Mode of action of β-glucuronidase from Aspergillus niger on the sugar chains of arabinogalactan-protein

Abstract

A β-glucuronidase purified from a commercial pectolytic enzyme preparation of Aspergillus niger hydrolyzed about half of the 4-O-methyl-glucuronic acid (4-Me-GlcA) residues located at the nonreducing terminals of (1→6)-linked β-galactosyl side chains of the carbohydrate portion of a radish arabinogalactan-protein (AGP) modified by treatment with fungal α-L-arabinosidase. Digestion of the α-L-arabinosidase-treated AGP with exo-β-(1→3)-galactanase released, by exo-fission of β-(1→3)-galactosidic bonds in the backbone chains of the AGP, neutral β-(1→6)-galactooligosaccharides with various chain lengths and their acidic derivatives substituted at their nonreducing terminals with 4-Me-β-GlcA groups. In contrast, successive digestion of the α-L-arabinosidase-treated AGP with β-glucuronidase followed by exo-β-(1→3)-galactanase liberated much higher amounts of β-(1→6)-galactooligomers together with a small portion of short acidic oligomers, mainly 4-Me-β-GlcA-(1→6)-Gal and 4-Me-β-GlcA- (1→6)-β-Gal-(1→6)-Gal. These results indicate that β-glucuronidase acts upon 4-Me-β-GlcA residues in long (1→6)-linked β-galactosyl side chains of the AGP, whereas short acidic side chains survive the attack of the enzyme.