A stereospecific enzyme of the putative biosynthetic pathway of cardenolides

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Abstract
Leaves of Digitalis purpurea contain an enzyme activity which catalyzes the conversion of progesterone to 5β-pregnane-3,20-dione. Since cardenolides without exception possess a 5β-configuration, 5β-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 K m value of 6 μM for progesterone. It required NADPH as a co-substrate with an apparent K m value of 22 μM. The optimum temperature in vitro was 30°C. The activity was not dependent on monovalent and bivalent cations.