Biosynthesis of serotonin and β-carboline alkaloids in hairy root cultures of Peganum harmala

Tryptophan decarboxylase (TDC) activity, and serotonin and harmane alkaloids levels of a Peganum harmala root culture were followed over a growth cycle. A close relationship between the peak of TDC activity and the highest specific content of serotonin was found, while such a correlation was not observed for the β-carboline alkaloids, the main constituents of the root cultures. The content of serotonin, but not of the alkaloids was greatly enhanced by feeding tryptamine. Trials to identify individual biosynthetic steps of alkaloid biosynthesis by feeding were only partially successful, because only the oxidation of dihydro-β-carbolines to aromatic harmane alkaloids was observed when the various alkaloids were added to the root cultures. Tracer experiments with tryptophan revealed the presence of 5− and 6-hydroxytetrahydronorharmane in the root cultures which, however, should not be biosynthetic intermediates of the major alkaloids, harmalol and harmine.