Milk proteins and iron absorption: contrasting effects of different caseinophosphopeptides
Résumé
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 β-casein (β-CPP) and αs-caseins (αs1-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 αS1-caseins were significantly lower than from Fe complexed to β-CPP. Inhibition of the intestinal phosphatase significantly increased the uptake and the absorption of Fe complexed to β-CPP without effect on the other forms of Fe. These results confirm the enhancing effect of β-casein and its CPP on Fe absorption. The differences between CPP could be explained by their structural and/or conformational features: binding Fe to αS1-CPP could impair access to digestive enzymes, whereas β-CPP-bound Fe is better absorbed than its free form. The differences in protein composition between cow and breast milk, which does not contain α-casein, could explain some of their differences in Fe bioavailability. Abbreviations: CPP, caseinophosphopeptides Fe, iron Iron (Fe) deficiency is one of the most common nutritional problems around the world, both in developed and in developing countries. It is responsible for a long-lasting impairment of neurologic development in infants; Fe is also essential for immunity and growth (1,2). Cow milk products and cow milk-based infant formulas are two main components of infant diet; however, Fe absorption from cow milk is low compared with breast milk (3); the high calcium concentration and the kind of proteins of cow milk could explain these differences (4,5). The main cow milk proteins are caseins, which bind cations, including Fe, by clusters of phosphoserine (6,7). This strong binding keeps Fe soluble at the alkaline gut pH but prevents its release in a free form available for absorption by the duodenal mucosa (8). Hydrolysis of whole caseins improves Fe absorption (5), but caseinophosphopeptides (CPP) released by the enzyme digestion of different caseins give conflicting results; the two main cow milk caseins are αS- and β-caseins; binding Fe to CPP of β-casein or to hydrolyzed β-casein improves its absorption and its bioavailability in rats and in humans (9-11); however, CPP derived from the hydrolysis of whole casein or fractions enriched with αS-casein CPP inhibit Fe absorption (12-14). One of the key steps of the absorption of CPP-bound Fe seems to be the release of phosphoserine residues by brush border phosphatase: it could be assumed that releasing free Fe could increase its absorption from a poorly absorbed form; however, releasing Fe from a stable, efficiently absorbed complex, such as Fe-βCPP (1-25) could impair its absorption by increasing the amount of free Fe subject to gut interactions. These functional differences between caseins and casein-derived CPP could give some clues to improve the bioavailability of cow milk Fe. The objectives of this study were to evaluate the absorption of Fe complexed to CPP purified either from β-casein or from αs1-casein and to determine the effect of an inhibitor of intestinal alkaline phosphatase (Na2WO4) (12,15). The study used two models, the in vivo perfused rat intestinal loop and a human colon adenocarcinoma cell line culture (Caco-2), which is predictive of human Fe bioavailability (16).