Purification and characterization of a NADH oxidase from the thermophile Thermus thermophilus HB8

A NADH oxidase has been purified from the extreme thermophile Thermus thermophilus HB8 by several chromatographic steps. The purified enzyme was essentially homogeneous as judged by gel electrophoresis under denaturing conditions and by determination of the N‐terminal amino acids sequence. It is a monomeric flavin‐adenine‐dinucleotide‐containing flavoprotein with an apparent molecular mass of 25 kDa and an 1:1 ratio of FAD to the polypeptide chain. The purified enzyme catalyzes the oxidation of reduced NADH or NADPH with the formation of H 2 0 2 . The apparent K m , values for NADH and NADPH are 4.14 μM and 14.0 μM (pH 7.2 at room temperature), respectively, with a sixfold greater k cat / K m values for NADH compared to NADPH. The enzyme uses O 2 as an electron acceptor in the presence of either FAD, riboflavin 5′‐phosphate or riboflavin as cofactor. In addition, the enzyme is able to catalyze electron transfer from NADH to various other electron acceptors (methylene blue, cytochrome c, p ‐nitroblue tetrazolium, 2,6‐dichloroindophenol and potassium ferricyanide), even in the absence of flavin shuttles. No significant inhibition of the NADH oxidoreductase activity by superoxide dismutase was observed with these artificial electron acceptors, indicating that electron transfer occurs mainly from NADH directly to the electron acceptors, not via O 2 − as an intermediate. The purified NADH oxidase exhibits highest activity at pH 5.0 and is stable at elevated temperatures of up to 80°C.