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

Using glycosylated beta-galactosidase (beta-gal) as a glycoprotein model, binding of glycoconjugates to human brain synaptosomes was studied. Out of beta-gal modified with a series of p-aminophenyl alpha- or beta-glycosides, beta-gal modified with p-aminophenyl beta-D-glucopyranoside (beta-D-Glc beta-gal) was bound to the synaptosomes most effectively, then beta-gal modified with beta-D-galactoside and with alpha-D-mannoside. Kinetic studies on the binding of beta-D-Glc beta-gal indicated the presence of saturable binding on human brain synaptosomes. The values of the apparent Km and of the maximal binding velocity were obtained to be 248 +/- 32.9 microM and 43.8 +/- 1.43 fmol/min/mg synaptosome protein, at 4 degrees C and pH 7.5. The specificity of the sugar recognition site proved by the competitive inhibition of the binding of beta-D-Glc beta-gal by bovine serum albumin modified with the same glycoside. The binding of beta-gal modified with beta-D-galactoside was inhibited by treatment of the synaptosomes with trypsin, phospholipase A2, C and D, and with neuraminidase, while the binding of beta-D-Glc beta-gal was inhibited by neuraminidase treatment of synaptosomes. In subcellular fractions of human brain the binding protein was localized mainly in synaptosomes.