Effects of electron transport inhibitors and uncouplers on the oxidation of ferrous iron and compounds interacting with ferric iron in Acidithiobacillus ferrooxidans

Oxidation of Fe 2+ , ascorbic acid, propyl gallate, tiron, L-cysteine, and glutathione by Acidithiobacillus ferrooxidans was studied with respect to the effect of electron transport inhibitors and uncouplers on the rate of oxidation. All the oxidations were sensitive to inhibitors of cytochrome c oxidase, KCN, and NaN 3 . They were also partially inhibited by inhibitors of complex I and complex III of the electron transport system. Uncouplers at low concentrations stimulated the oxidation and inhibited it at higher concentrations. The oxidation rates of Fe 2+ and L-cysteine inhibited by complex I and complex III inhibitors (amytal, rotenone, antimycin A, myxothiazol, and HQNO) were stimulated more extensively by uncouplers than the control rates. Atabrine, a flavin antagonist, was an exception, and atabrine-inhibited oxidation activities of all these compounds were further inhibited by uncouplers. A model for the electron transport pathways of A. ferrooxidans is proposed to account for these results. In the model these organic substrates reduce ferric iron on the surface of cells to ferrous iron, which is oxidized back to ferric iron through the Fe 2+ oxidation pathway, leading to cytochrome oxidase to O 2 . Some of electrons enter the uphill (energy-requiring) electron transport pathway to reduce NAD + . Uncouplers at low concentrations stimulate Fe 2+ oxidation by stimulating cytochrome oxidase by uncoupling. Higher concentrations lower Δp to the level insufficient to overcome the potentially uphill reaction at rusticyanin-cytochrome c 4 . Inhibition of uphill reactions at complex I and complex III leads to Δp accumulation and inhibition of cytochrome oxidase. Uncouplers remove the inhibition of Δp and stimulate the oxidation. Atabrine inhibition is not released by uncouplers, which implies a possibility of atabrine inhibition at a site other than complex I, but a site somehow involved in the Fe 2+ oxidation pathway.Key words: Acidithiobacillus ferrooxidans, electron transport, uncouplers, uphill electron ...