Allegra M, Furtmüller P G, Regelsberger G, Turco-Liveri M L, Tesoriere L, Perretti M, Livrea M A, Obinger C
Department of Pharmaceutical Toxicological and Biological Chemistry, University of Palermo, Via Carlo Forlanini, Palermo, 90123, Italy.
Biochem Biophys Res Commun. 2001 Mar 30;282(2):380-6. doi: 10.1006/bbrc.2001.4582.
Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving myeloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myeloperoxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MPO, we investigated the oxidation of melatonin by its redox intermediates compounds I and II using transient-state spectral and kinetic measurements at 25 degrees C. Spectral and kinetic analysis revealed that both compound I and compound II oxidize melatonin via one-electron processes. The second-order rate constant measured for compound I reduction at pH 7 and pH 5 are (6.1 +/- 0.2) x 10(6) M(-1) s(-1) and (1.0 +/- 0.08) x 10(7) M(-1) s(-1), respectively. The rates for the one-electron reduction of compound II back to the ferric enzyme are (9.6 +/- 0.3) x 10(2) M(-1) s(-1) (pH 7) and (2.2 +/- 0.1) x 10(3) M(-1) s(-1) (pH 5). Thus, melatonin is a much better electron donor for compound I than for compound II. Steady-state experiments showed that the rate of oxidation of melatonin is dependent on the H(2)O(2) concentration, is not affected by superoxide dismutase, and is quickly terminated by sodium cyanide. Melatonin can markedly inhibit the chlorinating activity of MPO at both pH 7 and pH 5. The implication of these findings in the activated neutrophil is discussed.
最近，有人提出褪黑素（N - 乙酰 - 5 - 甲氧基色胺）在一个很可能涉及髓过氧化物酶的反应中被活化的中性粒细胞氧化（《生物化学与生物物理研究通讯》(2000) 279, 657 - 662）。髓过氧化物酶（MPO）是中性粒细胞中含量最丰富的蛋白质，参与杀灭入侵的病原体。为了阐明褪黑素是否是MPO的底物，我们在25℃下使用瞬态光谱和动力学测量方法研究了褪黑素被其氧化还原中间体化合物I和化合物II的氧化情况。光谱和动力学分析表明，化合物I和化合物II均通过单电子过程氧化褪黑素。在pH 7和pH 5条件下测得的化合物I还原的二级速率常数分别为(6.1 ± 0.2) × 10⁶ M⁻¹ s⁻¹和(1.0 ± 0.08) × 10⁷ M⁻¹ s⁻¹。化合物II单电子还原回铁酶的速率分别为(9.6 ± 0.3) × 10² M⁻¹ s⁻¹（pH 7）和(2.2 ± 0.1) × 10³ M⁻¹ s⁻¹（pH 5）。因此，褪黑素作为化合物I的电子供体比作为化合物II的电子供体要好得多。稳态实验表明，褪黑素的氧化速率取决于H₂O₂浓度，不受超氧化物歧化酶影响，且能被氰化钠迅速终止。褪黑素在pH 7和pH 5时均能显著抑制MPO的氯化活性。本文讨论了这些发现对活化中性粒细胞的意义。
