Submitted to: Journal of Nutritional Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/9/2001
Publication Date: N/A
Citation: Interpretive Summary: Casein is a milk protein that inhibits iron absorption in humans. Partial breakdown of casein prior to ingestion diminishes this inhibition, and makes the iron more available for absorption. To explain this observation, we hypothesized that partial breakdown of casein exposes phosphoserine residues in casein. An enzyme in our intestine, alkaline phosphatase then attacks the phosphoserine residues that bind iron and prevent its absorption. We tested this observation using a model that mimics the human digestive tract. This model incorporates human cell cultures (intestinal epithelial cells) which take up the iron from the digest, thus providing a measure of absorption. Our results provided support for our hypothesis that the phosphoserine residues of casein bind iron and thereby inhibit absorption. Breakdown of the casein , thus exposing the phosphoserine to intestinal alkaline phosphatase inhibits iron binding, thereby allowing its sabsorption. These observations may result in ways to improve the absorption of iron from foods such as milk and milk based products.
Technical Abstract: Clusters of phosphoserine residues in casein bind iron with high affinity. Casein inhibits iron absorption in humans but partial hydrolysis of casein prior to ingestion diminishes this inhibition. The objective of this study was to test two hypotheses: 1. Partial hydrolysis of the peptide bonds in casein exposes phosphoserine residues to attack by intestinal alkaline phosphatase (IAP). 2. Hydrolysis of the phospho-ester linkage in phosphoserine residues in casein by IAP releases bound iron or inhibits iron chelation, thereby allowing its absorption. Test of hypothesis 1: Suspensions of sodium caseinate (SC), enzymatically hydrolyzed casein (EHC), and casein phosphopeptides (CPP) were subjected to an in vitro pepsin/pancreatin digestion and subsequently incubated in the presence of calf IAP. The rate of release of inorganic phosphate was measured with the following results (expressed as umol phosphate released/unit of IAP/min): 0.081, 0.104, 0.139 for SC, EHC, and CPP, respectively. These results are consistent with hypothesis 1. Test of hypothesis 2: 59Fe-citrate or 59Fe-citrate + CPP in minimum essential media were spiked with a Na2WO4 solution or water (Na2WO4 is a known inhibitor of IAP) and placed on Caco-2 cell monolayers. Uptake of 59Fe by the cells was used as an index of iron bioavailability. Na2WO4 did not affect 59Fe uptake from samples containing only iron but did slightly inhibit (by 10%) uptake from samples containing iron + CPP. These results are consistent with hypothesis 2 and provide a possible explanation for the observation that partial hydrolysis of casein improves iron bioavailability.