Transport across the Enterocyte

As discussed for the hepatocyte it is unlikely that bile acids traverse the enterocyte as free monomers, and a binding protein has been identified. 

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Reboul, E, Klein, A, Bietrix, F, Gleize, B, Malezet-Desmoulins, C, Schneider, M, Margotat, A, Lagrost, L, Collet, X, and Borel, P, 2006. Scavenger receptor class B type I (SR-BI) is involved in vitamin E transport across the enterocyte. J Biol Chem 281, 4739 4745. 

Enzymic processes in the enterocyte which handle a number of lipid species other than triglycerides are also known to be influenced by bile salts. For example, it has been postulated that pancreatic cholesterol esterase is absorbed by the mucosal cells and catalyzes the esterification of cholesterol in the cell [87]. This postulate has been challenged by Watt and Simmonds [88] although Bhat and Brockman [30] recently produced evidence for the importance of pancreatic cholesterol esterase for transport across the enterocyte membrane. Another enzyme, acyl-CoA cholesterol acyltransfera.se (ACAT) of microsomal origin, has been ascribed a major role in mucosal esterification of cholesterol [89], Unlike pancreatic cholesterol esterase, this enzyme is inhibited by taurocholate in vitro. 

The expression of metabolic enzymes in the enterocyte can lead to a profound gut wall first-pass extraction ratio for substrate drugs. In addition, efflux transporters can slow the passage of drugs across the enterocyte in a cycling fashion. This allows the metabolic enzymes several opportunities to metabolize their substrates, and in this way a low expression level of an enzyme can exhibit a significant extraction. 

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. 

Another means of transport across the intestine is via the paracellular route, that is between the adjacent enterocytes. Water can enter the intestinal space throngh this ronte and take with it small molecnles inclnding glncose, amino acids and small peptides. This is known as solvent drag (Figure 4.7). Unfortunately, the qnantitative importance of this route is not known. 

It is well established today that drug absorption through the alimentary canal walls is a complex event, which involves, in many cases, parallel or sequent microprocesses at the apical membrane of the absorptive cell (enterocyte) or between them (paracellular absorption). In addition to the various types of diffusion processes across the enterocyte membrane, numerous specific proteins—transporters and efflux pumps—are involved in the intricate drug absorption process. In the following sections the various epithelial tissues of the different organs of the GI tract will be looked at briefly. A review of major drug absorption mechanisms across epithelial cells, as they are customary today will follow. 

Iron levels are tightly regulated through control of dietary absorption of iron. The duodenum and upper jejunum are the only areas of the body where this occurs. Since nonheme iron forms insoluble complexes when ingested, it must first be converted into soluble complexes. This is accomplished on the apical surface of duodenal villus enterocytes by duodenal ferric reductase, which converts insoluble duodenal ferric (Fe3+) iron into soluble and absorbable ferrous (Fe2+) iron. Iron is then transported across the membrane to the cytoplasm through a transporter known as the divalent metal transporter 1 (DMT-l), a proton sym-porter (Harrison and Bacon, 2003). 

Once absorbed, iron becomes part of the cellular iron pool, either stored as ferritin or transported across the basolateral membrane of the enterocyte into the circulation by an iron transporter called ferroportin 1. Hephaestin, a basolateral membrane ferroxidase, oxidizes the ferrous iron back to its ferric form, thus completing the absorption process (Harrison and Bacon, 2003). 

Iron transport across the intestinal cell occurs at both the apical and basolateral interfaces. Figure 31-1 highlights the importance of this polarity in both iron transport into the cell and the sensing of iron stores in both the villus and crypt enterocytic apical and basolateral interfaces. The apical membrane is specialized to transport heme and ferrous iron into the cell through three major pathways. The first is via DMT-l, which transports ferrous iron and divalent metal ions into the enterocyte. Iron can also be absorbed as the intact heme moiety,... 

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. 

For the absorption of carbohydrates, amino acids, and peptides, a variety of transport systems following facilitated diffusion and active mechanisms have been identified on a molecular and functional level. D-Glucose is mainly absorbed via the Na -dependent transporter SGLTl in the brush-border membrane of enterocytes [18-20]. It is transported across the basolateral membrane by facilitated diffusion via the hexose transporter GLUT-2. Besides SGLTl, the Na +-independent transport protein GLUT-5 is localized in the apical enterocyte membrane, recognizing fructose as a substrate [21]. 

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-... 

Inhibition of riboflavin transport across the Caco-2 monolayer by 2,4-dini-trophenol (DNP), reducing the cellular ATP agent, points toward the necessity of delivering energy. However, the initial phase of whole absorption process i.e. transport across the epithelium membrane) does not require a supply of metabolic energy and is a consequence of riboflavin s association with carrier protein which is presented on the brush-border membrane. In the next step, some of the riboflavin absorbed inside the enterocytes cytosol is phosphory-lated by flavokinase and converted by FAD synthetase—enzymes which require ATP molecules for their action (Gastaldi et al. 1999). 

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. 

A number of additional flavonoids have been smdied by using the Caco-2 cell model. These include the simplest form of the flavone class of flavonoids, the highly UpophUic unsubstimted flavone, which has been shown to diffuse readily across the enterocyte monolayer [45]. It also includes the highly polar hesperidin glycosides, which are suggested to be transported at a low rate via the para-cellular pathway [46]. Another citms flavonoid, 7-geranyloxycoumarin, has been shown to have a low transcellular permeation rate but was also shown to... 

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