Abstract
Digoxin and other cardiotonic steroids (CTS) exert their effect by inhibiting Na,K-ATPase (NKA) activity. CTS bind to the various NKA isoforms that are expressed in different cell types, which gives CTS their narrow therapeutic index. We have synthesised a series of digoxin derivatives (γ-Benzylidene digoxin derivatives) with substitutions in the lactone ring (including non-oxygen and ether groups), to obtain CTS with better NKA isoform specificity. Some of these derivatives show some NKA isoform selective effects, with BD-3, BD-8, and BD-13 increasing NKA α2 activity, BD-5 inhibiting NKA α1 and NKA α3, BD-10 reducing NKA α1, but stimulating NKA α2 and α3; and BD-14, BD-15, and BD-16 enhancing NKA α3 activity. A molecular-docking approach favoured NKA isoform specific interactions for the compounds that supported their observed activity. These results show that BD compounds are a new type of CTS with the capacity to target NKA activity in an isoform-specific manner.
Highlights
Na,K-ATPase (NKA) is an enzyme responsible for the transport of Naþ and Kþ across the plasma membrane of most animal cells
Homogenates from the cells containing the different NKA isoforms were used to test the effects of the digoxin derivatives
BD-4 shows no selectivity for the NKA isoforms, but it is able to inhibit all isoforms at micromolar concentrations (Figure 2(C))
Summary
Na,K-ATPase (NKA) is an enzyme responsible for the transport of Naþ and Kþ across the plasma membrane of most animal cells. In E2-P, NKA is phosphorylated, releases Naþ ions to the extracellular side of the cell plasma membrane, and is ready to bind and transfer Kþ ions to the cytosol. NKA is constituted by three different subunits: the a, b, and a smaller polypeptide, which depending on the tissue, consists of one of several members of the FXYD family of polypeptides[4,5]. The a subunit is responsible for the catalytic activity of NKA and contains the ATP, Naþ, and Kþ binding sites. The b subunit is a glycosylated polypeptide responsible for the folding and functional competence of the NKA a subunit. The FXYD peptide functions as a modulator of the catalytic properties of NKA. Four isoforms of Supplemental data for this article can be accessed here
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