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 fractio ns, 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 follow ing enzyme activities were found and are indicative of an oxidative adenyly lsulfate pathway: adenylylsulfate reductase [138 mU (mg protein)(-1)], aden ylylsulfate:phosphate adenyltransferase ["ADP sulfurylase"; 86 mU (mg prote in)(-1)], adenylate kinase [650 mU (mg protein)(-1)], and rhodanese [thiosu lfate 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 pyrophosp hohydrolase activities were detected.