SUBMICROMOLAR AG+ INCREASES PASSIVE NA+ PERMEABILITY AND INHIBITS THE RESPIRATION-SUPPORTED FORMATION OF NA+ GRADIENT IN BACILLUS FTU VESICLES

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+. © 1990.