Myeloperoxidase as an effective inhibitor of hydroxyl radical production. Implications for the oxidative reactions of neutrophils.

Abstract

Hydroxyl radicals have been generated from hydrogen peroxide and superoxide (produced with xanthine oxidase), and an iron (EDTA) catalyst, and detected with deoxyribose, or in some cases with benzoate or a-keto-'y-methiolbutyric acid. Purified myeloperoxidase, and neutrophils stimulated with fMet-Leu-Phe and cytochalasin B, strongly inhibited this hydroxyl radical production in a concentration-dependent manner. Supernatants from stimulated cells also inhibited, and inhibition by cells or supernatant was prevented by azide. There was much less inhibition by myeloperoxidase-deficient neutrophils. Inhibition thus was due to myeloperoxidase released by the cells. With neutrophils stimulated with phorbol myristate acetate, which release very little myeloperoxidase, hydroxyl radical production was enhanced due to the additional superoxide produced by the cells. It is concluded that under conditions where neutrophils release myeloperoxidase as well as superoxide and hydrogen peroxide, breakdown of hydrogen peroxide by myeloperoxidase would make conditions unfavorable for hydroxyl radical production.

Neutrophil degranulation inhibits potential hydroxyl-radical formation. Relative impact of myeloperoxidase and lactoferrin release on hydroxyl-radical production by iron-supplemented neutrophils assessed by spin-trapping techniques

“…Recently the possibility was raised [39] that neutrophil-mediated destruction of spin-trapped adducts used to monitor GOH formation could have confounded interpretation of some of these data. In contrast with our [27] results, Winterbourn [30] reported that myeloperoxidase (MPO) released during neutrophil stimulation could inhibit the magnitude of OH formed (thiobarbituric acid-reactive 2-deoxyribose oxidation products) by the addition of iron-EDTA to a xanthine/ xanthine oxidase O2-generating system. This presumably occurred as a consequence of MPO removing H202 and/or 02 from the system [40,41].…”

“…Recent work in several laboratories has forced re-evaluation of original assumptions regarding the endogenous capacity of neutrophils to generate GOH [25][26][27][28][29][30][31][32][33][34]. Studies employing m.s.…”

“…Studies employing m.s. [32,33], deoxyribose oxidation ( [30,31]; B. E. Britigan, unpublished work), phenylalanine hydroxylation [34], and spin trapping [25][26][27][28][29] have failed to detect production of 'OH by neutrophils unless iron salts and a chelating-agent were included in the system. These studies suggest that generation of *OH in vivo in association with neutrophil stimulation requires the presence of an exogenous catalyst.…”

Possible role of bacterial siderophores in inflammation. Iron bound to the Pseudomonas siderophore pyochelin can function as a hydroxyl radical catalyst.

“…Previous reports have found that at least two of the granular components affect the potential for * OH formation in association with the neutrophil respiratory burst. Lactoferrin and myeloperoxidase inhibit * OH formation by iron-supplemented neutrophils by sequestering iron in a noncatalytic form and consuming H202, respectively (15,20,54). In contrast, it seemed possible that neutrophil proteases through their action on the peptide PVD could alter the potential of iron bound to PVD to participate in the Haber-Weiss reaction.…”

“…Although it has been reported that neutrophils have the endogenous capacity for *OH formation (8)(9)(10)(11)(12)(13), the experimental techniques used in these studies have been criticized for a lack of specificity (14). Recent studies using spin trapping and other techniques have strongly suggested that an exogenous transition metal catalyst must be present for neutrophil activation to result in -OH generation (3,(15)(16)(17)(18)(19)(20).…”