SUMMARY: Suspensions of Chromatium D oxidized thiosulphate completely to sulphate (in the light) under anaerobic conditions in the presence of carbon dioxide. During the oxidation, intracellular sulphur accumulated transiently and sulphate production followed a biphasic pattern. Using inner- S-thiosulphate, the initial burst of sulphate production, which accounted for about half of the total yield, occurred principally at the expense of the inner (SO-) atom of thiosulphate; there was no intracellular accumulation of labelled sulphur. The radioactivity from outer-S-thiosulphate accumulated transiently within the organisms and was transferred to sulphate at a rate which was similar to the second phase of sulphate production; most of the outer (S-) atom therefore passed through the stage of endogenous sulphur. Extracts of the organism catalysed the cyanolysis of thiosulphate to give sulphite and thiocyanate. An assay for this enzyme, rhodanese, based on the spontaneous reduction of 2,6-dichlorophenol-indophenol by sulphite was developed. The enzyme was also detected in extracts of Athiorhodaceae. The enzyme was partially purified from extracts of Chromatium D and resolved into two active fractions. It is concluded that endogenous sulphur is an intermediate in the oxidation of thiosulphate by Chromatium D and that the cleavage of the S—S bond in the molecule is a key step in the oxidation process. Rhodanese, which catalyses this type of reaction, may be concerned in this cleavage.


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