Seasonal changes of glucosinolates in Isatis leaves, and effect of harvest regimen and post-harvest treatment

Woad, Isatis tinctoria L. (Brassicaceae) is an ancient indigo dye and medicinal plant, which has been used and cultivated in Europe since antiquity. The anti-inflammatory potential of Isatis tinctoria had been analyzed in a broad-based pharmacological profiling, and inhibitors of cyclooxygenase-2 (COX-2), 5-lipoxygenase (5-LOX), human neutrophil elastase, and histamine release were identified [1]. Animal pharmacological studies and a clinical pilot study corroborated the anti-inflammatory potential of lipophilic Isatis extracts [2]. We observed that the phytochemical profile of woad undergoes profound changes during post-harvest treatment. For example, the indigo precursors are largely transformed, whereas the dual COX-2/5-LOX inhibitor tryptanthrin is formed only during the drying process [3]. The precursors of tryptanthrin are presently unknown, but the tetracyclic ring system contains at least one indole unit. We believe that some additional alkaloids with indole moieties may also be formed only during the drying process. Indole glucosinolates occur in rather high concentration in fresh Isatis leaves and are thus potential precursors for such alkaloids. As parts of a comprehensive metabolite profiling, we investigated the seasonal fluctuation of these glucosinolates of five Isatis tinctoria and Isatis indigotica accessions (first year, rosette stage) grown on field plots under identical conditions. The effects of repeated harvesting during the growth season on glucosinolate concentrations, and the influence of post-harvest processing were studied. Analysis of the non-derivatized glucosinolates was carried out with a recently developed and validated PLE (pressurized liqiud extraction) protocol and ion-pair HPLC coupled with ESI-MS detection in the negative ion mode. References: [1] Hamburger, M. (2002) Phytochem. Rev. 1: 333–344. [2] Recio, C. et al. (2006) Planta Med. 72: 715–720. [3] Oberthür, C., Hamburger, M. (2004) Planta Med. 70: 642–645.