Enzymes in cardenolide-accumulating shoot cultures of Digitalis purpurea L

Abstract

In contrast to undifferentiated cell suspension cultures of Digitalis lanata, photomixotrophic shoot cultures of Digitalis purpurea accumulate cardiac glycosides in substantial concentrations. They are used to investigate enzymes of the cardenolide pathway. All cardenolides are 5β-configurated. The progesterone 5β-reductase and the 3β-hydroxysteroid-5β-oxidoreductase are present in shoot cultures but not in undifferentiated cell cultures. These enzymes provide precursors for cardenolides, whereas the presence of the progesterone 5α-reductase, also present in shoot cultures, is discussed with regard to its role in phytosterol biosynthesis and may be attributed to the general steroid pathway. The progesterone 5α-reductase had an activity maximum during the early growth period seven days after onset of cultivation, whereas the corresponding progesterone 5β-reductase activity was highest on day 11. The maximum cardenolide accumulation was after 24 days. The enzyme activities present in crude extracts from shoot cultures were characterized with regard to their requirements for NADPH and NADH, pH-optimum, temperature optimum, affinity to their substrates and their localization in the cell. The progesterone 5β-reductase was purified 769-fold.