Endogenous ligands of rat lung β-galactoside-binding protein (galaptin) isolated by affinity chromatography on carboxyamidomethylated-galaptin-Sepharose

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Abstract
Rat lung .beta.-glactoside-binding protein (galaptin) is developmentally regulated during postnatal lung development. In common with other vertebrate galaptins, it is very labile when purified and dependent on the presence of exogenous thiol reagents. Reaction of rat lung galaptin with iodoacetamide resulted in a stable activity carboxyamidomethylated galaptin that could be coupled to Sepharose. The resultant affinity matrix bound asialoglycoproteins, and these could be quantitatively eluted with disaccharide haptens. The carboxyamidomethylated-galaptin-Sepharose affinity matrix was used to search for endogenous ligands in 13-day-rat lung. Cytosolic fractions of developing rat lung contained no moieties that could be specifically eluted with disaccharide hapten. Only when membranous fractions were extracted with 1% Triton were glycoproteins solubilized that bound to the affinity matrix and could be specifically eluted with disaccharide hapten. The eluted glycoproteins were potent inhibitors of galaptin binding to asialo-orosomucoid. Sodiu dodecyl sulfate/polyacrylamide-gel electrophoresis identified these glycoproteins as being of high MW with 3 components of MW 160,000-200,000 and a smaller component of MW 75,000. This is the first evidence for specific membrane-associated glycoproteins being the ligands of rat lung galaptin.