Plant progesterone 5β-reductase is not homologous to the animal enzyme.: Molecular evolutionary characterization of P5βR from Digitalis purpurea

Plants of the genus Digitalis produce cardiac glycosides, i.e. digoxin, which are widely used for congestive heart failure. Progesterone 5 beta-reductase (P5 beta R) is a key enzyme in the biosynthesis of these natural products. Here, we have carried out the purification and partial amino acid sequencing of the native P5 beta R from foxglove (Digitalis purpurea), and isolated a cDNA encoding this enzyme. Similarly to other steroid 5 beta-reductases, the recombinant P5 beta R catalyzes the stereospecific reduction of the Delta(4)-double bond of several steroids with a 3-oxo,Delta(4.5) structure. The gene encoding P5 beta R is expressed in all plant organs, and maximally transcribed in leaves and mature flowers. P5 beta R belongs to the short-chain dehydrogenase/reductase (SDR) superfamily, bearing no structural homology to its mammalian counterpart, which is a member of the aldo-keto reductase (AKR) superfamily. A similar situation occurs with 3 beta-hydroxy-Delta(5)-steroid dehydrogenase (3 beta HSD), the gene immediately preceding P5 beta R in the cardenolide pathway, which suggests that the entire route has evolved independently in animals and plants. P5 beta R is retained only in plants, where it is ubiquitous, and a few distantly related bacterial lineages after its diversification from the last universal common ancestor. Evolutionary conserved changes in its putative active site suggest that plant P50R is a member of a novel subfamily of extended SDRs, or a new SDR family. (c) 2006 Elsevier Ltd. All rights reserved.