Haem in the gut. I. Fate of haemoproteins and the absorption of haem.

Haem (FeII-protoporphyrin-IX) is presented to the gut lumen as haemoproteins derived from exogenous dietary) and endogenous (mucosal cell desquamation and bleeding) sources. Haemoproteins such as haemoglobin, myoglobin and catalase undergo hydrolysis by luminal proteases to release the haem. Released haem is maintained in a soluble form in the gut lumen by the products of haemoprotein digestion. Chelators of elemental iron do not bind haem-iron and so haem-iron is better absorbed than elemental iron. Haem-iron does not exchange with luminal elemental iron. Mucosal uptake of haem is limited. Less than 10% binds to the brush border of the villus cell. Although the mechanisms by which haem binds to the brush border and is transported to the intracellular environment are poorly understood, it is known that some haem is transferred to secondary lysosomes where the porphyrin ring is split to release iron and form bilirubin. Depending upon the composition of the diet, the iron released from haem within the villus cell can be the major physiological source of iron. In iron-deficiency in humans, absorption of haem-iron can increase threefold whereas absorption of elemental-iron can increase tenfold. These observations indicate that haem-iron and elemental-iron are absorbed via different mechanisms which are subject to different regulation. For haem-iron to be absorbed, the haem itself must be taken up by the mucosa.