Cytochrome c oxidase purified from a mercury-resistant strain of Acidithiobacillus ferrooxidans volatilizes mercury

We suggested in our previous study that the plasma membrane cytochrome c oxidase of the mercury-resistant iron-oxidizing bacterial strain Acidithiobacillus ferrooxidans, SUG 2-2, is involved in Fe²⁺-dependent mercury volatilization. To study the involvement of A. ferrooxidans cytochrome c oxidase in mercury reduction, the cytochrome c oxidase was extracted from mercury-resistant and mercury-sensitive strains and purified. The Fe²⁺-dependent mercury volatilization activities of the oxidases from these strains were compared. The cytochrome c oxidase from strain SUG 2-2 volatilized 39% of the total Hg²⁺ (7 nmol) that had been added to a 10-li reaction mixture (pH 3.8) in the presence of 10 μmol of Fe²⁺ after a 7-d incubation period at 30°C. In contrast, the enzyme purified from the mercury-sensitive strain AP19-3 volatilized 3.5% of the total mercury under the same conditions. The boiled SUG 2-2 oxidase did not exhibit activity to volatilize mercury. Fe²⁺ reduced the oxidase from SUG 2-2 and Hg²⁺ oxidized the reduced enzyme. The purified SUG 2-2 oxidase is composed of three protein subunits with apparent molecular weights of 56,000 Da (α), 24,000 Da (β), and 19,000 Da (γ). The amount of mercury bound to the purified SUG 2-2 oxidase was 6.2 μg/mg protein and those bound to α-, β- and γ-subunits of the cytochrome c oxidase were 3.5, 2.6 and 0.7 μg/mg protein, respectively.