Biotransformation of digitoxigenin – a plant secondary metabolite – by the fungus Cochliobolus lunatus

Abstract

Cardiac glycosides are plant secondary metabolites used to treat congestive heart failure. They also exhibit a wide spectrum of biological activities, including anti-carcinogenic, acaricidal, antifilarial, molluscicidal and antibacterial properties. Biotransformation of cardenolides has been investigated either as a strategy to obtain new derivatives or to convert the A-type cardenolides into the corresponding C-type compounds, which have clinical relevance [1]. The fungus Cochliobolus lunatus and its conidial anamorphous form Curvularia lanata are known for their capacity of hydroxylating Δ4–5 steroids at position 11β, 14α and 7α [2]. The biotransformation of digitoxigenin 1, the aglycone of digitoxin, by C. lunatus was investigated. The biotransformation reaction was carried out in a 4-day process, resulting in the isolation of four products. Their structures were elucidated by 1D and 2D NMR data as 1β-hydroxydigitoxigenin 2, 7β-hydroxydigitoxigenin 3, 8β-hydroxydigitoxigenin 4 and digitoxigenone 5 (Fig.1). The observed hydroxylation sites are distinct from those previously described for Δ4–5 steroids in reactions with C. lunatus. The production of 4 by a biotransformation reaction is clear evidence that C. lunatus hydroxylases involved in the reaction are not affected by the steric hindrance of the 14β-OH group [3]. Therefore, it is feasible to obtain a product with two vicinal hydroxyls at 8β and 14β-positions. Hydroxylation of 1 at 1β-position is of special interest, since some glycosides of 1β-hydroxydigitoxigenin have been reported to exhibit potent in vitro activity against ovarian adenocarcinoma and lung carcinoma [4].