GLYCOSYLATION IN CARDENOLIDE BIOSYNTHESIS

The glycosylation and deglycosylation of cardiac glycosides was invest igated using cell suspension cultures and shoot cultures, both establi shed from Digitalis lanata EHRH, plants, as well as isolated enzymes. Shoots were capable of glucosylating digitoxigenin, evatromonoside, di giproside, glucodigitoxigenin and digitoxin. Suspension cultured Digit alis cells glucosylated all the substrates mentioned but digiproside, whereas the UDP-glucose-dependent cardenolide glucosyltransferase isol ated from that source did not accept digitoxigenin and digiproside as substrates. It is concluded that at least three different glucosyltran sferases are involved in cardiac glycoside formation in Digitalis. Sim ilar experiments carried out with glucosylated cardenolides which were administered to cultured cells, shoots and a cardenolide beta-glucosi dase isolated from young leaves revealed that at least two different g lucosidases occur in Digitalis lanata, albeit in different tissues or during different phases of development. The biotransformation of gluco evatromonoside was investigated using unlabelled compound and [C-14-gl ucose]glucoevatromonoside synthesized enzymatically. After 7 d of incu bation almost no radioactivity could be recovered from the cardenolide fraction, indicating that the terminal glucose of glucoevatromonoside was now incorporated into volatile, hydrophilic and insoluble compoun ds. Since, on the other hand, large amounts of cardenolides were found in the experiments with unlabelled glucoevatromonoside it is assumed that steady state or pool size regulation is achieved by the coordinat ed action of a cardenolide glucosidase and a glucosyltransferase.