The glutathione S-transferase-CsGT45 fusion protein shows activity toward flavonoids. (A) The recombinant CsGT45 was analyzed using 10% (w/v) SDS-PAGE, and visualized with Coomassie staining. (B) Chemical structures of the flavonoids kaempferol and quercetin. (C) HPLC analysis of CsGT45 activity toward kaempferol. (D) HPLC analysis of CsGT45 activity toward quercetin. The obtained products, P1, P2, and P3 are denoted by arrows. (E) Positive ion mass spectrum of kaempferol 7-O-glucoside acquired during the HPLC-ESI-MS analysis. (F) Representative positive ion mass spectrum obtained for quercetin 7-O-glucoside, quercetin 4'-O-glucoside and 3'-O-glucoside acquired during the HPLC-ESI-MS analysis of each reaction product. Abbreviations: St, flavonol standard; -E, minus enzyme; and +E plus enzyme.