A stereospecific enzyme of the putative biosynthetic pathway of cardenolides

Leaves of Digitalis purpurea contain an enzyme activity which catalyzes the conversion of progesterone to 5 beta-pregnane-3,20-dione. Since cardenolides without exception possess a 5 beta-configuration, 5 beta-pregnane-3,20-dione can serve as a precursor for this class of secondary metabolites. It is assumed that the enzyme is part of the putative biosynthetic pathway of cardenolides. This enzyme activity was spotted in the soluble fraction of a crude homogenate. Product formation was detected by gas chromatography and by gas chromatography/mass spectroscopy (g.c./m.s.). The enzyme had a pH optimum at 8.0 and an apparent Km value of 6 microM for progesterone. It required NADPH as a co-substrate with an apparent Km value of 22 microM. The optimum temperature in vitro was 30 degrees C. The activity was not dependent on monovalent and bivalent cations.