Specific binding of 125I-labeled beta-hexosaminidase A to rat brain synaptosomes

Abstract

Purified human beta-hexosaminidase A (beta-N-acetylgulcosaminidase; 2-acetamido-2-deoxy-beta-D-glucoside acetamidodeoxyglucohydrolase, EC 3.2.1.30) has been labeled with 125I to high specific activity with the retention of 80% of its enzyme activity. The binding of this enzyme to sonicated synaptosomes from rat brain was shown to be a saturable and specific process. Glycoproteins containing a sialic acid-terminal oligosaccharide or a galactose-terminal oligosaccharide (i.e., alpha 1-acid glycoprotein and fetuin and their asialo derivatives) were strong inhibitors of the binding. In contrast, ovalbumin, which contains a mannose-rich oligosaccharide, and mannans were poor inhibitors of the binding. Of the monosaccharides tested, sialic acid, galactosamine, mannose, galactose, and lactose were inhibitory in decreasing potency of inhibition. Optimal binding occurred at pH 7.0 in the presence of 3 mM calcium ions. The binding was a linear function of synaptosomal protein concentration between 25 and 200 microgram of protein per assay and was directly proportional to time up to 3 hr, beyond which there was no further increase in specific binding. The data suggest a unique but complex mode of interaction of glycoproteins with receptors on synaptic membranes.