血红素加氧酶(HO)通过将血红素氧化降解为一氧化碳来解毒,铁,和胆绿素,被还原成胆红素排出体外.人类表达HO的两种同工型:诱导型HO-1,其响应于过量血红素和其他应激源而上调,和组成型HO-2。对HO-1的调节和生理功能了解很多,而对HO-2在调节血红素稳态中的作用知之甚少。表达组成型HO-2的生化必要性取决于血红素在HO-1不被诱导的条件下是否足够丰富和可作为底物。通过测量不稳定的血红素,总血红素,和胆红素在人胚肾HEK293细胞沉默或过表达的HO-2,以及各种HO-2突变等位基因,我们发现内源性血红素太限制了底物,无法观察到HO-2依赖性血红素降解。相反,我们发现了HO-2在血红素的结合和缓冲中的新作用。一起来看,在没有过量血红素的情况下,我们建议HO-2通过充当控制血红素生物利用度的血红素缓冲因子来调节血红素稳态。当血红素过量时,HO-1被诱导,HO-2和HO-1均可通过酶促降解保护血红素毒性。我们的结果解释了为什么HO-2的无催化活性突变体对氧化应激具有细胞保护作用。此外,HO-2过度表达导致的生物可利用血红素的变化,选择性结合三价铁血红素,与不稳定血红素被氧化一致,从而为血红素贩运和信号传递提供了新的见解。
Heme oxygenases (HOs) detoxify heme by oxidatively degrading it into carbon monoxide, iron, and biliverdin, which is reduced to bilirubin and excreted. Humans express two isoforms of HO: the inducible HO-1, which is upregulated in response to excess heme and other stressors, and the constitutive HO-2. Much is known about the regulation and physiological function of HO-1, whereas comparatively little is known about the role of HO-2 in regulating heme homeostasis. The biochemical necessity for expressing constitutive HO-2 is dependent on whether heme is sufficiently abundant and accessible as a substrate under conditions in which HO-1 is not induced. By measuring labile heme, total heme, and bilirubin in human embryonic kidney HEK293 cells with silenced or overexpressed HO-2, as well as various HO-2 mutant alleles, we found that endogenous heme is too limiting a substrate to observe HO-2-dependent heme degradation. Rather, we discovered a novel role for HO-2 in the binding and buffering of heme. Taken together, in the absence of excess heme, we propose that HO-2 regulates heme homeostasis by acting as a heme buffering factor that controls heme bioavailability. When heme is in excess, HO-1 is induced, and both HO-2 and HO-1 can provide protection from heme toxicity via enzymatic degradation. Our results explain why catalytically inactive mutants of HO-2 are cytoprotective against oxidative stress. Moreover, the change in bioavailable heme due to HO-2 overexpression, which selectively binds ferric over ferrous heme, is consistent with labile heme being oxidized, thereby providing new insights into heme trafficking and signaling.