Transition Metals Are Absorbed by Diverse Mechanisms

 In general, intestinal uptake of most transition metals is relatively inefficient. Only a small portion of the transition metals ingested each day is absorbed into our bodies. In addition, some transition metals, such as Ni and V, can be readily absorbed through the lungs when present in contaminated air or as a component of cigarette smoke. The perceived “inefficiency” of intestinal absorption may reflect the combination of the human body’s modest requirements for these elements and the need to buffer against the accumulation of excessive amounts of these potentially toxic heavy metals. While we understand the pathways by which some transition metals, such as Fe , are taken up in great detail, in other instances, little hard evidence has been uncovered.

Fe²⁺ is absorbed directly via a transmembrane protein, the divalent metal ion transport protein (DMT-1), in the proximal duodenum. DMT-1 is also postulated to constitute the primary vehicle for the uptake of Mn^2+, Ni²⁺, and—to a lesser extent—Cu²⁺. As most of the iron in the stomach is in the ferric state, Fe³⁺, it must be reduced to the ferrous, Fe²⁺, state in order to be absorbed. This reaction is catalyzed by a ferric reductase that is also present on the cell surface, duodenal cytochrome b (Dcytb). In addition, Dcytb is responsible for reducing Cu²⁺ to Cu¹⁺ prior to transport by the high affinity Cu transport protein Ctr1. However, excess levels of Zn can inhibit Cu absorption, which can lead to a potentially lethal anemia. Molybdenum and vanadium are absorbed in the gut as the oxyanions vanadate, HVO₄²⁻, and molybdate, HMoO₄ ²⁻, by the same nonspecific anion transporter responsible for the absorption of their structural analogs phosphate, HPO₄ ²⁻, and sulfate, SO₄ ²⁻.

Cobalt is absorbed as the organometallic complex cobalamin, that is, vitamin B12 , via a dedicated pathway involving two secreted cobalamin-binding proteins, haptocorrin and intrinsic factor, and a cell surface receptor, cubilin. In the stomach, cobalamin released from ingested foods binds to haptocorrin, which protects the coenzyme from the extreme pH of its gastric surroundings. As the cobalamin-haptocorrin complex moves into the duodenum, the pH rises, inducing dissociation of the complex. The released cobalamin is then bound by a haptocorrin homolog known as intrinsic factor. The resulting cobalamin-intrinsic factor complex is then recognized and internalized by cubilin receptors present on the surface of intestinal epithelial cells.