Caseins bind strongly to Fe by their phosphoseryl residues (Hegenauer et al. 1979; Brulé & Fauquant, 1982; Bouhallab et al. 1991; Emery, 1992) and keep it soluble at the alkaline pH of the duodenum (Manson & Annan, 1971; Bouhallab et al. 1991). It has been suggested that this strong binding prevents the release of free Fe during digestion and impairs its absorption (West, 1986; Hurrell, 1997), but in human studies hydrolysis by digestive enzymes to give low molecular mass peptides improves Fe absorption (Hurrell et al. 1988, 1989).

β-Casein peptide 1–25 (β-CN(1–25)) is the phosphorylated N-terminal fragment of β-CN. Its molecular mass is 3124 Da and it contains four phosphoseryl residues that bind four Fe atoms and keep them soluble (Bouhallab et al. 1991).

Preliminary results showed that binding Fe to β-CN(1–25) enhances its bioavailability in the rat (Aît-Oukhatar et al. 1997) and its absorption by the duodenal rat loop model (Pérès et al. 1997). We recently showed that the β- CN(1–25)–Fe complex is hydrolysed to a lesser extent than free β-CN(1–25) during duodenal digestion, and that the phosphorylated region of the peptide to which Fe is bound, β-CN(15–24), cannot be detected in the digestive lumen (our unpublished results).

These previous studies did not examine the effect of luminal digestion that could release free inorganic Fe. We considered the possibility that Fe bound to β-CN(1–25) is not released in the digestive lumen, and that it reaches the absorbing surface of the small intestine brush border membrane as a complex. To test this hypothesis we examined in vitro the influence of pH and digestive enzymes on the dialysability of the β-CN(1–25)–Fe complex.