“Chelatable iron pool”: inositol 1,2,3-trisphosphate fulfils the conditions required to be a safe cellular iron ligand

Abstract

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.

Glutathione: a key component of the cytoplasmic labile iron pool

“…What compounds are available to coordinate cytoplasmic iron(II)? Many ligands, previously linked with the cytoplasmic labile iron pool, will chelate iron(III) but not iron(II) at pH 7.0; namely aminoacids (Miller and Perkins 1969), ATP/AMP (Weaver and Pollack 1989), inositol phosphates (Veiga et al 2009) and 2,5-dihydroxybenzoic acid (Devireddy et al 2010). In contrast citrate does bind iron(II) under cytoplasmic conditions and has been suggested to be the major component of the labile iron pool (Morley and Bezkorovainy 1983).…”

New Horizons in Cellular Regulation by Inositol Polyphosphates: Insights from the Pancreaticβ-Cell

“…This latter observation may be important, because Ins(1,2,3)P 3 has antioxidant properties (Spiers et al, 1996;Veiga et al, 2009). Thus, the inositol depletion observed in diabetes will lead to the suppression of an important protective antioxidant in a number of tissues, including pancreatic b-cells.…”

“…However, it still may fulfill this role in other cellular contexts in vesicles or other subcellular organelles and more widely in nutrition, in which its abundance and chelating properties are known to cause malnutrition in developing countries (Zhou and Erdman, 1995). Furthermore, the InsP 6 breakdown product Ins(1,2,3)P 3 (Barker et al, 1995) has now been subject to the same rigorous assessment as InsP 6 (Veiga et al, 2009) but, in contrast, remains one of the best candidates in the mammalian cell for both complexing iron and preventing iron-catalyzed free radical cascades, acting therefore as a potent antioxidant. The importance of InsP 6 as an antioxidant then becomes as the reservoir from which Ins(1,2,3)P 3 is derived.…”

Insight into the protonation and K(I)-interaction of the inositol 1,2,3-trisphosphate as provided by 31P NMR and theoretical calculations

“…Over the past years, we have carried out a systematic study of the unusual and often non-intuitive chemical behaviour displayed by these highly charged compounds, in particular in the presence of multivalent cations [3][4][5][6][7][8]. These reports have shed light over the acid-base and complexation equilibria established by InsP 6 , Ins(1,3,4,5,6)P 5 and Ins(1,2,3)P 3 in various biological systems.…”

“…doi:10.1016/j.molstruc.2010.11.050 metal complexation properties of myo-inositol phosphates [9][10][11][12]. In particular for Ins(1,2,3)P 3 , a molecule whose biological function is still controversial, some information regarding the intrinsic acid-base properties has been reported [12], although the data were acquired in the presence of large amounts of K(I), which substantially interacts with this ligand in weakly acid to basic media [4]. This is expected to alter the 31 P NMR signals and, consequently, the macro and micro protonation constants obtained.…”