We have cloned NADH oxidase homologues fromPyrococcus 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 +fi2CoA-SH + NAD(P) + ). The P. horikoshiienzyme shows a kcat app of s )1 with NADPH at 75 C.