Kettle A J, Winterbourn C C
Department of Pathology, Christchurch School of Medicine, New Zealand.
J Biol Chem. 1992 Apr 25;267(12):8319-24.
Myeloperoxidase (MPO) is a prime candidate for mediating the inflammatory tissue damage of neutrophils because it converts Cl- to the potent oxidant hypochlorous acid. It also oxidizes xenobiotics to reactive free radicals. We have found that the kinetics of oxidation of hydroquinone by myeloperoxidase are inadequately explained by the classical peroxidase mechanism. Peroxidation of hydroquinone displayed a distinct lag phase, which was practically abolished by excluding O2 and was eliminated by adding benzoquinone at the start of the reaction. Superoxide dismutase increased the rate of peroxidation by 40% but did not eliminate the lag phase. Spectral investigations revealed that during the initial phase of the reaction, MPO was converted to oxy-MPO, or compound III, by a mechanism that was not reliant on superoxide. Benzosemiquinone, however, was able to convert ferric-MPO to compound III. Both compound III and ferro-MPO reacted with benzoquinone to regenerate ferric-MPO. We propose that the lag phase occurs because benzosemiquinone reduces ferric-MPO to ferro-MPO, which rapidly binds O2 to form compound III. Since compound III is outside the peroxidation cycle, conversion of hydroquinone to benzoquinone is retarded. However, as benzoquinone accumulates, it oxidizes ferro-MPO and compound III to ferric-MPO, thereby increasing the rate of peroxidation. There is a minimal lag phase under an atmosphere of N2 because ferro-MPO would be rapidly oxidized by benzoquinone, without formation of compound III. We conclude that when substrates produce radicals capable of reducing ferric-MPO, they will be peroxidized efficiently only if oxy-MPO is readily recycled. Furthermore, these radicals will prevent MP3+ from reacting with H2O2, and thereby prevent the enzyme from oxidizing Cl- to hypochlorous acid. Thus, this mechanism could be exploited to prevent hypochlorous acid-mediated inflammatory tissue damage.
髓过氧化物酶（MPO）是介导中性粒细胞炎症性组织损伤的主要候选物质，因为它能将Cl-转化为强效氧化剂次氯酸。它还能将外源性物质氧化为活性自由基。我们发现，髓过氧化物酶氧化对苯二酚的动力学过程无法用经典的过氧化物酶机制充分解释。对苯二酚的过氧化呈现出明显的滞后阶段，通过排除O2可几乎消除该阶段，并且在反应开始时加入苯醌可消除该阶段。超氧化物歧化酶使过氧化速率提高了40%，但并未消除滞后阶段。光谱研究表明，在反应的初始阶段，MPO通过一种不依赖超氧化物的机制转化为氧合MPO或化合物III。然而，苯半醌能够将三价铁MPO转化为化合物III。化合物III和二价铁MPO都与苯醌反应以再生三价铁MPO。我们提出滞后阶段的出现是因为苯半醌将三价铁MPO还原为二价铁MPO，后者迅速结合O2形成化合物III。由于化合物III不在过氧化循环中，对苯二酚向苯醌的转化会受到阻碍。然而，随着苯醌的积累，它会将二价铁MPO和化合物III氧化为三价铁MPO，从而提高过氧化速率。在N2气氛下存在最小的滞后阶段，因为二价铁MPO会被苯醌迅速氧化，而不会形成化合物III。我们得出结论，当底物产生能够还原三价铁MPO的自由基时，只有当氧合MPO能够容易地循环利用时，它们才会被有效地过氧化。此外，这些自由基会阻止MP3+与H2O2反应，从而阻止该酶将Cl-氧化为次氯酸。因此，这种机制可被用于预防次氯酸介导的炎症性组织损伤。
