Milk osteopontin has high iron-binding capacity and facilitates iron absorption in intestinal cells

Insufficient absorption of iron and the consequent development of iron deficiency have serious health consequences. Hence, identification and development of iron delivery systems that can increase the bioavailability and uptake of dietary iron are important. Osteopontin (OPN) is an acidic and highly phosphorylated integrin-binding protein found in milk where it exists as a full-length protein and as N-terminally derived fragments. Milk OPN can be taken up by enterocytes and transported across the intestinal barrier into the circulation. Milk OPN has previously been shown to bind calcium and magnesium. This study investigates milk OPN as a carrier of iron and its potential to increase iron absorption in intestinal cells. Full-length OPN and N-terminal fragments of OPN were shown to bind ∼30 and ∼10 mol of iron, respectively, and the phosphorylated residues were crucial for iron binding. Osteopontin retained iron bound after simulated gastrointestinal digestion. Immunodetection of digested OPN and OPN-Fe complexes showed that the OPN-Fe complexes were more resistant to pepsin digestion than OPN without bound iron. The cellular uptake of iron was investigated by measuring intracellular ferritin formation and mRNA expression of divalent metal transporter 1 in Caco-2 cells. Osteopontin increased the uptake of iron even in the presence of phytic acid, a dietary inhibitor of iron absorption. These data indicate that OPN can function as an iron carrier for use in alternative strategies for delivering iron in a bioavailable form for intestinal uptake.