title: The behaviour of myo-inositol hexakisphosphate in the presence of magnesium(II) and calcium(II): protein-free soluble InsP6 is limited to 49 microM under cytosolic/nuclear conditions.
abstract: Progress in the biology of myo-inositol hexakisphosphate (InsP(6)) has been delayed by the lack of a quantitative description of its multiple interactions with divalent cations. Our recent initial description of these [J. Torres, S. Dominguez, M.F. Cerda, G. Obal, A. Mederos, R.F. Irvine, A. Diaz, C. Kremer, J. Inorg. Biochem. 99 (2005) 828-840] predicted that under cytosolic/nuclear conditions, protein-free soluble InsP(6) occurs as Mg(5)(H(2)L), a neutral complex that exists thanks to a significant, but undefined, window of solubility displayed by solid Mg(5)(H(2)L).22H(2)O (L is fully deprotonated InsP(6)). Here we complete the description of the InsP(6)-Mg(2+)-Ca(2+) system, defining the solubilities of the Mg(2+) and Ca(2+) (Ca(5)(H(2)L).16H(2)O) solids in terms of K(s0)=[M(2+)](5)[H(2)L(10-)], with pK(s0)=32.93 for M=Mg and pK(s0)=39.3 for M=Ca. The concentration of soluble Mg(5)(H(2)L) at 37 degrees C and I=0.15M NaClO(4) is limited to 49muM, yet InsP(6) in mammalian cells may reach 100muM. Any cytosolic/nuclear InsP(6) in excess of 49muM must be protein- or membrane-bound, or as solid Mg(5)(H(2)L).22H(2)O, and any extracellular InsP(6) (e.g. in plasma) is surely protein-bound.
authors.affiliation: Cátedra de Química Inorgánica, Departamento Estrella Campos, Facultad de Química, Universidad de la República, Montevideo, Uruguay.
authors.forename: Nicolás
authors.initials: N
authors.lastname: Veiga
authors.forename: Julia
authors.initials: J
authors.lastname: Torres
authors.forename: Sixto
authors.initials: S
authors.lastname: Domínguez
authors.forename: Alfredo
authors.initials: A
authors.lastname: Mederos
authors.forename: Robin F
authors.initials: RF
authors.lastname: Irvine
authors.forename: Alvaro
authors.initials: A
authors.lastname: Díaz
authors.forename: Carlos
authors.initials: C
authors.lastname: Kremer
mesh_terms: D002118:Calcium / Q000737:chemistry*; D002467:Cell Nucleus / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D008274:Magnesium / Q000737:chemistry*; D010833:Phytic Acid / Q000737:chemistry* / Q000378:metabolism; D011506:Proteins / Q000032:analysis; D012995:Solubility
chemical_list: D011506:Proteins; D010833:Phytic Acid; D008274:Magnesium; D002118:Calcium