SUMMARY: Sulphate was progressively replaced by tetrathionate as the end product of thiosulphate oxidation by suspensions of Chromatium D when the pH value was decreased from 7.3 to 6.25; tetrathionate was not itself oxidized to sulphate within this range. The effect of pH value on the oxidation of endogenous sulphur was less marked than on the production of sulphate from thiosulphate. Extracts of Chromatium D catalysed the oxidation of thiosulphate to tetrathionate in the presence of ferricyanide; pH 5.0 was the optimum of the purified enzyme. Tetrathionate inhibited growth and the complete oxidation of thiosulphate by suspensions. At pH 6.75 where sulphur accumulated during thiosulphate utilization and sulphate and tetrathionate were formed, added tetrathionate inhibited the accumulation of endogenous sulphur but not the production of tetrathionate from thiosulphate. Tetrathionate had no effect on the oxidation of endogenous sulphur by suspensions. It is concluded that in Chromatium D thiosulphate is metabolized by two alternative pathways depending on the conditions: (1) the cleavage of one molecule of thiosulphate in a reaction similar to that catalysed by thiosulphate sulphur-transferase E.C. (rhodanese); (2) the oxidative coupling of two molecules of thiosulphate to give tetrathionate, catalysed by the thiosulphate-oxidizing enzyme which only operates at low pH values.


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