Solution behaviour of myo-inositol hexakisphosphate in the presence of multivalent cations. Prediction of a neutral pentamagnesium species under cytosolic/nuclear conditions Julia Torres a , Sixto Domı ´nguez b , M. Fernanda Cerda ´ c , Gonzalo Obal c , Alfredo Mederos b , Robin F. Irvine d , Alvaro Dı ´az e, * , Carlos Kremer a, * a Ca ´ tedra de Quı ´mica Inorga ´ nica, Departamento Estrella Campos, Facultad de Quı ´mica, Universidad de la Repu ´ blica, CC 1157, Montevideo, Uruguay b Departamento de Quı ´mica Inorga ´ nica, Universidad de La Laguna, Tenerife, Canary Islands, Spain c Laboratorio de Biomateriales, Facultad de Ciencias, Universidad de la Repu ´ blica, Igua ´ 4225, CP 11400 Montevideo, Uruguay d Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, UK e Ca ´ tedra de Inmunologı ´a, Facultad de Quı ´mica/Ciencias, Universidad de la Repu ´ blica, Avenida Alfredo Navarro 3051, CP 11600, Montevideo, Uruguay Received 7 September 2004; received in revised form 20 December 2004; accepted 21 December 2004 Abstract myo-Inositol hexakisphosphate (InsP 6 ) is an ubiquitous and abundant molecule in the cytosol and nucleus of eukaryotic cells whose biological functions are incompletely known. A major hurdle for studying the biology of InsP 6 has been a deficiency of a full understanding of the chemistry of its interaction with divalent and trivalent cations. This deficiency has limited our appreciation of how it remains in solution within cells, and the likely degree to which it might interact in vivo with physiologically important cations such as Ca 2+ and Fe 3+ . We report here the initial part of the description of the InsP 6 -multivalent cation chemistry, including its solu- tion equilibria studied by high resolution potentiometry and (for the Fe(III)/Fe(II) couple) cyclic voltammetry. InsP 6 forms anionic complexes of high affinities and 1:1 stoichiometry with Mg(II), Ca(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II). Of particular importance is the observation that, in the exceptional case of Mg(II), InsP 6 forms the species [Mg 5 (H 2 L)] (L representing fully deprotonated InsP 6 ); this soluble neutral species is predicted to be the predominant form of InsP 6 under nuclear or cytosolic conditions in animal cells. Contrary to previous suggestions, InsP 6 is predicted not to interact with cytosolic calcium even when cal- cium is increased during signalling events. In vitro, InsP 6 also forms high affinity 1:1 complexes with Fe(III) and Al(III). However, our data predict that in the biological context of excess free Mg(II), neither Fe(III) nor Fe(II) are complexed by InsP 6 . Ó 2004 Elsevier Inc. All rights reserved. Keywords: Inositol; Inositol polyphosphates; Iron; Calcium; Magnesium 1. Introduction myo-Inositol hexakisphosphate or phytate (InsP 6 ; its different protonated forms are denoted in this work by H 12 L, H 11 L  , ...,L 12 ) is an ubiquitous and abundant molecule in eukaryotic cells (reviewed in [1–3]). InsP 6 can act as a precursor of diphosphoinositol pentakis- phosphates (PPInsP 5 ) and bis(diphospho)inositol tetra- kisphosphates ((PP) 2 InsP 4 ) (reviewed in [4]). The rate of metabolic turnover between InsP 6 and the derived pyro- phosphates is very high [5–7], especially when consid- ered in relation to the lower concentrations of the latter. It is thus easy to envisage the pyrophosphates as molecular switches regulating cellular processes 0162-0134/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2004.12.011 * Corresponding authors. Tel.: + 5982 9249739; fax: + 5982 9241906 (C. Kremer for chemical aspects); fax: + 5982 4874320 (A. Dı ´az for biological aspects). E-mail addresses: adiaz@fq.edu.uy (A. Dı ´az), ckremer@fq.edu.uy (C. Kremer). www.elsevier.com/locate/jinorgbio Journal of Inorganic Biochemistry 99 (2005) 828–840 JOURNAL OF Inorganic Biochemistry