A NAD(P)H oxidase isolated from the archaeon Sulfolobus solfataricus is not homologous with another NADH oxidase present in the same microorganism

A NAD(P)H oxidase has been isolated from the ar- chaeon Sulfolobus solfataricus. The enzyme is a ho- modimer with Mr 38,000 per subunit (SsNOX38) contain- ing 1 FAD molecule/subunit. It oxidizes NADH and, less efficiently, NADPH with the formation of hydrogen per- oxide. The enzyme was resistant against chemical and physical denaturating agents. The temperature for its half-denaturation was 93 and 75 °C in the absence or presence, respectively, of 8 M urea. The enzyme did not show any reductase activity. The SsNOX38 encoding gene was cloned and sequenced. It accounted for a prod- uct of 36.5 kDa. The translated amino acid sequence was made of 332 residues containing two putative -fold regions, typical of NAD- and FAD-binding proteins. The primary structure of SsNOX38 did not show any homol- ogy with the N-terminal amino acid sequence of a NADH oxidase previously isolated from S. solfataricus (Ss- NOX35) (Masullo, M., Raimo, G., Dello Russo, A., Boc- chini, V. and Bannister, J. V. (1996) Biotechnol. Appl. Biochem. 23, 47–54). Conversely, it showed 40% sequence identity with a putative thioredoxin reductase from Ba- cillus subtilis, but it did not contain cysteines, which are essential for the activity of the reductase.