Iron metabolism is normal

The normal daily human diet contains about 10-20 mg of iron (90% in a free state, 10% in combination with heme), of which 1-1.5 mg is absorbed. The amount of absorbed iron depends on its reserves in the body: the higher the need, the more iron is absorbed. Absorption occurs in the upper parts of the small intestine and is an active process in which iron can be transferred even against the concentration gradient. However, the mechanisms of transfer are unknown. Proteins that can be iron carriers have been identified, but their exact role has not yet been established.

In mucosal cells, iron is found in the cytosol. Some of it is bound and stored as ferritin, which is then either used or lost through cell shedding. Some of the iron destined for metabolism in other tissues is transported across the basolateral membrane of the cell (the mechanism is not understood) and binds to transferrin, the main iron transport protein in the blood.

Transferrin (molecular weight 77,000 Da) is a glycoprotein synthesized mainly in the liver. It can bind 2 molecules of iron. The total iron-binding capacity of serum due to transferrin is 250-370 μg%. Normally, transferrin is approximately one-third saturated with iron. Physiological iron absorption by reticulocytes and hepatocytes depends on transferrin receptors on the cell surface, which have an affinity primarily for iron-bound transferrin. The iron-receptor complex enters the cell, where the iron is released. When the cell is saturated with iron, the cellular transferrin receptors are inhibited. When transferrin is completely saturated, as occurs, for example, in severe hemochromatosis, iron circulates in forms not bound to transferrin, in the form of compounds with low-molecular chelators. In this form, iron readily enters the cells regardless of their degree of iron saturation.

In cells, iron is deposited in the form of ferritin (molecular weight 480,000 Da) - a complex of the protein apoferritin (subunits H and L) with iron, which, under electron microscopy, looks like particles with a diameter of 50 A, freely located in the cytoplasm. One molecule of ferritin can contain up to 4,500 iron atoms. At high concentrations of iron, the synthesis of apoferritin is enhanced.

Clusters of broken ferritin molecules are hemosiderin, which is stained blue by ferrocyanide. Approximately one-third of the body's iron stores are in the form of hemosiderin, and the amount increases in diseases associated with excess iron accumulation.

Lipofuscin, or wear pigment, accumulates due to iron overload. It is yellow-brown in color and does not contain iron.

When the need for hemoglobin synthesis increases, iron deposited in the form of ferritin or hemosiderin is mobilized.

Normally, the body contains about 4 g of iron, of which 3 g is in hemoglobin, myoglobin, catalase and other respiratory pigments or enzymes. Iron reserves are 0.5 g, of which 0.3 g are in the liver, but they are not visible during a routine histological examination with iron staining using conventional methods. The liver is the main storage site for iron absorbed in the intestine. When it is fully saturated, iron is deposited in other parenchymatous organs, including the acinar cells of the pancreas and the cells of the anterior pituitary gland. The reticuloendothelial system plays a limited role in iron accumulation and becomes the site of predominant iron deposition only when it is administered intravenously. Iron from destroyed red blood cells accumulates in the spleen.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ], [ 8 ], [ 9 ]