A crystallographic, conformational energy, and biological study of actodigin (AY-22,241) and its genin

A multidisciplinary (crystallographic, conformational energy, biological) study of Actodigin and digitoxigenin was completed, and the data analyzed using the NIH PROPHET computer system. These data were compared to Na ⁺,K ⁺-ATPase inhibition studies on Actodigin genin, digitoxigenin β-D-glucoside, digitoxin, and digitoxigenin β-D-digitoxide. This work has shown that Actodigin genin's ability to inhibit Na ⁺,K ⁺-ATPase can be largely explained by its lactone carbonyl oxygen position (5.22 Å displaced from the carbonyl oxygen of digitoxigenin, both molecules in their crystallographically observed energy minima) and molecular conformation. The ring D of Actodigin, for example, was found to be in a half chair, unlike those of natural digitalis ring D's, which exist in an envelope. However, the β-D-glucose makes an unexpectedly large contribution to Actodigin's activity - much larger than with digitoxigenin glucoside. Actodigin genin has very low activity (I ₅₀ = 7 x 10 ^-%5 M), nearly the least active genin we have studied. These findings were not predicted by a recently proposed Actodigin binding model, and they give new insight into the glycoside binding model proposed by Yoda and Yoda.