Speciation of trace metals, with special reference to intestinal iron absorption

Abstract

Mechanisms of intestinal iron absorption are reviewed: there are three principal pathways of mucosal inorganic iron uptake. These include the electrogenic-carrier and fatty-acid-mediated transcellular pathways and a nonspecific paracellular route. A novel method is described for the speciation of dietary and luminal iron during its transit along the gastro-intestinal tract. Extrinsic labelling of the diet with 59FeCl 3 revealed a differential distribution of chemical and radio-iron in the various fractions, indicating that absorption and bioavailability data obtained with extrinsically labelled diets must be interpreted with caution. In standard laboratory chow, two-thirds of the iron is in a residual (acid-soluble) fraction, with only 1% in an exchangeable (aqueous MgCl 2-soluble) fraction. However, analysis of gastric contents after feeding the diet indicated that most of the iron had become redistributed from the residual to the exchangeable fraction. This process could only be partially mimicked by acid incubation of the diet. In the duodenal lumen there was a significant decrease in the exchangeable fraction consistent with its availability for absorption. In the jejunum and ileum most of the remaining exchangeable iron had become redistributed to carbonate and oxide-bound fractions: this redistribution could be mimicked in vitro by neutralisation of gastric contents with sodium bicarbonate. Use of Fe(II)-specific indicators shows that the gastric lumen is a major site of reduction of dietary Fe(III). Recent studies have demonstrated, additionally duodenal mucosal ferri-reductase activity, highlighting the key role of Fe(II) as an intermediate in iron absorption.