A xyloglucan-oligosaccharide-specific α-D-xylosidase or exo-oligoxyloglucan-α-xylohydrolase from germinated nasturtium (Tropaeolum majus L.) seeds: Purification, properties and its interaction with a xyloglucan-specific endo-(1→4)-β-D-glucanase and other hydrolases during storage-xyloglucan mobilisation

The α-xylosidase which is involved in the post-germinative mobilisation of xyloglucan in nasturtium seed cotyledons has now been purified to apparent homogeneity by a facile procedure involving lectin affinity chromatography. The purified enzyme, a glycoprotein, moved as a single band (apparent molecular weight 85000) on sodium dodecyl sulphate-gel electrophoresis, whilst isoelectric focusing gave a number of enzymatically active protein bands spanning the range pI = 5.0 to 7.1 (maximum activity at pI = 6.1). The enzyme did not hydrolyse the simple α-xylosides p-nitrophenyl-α-D-xylopyranoside and isoprimeverose (α-D-Xyl(1→6)-D-Glc), or polymeric tamarind-seed xyloglucan. It released xylose from a complex mixture of oligosaccharides produced by exhaustive hydrolysis of tamarind seed xyloglucan using the xyloglucan-specific endo-(1→4)-β-D-glucanase from germinated nasturtium seeds (M. Edwards et al. 1986, J. Biol. Chem., 261. 9489—9494). The three xyloglucan oligosaccharides of lowest molecular size were purified from this mixture and were shown by 1H-nuclear magnetic resonance (1H—NMR) and enzymatic analysis to have the structures: $\stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\left(\mathrm{A}\right)}{}}{\mathrm{G}\mathrm{l}\mathrm{c}},\stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\stackrel{\mathrm{G}\mathrm{a}\mathrm{l}}{\downarrow }}{?}}{\stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}}\to \stackrel{\stackrel{\left(\mathrm{B}\right)}{}}{\mathrm{G}\mathrm{l}\mathrm{c}},\text{\hspace{0.17em}}\stackrel{\stackrel{\mathrm{X}\mathrm{y}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\stackrel{\stackrel{\mathrm{G}\mathrm{a}\mathrm{l}}{\downarrow }}{\mathrm{X}\mathrm{y}}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\stackrel{\stackrel{\mathrm{G}\mathrm{a}\mathrm{l}}{\downarrow }}{\mathrm{X}\mathrm{y}}\mathrm{l}}{\downarrow }}{\mathrm{G}\mathrm{l}\mathrm{c}}\to \stackrel{\stackrel{\left(\mathrm{C}\right)}