The Brucella abortus alkyl hydroperoxide reductase complex (AhpCD) is the primary scavenger of endogenous hydrogen peroxide and the ahpCD operon is regulated in response to endogenous hydrogen peroxide.

Abstract

Brucella abortus is a facultative intracellular pathogen that causes abortion and infertility in cattle and relapsing fever in humans. These bacteria reside and replicate inside macrophages during infection, and the ability of Brucella to survive within this intracellular niche is critical for the establishment of disease in host animals. Exposure to the oxidative burst of host phagocytes is one of the environmental stresses the brucellae must deal with during their intracellular residence. Because they are aerobes, these bacteria must also detoxify reactive oxygen species (ROS) generated as a by-product of their respiratory metabolism. Production of Brucella's sole catalase, KatE, enhances viability of the brucellae exposed to high levels of exogenous hydrogen peroxide (H₂O₂), but Brucella katE mutants are virulent in both the experimental and natural hosts. So it is currently unknown how the brucellae detoxify hydrogen peroxide produced from the macrophage and respiratory metabolism. The genes designated as BAB2_0531 and BAB2_0532 in the B. abortus 2308 genome sequence are predicted to encode the components (AhpC and AhpD, respectively) of an alkyl hydroperoxide reductase complex. Peroxiredoxins of the AhpC family are important enzymes that detoxify H₂O₂, organic peroxides, and peroxynitrite in bacterial cells. The research described in this dissertation shows that AhpC is the primary detoxifier of endogenous H₂O₂ generated by aerobic metabolism. KatE, on the other hand, plays a major role in scavenging exogenous H₂O₂, although this enzyme can play a supporting role in the detoxification of H₂O₂ of endogenous origin if AhpC is absent. Our data also show that ahpC expression is H2O2-responsive, and AhpCD is part of the Brucella oxidative stress response. We have found three regulators--OxyR, BAB2_0530, Irr--that affect ahpC expression in some manner in B. abortus 2308, but only one, the iron-responsive regulator Irr, affects the H₂O₂-responsive expression of ahpC.