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SUMMARY: The enzyme oxidizing thiosulphate to tetrathionate (‘tetrathionate synthase’) has been purified: the native enzyme has a Mr of 138000 and subunit Mr of 45000, contains no haem and shows Km values for thiosulphate of 4 and 110 μM respectively with cytochrome c or ferricyanide as electron acceptor. Most of the enzyme was recovered from the periplasm of the cell. Evidence is also presented for a trithionate hydrolyase, catalysing the hydrolytic cleavage of trithionate to thiosulphate and sulphate. This activity was partially purified and assayed in a coupled system in which cytochrome c reduction by the purified tetrathionate synthase was measured. Tetrathionate oxidation by cell-free preparations could not be obtained, but a sulphite dehydrogenase (with ferricyanide as electron acceptor) was demonstrated and sulphite-dependent reduction of cytochrome c in T. tepidarius membrane preparations occurred. The latter was inhibited by HQNO (2-heptyl-4-hydroxyquinoline-N-oxide), indicating the involvement of cytochrome b. Sulphite oxidation by APS reductase (adenylylsulphate reductase) could not be detected, which was consistent with the earlier demonstration of complete inhibition of ATP synthesis in intact organisms by FCCP (carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone). A scheme is presented which is consistent with all the whole organism and enzyme data available to date.

Cellular Location and Partial Purification of the 'Thiosulphate-oxidizing Enzyme' and 'Trithionate Hydrolyase' from Thiobacillus tepidarius