Binding of specific glycoconjugates to human brain synaptosomes: studies using glycosylated β-galactosidase

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Using glycosylated β-galactosidase (β-gal) as a glycoprotein model, binding of glycoconjugates to human brain synaptosomes was studied. Out of β-gal modified with a series of p-aminophenyl α- or β-glycosides, β-gal modified with p-aminophenyl β- d-glucopyranoside (β- d-Glc β-gal) was bound to the synaptosomes most effectively, then β-gal modified with β- d-galactoside and with α- d-mannoside. Kinetic studies on the binding of β- d-Glc β-gal indicated the presence of saturable binding on human brain synaptosomes. The values of the apparent K m and of the maximal binding velocity were obtained to be 248 ± 32.9 μM and 43.8 ± 1.43 fmol/min/mg synaptosome protein, at 4°C and pH 7.5. The specificity of the sugar recognition site proved by the competitive inhibition of the binding of β- d-Glc β-gal by bovine serum albumin modified with the same glycoside. The binding of β-gal modified with β- d-galactoside was inhibited by treatment of the synaptosomes with trypsin, phospholipase A 2, C and D, and with neuraminidase, while the binding of β- d-Glc β-gal was inhibited by neuraminidase treatment of synaptosomes. In subcellular fractions of human brain the binding protein was localized mainly in synaptosomes.