Weber Günther, von Wirén Nicolaus, Hayen Heiko
ISAS-Institute for Analytical Sciences, Bunsen-Kirchhoff-Str. 11, 44139 Dortmund, Germany.
Biometals. 2008 Oct;21(5):503-13. doi: 10.1007/s10534-008-9137-8. Epub 2008 Mar 6.
Phytosiderophores (PS) are strong iron chelators, produced by graminaceous plants under iron deficiency. The ability of released PS to chelate iron(III), and subsequent uptake of this chelate into roots by YS1-type transport proteins, are well-known. The mechanism of iron release from the stable chelate inside the plant cell, however, is unclear. One possibility involves the reduction of ferric PS in the presence of an iron(II) chelator via ternary complex formation. Here, the conversion of ferric PS species by ascorbate in the presence of the intracellular ligand nicotianamine (NA) has been investigated at cytosolic pH (pH 7.3), leading to the formation of a ferrous NA chelate. This reaction takes place when supplying Fe(III) as a chelate with 2'-deoxymugineic acid (DMA), mugineic acid (MA), and 3-epi-hydroxymugineic acid (epi-HMA), with the reaction rate decreasing in this order. The progress of the conversion of ferric DMA to ferrous NA was monitored in real-time by high resolution mass spectrometry (FTICR-MS), and the results are complemented by electrochemical measurements (cyclic voltammetry), which allows detecting reactive intermediates and their change with time at high sensitivity. Hence, the combined results of electrochemistry and mass spectrometry indicate an ascorbate-mediated mechanism for the iron release from ferric PS, which highlights the role of ascorbate as a simple, but effective plant reductant.
植物铁载体（PS）是禾本科植物在缺铁条件下产生的强铁螯合剂。释放出的PS螯合铁（III）的能力，以及随后这种螯合物通过YS1型转运蛋白被根系吸收的过程，都是众所周知的。然而，植物细胞内稳定螯合物中铁的释放机制尚不清楚。一种可能性是在存在铁（II）螯合剂的情况下，通过三元复合物的形成使三价铁的植物铁载体还原。在此，研究了在细胞溶质pH值（pH 7.3）下，抗坏血酸在细胞内配体烟酰胺（NA）存在时对三价铁植物铁载体物种的转化，导致形成亚铁-NA螯合物。当以与2'-脱氧 mugineic 酸（DMA）、mugineic 酸（MA）和3-表-羟基 mugineic 酸（epi-HMA）形成的螯合物形式提供Fe（III）时，该反应发生，反应速率按此顺序降低。通过高分辨率质谱（FTICR-MS）实时监测三价铁DMA向亚铁NA的转化过程，电化学测量（循环伏安法）对结果进行补充，这使得能够高灵敏度地检测反应中间体及其随时间的变化。因此،电化学和质谱的综合结果表明了一种抗坏血酸介导的从三价铁植物铁载体释放铁的机制，这突出了抗坏血酸作为一种简单但有效的植物还原剂的作用。
