Cell lines of Catharcmthus roseus were selected for resistance to 4-methyltryptophan (4-MT) in order to get strains with increased activities of tryptophan decarboxylase (TDC). This enzyme may exert a decisive regulatory control in indole alkaloid biosynthesis. 4-Methyltryptophan is a substrate for TDC and is detoxified by this enzyme. All 4-MT resistant cell lines showed increased TDC activity (3-to 10-fold) and higher accumulation of tryptamine (3- to 5-fold). Two of these cell lines with increased TDC activity accumulated indole alkaloids in the growth medium, whereas in wild type cultures and other resistant cell lines this was not the case. All the cell lines obtained were not only resistant to 4-MT (20- to 70-times more than wild type cells) but were also cross-resistant to other tryptophan analogues which are not metabolized by TDC. Therefore the resistance was not only due to a better detoxification of the analogue. The selected cell lines also contained 2 to 6 times higher levels of ʟ-tryptophan but the feedback sensitivity of the anthranilate synthetase of the resistant cells was not alteref

Fourteen tryptophan analogues have been characterized with regard to their growth inhibitory effects on Catharanthus roseus cell cultures, their inhibitory effect on anthranilate synthetase, their incorporation into protein and their substrate affinity to tryptophan decarboxylase. The toxicity of 12 of the analogues was directly correlated with their inhibitory effect on anthranilate synthetase activity. None of the analogues was efficiently incorporated into protein. Only 4 of the analogues (4-methyl-, 4-fluoro-, 5-fluoro- and 5-hydroxytryptophan) were good substrates for tryptophan decarboxylase while all other analogues were rather poor substrates or were not converted at all. Alpha-methyltryptophan was a competitive inhibitor of tryptophan de­carboxylase