Active ingredients in Chinese medicines promoting blood circulation as Na+/K+ -ATPase inhibitors

Abstract

The positive inotropic effect of cardiac glycosides lies in their reversible inhibition on the membrane-bound Na(+)/K(+)-ATPase in human myocardium. Steroid-like compounds containing a core structure similar to cardiac glycosides are found in many Chinese medicines conventionally used for promoting blood circulation. Some of them are demonstrated to be Na(+)/K(+)-ATPase inhibitors and thus putatively responsible for their therapeutic effects via the same molecular mechanism as cardiac glycosides. On the other hand, magnesium lithospermate B of danshen is also proposed to exert its cardiac therapeutic effect by effectively inhibiting Na(+)/K(+)-ATPase. Theoretical modeling suggests that the number of hydrogen bonds and the strength of hydrophobic interaction between the effective ingredients of various medicines and residues around the binding pocket of Na(+)/K(+)-ATPase are crucial for the inhibitory potency of these active ingredients. Ginsenosides, the active ingredients in ginseng and sanqi, substantially inhibit Na(+)/K(+)-ATPase when sugar moieties are attached only to the C-3 position of their steroid-like structure, equivalent to the sugar position in cardiac glycosides. Their inhibitory potency is abolished, however, when sugar moieties are linked to C-6 or C-20 position of the steroid nucleus; presumably, these sugar attachments lead to steric hindrance for the entrance of ginsenosides into the binding pocket of Na(+)/K(+)-ATPase. Neuroprotective effects of cardiac glycosides, several steroid-like compounds, and magnesium lithospermate B against ischemic stroke have been accordingly observed in a cortical brain slice-based assay model, and cumulative data support that effective inhibitors of Na(+)/K(+)-ATPase in the brain could be potential drugs for the treatment of ischemic stroke.

Figure 1

(A) Crystal structure of ouabain binding to the extracellular pocket of shark rectal gland Na⁺/K⁺-ATPase (PDB code 3A3Y) α subunit. Amino acid residues of Na⁺/K⁺-ATPase α subunit are shown in ribbon structure, and ouabain in scaled ball and stick. K⁺ binding sites are shown in purple balls. (B) Enlarged diagram without the membrane bilayer shown in the blue box of (A).

Figure 2

(A) Chemical structures of ouabain and 11 steroid-like compounds found in Chinese medicinal products used for the promotion of blood circulation. (B) Inhibition of porcine Na⁺/K⁺-ATPase by 0.1 mmol/L of ouabain and the selected 11 steroid-like compounds. Data represent mean±SEM of 5 replicates. ^bP<0.05, ^cP<0.01 vs control group (CON: deionized water only). (Adopted and modified from Figures 1 and 2 of Chen et al, Acta Pharmacol Sin 2010; 31: 696–702).

Figure 3

(A) Chemical structures of ouabain and MLB. The 3D structures of ouabain and MLB (in dark background) were displayed using RasWin Molecular Graphics Windows Version 2.6. Gray, red, and green colors represent C, O, and Mg²⁺ atoms, respectively. (B) Proposed molecular mechanism responsible for the therapeutic effects of cardiac glycosides, ginsenosides, MLB, and other steroid-like compounds in cardiac cells. Step 1: Inhibiting the cellular exchange of Na⁺ and K⁺ by drugs binding to Na⁺/K⁺-ATPase. Step 2: Accumulation of Na⁺ in the intracellular space due to the inhibition of Na⁺/K⁺-ATPase activity. Step 3: Promotion of the cellular exchange of Na⁺ and Ca²⁺ via the Na⁺/Ca²⁺ exchanger system. Step 4: Increasing the intracellular Ca²⁺ concentration owing to the activation of the Na⁺/Ca²⁺ exchanger system. Step 5: The elevated intracellular Ca²⁺ concentration leads to an increased inotropism and accentuates the force of myocardial contraction. (Adopted from Figure 1 and the cover page of Tzen et al, Acta Pharmacol Sin 2007; 28: 609–15).

Figure 4

Detailed molecular interactions between the extracellular binding pocket of Na⁺/K⁺-ATPase and ouabain, bufalin, ginsenoside Rh2, ursolic acid, or MLB. (Left panels) Modeling is displayed for ligand compounds, ouabain, bufalin, ginsenoside Rh2, ursolic acid, and MLB binding to the extracellular pocket of Na⁺/K⁺-ATPase α subunit. Amino acid residues around the binding pocket of Na⁺/K⁺-ATPase are shown in ribbon structure, and ligand compounds in stick structure. (Right panels) Amino acid residues of Na⁺/K⁺-ATPase close to ligand compounds (ball-and-stick structure) are shown in line structure. Green box or oval represents one or two hydrogen bonds formed between Na⁺/K⁺-ATPase and ligand compounds. (Adopted and modified from Figure 6 of Chen et al, Acta Pharmacol Sin 2010; 31: 696–702 and Figure 5 of Chen et al, Acta Pharmacol Sin 2010; 31: 923–9).

Figure 5

A summarized diagram for the effects of sugar attachments in three different positions of the steroid nucleus of ginsenosides. Detailed 3D diagrams are shown in Figures 4, 5, and 6 of Chen et al, Acta Pharmacol Sin 2009; 30: 61–9.