Examples are phytates, phosphates, tannates, oxalates, and carbonates. Heme is maintained soluble and available for absorption by globin degradation products produced by pancreatic enzymes. Heme iron and nonheme iron are absorbed into the enterocyte noncompetitively. Heme enters the cell as an intact metalloporphyrin, presumably by a vesicular mechanism. Heme is degraded within the enterocyte by heme oxygenase with release of iron so that it traverses the basolateral cell membrane in competition with nonheme iron to bind transferrin in the plasma.
Ferric iron utilizes a different pathway to enter cells than ferrous iron.
Which pathway transports most nonheme iron in humans is not known. Most nonheme dietary iron is ferric iron. Iron absorption in mice and rats may involve more ferrous iron because they excrete moderate quantities of ascorbate in intestinal secretions. Contrariwise, humans are a scorbutic species and are unable to synthesize ascorbate to reduce ferric iron. Other proteins are described that appear related to iron absorption.