Mechanism of Growth Inhibition by Tungsten in Acidithiobacillus ferrooxidans

Abstract

Cell growth of three hundred iron-oxidizing bacteria isolated from natural environments was inhibited strongly by 0.05 mM, and completely by 0.2 mM of sodium tungstate (Na₂WO₄), respectively. Since no great difference in the level of tungsten inhibition was observed among the 300 strains tested, the mechanism of inhibition by Na₂WO₄ was studied with Acidithiobacillus ferrooxidans strain AP19-3. When resting cells of AP19-3 were incubated in 0.1 M β-alanine-SO^2-₄ buffer (pH 3.0) with 0.1 mM Na₂WO₄ for 1 h, the amount of tungsten bound to the cells was 12 μg/mg protein. The optimum pH for tungsten binding to the resting cells was 2∼3. Approximately 2 times more tungsten bound to the cells at pH 3.0 than at pH 6.0 The tungsten binding was specifically inhibited by sodium molybdenum. However, copper, nickel, cadmium, zinc, manganese, cobalt, and vanadate did not disturb tungsten binding to the resting cells. The iron-oxidizing activity of AP19-3 was inhibited 24, 62, and 77% by 1, 5, and 10 mM of Na₂WO₄, respectively. Among the components of iron oxidation enzyme system, iron: cytochrome c oxidoreductase activity was not inhibited by 10 mM of Na₂WO₄. In contrast, the activity of cytochrome c oxidase purified highly from the strain was inhibited 50 and 72%, respectively, by 0.05 and 0.1 mM of Na₂WO₄. The amounts of tungsten bound to plasma membrane, cytosol fraction, and a purified cytochrome c oxidase were 8, 0.5, and 191 μg/mg protein, respectively. From the results, the growth inhibition by Na₂WO₄ observed in A. ferrooxidans is explained as follows: tungsten binds to cytochrome c oxidase in plasma membranes and inhibits cytochrome c oxidase activity, and as a results, the generation of energy needed for cell growth from the oxidation of Fe²⁺ is stopped.