Binding of Cu2+, Zn2+, and Cd2+to Inositol Tri-, Tetra-, Penta-, and Hexaphosphates

Abstract

myo-Inositol hexaphosphate, the salt of myo-inositol hexaphosphoric acid (IP6), is a common constituent of many plant foods, such as cereals and legumes. IP6 interacts with mineral elements, influencing their bioavailability. Processed foods contain a mixture of different inositol phosphates, i.e., IP6 and its degradation products with five or less phosphate groups (IP5-IP1). The interaction of the lower inositol phosphates with mineral elements is not well-known. In this study, the interaction between metal ions (Cu2+, Zn2+, and Cd2+) and isolated fractions of inositol phosphates with 6, 5, 4 and 3 phosphate groups (IP6-IP3) was investigated by using a potentiometric technique. The study was performed at pH 3-7, which is the pH range in the upper part of the duodenum, where mineral absorption takes place. The inositol phosphate fractions studied had a pronounced binding capacity between pH 5 and 7. Thus, mineral complex formation with lower inositol phosphates is likely to occur in the duodenum, which would be important from a nutritional point of view. The mineral binding capacity as calculated per phosphate group was similar for IP6, IP5, IP4, and IP3, but the binding strength was lower for the lower inositol phosphates (IP4 and IP3). At increasing pH, within the range (pH 3-7), the metal complex formation generally began in the order copper, zinc, cadmium for all inositol phosphates indicating the same order of binding strength, i.e., Cu>Zn>Cd. For IP6 the difference was small between Cu and Zn. (Less)