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

The nature of the electron transfer and terminal oxidase(s)in the sulfite-oxidizing system of Thiobacillus thiooxidnaswas studied in detail with various artificial electron donorsand inhibitors. Thionine, when reduced by ascorbate, was mosteffectively oxidized by whole cells and the particulate fractionof the various artificial electron donors. p-PD and TMPD werescarcely oxidized by either intact cells or the particulatefraction. The optimum pH of the thionine-oxidizing activity by the particulatefraction was 7.0 and that of the sulfite-oxidizing activitywas 6.8. The K_m values for thionine and sulfite were 7.6x10^–5Mand 1.6xl0^–4M, respectively. Sulfite oxidase activity in the particulate fraction was markedlyinhibited by amytal, rotenone, quinacrine-HGl and 2,4-DNP. HOQNOinhibited sulfite oxidase activity completely, but had no effecton thionine oxidase activity. Cyanide- and azide-insensitive respirations were present inthe particulate fraction. Thionine oxidase activity was inhibitedphoto-irreversibly with carbon monoxide, while sulfite oxidaseactivity showed photo-reversible carbon monooxide inhibition.The presence of two carbon monoxide-binding pigments was confirmedin the particulate fraction by a spectrophotometric study. (Received May 16, 1975; )