Interaction of Odoroside A, A Known Natural Cardiac Glycoside, with Na+/K+-ATPase

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Abstract
The nature of odoroside A, a cardiac glycoside (CG) extracted from Nerium oleander, as well as its chemical structure is quite similar to a well-known CG, ouabain possessing a steroid skeleton, a five-membered unsaturated lactone ring, and a sugar moiety as a common structure. Like ouabain, odoroside A inhibits the activity of Na+/K+-ATPase (NKA) and shows significant anticancer activity, however its inhibitory mechanism remains unknown. CGs show various physiological activities, including cardiotonic and anticancer activities, through the inhibition of NKA by direct interaction. Additionally, X-ray crystallographic analysis revealed the inhibitory mechanism of ouabain and digoxin in relation to NKA. By using different molecular modeling techniques, docking simulation of odoroside A and NKA was conducted based on the results of these X-ray crystallographic analyses. Furthermore, a comparison of the results with the binding characteristics of three known CGs (ouabain, digoxin, and digitoxin) was also conducted. Odoroside A fitted into the CG binding pocket on the α-subunit of NKA revealed by X-ray crystallography. It had key interactions with Thr797 and Phe783. Also, three known CGs showed similar interactions with Thr797 and Phe783. Interaction modes of odoroside A were quite similar to those of ouabain, digoxin, and digitoxin. Docking simulations indicated that the sugar moiety enhanced the interaction between NKA and CGs, but did not show enhanced NKA inhibitory activity because the sugar moiety was placed outside the entrance of active site. Thus, these results suggest that the inhibitory mechanism of odoroside A to NKA is the same as the known CGs. Graphical Abstract

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