Ascorbate offsets the inhibitory effect of inositol phosphates on iron uptake and transport by Caco-2 cells

Differentiated monolayer cultures of Caco-2 human intestinal cells were used as a model to examine interactions between various dietary factors related to the intestinal uptake and absorption of nonheme Fe. Caco-2 cells accumulated 91-98 pmol Fe/mg protein from uptake buffer containing 12 nmol of Fe(III)-nitrilotriacetate during a 1-hr incubation at 37 degrees C. Addition of a 10-fold molar excess of inositol hexaphosphate (IP6) and its lesser phosphorylated derivatives (IP3, IP4, and IP5) decreased cellular uptake and transport of Fe from the lumenal compartment. Addition of ascorbic acid (AA) to the solution containing IPs stimulated Fe uptake and transport in a manner dependent upon the ratio of AA to IP and inversely proportional to the degree of phosphorylation of inositol (i.e., IP3 > IP4 > IP5 > IP6). A mixture of essential amino acids had minimal impact on Fe uptake in either the absence or presence of IPs. Cellular acquisition of Fe from solutions containing IPs was further enhanced by simultaneous addition of essential amino acids and AA. The stimulatory influence of ascorbic acid on Fe uptake from solutions containing IP6 was associated with an increase in the level of ferrous ion. These data further support the usefulness of Caco-2 cells as a model for investigating the effects of various dietary factors on mineral bioavailability.