Purified thioredoxin reductase from O2-sensitive Bifidobacterium bifidum degrades H2O2 by interacting with alkyl hydroperoxide reductase

Abstract

Bifidobacterium is beneficial for host health and exhibits different O₂ sensitivity levels among species or strains via unknown mechanisms. Bifidobacterium bifidum JCM1255^T, a type species of Bifidobacterium, is an O₂-sensitive bacterium that can grow under low-O₂ (5%) conditions, and the growth of this species is inhibited under high-O₂ conditions (10% ∼) with accumulation of H₂O₂. 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 H₂O₂ 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 H₂O₂ 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 O₂ 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 O₂ and protects B. bifidum from oxidative stress by degrading H₂O₂ via the TrxR-AhpC system.

Keywords: Alkyl hydroperoxide reductase; Bifidobacterium; H(2)O(2); O(2); Thioredoxin reductase.