Effects the of nitrogen source on alkaloid metabolism in callus culture ofCatharanthus roseus (L.)G Don

The effects of nitrogen sources on primary and secondary metabolism were st udied in Catharanthus roseus calli throughout a 20 day culture cycle. The a ctivity profiles of enzymes involved in nitrogen assimilation, such as glut amine synthetase (GS) and glutamate dehydrogenase (GDH), were different in calli cultured on media with either nitrate or ammonium as the sole nitroge n source. However, important similarities occurred when such profiles were compared to those from calli grown on a nitrate/ammonium mixture. These res ults could be explained by the presence of nitrate and ammonium regulated i soforms, both for GS and GDH. No quantitative changes were observed in alka loid metabolism since the levels of total alkaloids were similar in calli c ultured on the different nitrogen sources. The activity of tryptophan decar boxylase (TDC), which catalyzes the first committed step in indole alkaloid biosynthesis, remained unaffected. However, the alkaloid qualitative profi le changed depending on the nitrogen source; indicating that alteration of the primary N metabolism does not affect the total amount of nitrogen and c arbon funneled into alkaloid metabolism, but rather on its distribution.