Submicromolar Ag+ increases passive Na+ permeability and inhibits the respiration‐supported formation of Na+ gradient in Bacillus FTU vesicles

Abstract

The effect of Ag⁺ on Na⁺ pumping by Na⁺‐motive NADH‐quinone reductase and terminal oxidase has been studied in Bacillus FTU inside‐out vesicles. Very low concentrations of Ag⁺ (C = 1 × 10⁻⁸M or 2 × 10⁻¹² g ion · mg protein⁻¹) are shown to inhibit the uphill Na⁺ uptake coupled to the oxidation of NADH by fumarate or of ascorbate + TMPD by oxygen but exert no effect on the H⁺ uptake by the H⁺‐motive respiratory chain. Low Ag⁺ also induces a specific increase in the Na⁺ permeability of the vesicles. HQNO, added before and not after Ag⁺, prevents the Ag⁺‐induced permeability increase, with effective HQNO concentrations being similar to those inhibiting the uphill Na⁺‐uptake coupled to the NADH‐fumarate oxidoreduction. Reduction of terminal oxidase by ascorbate + TMPD in the presence of cyanide sensitizes the Na⁺ permeability to Ag⁺. It is suggested that low [Ag⁺], known as a specific inhibitor of electron transport by the Na⁺‐motive NADH‐quinone reductase, uncouples the electron and Na⁺ transports so that the Ag⁺‐modified NADH‐quinone reductase operates as an Na⁺ channel rather than an Na⁺ pump. This effect is discussed in connection with the antibacterial action of Ag⁺.