Which of the following involved in iron absorption is synthesized by liver and secreted through bile in the intestine?

Ans. c. (Apotransferrin). (Ref. Guyton physiology 11th ed., 425-426).Mechanism of absorption of Iron from the small Intestinal Tract# The liver secretes moderate amounts of apotransferrin into bile, which flows via bile duct into duodenum, where the apotransferrin binds with free iron and with certain iron compounds, such as hemoglobin and myoglobin from meat, two of the most important sources of iron in the diet, to form transferrin.# Trasnsferrin, in turn, is attracted to and binds with receptors in the membranes of the intestinal epithelial cells. Then, by pinocytosis, the transferrin molecule, carrying its iron store, is absorbed into the epithelial cells and later released into the blood capillaries beneath these cells in the form of plasma transferrin.# Iron is important for the formation not only of hemoglobin but also of other essential elements in the body (e.g., myoglobin, cytochromes, cytochrome oxidase, peroxidase, catalase).# The total quantity of iron in the body averages 4 to 5 grams, about 65% of which is in the form of hemoglobin. About 4% is in the form of myoglobin, 1% is in the form of the various heme compounds that promote intracellular oxidation, 0.1% is combined with the protein transferrin in the blood plasma, and 15 to 30% is stored for later use, mainly in the reticuloendothelial system and liver parenchymal cells, principally in the form of ferritin.Educational Points:# In the cytoplasm, iron combines mainly with a protein, apoferritin, to form ferritin. Apoferritin has a molecular weight of 460,000, and varying quantities of iron can combine in clusters of iron radicals with this large molecule; therefore, ferritin may contain only a small amount of iron or a large amount. This iron stored as ferritin is called storage iron.