Inhibition of myeloperoxidase- and neutrophil-mediated oxidant production by tetraethyl and tetramethyl nitroxides

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Tracey B Kajer, Kathryn E Fairfull-Smith, Toshihide Yamasaki, Ken-ichi Yamada, Shanlin Fu, Steven E Bottle, Clare Louise Hawkins, Michael Jonathan Davies

The powerful oxidant HOCl (hypochlorous acid and its corresponding anion, (-)OCl) generated by the myeloperoxidase (MPO)-H2O2-Cl(-) system of activated leukocytes is strongly associated with multiple human inflammatory diseases; consequently there is considerable interest in inhibition of this enzyme. Nitroxides are established antioxidants of low toxicity that can attenuate oxidation in animal models, with this ascribed to superoxide dismutase or radical-scavenging activities. We have shown (M.D. Rees et al., Biochem. J. 421, 79-86, 2009) that nitroxides, including 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-1-yloxyl radical), are potent inhibitors of HOCl formation by isolated MPO and activated neutrophils, with IC50 values of ~1 and ~6 µM respectively. The utility of tetramethyl-substituted nitroxides is, however, limited by their rapid reduction by biological reductants. The corresponding tetraethyl-substituted nitroxides have, however, been reported to be less susceptible to reduction. In this study we show that the tetraethyl species were reduced less rapidly than the tetramethyl species by both human plasma (89-99% decreased rate of reduction) and activated human neutrophils (62-75% decreased rate). The tetraethyl-substituted nitroxides retained their ability to inhibit HOCl production by MPO and activated neutrophils with IC50 values in the low-micromolar range; in some cases inhibition was enhanced compared to tetramethyl substitution. Nitroxides with rigid structures (fused oxaspiro rings) were, however, inactive. Overall, these data indicate that tetraethyl-substituted nitroxides are potent inhibitors of oxidant formation by MPO, with longer plasma and cellular half-lives compared to the tetramethyl species, potentially allowing lower doses to be employed.

Original languageEnglish
JournalFree Radical Biology & Medicine
Volume70
Pages (from-to)96-105
Number of pages10
ISSN0891-5849
DOIs
Publication statusPublished - May 2014
Externally publishedYes

    Research areas

  • Animals, Antioxidants, Cyclic N-Oxides, Humans, Hydrogen Peroxide, Hypochlorous Acid, Inflammation, Neutrophil Activation, Neutrophils, Peroxidase, Superoxide Dismutase, Superoxides

ID: 128973689