While surveying the genomes of hyperthermophilic and thermophilic Archaea for homologues of the flavoprotein disulfide reductases, many homologues with a high degree of identity to the branch of this family represented by glutathione reductase were found [1]. Most of the homologues appear to belong to the subfamily that depend on a redox-active single cysteine, analogous to the NADH oxidase and per-oxidase of Enterococcus and the coenzyme A disulfide reductase (CoADR; EC 1.8.1.14) of Staphylococcus Correspondence E. J. Crane III, We have cloned NADH oxidase homologues from Pyrococcus horikoshii and P. furiosus, and purified the recombinant form of the P. horikoshii enzyme to homogeneity from Escherichia coli. Both enzymes (previously referred to as NOX2) have been shown to act as a coenzyme A disulfide reductases (CoADR: CoA-S-S-CoA + NAD(P)H + H + fi 2CoA-SH + NAD(P) +). The P. horikoshii enzyme shows a k cat app of 7.2 s)1 with NADPH at 75 °C. While the enzyme shows a preference for NADPH, it is able to use both NADPH and NADH efficiently, with both giving roughly equal k cat s, while the K m for NADPH is roughly eightfold lower than that for NADH. The enzyme is specific for the CoA disulfide, and does not show significant reductase activity with other disulfides, including dephos-pho-CoA. Anaerobic reductive titration of the enzyme with NAD(P)H proceeds in two stages, with an apparent initial reduction of a nonflavin redox center with the first reduction resulting in what appears to be an EH 2 form of the enzyme. Addition of a second of NADPH results in the formation of an apparent FAD-NAD(P)H complex. The behavior of this enzyme is quite different from the mesophilic staphylococcal version of the enzyme. This is only the second enzyme with this activity discovered, and the first from a strict anaerobe, an Archaea, or hyperthermophilic source. P. furio-sus cells were assayed for small molecular mass thiols and found to contain 0.64 lmol CoAAEg dry weight)1 (corresponding to 210 lm CoA in the cell) consistent with CoA acting as a pool of disulfide reducing equivalents. Abbreviations CoADR, coenzyme A disulfide reductase (EC# 1.8.1.14); pfCoADR, P. furiosus coenzyme A disulfide reductase; phCoADR, P. horikoshii coenzyme A disulfide reductase; DTNB, 5,5¢ dithiobis(2-nitrobenzoic acid); EH 2 , two-electron reduced enzyme; EH 4 , four-electron reduced enzyme; HEPPS, N-(2-hydroxyethyl)piperazine-N¢-3-propanesulfonic acid; NOX, NADH oxidase; NPX, NADH peroxidase; TCA, trichloroacetic acid.