The mobilization of ferritin iron by liver cytosol. A comparison of xanthine and NADH as reducing substrates.
The mobilization of ferritin iron by liver cytosol. A comparison of xanthine and NADH as reducing substrates.
复制标题
肝细胞质对铁蛋白的动员。
作者：
R. Topham;M. Goger;K. Pearce;P. Schultz
大量证据表明，铁蛋白中铁的释放是一个还原过程。已经提出两种肝酶在这一过程中发挥作用，即黄嘌呤氧化还原酶和 NADH 氧化还原酶。将黄嘌呤和 NADH 作为酶介导的铁蛋白铁释放（铁还原酶活性）的还原能力来源的能力与火鸡肝脏和大鼠肝脏匀浆进行比较。与NADH反应的最大速度（Vmax.）是与黄嘌呤反应的50倍；然而，黄嘌呤达到半最大速度 (Km) 所需的底物浓度比 NADH 低 1000 倍。 NADPH 可以替代 NADH，活性几乎没有损失。双香豆素不会抑制与 NADH 或 NADPH 的反应，表明这些底物的铁还原酶活性不是肝酶“DT-心肌黄酶”[NAD(P)H 脱氢酶（醌）] 的结果。当使用黄嘌呤、NADH 或 NADPH 作为还原底物时，大鼠和火鸡肝细胞质中的铁还原酶活性需要黄素核苷酸。 FMN 使用 NADH 或 NADPH 产生的活性是其两倍，而 FAD 使用黄嘌呤作为底物时的活性是其两倍。两种还原底物的铁还原酶活性的动力学比较、不稳定性和部分色谱分辨率的差异强烈表明，黄嘌呤和NADH的铁还原酶活性是由肝细胞质中包含的单独的酶系统催化的。别嘌呤醇对未透析肝胞质制剂内源性铁还原酶活性的完全抑制以及黄嘌呤恢复与透析制剂同等活性的能力表明内源性活性的还原能力的来源是黄嘌呤。这些研究表明，黄嘌呤、NADH 或 NADPH 可以作为酶介导的铁蛋白铁还原的还原力来源，黄素核苷酸充当从酶到铁蛋白铁的电子穿梭。
Considerable evidence suggests that the release of iron from ferritin is a reductive process. A role in this process has been proposed for two hepatic enzymes, namely xanthine oxidoreductase and an NADH oxidoreductase. The abilities of xanthine and NADH to serve as a source of reducing power for the enzyme-mediated release of ferritin iron (ferrireductase activity) were compared with turkey liver and rat liver homogenates. The maximal velocity (Vmax.) for the reaction with NADH was 50 times greater than with xanthine; however, the substrate concentration required to achieve half-maximal velocity (Km) was 1000 times less with xanthine than with NADH. NADPH could be substituted for NADH with little loss in activity. Dicoumarol did not inhibit the reaction with NADH or NADPH, demonstrating that the ferrireductase activity with those substrates was not the result of the liver enzyme 'DT-diaphorase' [NAD(P)H dehydrogenase (quinone)]. A flavin nucleotide was required for ferrireductase activity with rat and turkey liver cytosol when xanthine, NADH or NADPH was used as the reducing substrate. FMN yielded twice the activity with NADH or NADPH, whereas FAD was twice as effective with xanthine as substrate. Kinetic comparisons, differences in lability and partial chromatographic resolution of the ferrireductase activities with the two types of reducing substrates strongly indicate that the ferrireductase activities with xanthine and NADH are catalysed by separate enzyme systems contained in liver cytosol. Complete inhibition by allopurinol of the ferrireductase activity endogenous to undialysed liver cytosol preparations and the ability of xanthine to restore equivalent activity to dialysed preparations indicate that the source of reducing power for the endogenous activity is xanthine. These studies suggest that xanthine, NADH or NADPH can serve as a source of reducing power for the enzyme-mediated reduction of ferritin iron, with a flavin nucleotide serving as the shuttle of electrons from the enzymes to the ferritin iron.