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 2 = 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 +.