Fig. 7.

Substrate specificity assay of sugar acceptors for GAGT. The purified plant enzyme was tested for substrate specificity using UDP-[¹⁴C]-xylose and the following phenolics were used as sugar acceptors: benzoic acid (BA), 2-hydroxybenxoic acid (salicylic acid, SA), 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 2,3-dihydroxybenzoic acid, 2,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid (gentisic acid, GA), 2,6-dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid, caffeic acid, ferulic acid, coumaric acid, escopoletin, esculetin, and umbelliferone. Samples were HPLC-analysed and the eluted radioactivity was monitored. (A) Typical elution profiles obtained with UDP-[¹⁴C]-xylose alone (top panel) or in the presence of GA (middle panel) or SA (bottom panel). A distinctive radioactive peak was detected using GA as a sugar acceptor, but not when the acceptor was SA. The same negative result was obtained with the rest of the phenolics assayed. The qualitative results for some representative phenolics are shown in (B).