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Adenylyl sulfate reductase was purified from Thiocapsa roseopersicina 60- to 80- fold, and the properties were studied. The molecular weight is 180,000. The enzyme contains, per molecule; one flavine group, two heme groups of cytochrome c character, four atoms of nonheme iron, and six labile sulfide groups. Cytochrome c and ferricyanide serve as electron acceptors. With ferricyanide as the electron acceptor, the pH optimum of the enzyme is at 8.0; with cytochrome c, the pH optimum is at 9.0. Of the nucleotides studied, adenosine 5'-monophosphate is most effective. The influence of substrate concentrations on the activity of the enzyme was studied, and the K(m) values for sulfite, adenosine 5'-monophosphate, ferricyanide, and cytochrome c were determined. The properties of the enzyme are compared with those of adenylyl sulfate reductases purified from sulfate-reducing bacteria and thiobacilli.