TAILIEUCHUNG - Chapter 100. Megaloblastic Anemias (Part 2)

IF is produced in the gastric parietal cells of the fundus and body of the stomach, and its secretion parallels that of hydrochloric acid. The IF-cobalamin complex passes to the ileum, where IF attaches to a specific receptor (cubilin) on the microvillus membrane of the enterocytes. Cubilin is also present in yolk sac and renal proximal tubular epithelium. Cubulin appears to traffic by means of amnionless (AMN), an endocytic receptor protein that directs sublocalization and endocytosis of cubulin with its ligand IF-cobalamin complex. The cobalamin-IF complex enters the ileal cell where IF is destroyed. After a delay of about. | Chapter 100. Megaloblastic Anemias Part 2 IF is produced in the gastric parietal cells of the fundus and body of the stomach and its secretion parallels that of hydrochloric acid. The IF-cobalamin complex passes to the ileum where IF attaches to a specific receptor cubilin on the microvillus membrane of the enterocytes. Cubilin is also present in yolk sac and renal proximal tubular epithelium. Cubulin appears to traffic by means of amnionless AMN an endocytic receptor protein that directs sublocalization and endocytosis of cubulin with its ligand IF-cobalamin complex. The cobalamin-IF complex enters the ileal cell where IF is destroyed. After a delay of about 6 h the cobalamin appears in portal blood attached to transcobalamin TC II. Between and pg of cobalamin enters the bile each day. This binds to IF and a major portion of biliary cobalamin is normally reabsorbed together with cobalamin derived from sloughed intestinal cells. Because of the appreciable amount of cobalamin undergoing enterohepatic circulation cobalamin deficiency develops more rapidly in individuals who malabsorb cobalamin than it does in vegans in whom reabsorption of biliary cobalamin is intact. Transport Two main cobalamin transport proteins exist in human plasma they both bind cobalamin one molecule for one molecule. One HC known as TC I is closely related to other cobalamin-binding HCs in milk gastric juice bile saliva and other fluids. These HCs differ from each other only in the carbohydrate moiety of the molecule. TC I is derived primarily from the specific granules in neutrophils. Normally it is about two-thirds saturated with cobalamin which it binds tightly. TC I does not enhance cobalamin entry into tissues. Glycoprotein receptors on liver cells are involved in the removal of TC I from plasma and TC I may have a role in the transport of cobalamin analogues to the liver for excretion in bile. The other major cobalamin transport protein in plasma is TC II. This is synthesized by

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