Tryptophan is a precursor for melatonin and serotonin biosynthesis in in vitro regenerated St. John's wort (Hypericum perforatum L. cv. Anthos) plants

Evidence of a pathway for the biosynthesis of the mammalian neurohormones m elatonin and serotonin in in vitro regenerated plantlets of St. John's wort (Hypericum perforatum cv. Anthos) is presented. Isotope tracer experiments were performed on plantlets regenerated from thidiazuron-induced stem expl ants and grown on MS basal medium for 2 months. Radiolabel from C-14-trypto phan was recovered as C-14-indoleacetic acid, C-14-tryptamine, C-14-5-hydro xytryptophan, C-14-serotonin and C-14-melatonin in the treated St. John's w ort plantlets. Chromatographic peak identity was confirmed by high performa nce liquid chromatography-mass spectrometry-mass spectrometry and quantific ation of melatonin by radioimmunoassay. Significantly more radiolabel was r ecovered in serotonin relative to melatonin under low light conditions with this ratio being reversed under increased lighting, indicating that the ra te of flow through this biosynthetic pathway is regulated, at least in part , by light.