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A Non-catalytic Antioxidant Role for Helicobacter pylori Urease.

The well-studied catalytic role of urease, the Ni-dependent conversion of urea into carbon dioxide and ammonia, has been shown to protect Helicobacter pylori against the low pH environment of the stomach lumen. We hypothesized that the abundantly expressed urease protein can play another, non-catalytic role in combating oxidative stress via Met-residue mediated quenching of harmful oxidants. Three catalytically inactive urease mutant strains were constructed by single substitutions of Ni binding residues. The mutant versions synthesize normal levels of urease and the altered versions retain all methionine residues. The three site-directed urease mutants were able to better withstand a hypochlorous acid (HOCl) challenge than a Δ ureAB deletion strain. The capacity of purified urease to protect whole cells via oxidant quenching was assessed by adding urease enzyme to non-growing HOCl-exposed cells. No wild type cells were recovered with oxidant alone, whereas urease addition significantly aided viability. These results suggest that urease can protect H. pylori against oxidative damage and that the protective ability is distinct from the well characterized catalytic role. In order to determine the capability of Msr to reduce oxidized Met residues in urease, purified H. pylori urease was exposed to HOCl and a previously-described Msr peptide repair mixture was added. Of the 25 methionine residues in urease, 11 were subject to both oxidation and to Msr-mediated repair, as identified by MS analysis; therefore the oxidant quenchable Met pool comprising urease can be recycled by the Msr repair system. Non-catalytic urease appears to play an important role in oxidant protection. Importance Chronic Helicobacter pylori infection can lead to gastric ulcers and gastric cancers. The enzyme urease contributes to the survival of the bacterium in the harsh environment of the stomach through increasing the local pH. In addition to combating acid, H. pylori must survive host-produced reactive oxygen species in order to persist in the gastric mucosa. We describe a cyclic amino acid based antioxidant role of urease, whereby oxidized methionine residues can be recycled by methionine sulfoxide reductase to again quench oxidants. This work expands our understanding of the role of an already-acknowledged pathogen virulence factor and specifically expands our knowledge of H. pylori survival mechanisms.

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