Proteases brush border

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. 

Riepe S, Goldstein J, Alpers DH Effect of secreted Bacteroides proteases on human intestinal brush border hydrolases. J Clin Invest 1980 66 314-322. 

In addition, renal tubular cells contain various proteases for the degradation of proteins and oligopeptides. These enzymes are located predominantly in the lysosomes and micro-somes of these cells, but some have been reported on the brush-border membranes [16]. Degradative enzymes include various endopeptidases, exopeptidases and esterases [17]. 

Therapeutic peptides, proteins, and peptidomimetics are getting increasingly important. These compounds are degraded in the first instance by luminally secreted and brush border membrane-bound proteases. Because of their hydrophilic character and comparatively large size, they are usually uptaken via the paracellular route after oral administration. 

Regardless of toxic risks, various inhibitors of pancreatic and brush border membrane-bound proteases are listed in Table 5.2 and Table 5.3, respectively. Beside this overview, a classification of inhibitory agents based on their chemical structure is shown below. 

Mucoadhesive polymers exhibiting strong complexing properties are capable of inhibiting intestinal brush border membrane-bound proteases through a far distance inhibitory effect [65]. In vivo, the mucoadhesive polymer is separated from the brush border membrane by a mucus layer [30]. Although there is no direct contact between polymer- and membrane-bound enzymes, it could be shown that inhibition takes place. The exploitation of this far distance effect seems to be a very promising alternative to small molecular mass inhibitors, which are currently used as inhibitors of brush border membrane-bound proteases. 

Pancreatic amylase hydrolyses starches to maltose and oligosaccharides. Final digestion of carbohydrates takes place in the brush border of the epithelial cells. Oligosaccharides are hydrolysed to monosaccharides and reabsorbed. Proteins are hydrolysed to peptides by pepsin, trypsin and several other proteolytic enzymes from the pancreas. The brush border attaches proteases which hydrolyse the di- and tripeptides to amino acids and absorbs them. 

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

The zymogen trypsinogen is cleaved to form trypsin by enteropeptidase (a protease, formerly called enterokinase) secreted by the brush-border cells of the small intestine. Trypsin catalyzes the cleavages that convert chymotrypsinogen to the active enzyme chymotrypsin, proelastase to elastase, and the procarboxypeptidases to the car-boxypeptidases. Thus, trypsin plays a central role in digestion because it both cleaves dietary proteins and activates other digestive proteases produced by the panaeas. 

The jS-D-fructofuranosidase (see also sucrose a-D-glucohydrolases) released from the vesicles of human intestinal brush-border membranes by various enzymes, particularly pancreatic proteases, has been studied. The results were discussed in terms of the location and chemical binding of the jS-o-fructofuranosidase. 

Macrocyclization can potentially have an even greater impact on oral bioavailability by increasing the fraction absorbed (Fg). For peptides, cyclization can reduce or eliminate susceptibility to proteolytic degradation by gastric, intestinal, or brush border proteases,thereby significantly increasing the fraction of peptides available for permeation. Table 10.2 shows the stability of two cyclic peptides in brush border membrane vesicles. Cyclic peptide 6 is only 25% degraded after 90 minutes. N-methylation... 

GI stability (For peptides) provide resistance to gastric, intestinal, and brush border proteases 46... 

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