β-Glucuronosyltransferase from Radish Primary Roots Involving in the Synthesis of Arabinogalactan-proteins

Abstract

We investigated β-glucuronosyltransferase (GlcAT) involved in the synthesis of radish arabinogalactan-proteins (AGPs) whose basic carbohydrate structure comprises a β-(1→3)-galactan, branched with consecutive (1→6)-linked β-Gal residues, to which GlcA residues are attached through β-(1→6)-linkages.
The reaction was carried out in a mixture composed of a microsome fraction prepared from 6-d-old radish roots, UDP-[¹⁴C]GlcA, and β-(1→3)-galactan as an acceptor at 25ºC. The GlcA transfer occurred maximally at pH 6.0 in the presence of 30 mM Mn²⁺ and 0.75% triton X-100. Digestion of rdiolabeled product with exo-β-(1→3)-galactanase released GlcA-Gal and GlcA-Gal-Gal as the main products, which were further degraded into GlcA by the treatment with β-glucuronidase [1] purified from Aspergillus niger. GlcA transfer for mature root AGP was insignificant but increased by prior enzymatic digestion with α-L-arabinosidase and endo-β-(1→6)-galactanase [2].
[1] Carbohydr. Res., 333 (2001) 27-39
[2] Carbohydr. Res., in press