Enterocytes plasma membrane

Transport of amino acids into cells is mediated by specific membrane-bound transport proteins, several of which have been identified in mammalian cells. They differ in their specificity for the types of amino acids transported and in whether the transport process is linked to the movement of Na+ across the plasma membrane. (Recall that the gradient created by the active transport of Na+ can move molecules across membrane. Na+-dependent amino acid transport is similar to that observed in the glucose transport process illustrated in Figure 11.28.) For example, several Na+-dependent transport systems have been identified within the lumenal plasma membrane of enterocytes. Na+-independent transport systems are responsible for transporting amino acids across the portion of enterocyte plasma membrane in contact with blood vessels. The y-glutamyl cycle (Section 14.3) is believed to assist in transporting some amino acids into specific tissues (i.e., brain, intestine, and kidney). 

The binding of enterotoxin (produced by several bacterial species) to another type of guanylate cyclase found in the plasma membrane of intestinal cells causes diarrhea. For example, one form of traveler s diarrhea is caused by a strain of E. coli that produces heat stable enterotoxin. The binding of this toxin to an enterocyte plasma membrane receptor linked to guanylate cyclase triggers excessive secretion of electrolytes and water into the lumen of the small intestine. 

The mechanism of DHAA uptake by luminal membranes of human jejunum has pharmacological characteristics that clearly differ from those of ascorbate uptake. Sodium-independent carriers take up DHAA by facilitated diffusion, and these are distinct from the sodium-dependent transporters of ascorbate. Glucose inhibits ascorbate uptake but not DHAA uptake, which raises the possibility that glucose derived from food may increase the bioavailability of DHAA relative to ascorbate (Malo and Wilson, 2000). Human enterocytes contain reductases that convert DHAA to ascorbate (Buffinton and Doe, 1995). This conversion keeps the intracellular level of DHAA low, and the resulting concentration gradient favors uptake of oxidized AA across the enterocyte plasma membrane. 

Bundles of parallel actin filaments with uniform polarity. The microvilli of intestinal epithelial cells (enterocytes) are packed with actin filaments that are attached to the overlying plasma membrane through a complex composed of a 110-kD protein and calmodulin. The actin filaments are attached to each other through fimbrin (68 kD) and villin (95 kD). The actin bundles that emerge out of the roots of microvilli disperse horizontally to form a filamentous complex, the terminal web, in which several cytoskeletal proteins, spectrin (fodrin), myosin, actinin, and tropomyosin are present. Actin in the terminal web also forms a peripheral ring, which is associated with the plasma membrane on the lateral surfaces of the enterocyte (see Figure 5, p. 24). 

The third mucosal layer is that lining the entire length of the small intestine and which represents a continuous sheet of epithelial cells. These epithelial cells (or enterocytes) are columnar in shape, and the luminal cell membrane, upon which the microvilli reside, is called the apical cell membrane. Opposite this membrane is the basal (or basolateral) plasma membrane, which is separated from the lamina propria by a basement membrane. A sketch of this cell is shown in Fig. 5. The primary function of the villi is absorption. 

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

Final hydrolysis of di- and oligosaccharides is carried out by surface enzymes of the small intestinal epithelial cells, called the brush border, a term that comes from the appearance of the enterocytes, in which the luminal plasma membrane is enlarged by a regular array of projections called microvilli. The enzymes are not secreted into the lumen, but are embedded in the cell membrane, many of these enzymes can protrude into the intestinal lumen up to 10 gm, as they are attached to the plasma membrane by an anchoring polypeptide that has no role by itself in the hydrolysis. 

A variety of membrane-bound proteins are of vital interest to the medical and nutritional scientist, because defects or changes in these proteins can cause such problems as lactose intolerance, cardiovascular disease, cystic fibrosis, and diabetes. Sucrase-isomaltase, an enz)mie of the small intestine, is a membrane-bound protein, bound to the plasma membrane of the enterocyte (gut cell). Part of the production of this enzyme is depicted in Figure 1.26. In Step 1, the pol) ptide chain is polymerized on the ribosome (shown in black). In Step 2, part of the amino acid chain near the N terminus crosses the membrane of the ER into the lumen but some of the amino acids at the N terminus remain outside. Step 3 shows the protein assuming a three-dimensional shape within the lumen both the C and N... 

FIGURE 2.23 The absorption of vitamin B12, an event dependent on intrinsic factor. The absorption of vitamin B12 from the gut and its delivery to various cells of the body involves the following steps. (1) Vitamin B12 is liberated from meat. (2) Intrinsic factor is secreted from parietal cells in the stomach. (3) A tight complex between intrinsic factor and the vitamin is formed. (4) The complex travels to the ileum, where the vitamin dissociates from the intrinsic factor at the membrane of the enterocyte. (5) Transcobalamin, a protein circulating in tire blood, forms a complex with the vitamin. (6) The complex travels to various organs, diffuses from the capillary to the plasma membrane of a cell, and is boimd by receptor proteins. The complex is then taken into the cell by endocytosis. 

The ZIP family are involved in Zn transport into the cytosol, mostly across the plasma membrane. Although the human genome encodes 14 ZIP-related proteins, Z1P4 appears to mediate Zn uptake. It s involvement in dietary Zn uptake into intestinal enterocytes is well estabhshed, and mutations in Z1P4 have been found in patients with acrodermatitis enteropathica, a recessive disorder of Zn absorption which results in Zn deficiency. DMTl is probably also involved in the transport of dietary zinc across the brush border membrane of the intestine. 

Coupled transport systems frequently exhibit an asymmetric localization within plasma membranes. In enterocytes, the Na + -dependent glucose transporter and the a -dependent amino acid uptake systems are localized in apical (luminal) membrane, whereas the Na + / K+-ATPase is localized within the basolateral (blood-sided) membrane. Thus, the secondary active Na + - or H + -dependent transport systems are key elements for nutrient absorption, whereas subsequent transport across the basolateral membrane frequently follows the facilitated diffusion. 

The isolated brush border vesicles from the plasma membrane of the microvilU is the simplest in vitro system used so far. The interaction of lipid with rabbit intestinal brush border vesicles has been investigated by Proulx et al. [50] who found that PC, phosphatidylethanolamine, cholesterol, diglyceride as well as fatty acids were taken up by vesicles. Barsukov et al. [51] have shown that transfer of PC from PC vesicles to isolated brush border vesicles can occur in the presence of PC-exchange protein. The use of brush border vesicles is an interesting new approach that permits detailed studies of rate of transfer of specific lipids into the plasma membrane of the enterocyte. The model is seriously hmited by the fact that incubation with solutions containing bile salts at a concentration above the critical micellar concentration will result in partial or total solubilization of the membrane vesicles. 

The enterocytes of the small intestine can be isolated and used for study of intracellular aspects of intestinal lipid transport like triglyceride synthesis [52]. The disappearance of the mucus barrier during isolation of the epithelial cells results in plasma membrane disintegration and loss of cellular integrity when the cells are exposed to bile salts. As is the case for brush border vesicles, the system of isolated cells does not allow study of interaction between enterocytes and lipids dispersed in a form that resembles physiological conditions, i.e. solubilized in mixed bile salt micelles. 

In summary, a lipid molecule on its route from the luminal bulk phase into the intracellular compartment of an enterocyte has to overcome two unstirred water layers and one plasma membrane of lipid bilayer structure. The unstirred water layer on the luminal side partly coincides with the mucus gel and the glycocalyx relatively little is known of the importance of these diffusional barriers. 

The uptake of lipids across the brush border plasma membrane into the enterocyte is considered to be a transport process that requires no energy [8], The route of transfer of lipids thus has to take place via a process of passive diffusion. A theoretical model for passive lipid solute transfer that takes into account the different factors that affect the rate of transport has been worked out by Dietschy and collaborators [8,68] (cf. Chapter 5). 

The factors that are mainly responsible for the relative rate of uptake of a particular lipid are the resistance of the luminal unstirred water layer and the permeability of the plasma membrane of the enterocyte. Depending on the properties of a specific lipid, the relative importance of these two factors can be predicted. If a lipid is rapidly transported across the luminal unstirred water layer, i.e. it has a relatively high aqueous diffusion constant, then the permeability of the membrane will be the key factor determining the rate of transport into the cytosol. The concentration gradient will be high across the lipid membrane, whereas the con-... 

Fig. 1. Profile of the concentration gradient (C,-C4) from the luminal bulk phase (L) across the brush border plasma membrane (M) to the intracellular compartment (IC) of the enterocyte. Adjacent to the membrane, on both the luminal and the intracellular side there is an unstirred water layer (UWL). It should be noted that this diagram does not attempt to present the relative dimensions of the two unstirred water layers and the plasma membrane. The concentration gradient will have a different appearance in the case of a lipid towards which the membrane permeability is low (panel A) compared to the case where the resistance of the unstirred water layer against diff.ision of the lipid is high while the lipid readily transverses the plasma membrane (panel B). After Thomson and Dietschy [8).

NPCILI is a protein with 42% identity with NPCl (J.P. Davies, 2000). Several lines of evidence indicate that NPCILI plays a role in the intestinal absorption of cholesterol and plant sterols (S.W. Altmann, 2004) [22]. NPCILI is found on the plasma membrane of enterocytes in the proximal jejunum. NPCILI homozygous knockout mice absorb significantly less cholesterol than control mice and are insensitive to the effects of ezetimibe, a cholesterol absorption inhibitor that lowers plasma LDL cholesterol levels. 

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