Hemoproteins constitute some of the most important normal endogenous pigments because they include hemoglobin, the cytochromes, and a variety of enzymes.

Central to an understanding of disorders involving these pigments is a knowledge of the uptake, metabolism, excretion, and storage of iron.

A normal adult male requires approximately 1 mg of iron per day to balance the average net loss each day through bile, sweat, minute episodic blood loss in the gut, and cell turnover in the gastrointestinal tract and skin.

During periods of rapid body growth (infancy and puberty), menstrual years of women, and the last two trimesters of pregnancy, daily iron requirements are increased. Iron in food, which is in the ferric state, is reduced to the ferrous form by reducing substances present in food and is absorbed across the duodenal and jejunal mucosa.

The sites of absorption should be mentioned because of their important function of closely regulating the uptake of iron into the body.

Clinical and experimental studies indicate that some of the iron entering the mucosal cell may be complexed to transferring, a beta-globulin, which is the major iron transport protein and transferred directly and rapidly (within 8 hours after ingestion) into the plasma. The remainder of iron is oxidized back to the ferric form and unites with a beta-globulin called apoferritin to form ferritin, a compound containing about 17 to 23% iron.

Ferritin is then slowly absorbed onto the blood, taking several days. Since it is in equilibrium with ferrous iron in the cell, its slow removal tends to maintain ferrous iron at a high saturated level.

Such intracellular levels of ferrous iron inhibit further uptake of iron from the lumen of the intestine and enhance the movement of ferritin from the cell into the blood.

Iron absorption is also regulated by the level of plasma iron, being increased when plasma iron is low, as in individuals with sustained blood loss or anemia.

A variety of dietary factors affect iron absorption - alcohol, ascorbic acid, and fructose tend to enhance absorption, where as phytates (plant salts), phosphates, fats, and calcium impair it.

Conditions such as achlorhydria (very low levels of gastric acid) and the altered gastric mucosa associated with it also tend to diminish absorption.

Tissue Iron:

Iron enters the plasma from tissue stores, from the intestinal mucosa and from reticuloendothelial cells, which remove and destroy damaged red cells.

Plasma iron then enters the bone marrow where it is used for synthesis of hemoglobin.

Iron is stored in tissues in essentially two forms - ferritin, which is not apparent with light microscopy but is visualized with electron microscopy as a tetrad aggregation of intensely electron-dense particles, and hemosiderin, which is composed of large irregular aggregates of ferritin that are insoluble and appear as coarse, brown cytoplasmic granules.

The granules can be demonstrated to contain ferric iron because they form a deep blue product, ferric ferrocyanide, on reaction of tissue with an acid solution of potassium ferrocyanide (Prussian blue reaction).

The equilibrium between storage and plasma iron depends on the degree of transferrin saturation by iron.

Low plasma iron levels and reduced saturation shift the equilibrium so that iron is mobilized from stores to the plasma. When the converse is true, iron moves from plasma to tissue stores. Hemosiderosis: click here