Reduction of digoxin to 20R-dihydrodigoxin by cultures of Eubacterium lentum

The anaerobic bacterium Eubacterium lentum, a common constituent of the intestinal microflora, inactivates digoxin by reducing the unsaturated lactone ring. Reduction of the cardiac glycoside by growing cultures of E. lentum ATCC 25559 proceeded in a stereospecific manner, with the 20R-dihydrodigoxin constituting more than 99% of the product formed. This is in contrast to the 3:1 ratio of 20R and 20S epimers formed in the chemical catalytic hydrogenation. Formation of the reduced glycosides proceeded quantitatively when an overall concentration of 10 micrograms/ml was added to the cultures. E. lentum did not hydrolyze the digitoxose sugars from C-3 of the parent glycoside. However, the synthetically prepared sugar-hydrolyzed metabolites (digoxigenin, digoxigenin monodigitoxoside, and digoxigenin bisdigitoxoside) were reduced to the corresponding dihydro metabolites. Repetition of the experiments with a feces sample from a volunteer who was known to be a converter of digoxin to dihydrodigoxin gave results identical to those obtained with pure E. lentum cultures.