Two modes of sulfite oxidation in the extremely thermophilic and acidophilic archaeon Acidianus ambivalens

Sulfite-oxidizing enzyme activities were analyzed in cell-free extracts of aerobically frown cells of Acidianus ambivalens, an extremely thermophilic and chemolithoautotrophic archaeon. In the membrane and cytoplasmic fractions, two distinct enzyme activities were found. In the membrane fraction, a sulfite:acceptor oxidoreductase activity was found [530 mU (mg protein)(-1); apparent K-m for sulfite, 3.6 mM]. In the cytoplasmic fraction the following enzyme activities were found and are indicative of an oxidative adenylylsulfate pathway: adenylylsulfate reductase [138 mU (mg protein)(-1)], adenylylsulfate:phosphate adenyltransferase ["ADP sulfurylase"; 86 mU (mg protein)(-1)], adenylate kinase [650 mU (mg protein)(-1)], and rhodanese [thiosulfate sulfur transferase, 9.2 mU (mg protein)(-1)]. In addition, 5',5"'-P-1,P-4-di(adenosine-5') tetraphosphate (Ap(4)A) synthase and Ap(4)A pyrophosphohydrolase activities were detected.