Studies on the metabolism of a sulfur-oxidizing bacterium IX. Electron transfer and the terminal oxidase system in sulfite oxidation of Thiobacillus thiooxidans

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The nature of the electron transfer and terminal oxidase(s) in the sulfite-oxidizing system of Thiobacillus thiooxidnas was studied in detail with various artificial electron donors and inhibitors. Thionine, when reduced by ascorbate, was most effectively oxidized by whole cells and the particulate fraction of the various artificial electron donors. p-PD and TMPD were scarcely oxidized by either intact cells or the particulate fraction. The optimum pH of the thionine-oxidizing activity by the particulate fraction was 7.0 and that of the sulfite-oxidizing activity was 6.8. The K_m values for thionine and sulfite were 7.6 × 10^<-5> M and 1.6 × 10^<-4> M, respectively. Sulfite oxidase activity in the particulate fraction was markedly inhibited by amytal, rotenone, quinacrine-HCl and 2,4-DNP. HOONO inhibited sulflte oxidase activity completely, but had no effect on thionine oxidase activity. Cyanide- and azide-insensitive respirations were present in the particulate fraction. Thionine oxidase activity was inhibited photo-irreversibly with carbon monoxide, while sulfite oxidase activity showed photo reversible carbon monooxide inhibition. The presence of two carbon monoxide-binding pigments was confirmed in the particulate fraction by a spectrophotometric study.
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