Mechanism of growth inhibition by tungsten in Acidithiobacillus ferrooxidans

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 ferro oxidans strain AP19-3. When resting cells of AP19-3 were incubated in 0.1M beta -alanine-SO42- buffer (pH 3.0) with 0.1 mM Na2WO4 for 1 h, the amount of tungsten bound to the cells was 12 mug/mg protein. The optimum pH for tu ngsten binding to the resting cells was 2 similar to3. Approximately 2 time s more tungsten bound to the cells at pH 3.0 than at pH 6.0. The tungsten b inding was specifically inhibited by sodium molybdenum. However, copper, ni ckel, cadmium, zinc, manganese, cobalt, and vanadate did not disturb tungst en 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 oxidored uctase activity was not inhibited by 10 mM of (NaWO4)-W-2. In contrast, the activity of cytochrome c oxidase purified highly from the strain was inhib ited 50 and 72%, respectively, by 0.05 and 0.1mM of Na2WO4. The amounts of tungsten bound to plasma membrane, cytosol fraction, and a purified cytochr ome c oxidase were 8, 0.5, and 191 mug/mg protein, respectively. From the r esults, the growth inhibition by Na2WO4 observed in A. ferrooxidans is expl ained as follows: tungsten binds to cytochrome c oxidase in plasma membrane s and inhibits cytochrome c oxidase activity, and as a results, the generat ion of energy needed for cell growth from the oxidation of Fe2+ is stopped.