Milk proteins and iron absorption: contrasting effects of different caseinophosphopeptides

Clusters of phosphoserine residues in cow milk caseins bind iron (Fe) with high affinity. Casein inhibits Fe absorption in humans, but protein hydrolysis lessens this effect. Phosphopeptides from different caseins gave conflicting results on Fe absorption; release of phosphate residues by intestinal alkaline phosphatase could be a key point of that metabolism. The objectives of this study were to compare the absorption of Fe complexed to caseinophosphopeptides (CPP) of the main cow milk caseins beta-casein (beta-CPP) and alpha(s)-caseins (alpha(s1)-CPP) and to assess the role of alkaline phosphatase on this absorption. Two experimental models were used: an in vivo perfused rat intestinal loop and an in vitro Caco-2 cell culture model. In addition, we determined the effect of an intestinal phosphatase inhibitor on these various forms of Fe. Gluconate Fe was used as control. In both models, uptake and net absorption of Fe complexed to CPP from alpha(S1)-caseins were significantly lower than from Fe complexed to beta-CPP. Inhibition of the intestinal phosphatase significantly increased the uptake and the absorption of Fe complexed to beta-CPP without effect on the other forms of Fe. These results confirm the enhancing effect of beta-casein and its CPP on Fe absorption. The differences between CPP could be explained by their structural and/or conformational features: binding Fe to alpha(S1)-CPP could impair access to digestive enzymes, whereas beta-CPP-bound Fe is better absorbed than its free form. The differences in protein composition between cow and breast milk, which does not contain alpha-casein, could explain some of their differences in Fe bioavailability.