IRON METABOLISM : THE PATHOPHYSIOLOGY OF IRON STORAGE

On the basis of experimental and clinical observations and a review of the literature, a concept of the behavior of storage iron in relation to body iron metabolism has been formulated.

Storage iron is defined as tissue iron which is available for hemoglobin synthesis when the need arises. This iron is stored intracellularly in protein complex as ferritin and hemosiderin. It would appear that wherever the cell is functionally intact, such iron is available for general body needs.

Iron is transported by a globulin of the serum to and from the various tissues of the body to satisfy their metabolism. Surplus iron carried by this iron-binding protein is deposited chiefly in the liver.

Storage iron may be increased in two ways. The first mechanism results from the inability of the body to excrete significant amounts of iron. Because of this, any decrease in circulating red cell iron (any anemia other than blood loss or iron deficiency anemia) is accompanied by a shift of iron to the tissue compartment. The total amount of body iron remains constant and is merely redistributed.

This is to be contrasted with the absolute increase in body iron and enlarged iron stores which follow excessive iron absorption or parenteral iron administration. Enlarged iron stores in either instance may be evaluated by examination of sternal marrow or determination of the serum iron and saturation of the iron binding protein

In states of iron excess, differences in initial distribution are observed, depending on the route of administration and type of iron compound employed. Iron absorbed from the gastro-intestinal tract and soluble iron salts injected in small amounts are transported by the iron-binding protein of the serum and stored predominantly in the liver. Colloidal iron given intravenously is taken up by the reticulo-endothelial tissue. Erythrocytes appear to localize in greatest concentration in the spleen, while greater amounts of hemoglobin iron are found in the renal parenchyma. These latter differences in distribution reflect the capacity of various body tissues to assimilate different iron compounds, which while present in the plasma are not carried by the iron-binding protein.

Over a period of time an internal redistribution of iron from these various sites occurs through the serum iron compartment. The liver becomes progressively loaded with iron. When the capacity of the liver to store iron is exceeded, the serum iron increases and secondary tissue receptors begin to fill with iron. That iron in large amounts is toxic to tissues is suggested by the occurrence of fibrosis in the organs most heavily laden with iron. This sequence of events, whether following excessive iron absorption or parenteral iron administration is believed to be responsible for the clinical and pathologic picture of hemochromatosis.