Mucosal cells, intestinal

Lymphocytes, inflammatory cells, intestinal mucosal cells, cartilage cells and bone precursor cells Inhibition of proliferation... 

There are two main classes of proteolytic digestive enzymes (proteases), with different specificities for the amino acids forming the peptide bond to be hydrolyzed. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, fi"om the ends of polypeptides. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from rhe free carboxyl terminal, and aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. 

Although iron deficiency is a common problem, about 10% of the population are genetically at risk of iron overload (hemochromatosis), and elemental iron can lead to nonen2ymic generation of free radicals. Absorption of iron is stricdy regulated. Inorganic iron is accumulated in intestinal mucosal cells bound to an intracellular protein, ferritin. Once the ferritin in the cell is saturated with iron, no more can enter. Iron can only leave the mucosal cell if there is transferrin in plasma to bind to. Once transferrin is saturated with iron, any that has accumulated in the mucosal cells will be lost when the cells are shed. As a result of this mucosal barrier, only about 10% of dietary iron is normally absorbed and only 1-5% from many plant foods. 

Fluxes of iron from the plasma towards BM and other tissues can be quantified by ferrokinetic studies, using 59Fe and sophisticated computer models (Ricketts et ah, 1975 Ricketts and Cavill, 1978 Barosi et ah, 1978 Stefanelli et ah, 1980). Plasma iron turnover (PIT), erythroid iron turnover (EIT), non-erythroid iron turnover (NEIT), marrow iron turnover (MIT), and tissue iron turnover (TIT) could be calculated in many disorders of iron metabolism and in all kinds of anaemias. Iron is rapidly cleared from the plasma in iron deficiency and in haemolytic anaemias. If more iron is needed for erythropoiesis, more transferrin receptors (TfR) are expressed on erythroblasts, resulting in an increased flux of iron from intestinal mucosal cells towards the plasma. In haemolytic anaemias MPS, and subsequently hepatocytes, are overloaded. In hereditary haemochromatosis too much iron is absorbed by an intrinsic defect of gut mucosal cells. As this iron is not needed for erythropoiesis,... 

The peculiar thing in hereditary haemochromatosis (HH) is that the intestinal mucosal cell behaves essentially like an iron deficient cell. Iron absorption is always high if related to the body s iron needs. In HH subjects with normal plasma ferritin values, both mucosal uptake and mucosal transfer of iron often exceed values found in patients with uncomplicated iron deficiency (Marx, 1979b). In fact the situation with respect to iron absorption in mature intestinal mucosal cells, as depicted in Figure 9.4(b), is identical to that in iron deficiency, except for the difference in plasma iron saturation. It was already known that mucosal cells in HH contain no ferritin, explaining the high mucosal transfer of iron (Francanzani... 

Figure 7.12 Schematic representation of iron absorption in mature intestinal mucosal cells. (From Crichton, 2001. Reproduced with permission from John Wiley Sons., Inc.)...

Intrinsic factor is a glycopeptide secreted by cells in the pyloric region of the stomach, which is needed for the translocation of the very large vitamin Bi2 molecule across the intestinal mucosal cell membranes. 

Vibrio cholera exotoxin ADP-ribosylates G O, leading to an increase in cAMP and subsequently chloride secretion bom intestinal mucosal cells, causing the diarrhea of cholera. 

Excretion - The daily loss of iron from urine, sweat, and sloughing of intestinal mucosal cells amounts to approximately 0.5 to 1 mg in healthy men. In menstruating women, approximately 1 to 2 mg is the normal daily loss. 

All pathogenic E. coli stains follow a similar strategy of infection by colonizing the intestinal mucosal cells. The mode in which illness occurs varies between the different pathogenic E. coli types. ETEC and EaggEC produce enterotoxin, EIEC invades the epithelial cells with EPEC and EHEC adhering to the cell and modifying cellular activity. 

Disaccharides are cleaved to monosaccharides by a battery of disaccharidases after absorption into intestinal mucosal cells. 

B. Uptake of monosaccharides and disaccharides by intestinal mucosal cells is mediated by a variety of transporters. 

Iron is transported via transferrin. When body stores of iron are high, ferric iron combines with apoferritin to form ferritin. Ferritin is the protein of iron storage. About 80 percent iron in plasma goes to erythroid marrow. The excretion of iron is minimal. Only little amount of iron is lost by exfoliation of intestinal mucosal cells and trace amount is excreted in urine, sweat and bile. 

Iron crosses the luminal membrane of the intestinal mucosal cell by two mechanisms active transport of ferrous iron and absorption of iron complexed with heme (Figure 33-1). The divalent metal transporter, DMT1, efficiently transports ferrous iron across the luminal membrane of the intestinal enterocyte. The rate of iron uptake is regulated by mucosal cell iron stores such that more iron is transported when stores are low. Together with iron split from absorbed heme, the newly absorbed iron can be actively transported into the blood across the basolateral membrane by a transporter known... 

In addition to the storage of iron in intestinal mucosal cells, iron is also stored, primarily as ferritin, in macrophages in the liver, spleen, and bone, and in parenchymal liver cells (Figure 33-1). Apoferritin synthesis is regulated by the levels of free iron. When these levels are low, apoferritin synthesis is inhibited and the balance of iron binding shifts toward transferrin. When free iron levels are high, more apoferritin is produced to sequester more iron and protect organs from the toxic effects of excess free iron. 

There is no mechanism for excretion of iron. Small amounts are lost in the feces by exfoliation of intestinal mucosal cells, and trace amounts are excreted in bile, urine, and sweat. These losses account for no more than 1 mg of iron per day. Because the body s ability to excrete iron is so limited, regulation of iron balance must be achieved by changing intestinal absorption and storage of iron, in response to the body s needs. As noted below, impaired regulation of iron absorption leads to serious pathology. 

C. Final carbohydrate digestion by enzymes synthesized by the intestinal mucosal cells... 

D. Absorption of lipids by intestinal mucosal cells (enterocytes)... 

Absorption of lipids contained in a mixed micelle by an intestinal mucosal cell. 

Correct answer = A. Pancreatic lipase hydrolyzes dietary triacylglycerol primarily to 2-monoacylglycerol plus two fatty acids. These products of hydrolysis can be absorbed by the intestinal mucosal cells. Bile salts do not inhibit release of fatty acids from triacylglycerol, but rather are necessary for the proper solubilization and hydrolysis of dietary triacylglycerol in the small intestine. Short- and medium-chain length fatty acids enter the portal circulation after absorption from the small intestine. Synthesis of apolipoproteins, especially apo B-48, is essential for the assembly and secretion of chylomicrons. 

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