Influence of 16β formylation on Na, K-ATPase inhibition by cardiac glycosides

Summary

The inhibitory effect of formylated cardiac steroids (gitaloxin and its derivatives) on guinea-pig heart Na, K-ATPase was compared to that of other cardiac steroids with various hydroxy substituents. The decreasing order of potency of aglycones at equilibrium was as follows: gitaloxigenin > digitoxigenin > ouabagenin > digoxigenin > gitoxigenin > diginatigenin. This sequence was different to the sequence of drugs hydrophobic character. The compounds with hydroxy groups in the vicinity of the lactone ring (gitoxigenin, diginatigenin) were less potent than the hydrophilic compound ouabagenin. We propose that intramolecular bonding between 16β-OH and the lactone ring contributes to the relatively low potency of gitoxigenin and diginatigenin. The formylation of 16β-OH increased the potency of gitoxigenin by a factor of 41. The formylated compound (gitaloxigenin) was 5-fold more potent than digitoxigenin. The 3β-glycosylation of digoxigenin lead to pseudo-irreversible inhibitors of Na, K-ATPase. The half-time to achieve the equilibrium (for 5μmol/l) was equal to 54 s, 90 s and 108 s respectively for digoxigenin monodigitoxoside, digoxin and desacetyllanatoside C. However, at equilibrium the three glycosides were equipotent, suggesting the existence of steric effects at the sugar site of the receptor. The sequence of potency observed for monodigitoxosides, monodigitalosides and tridigitoxosides after 60 min incubation was similar to that observed for the corresponding aglycones. These results suggest that the strongly negative inductive group 16β-OCHO is tightly bound to Na, K-ATPase, possibly to the same receptor site than that which is thought forming hydrogen and ionic bonds with the lactone ring. They show that the high toxicity of gitaloxin in guineapig heart is likely due to its high potency as Na, K-ATPase inhibitor.