Purified thioredoxin reductase from O-sensitive Bifidobacterium bifidum degrades HO by interacting with alkyl hydroperoxide reductase.

Bifidobacterium is beneficial for host health and exhibits different O sensitivity levels among species or strains via unknown mechanisms. Bifidobacterium bifidum JCM1255, a type species of Bifidobacterium, is an O-sensitive bacterium that can grow under low-O (5%) conditions, and the growth of this species is inhibited under high-O conditions (10% ∼) with accumulation of HO. We previously reported that NADH or NAD(P)H oxidase-active fractions were detected during purification using microaerobically grown B. bifidum cells, and the active enzyme was purified from the NADH oxidase-active fraction. The purified enzyme was identified as b-type dihydroorotate dehydrogenase (DHODb) and characterized as a dominant HO producer in B. bifidum. In this study, we performed further purification of the enzyme from the NAD(P)H oxidase-active fraction and characterized the purified enzyme as a part of the HO degradation system in B. bifidum. This purified enzyme was identified as thioredoxin reductase (TrxR); the NAD(P)H oxidase activity of this enzyme was not expressed in anaerobically grown B. bifidum, and mRNA expression was induced by O exposure. Furthermore, the purified B. bifidum TrxR interacted with recombinant alkyl hydroperoxide reductase (rAhpC) and exhibited NAD(P)H peroxidase activity. These results suggest that TrxR responds to O and protects B. bifidum from oxidative stress by degrading HO via the TrxR-AhpC system.