Brush-border enzyme

Wali RK, Singh R, Dudeja PK, et al. 1982. Effect of a single oral dose of endosulfan on intestinal uptake of nutrients and on brush-border enzymes in rats. Toxicol Lett 12 7-12. 

The intestinal mucosal peptidases are distributed in the brush border and cytosol of the absorptive cell. There are, however, distinct differences between the brush border and cytosolic peptidases [75], The tetrapeptidase activity is associated exclusively with the brush border enzyme. Furthermore, brush border peptidases exhibit more activity against tripeptides than dipeptides, whereas the cytosolic enzymes show greater activity against dipeptides. Studies have demonstrated that more than 50% of dipeptidase activity was detected in the cytosol [76] and just 10% in the brush border membrane [77]. The brush border enzymes include... 

Cell monolayers grown on permeable culture inserts form confluent mono-layers with barrier properties and can be used for drug absorption experiments. The most well-known cell line for the in vitro determination of intestinal drug permeability is the human colon adenocarcinoma Caco-2 [20, 21], The utility of the Caco-2 cell line is due to its spontaneous differentiation to enterocytes under conventional cell culture conditions upon reaching confluency on a porous membrane to resemble the intestinal epithelium. This cell model displays small intestinal carriers, brush borders, villous cell model, tight junctions, and high resistance [22], Caco-2 cells express active transport systems, brush border enzymes, and phase I and II enzymes [22-24], Permeability models... 

Other cell lines used in permeability studies include the T84 human colonic adenocarcinoma colonic crypt cell model. This line has a reduced carrier expression, secrets mucus, and has very high resistance [31, 32], The IEC cell line is a rat fetal intestinal epithelium cell with higher permeabilities than Caco-2 cells [33], LLC PKi is a pig kidney epithelial cell line with low expression of efflux systems, but expression systems for transport proteins [32], 2/4/A1 cells are a conditionally immortalized rat fetal intestinal epithelium line with crypt cell-like morphology and temperature-sensitive differentiation [34], They form differentiated monolayers with tight junctions, increased brush border enzymes when grown on extracellular matrices with laminin. Transport of drugs with LP in 2/4/A1 monolayers was comparable to that in the human jejunum and up to 300 times faster than that in Caco-2 monolayers. In contrast, the permeability of HP drugs was comparable in both cell lines [34],... 

Fig. 6.11. Peptide prodrugs (6.20, 6.21, and 6.22) for the intestine-selective delivery of 5-aminosalicylic acid (6.23). The prodrugs undergo selective activation by intestinal brush border enzymes, namely aminopeptidase A and/or carboxypeptidases [39].

The brush border enzymes in the intestine play a major role in peptide hydrolysis. Thus, both aminopeptidase and endopeptidase activities were detected with [Leu5]enkephalin as the substrate [146],... 

R. Pellicciari, A. Garzon-Aburbeh, B. Natalini, M. Marinozzi, C. Clerici, G. Gentili, A. Morelli, Brush-Border-Enzyme-Mediated Intestine-Specific Drug Delivery. Amino Acid Prodrugs of 5-Aminosalicylic Acid , J. Med. Chem. 1993, 36, 4201 -4207. 

Alternatively, the pro-drug may be a substrate for brush-border enzymes of the proximal tubular cell, resulting in release of the active drug in the tubular lumen and subsequent reabsorption at distal sites or elimination in the urine. 

In general, ethanol in low to moderate amounts, is relatively benign to most body systems. A moderate amount of ethanol causes peripheral vasodilation, especially of cutaneous vessels, and stimulates the secretion of salivary and gastric fluids the latter action may aid digestion. On the other hand, ethanol consumption in high concentrations, as found in undiluted spirits, can induce hemorrhagic lesions in the duodenum, inhibit intestinal brush border enzymes, inhibit the uptake of amino acids, and limit the absorption of vitamins and minerals. In addition, ethanol can reduce blood testosterone levels, resulting in sexual dysfunction. 

Heizmann, J., et al. 1996. Enzymatic cleavage of thymopoietin oligopeptides by pancreatic and intestinal brush-border enzymes. Peptides 17 1083. 

Proteolysis of peptides and proteins commences in the stomach when pepsin is present. As a result, protein or peptide drugs will be hydrolyzed into smaller fragments like amino acids or oligopeptides, which are absorbed through the mucosa either by diffusion or by a carrier-mediated transport [18], In an average individual, 94-98% of the total protein is completely digested and absorbed [19], Proteolysis continues in the intestine with pancreatic enzymes like trypsine and brush-border enzymes. 

Figure 1.1 shows the major sites of enzyme activity in the GI tract, and we will consider each of these in turn. While most of the enzymes that hydrolyse macromolecules enter the gut in the pancreatic fluid and hence are found in the lumen of the gut, there is significant peptidase activity located on the membranes of the intestinal cells, the so-called brush border. Consideration should also be given to the enzymes that are located inside the cells of the intestinal mucosa, namely, the epithelial cells or enterocytes. This is for two reasons first, the intestinal mucosa has a turnover of 3-6 days in humans and this means that the enterocytes are constantly being sloughed-off into the lumen of the gut. Thus intracellular enzymes and brush border enzymes will be found in the lumen of the gut, though the precise quantity is difficult to assess (see later in Section 1.6). 

The lysosomal enzymes The lysosomes are membrane vesicles ubiquitous to mammalian cells and contain a panoply of hydrolytic enzymes, estimated to be over 60 in number, that function to digest practically any biological macromolecule. They are important to the discussion of oral macromolecular drug delivery for two reasons. First, any macromolecules that escape digestion by the pancreatic and brush border enzymes are likely to be taken up into the epithelial cells by the process of endocytosis. In this process, the apical membrane invaginates and the target molecules enter endocytic vesicles that then fuse with the lysosomes and are subjected to intracellular hydrolysis by the lysosomal enzymes. Second, the sloughing-off of the epithelial cells means that the lysosomal enzymes will be released into the lumen of the intestine. They may be... 

Limitations of Specific Enzymes NAG is not specific for the proximal tubule, but also increases with papillary injury and glomerular disorders. Concomitant evaluation of one of the proximal tubule specific brush border enzymes (LAP, GGT or IALP) allows correct interpretation of the NAG increase. There are also significant increases in basal excretion as a function of age in rats and gender in dogs (higher in males), so age and sex-matched controls must be used (reviewed in Emeigh Hart and Kinter 2005). 

Final digestion of dietary carbohydrates and proteins occurs on the lumen face of small intestinal enterocytes by so-caUed brush border enzymes , including maltase, sucrose, lactase and peptidases. 

Enterokinase, a brush-border enzyme, activates trypsinogen into trypsin, which in tnm converts a number of precursor pancreatic proteases into their active forms. 

Malabsorption is defined as an inadequate assimilation of dietary substances due to defects in digestion, absorption or transport. Malabsorption can affect macronutrients (proteins, carbohydrates, fats), micronutrients (vitamins, minerals) or both, causing excessive faecal excretion and producing nutritional deficiencies and GI symptoms. Digestion and absorption occur in three phases, namely (i) the intra-lumen hydrolysis of fats, proteins and carbohydrates by enzymes, and emulsification by bile salts, (ii) digestion by brush-border enzymes and uptake of end-products and (iii) lymphatic transport of nutrients. Malabsorption can occur when any of these phases is impaired. 

Enzymuria has been reported after the intravascular administration of high-osmolar or low-osmolar contrast media (180). The study suggested that the brush-border enzyme gamma-glutamyl transpeptidase is a better marker for tubular toxicity due to contrast media than alanine aminopeptidase. However, no relation has been established between a reduction in glomerular filtration rate, a rise in serum creatinine (the characteristic features of contrast nephrotoxicity), and the presence of enzymuria. It has been argued that the detection of urinary enzymes is of httle importance to the clinical assessment and management of contrast medium nephrotoxicity (161). 

The effects of CPH-treatment of rats (1200 mg/ kg/d for 3d) on the polypeptide composition of renal brush border from the proximal tubule cells enzymatic activities and transport systems of the brush border membrane vesicles (BBMV) were investigated [77]. The results of these studies showed that CPH-treatment induces a 20-30% decrease in the specific activities of renal brush border enzymes leucine aminopeptidase and D-glutamyltransferase. SDS-gel electrophoresis showed that CPH-treatment induced a decrease of the intensity of 3 brush border polypeptides of molecular weights of 72,000, 58,000 and 39,000 [77]. 

Not digested by brush border enzymes of the small intestine, pancreatic juices and enzymes... 

Imipenem, a carbapenem antimicrobial, also possesses nephrotoxic potential. In animal models, nephrotoxicity is dose dependent and characterized by tubular necrosis. Interestingly, imipenem nephrotoxicity is markedly attenuated by co-administration of cilastatin, an inhibitor of the cytosolic and brush border enzyme dehydropeptidase I (DHP). Although DHP is responsible for hydrolyzing imipenem to inactive metabolites, the major protective effect of cilastatin appears to be due to inhibition of renal imipenem accumulation rather than DHP inhibition. 

The brush border enzymes with disaccharidase and ohgosaccharidase activity are listed in Table 48-1. The sucrase-isomaltase complex comprises most of the sucrase, isomaltase, and maltase (80%) activity of the small intestine. It hydrolyzes sucrose to its constituent monosaccharides, cleaves glucose from a-limit dextrins with 1,6 bonds, and hydrolyzes maltose. The activity of the complex is fourfold to fivefold greater in the jejunum than in the ileum. Changes in diet have a marked effect on the expression of the complex starvation leads to a rapid decline in activity, which is rapidly restored on refeeding. AH small intestinal saccha-ridases may decrease with infection or inflammation of the small bowel to the extent that carbohydrate malabsorption... 

In addition to their actions on disaccharides, the brush border enzymes further hydrolyze the products of amylase action, including maltose, maltotriose, and a-limit dextrins. The brush border enzymes appear to act in an integrated manner in that there is a flow of substrate from glucoamy-lase and isomaltase to sucrase with the production of the monosaccharides glucose, galactose, and fructose. These monosaccharides are transported into the ehterocyte by... 

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