SPDP-Modified PE Lipid Derivatives

SPDP also is a popular choice for coupling sulfhydryl-containing molecules to liposomes. PE residues in vesicles may be activated with this crosslinker to form pyridyl disulfide derivatives that can react with sulfhydryls to form disulfide linkages. Unlike the iodoacetyl- and maleimide-based crosslinkers discussed previously, the linkage formed with SPDP is reversible by simple disulfide reduction. Pure PE may be activated with SPDP prior to its incorporation into a liposome, 

The following protocol is a suggested method for coupling sulfhydryl-containing proteins to SPDP-activated vesicles. 

Prepare a 5mg/ml liposome suspension containing a mixture of PC cholesterol PG PDP-PE in molar ratios of 8 10 1 1. The emulsification may be done by any established method (Section 1, this chapter). Suspend the vesicles in 50mM sodium phosphate, 0.15 M NaCl, 10 mM ethylenediamine triacetic acid EDTA, pH 7.2. 

Add at least 5 mg/ml of a sulfhydryl-containing protein or other molecule to the SPDP-modified vesicles to effect the conjugation reaction. Molecules lacking available sulf-hydryl groups may be modified to contain them by a number of methods (Chapter 1, Section 4.1). The conjugation reaction should be done in the presence of at least lOmM EDTA to prevent metal-catalyzed sulfhydryl oxidation. 

React overnight with stirring at room temperature. Maintain the suspension in a nitrogen or argon atmosphere to prevent lipid oxidation. 

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