CARDENOLIDE FORMATION AND ACTIVITY OF PREGNANE-MODIFYING ENZYMES IN CELL-SUSPENSION CULTURES, SHOOT CULTURES AND LEAVES OF DIGITALIS-LANATA

The activities of five enzymes involved in pregnane metabolism and the biosynthesis of cardenolides were determined in various tissues of Di gitalis lanata, i.e. young leaves of greenhouse plants, cardenolide-pr oducing light-grown shoot cultures, dark-grown shoot cultures which do not accumulate cardenolides and suspension cultures unable to form ca rdenolides. A correlation between the endogenous content of cardiac gl ycosides and enzyme activity was found only for the progesterone 5 bet a-reductase that catalyses the conversion of progesterone to 5 beta-pr egnane-3,20-dione in the putative cardenolide pathway. This enzyme was very active in young leaves, exhibited moderate activity in light-gro wn, green shoot cultures and was not seen in dark-grown, white shoots and suspension-cultured cells. The progesterone 5 alpha-reductase, on the other hand, showed only weak activity in young leaves but was very active in cultured shoots and cells. Progesterone 5 beta-reductase an d progesterone 5 alpha-reductase may compete for the same substrate in planta. A similar competitive situation may be assumed for another pa ir of stereospecific enzymes, namely the 3 beta-hydroxysteroid 5 beta- oxidoreductase and the 3 alpha-hydroxysteroid 5 beta-oxidoreductase. A ctually, an activity distribution was seen similar to that described f or the progesterone reductases. It is supposed that enzymes like the 3 alpha-hydroxysteroid 5 beta-oxidoreductase and the progesterone 5 alp ha-reductase remove precursors from the cardenolide pathway and this i n way contribute to the overall regulation of cardenolide biosynthesis .