Studies of Sulfate Utilization by Algae. 5. Identification of Thiosulfate as a Major Acid-Volatile Product Formed by a Cell-Free Sulfate-Reducing System From Chlorella

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Abstract
Separation of the products formed from sulfate-35S by cell-free extracts of Chlorella pyrenoidosa (Emerson Strain 3) has permitted the identification of thiosulfate as a major product which yields acid-volatile radioactivity. The products formed, as separated by Dowex-1-nitrate chromatography, are qualitatively the same whether extracts at pH 7.0 (using TPNH as the reductant) or extracts at pH 9 [using 2,3-dimercaptopropan-1-ol, (BAL) as reductant] are employed. While thiosulfate can be separated without the addition of carrier, the inclusion of carrier improves the recovery. High concentrations of ATP which have been shown previously to inhibit the formation of acid-volatile radioactivity from radioactive sulfate, inhibit the formation of thiosulfate almost completely. Degradation of the thiosulfate formed at normal ATP concentrations reveals that most of the radioactivity is in the SO3-sulfur of the molecule suggesting that the SH-sulfur is derived from the enzyme extracts. If carrier sulfite is present during thiosulfate formation from sulfate-35S, radioactive sulfite is recovered at the expense of radioactive thiosulfate. Reconstruction experiments utilizing specifically-labeled thiosulfates indicate that radioactive sulfite formation is probably not the result of trapping a normal intermediate, but can be attributed to non-enzymatic exchange between labeled thiosulfate formed from sulfate and the non-radioactive sulfite added, suggesting that free sulfite is not an intermediate in thiosulfate formation from sulfate.