to cells.