Plasma Proteins Associated with Hemoglobin Physiology

4 PLASMA PROTEINS ASSOCIATED WITH HEMOGLOBIN PHYSIOLOGY 

Haptoglobin is a glycoprotein that combines avidly with hemoglobin released from hemolyzed red blood cells according to the equation 

Transferrin is a single-chain glycoprotein that binds 2 g-atoms of ferric iron per mole of protein. The iron is chelated via tyrosyl and histidyl residues, and the complex is extremely stable at physiologic pH. The function of transferrin is to transport iron throughout the human organism, especially to the immature red cells, which cannot effectively acquire iron for the biosynthesis of hemoglobin unless it is presented to them in combination with transferrin. Specific transferrin receptors are present on the surface of such immature red cells as well as in all other tissues. Receptor-mediated endocytosis (see Chapter 9) is believed to be the main means of transferrin-bound iron entry into cells. Transferrin is also believed to be antimicrobial because it withholds iron from microorganisms. 

Under normal circumstances, transferrin is one-fourth to one-third saturated with iron. The level of saturation may decrease in systemic infection or cancer and in iron deficiency anemia, the most common nutritional deficiency in the United States. In individuals with iron deficiency anemia, transferrin levels are increased. The level of saturation with iron increases in iron overload syndromes such as hereditary hemochromatosis or as a result of repeated blood transfusions, as is the case in thalassemia patients. Determinations of total plasma iron (TI) and plasma total iron binding capacity (TIBC) are routinely performed in the clinical biochemistry laboratory. The TIBC value reflects transferrin levels in plasma the amount of iron that can be bound by transferrin is equal to TIBC x 0.7. Total plasma iron levels in iron deficiency anemia become abnormal before hemoglobin levels show any change. 

Iron is best absorbed in the ferrous form or as heme. It is believed that transferrin, transported into the small intestine from the liver via bile, carries iron into the intestinal mucosal cells. Though about 16 mg of iron enter these cells every day, only 1-2 mg finds its way into the bloodstream. The rest remains bound to an iron storage protein called ferritin and is eventually lost in the feces in the normal sloughing-off process. Iron absorption also depends on its bioavail- 

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