Guanidinium derivatives act as high affinity antagonists of Na⁺ ions in occlusion sites of Na⁺,K⁺-ATPase

We have screened various alkyl- and arylguanidinium derivatives as possible competitors of Na⁺ or Rb⁺ for the cation sites on renal Na⁺,K⁺-ATPase. Alkylmonoguanidinium or alkylbisguanidinium (BisG) compounds (chain lengths of C₃ to C₁₀) competitively inhibit the occlusion of Rb⁺ and Na⁺ with an order of affinities C₁₀ > C₈ > C₆ > C₄ > C₃. BisG compounds are approximately twice as effective as the equivalent alkylmonoguanidinium compounds. In media of high ionic strength, affinities of tens of micromolar are observed, e.g. 26 μM for BisG 8. m-(mXBG)- and p-xylylenebisguanidinium were synthesized and were found to compete with Rb⁺ or Na⁺ with intrinsic affinities of 7.7 and 8.2 μM, respectively. The hydrophobicity rather than the degree of proximity of the guanidinium groups in all BisG compounds appears to determine the binding affinity. A systematic search has been made of conditions in occlusion assays for which the inhibitor affinities are highest. When the pH is raised from 7.0 to 8.5, a 5-fold increase in affinity is observed, suggesting that the guanidinium derivatives compete with protons at sites of pK(a) ~ 7.5. Replacing Tris-HCl with choline chloride-containing media raised apparent affinities ~2-fold. All guanidinium derivatives stabilize the E₁ conformation of fluorescein-labeled Na⁺,K⁺-ATPase, acting as competitive Na⁺ analogues. In media containing only 1 mM Tris-HCl, pH 8.55, very high affinities were observed for binding to the fluorescein-labeled enzyme (e.g. 0.08 μM for mXBG). In very low ionic strength medium, the inhibition was still competitive with Rb⁺ ions. However, there was also evidence for nonspecific adsorption to the membranes. The following findings show that mXBG, a typical guanidinium derivative, behaves as a Na⁺-like antagonist. (a) It inhibits Na⁺,K⁺-ATPase activity, competing strongly with Na⁺ but only weakly with K⁺ ions. (b) It inhibits phosphorylation from ATP, competing with Na⁺ ions. (c) Like Na⁺ ions, it blocks phosphorylation from inorganic phosphate. Based on these results, we propose that the guanidinium group binds to a relatively wide vestibule at the cytoplasmic surface; but, unlike Na⁺ or K⁺ ions, it cannot pass into a narrower region of the cation transport path within the membrane. Therefore, it blocks the occlusion and active transport of cations. In the future, high affinity guanidinium derivatives may serve the purpose of locating cation-binding domains of the pump protein after being converted to reactive affinity or photoaffinity covalent labels.