Veiga Nicolás, Torres Julia, Mansell David, Freeman Sally, Domínguez Sixto, Barker Christopher J, Díaz Alvaro, Kremer Carlos
Departamento Estrella Campos, Facultad de Química, Universidad de la República, CC 1157, Montevideo, Uruguay.
J Biol Inorg Chem. 2009 Jan;14(1):51-9. doi: 10.1007/s00775-008-0423-2. Epub 2008 Sep 2.
Mammalian cells contain a pool of iron that is not strongly bound to proteins, which can be detected with fluorescent chelating probes. The cellular ligands of this biologically important "chelatable", "labile" or "transit" iron are not known. Proposed ligands are problematic, because they are saturated by magnesium under cellular conditions and/or because they are not "safe", i.e. they allow iron to catalyse hydroxyl radical formation. Among small cellular molecules, certain inositol phosphates (InsPs) excel at complexing Fe(3+) in such a "safe" manner in vitro. However, we previously calculated that the most abundant InsP, inositol hexakisphosphate, cannot interact with Fe(3+) in the presence of cellular concentrations of Mg(2+). In this work, we study the metal complexation behaviour of inositol 1,2,3-trisphosphate [Ins(1,2,3)P(3)], a cellular constituent of unknown function and the simplest InsP to display high-affinity, "safe", iron complexation. We report thermodynamic constants for the interaction of Ins(1,2,3)P(3) with Na(+), K(+), Mg(2+), Ca(2+), Cu(2+), Fe(2+) and Fe(3+). Our calculations indicate that Ins(1,2,3)P(3) can be expected to complex all available Fe(3+) in a quantitative, 1:1 reaction, both in cytosol/nucleus and in acidic compartments, in which an important labile iron subpool is thought to exist. In addition, we calculate that the fluorescent iron probe calcein would strip Fe(3+) from Ins(1,2,3)P(3) under cellular conditions, and hence labile iron detected using this probe may include iron bound to Ins(1,2,3)P(3). Therefore Ins(1,2,3)P(3) is the first viable proposal for a transit iron ligand.
哺乳动物细胞含有一批未与蛋白质紧密结合的铁，可用荧光螯合探针检测到。这种具有生物学重要性的“可螯合”“不稳定”或“转运”铁的细胞配体尚不清楚。提出的配体存在问题，因为它们在细胞条件下会被镁饱和，和/或因为它们“不安全”，即它们会使铁催化羟基自由基的形成。在小分子细胞中，某些肌醇磷酸酯（InsPs）在体外以这种“安全”方式与Fe(3+)络合方面表现出色。然而，我们之前计算得出，细胞中最丰富的肌醇磷酸酯——肌醇六磷酸，在细胞内镁离子浓度存在时无法与Fe(3+)相互作用。在这项研究中，我们研究了1,2,3-三磷酸肌醇[Ins(1,2,3)P(3)]的金属络合行为，它是一种功能未知的细胞成分，也是表现出高亲和力、“安全”铁络合的最简单的肌醇磷酸酯。我们报告了Ins(1,2,3)P(3)与Na(+)、K(+)、Mg(2+)、Ca(2+)、Cu(2+)、Fe(2+)和Fe(3+)相互作用的热力学常数。我们的计算表明，无论是在细胞质/细胞核中还是在酸性区室中，Ins(1,2,3)P(3)都有望以定量的1:1反应络合所有可用的Fe(3+)，据认为酸性区室中存在一个重要的不稳定铁亚池。此外，我们计算得出，荧光铁探针钙黄绿素在细胞条件下会从Ins(1,2,3)P(3)中夺取Fe(3+)，因此使用该探针检测到的不稳定铁可能包括与Ins(1,2,3)P(3)结合的铁。因此，Ins(1,2,3)P(3)是第一个关于转运铁配体的可行提议。
