Kettle A J, Candaeis L P
Free Radical Research Group, Christchurch School of Medicine, New Zealand.
Redox Rep. 2000;5(4):179-84. doi: 10.1179/135100000101535726.
The neutrophil enzyme myeloperoxidase catalyzes the oxidation of tyrosine to tyrosyl radicals, which cross-link to proteins and initiate lipid peroxidation. Tryptophan is present in plasma at about the same concentration as tyrosine and has a similar one-electron reduction potential. In this investigation, we have determined the ability of myeloperoxidase to catalyze the oxidation of tryptophan to assess whether or not this reaction may contribute to oxidative stress at sites of inflammation. We show that tryptophan is a poor substrate for myeloperoxidase because, even though it reacts rapidly with compound I (kI 2.1 x 10(6) M(-1)s(-1)), it reacts sluggishly with compound II (kII 7 M(-1)s(-1)). Tryptophan reversibly inhibited production of hypochlorous acid by purified myeloperoxidase by converting the enzyme to a mixture of compound II and compound III. It gave 50% inhibition (I50) at a concentration of 2 microM. In contrast, it was an ineffective inhibitor of hypochlorous acid production by human neutrophils (I50 80 microM) unless superoxide dismutase was present (I50 5 microM). We propose that compound I of myeloperoxidase will oxidize tryptophan at sites of inflammation. Enzyme turnover will result from the reaction of superoxide or tyrosine with compound II. Thus, tryptophan radicals are potential candidates for exacerbating oxidative stress during inflammation.
中性粒细胞酶髓过氧化物酶催化酪氨酸氧化为酪氨酰自由基，后者与蛋白质交联并引发脂质过氧化。色氨酸在血浆中的浓度与酪氨酸大致相同，且具有相似的单电子还原电位。在本研究中，我们测定了髓过氧化物酶催化色氨酸氧化的能力，以评估该反应是否可能在炎症部位导致氧化应激。我们发现色氨酸是髓过氧化物酶的不良底物，因为尽管它与化合物I反应迅速（kI 2.1×10(6) M(-1)s(-1)），但与化合物II反应缓慢（kII 7 M(-1)s(-1)）。色氨酸通过将纯化的髓过氧化物酶转化为化合物II和化合物III的混合物，可逆地抑制次氯酸的产生。在浓度为2 microM时，它产生50%的抑制作用（I50）。相比之下，它对人中性粒细胞产生次氯酸的抑制作用无效（I50 80 microM），除非存在超氧化物歧化酶（I50 5 microM）。我们提出，髓过氧化物酶的化合物I将在炎症部位氧化色氨酸。超氧化物或酪氨酸与化合物II的反应将导致酶的周转。因此，色氨酸自由基是炎症期间加剧氧化应激的潜在因素。
