Gohya Tomohiko, Sato Michihiko, Zhang Xuhong, Migita Catharina T
Department of Biological Chemistry, Faculty of Agriculture, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8515, Japan.
Biochem Biophys Res Commun. 2008 Nov 14;376(2):293-8. doi: 10.1016/j.bbrc.2008.08.141. Epub 2008 Sep 7.
Heme oxygenase (HO) converts hemin to biliverdin, CO, and iron applying molecular oxygen and electrons. During successive HO reactions, two intermediates, alpha-hydroxyhemin and verdoheme, have been generated. Here, oxidation state of the verdoheme-HO complexes is controversial. To clarify this, the heme conversion by soybean and rat HO isoform-1 (GmHO-1 and rHO-1, respectively) was compared both under physiological conditions, with oxygen and NADPH coupled with ferredoxin reductase/ferredoxin for GmHO-1 or with cytochrome P450 reductase for rHO-1, and under a non-physiological condition with hydrogen peroxide. EPR measurements on the hemin-GmHO-1 reaction with oxygen detected a low-spin ferric intermediate, which was undetectable in the rHO-1 reaction, suggesting the verdoheme in the six-coordinate ferric state in GmHO-1. Optical absorption measurements on this reaction indicated that the heme degradation was extremely retarded at verdoheme though this reaction was not inhibited under high-CO concentrations, unlike the rHO-1 reaction. On the contrary, the Gm and rHO-1 reactions with hydrogen peroxide both provided ferric low-spin intermediates though their yields were different. The optical absorption spectra suggested that the ferric and ferrous verdoheme coexisted in reaction mixtures and were slowly converted to the ferric biliverdin complex. Consequently, in the physiological oxygen reactions, the verdoheme is found to be stabilized in the ferric state in GmHO-1 probably guided by protein distal residues and in the ferrous state in rHO-1, whereas in the hydrogen peroxide reactions, hydrogen peroxide or hydroxide coordination stabilizes the ferric state of verdoheme in both HOs.
血红素加氧酶（HO）利用分子氧和电子将血红素转化为胆绿素、一氧化碳和铁。在连续的HO反应过程中，会生成两种中间体，即α-羟基血红素和胆绿血红素。在此，胆绿血红素-HO复合物的氧化态存在争议。为了阐明这一点，分别比较了大豆和大鼠HO同工型-1（分别为GmHO-1和rHO-1）在生理条件下（GmHO-1与铁氧还蛋白还原酶/铁氧还蛋白以及氧气和NADPH偶联，rHO-1与细胞色素P450还原酶以及氧气和NADPH偶联）和在非生理条件下（用过氧化氢）的血红素转化情况。对血红素-GmHO-1与氧气反应的电子顺磁共振（EPR）测量检测到一种低自旋铁中间体，而在rHO-1反应中未检测到，这表明GmHO-1中的胆绿血红素处于六配位铁状态。对该反应的光吸收测量表明，尽管在高浓度一氧化碳下该反应未受抑制，但与rHO-1反应不同，在胆绿血红素处血红素降解极其缓慢。相反，GmHO-1和rHO-1与过氧化氢的反应均产生了铁低自旋中间体，尽管它们的产率不同。光吸收光谱表明，反应混合物中铁和亚铁胆绿血红素共存，并缓慢转化为铁胆绿素复合物。因此，在生理氧气反应中，发现胆绿血红素在GmHO-1中可能受蛋白质远端残基引导而稳定在铁状态，在rHO-1中稳定在亚铁状态，而在过氧化氢反应中，过氧化氢或氢氧根配位使两种HO中的胆绿血红素的铁状态稳定。
