Hazen S L, Hsu F F, Duffin K, Heinecke J W
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Biol Chem. 1996 Sep 20;271(38):23080-8. doi: 10.1074/jbc.271.38.23080.
Oxidation of low density lipoprotein (LDL) may be of critical importance in triggering the pathological events of atherosclerosis. Myeloperoxidase, a heme protein secreted by phagocytes, is a potent catalyst for LDL oxidation in vitro, and active enzyme is present in human atherosclerotic lesions. We have explored the possibility that reactive intermediates generated by myeloperoxidase target LDL cholesterol for oxidation. LDL exposed to the myeloperoxidase-H2O2-Cl- system at acidic pH yielded a family of chlorinated sterols. The products were identified by mass spectrometry as a novel dichlorinated sterol, cholesterol alpha-chlorohydrin (6beta-chlorocholestane-(3beta,5alpha)-diol), cholesterol beta-chlorohydrin (5alpha-chlorocholestane-(3beta, 6beta)-diol), and a structurally related cholesterol chlorohydrin. Oxidation of LDL cholesterol by myeloperoxidase required H2O2 and Cl-, suggesting that hypochlorous acid (HOCl) was an intermediate in the reaction. However, HOCl failed to generate chlorinated sterols under chloride-free conditions. Since HOCl is in equilibrium with molecular chlorine (Cl2) through a reaction which requires Cl- and H+, this raised the possibility that Cl2 was the actual chlorinating intermediate. Consonant with this hypothesis, HOCl oxidized LDL cholesterol in the presence of Cl- and at acidic pH. Moreover, in the absence of Cl- and at neutral pH, Cl2 generated the same family of chlorinated sterols as the myeloperoxidase-H2O2-Cl- system. Finally, direct addition of Cl2 to the double bond of cholesterol accounts for dichlorinated sterol formation by myeloperoxidase. Collectively, these results indicate that Cl2 derived from HOCl is the chlorinating intermediate in the oxidation of cholesterol by myeloperoxidase. Our observations suggest that Cl2 generation in acidic compartments may constitute one pathway for oxidation of LDL cholesterol in the artery wall.
低密度脂蛋白（LDL）的氧化在引发动脉粥样硬化的病理过程中可能至关重要。髓过氧化物酶是一种由吞噬细胞分泌的血红素蛋白，在体外是LDL氧化的有效催化剂，且活性酶存在于人类动脉粥样硬化病变中。我们探讨了髓过氧化物酶产生的反应性中间体将LDL胆固醇作为氧化靶点的可能性。在酸性pH条件下，将LDL暴露于髓过氧化物酶-H2O2-Cl-体系中，产生了一系列氯化固醇。通过质谱鉴定产物为一种新型二氯化固醇、胆固醇α-氯醇（6β-氯胆甾烷-（3β,5α）-二醇）、胆固醇β-氯醇（5α-氯胆甾烷-（3β,6β）-二醇）以及一种结构相关的胆固醇氯醇。髓过氧化物酶对LDL胆固醇的氧化需要H2O2和Cl-，这表明次氯酸（HOCl）是该反应的中间体。然而，在无氯条件下，HOCl无法产生氯化固醇。由于HOCl通过一个需要Cl-和H+的反应与分子氯（Cl2）处于平衡状态，这就增加了Cl2是实际氯化中间体的可能性。与该假设一致的是，HOCl在有Cl-存在且处于酸性pH时能氧化LDL胆固醇。此外，在无Cl-且处于中性pH时，Cl2产生的氯化固醇家族与髓过氧化物酶-H2O2-Cl-体系相同。最后，直接将Cl2添加到胆固醇的双键上可解释髓过氧化物酶形成二氯化固醇的过程。总体而言，这些结果表明，由HOCl衍生的Cl2是髓过氧化物酶氧化胆固醇过程中的氯化中间体。我们的观察结果表明，在酸性区室中产生Cl2可能是动脉壁中LDL胆固醇氧化的一条途径。
