Studies on the metabolism of a sulfur-oxidizing bacterium VII. Oxidation of sulfite by a cell-free extract of Thiobacillus thiooxidans

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Properties of the cell-free extract, prepared from a strain of Thiobacillus thiooxidans by sonic disruption followed by fractionation with centrifugation, were investigated with reference to its sulfite-oxidizing activity. Without the addition of cofactors the particulate fraction(F-P) catalyzed oxidation of sulfite with oxygen or bacterial cytochrome c-552 obtained from Pseudomonas stutzeri as electron acceptor. TMPD reduced by ascorbic acid was also oxidized by F-P. The soluble fraction(F-S) showed no activity in oxidizing sulfite and TMPD, but stimulated TMPD oxidation by F-P. Oxygen uptake with either sulfite or TMPD as substrate was inhibited by KCN, NaN_3, CO and o-phenanthroline. CO-Inhibition was reversed by light. Reduction of cytochrome c-552 by sulfite was insensitive to these agents. Antimycin A markedly inhibited sulfite oxidation with either oxygen or cytochrome c-552 as electron acceptor, but was without effect on TMPD oxidation. DDC and SAO, both strong inhibitors of sulfur oxidation, did not affect sulfite and TMPD oxidations. Cytochromes of the a, b and c types were contained in F-P. These cytochromes were rapidly reduced when F-P was incubated with sulfite. Cytochrome(s) of the c type was present in F-S, too.
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