Question

1. Describe the movement of iron from plant vs. animal foods from the intestinal lumen onto transferrin in the blood.

Answer 1

Ans: Dietary iron is found in two basic forms, either as haem-found in meat and meat products-or non-haem iron-present in cereals, vegetables, pulses, beans, fruits etc in a number of forms ranging from simple iron oxides and salts to more complex organic chelates.

Both haem and non-haem iron are taken up in this proximal region of the small intestine, though their transport across the apical membrane of the enterocytes occurs through totally independent pathways.

Plant food Iron movement: The majority of dietary non-haem iron enters the gastrointestinal tract in the ferric form.  However, Fe3+ is thought to be essentially non-bioavailable (see below) and, therefore, it must first be converted to ferrous iron prior to absorption. There are numerous dietary components capable of reducing Fe3+ to Fe2+, including ascorbic acid, and amino acids such as cysteine and histidine. A number of studies have demonstrated that the brush-border surface of duodenal enterocytes and cultured intestinal cells possess ferric reductase enzymic activity.

Dcytb (for duodenal cytochrome b), a homologue of cytochrome b561, was identified as the enzyme responsible for this process. Dcytb is a haem-containing protein with putative binding sites for ascorbate and semi-dehydroascorbate. The protein is expressed on the brush border membrane of duodenal enterocytes, the major site for the absorption of dietary iron.

Following reduction either by Dcytb or dietary reducing agents, the resulting Fe2+ becomes a substrate for the divalent metal transporter, DMT1-also known as the divalent cation transporter, DCT1, and natural resistance associated macrophage protein, Nramp2. The relatively low pH of the proximal duodenum together with the acid microclimate present at the brush border membrane stabilises iron in the ferrous form and provides a rich source of protons that are essential for driving iron uptake across the apical membrane of the intestinal epithelium.

Animal food Iron movement: Non-vegetarian diets provide an additional and important source of iron in the form of haem (largely from haemoglobin and myoglobin). Haem binds to the duodenal brush border membrane and is absorbed as an intact molecule. HCP1 acts as a haem import protein and the high duodenal expression of HCP1 suggests that it may be the protein involved in haem uptake from the diet.

Following absorption, haem is detectable in membrane-bound vesicles within the cytoplasm[52,53]. Within these vesicles, it is thought that the iron contained with the protoporphyrin ring is excised by the action of haem oxygenase 1 yielding ferrous iron which enters a common intracellular pool along with the iron absorbed via the non-haem transport pathways.

Iron Export: Efflux of iron across the basolateral surface of enterocytes is achieved through the co-ordinated action of a transport protein IREG1-also known as ferroportin and MTP1-and a ferrioxidase, hephaestin.

This efflux function is up-regulated in the presence of ceruloplasmin, a copper binding ferrioxidase, and transferrin to bind the newly liberated iron. This suggests that while ferrous iron is released through IREG1 it must be oxidised to ferric iron to facilitate its loading onto transferrin for onward transport in the circulation.

The image enclosed below is representing the trasport of iron from both animal and plant food:

LIP stands for Labile Iron Pool, Fe+3 is represnting Iron from Plant food and haem is the iron content from animal food.