Inositol phosphates influence iron uptake in Caco-2 cells

Phytate, inositol hexaphosphate (IP ₆), may be hydrolyzed to inositol phosphates with lower degree of phosphorylation, i.e. inositol penta- to monophosphate (IP ₅-IP ₁), during food processing. Each of these lower inositol phosphates exists in different isomeric forms. The objective of this study was to determine if different isomers of IP ₅-IP ₃ and IP ₆ affect the uptake of iron. We studied the iron uptake in vitro using the human intestinal epithelial cell line, Caco-2. The effects of the inositol phosphate isomers Ins(1,2,4)P ₃, Ins(1,2,3)P ₃, Ins(1,2,6)P ₃, Ins(1,3,4)P ₃, Ins(1,2,3,4)P ₄, Ins(1,2,5,6)P ₄, Ins(1,2,4,5,6)P ₅, Ins(1,3,4,5,6)P ₅ and IP ₆ were investigated. Addition of a 2-fold molar excess of IP ₆ or IP ₅ in proportion to iron (1 h incubation at 37°C reduced iron uptake by 46-52% (P<0.001). Neither IP ₄ isomers nor IP ₃ isomers affected iron uptake significantly at 1 h incubation with a molar IP:Fe level of 2:1. Iron uptake was shown to not be a function of the isomeric form of inositol phosphates. The inositol phosphate isomers did not seem likely to interact with each other through iron to form more stable iron complexes. At an IP:Fe molar level of 20:1 an inhibitory effect of IP ₄ was found, while IP ₃ did not affect the iron absorption even at a 20-fold molar excess.