Thiosulfate Oxidation and Electron Transport in Thiobacillus novellus

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Aleem, M. I. H. (Research Institute for Advanced Studies, Baltimore, Md.). Thiosulfate oxidation and electron transport in Thiobacillus novellus. J. Bacteriol. 90:95–101. 1965.—A cell-free soluble enzyme system capable of oxidizing thiosulfate was obtained from Thiobacillus novellus adapted to grow autotrophically. The enzyme systems of autotrophically grown cells brought about the transfer of electrons from thiosulfate to molecular oxygen via cytochromes of the c and a types; the reactions were catalyzed jointly by thiosulfate oxidase and thiosulfate cytochrome c reductase. The levels of both of these enzymes were markedly reduced in the heterotrophically grown organism. Cell-free extracts from the autotrophically grown T. novellus catalyzed formate oxidation and enzymatically reduced cytochrome c with formate. Both formate oxidation and cytochrome c reduction activities were abolished under heterotrophic conditions. The thiosulfate-activating enzyme S2O3−2-cytochrome c reductase, as well as thiosulfate oxidase, was localized chiefly in the soluble cell-free fractions, and the former enzyme was purified more than 200-fold by ammonium sulfate fractionation and calcium phosphate gel adsorption procedures. Optimal activity of the purified enzyme occurred at pH 8.0 in the presence of 1.67 × 10−1m S2O3−2 and 2.5 × 10−4m cytochrome c. The thiosulfate oxidase operated optimally at pH 7.5 and thiosulfate concentrations of 1.33 × 10−3 to 3.33 × 10−2m in the presence of added cytochrome c at a concentration of 5 × 10−4m. Both enzymes were markedly sensitive to cyanide and to a lesser extent to some metal-binding agents. Although a 10−3m concentration of p-hydroxymercuribenzoate had no effect on S2O3−2-cytochrome c reductase, it caused a 50% inhibition of S2O3−2 oxidase, which was completely reversed in the presence of 10−3m reduced glutathione. Carbon monoxide also inhibited S2O3−2 oxidase; the inhibition was completely reversed by light.

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