INTERACTION OF INOSITOL PHOSPHATES WITH CALCIUM, ZINC, AND HISTIDINE

An in vitro study was conducted to investigate the interaction of inositol phosphates with Ca, Zn, and histidine. Inositol tri- to pentaphosphates (IP3-IP5) were prepared by hydrolyzing the hexaphosphate, phytic acid, and were separated by anion exchange chromatography. Elution of the inositol phosphates was monitored using metal-dye detection and measurement of inorganic phosphate after persulphate oxidation. The effect of inositol phosphates on the solubility of Ca and Zn was studied in vitro at molar ratios of 1:1 to 1:40 for IP:Zn and 15:1 to 500:1 for Ca:Zn, reflecting the concentrations of these constituents in human diets. The solubilities of Ca-Zn-IP complexes were high at pH 3 to 4 but decreased rapidly when the pH was raised to between 5 and 6. At pH 7 or above, the complexes were almost completely insoluble when the Ca:IP:Zn ratio was 100:10:1. The solubility of both Ca and Zn decreased as the IP:Zn and Ca:Zn ratios were increased. Inositol phosphates showed a greater binding capacity for Zn than for Ca. Calcium potentiated the binding of Zn to inositol phosphates at high molar IP:Zn ratios (5:1 to 40:1), but competed with Zn for binding sites when the IP:Zn ratio was low (1:1). Although histidine increased the solubility of zinc in the presence of the inositol phosphates and calcium (histidine:Ca:IP:Zn = 250:100:10:1), the effect was markedly dependent on the order in which the components were mixed. The formation of insoluble Ca-IP-Zn complexes was also influenced by changes in the ionic strength of the equilibration medium brought about by addition of sodium or potassium chloride. In general, the solubility of Ca and Zn decreased with increasing degree of phosphate substitution of the inositol molecule, i.e., following the order IP3 > IP4 > IP5 > IP6.