Pancreas proteases

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. 

Pancreatic secretion for many, if not most, species is regulated in order to insure adequate protein digestion. Correspondingly, protease inhibitors have a greater impact on pancreatic secretion than do inhibitors of amylase and lipase (Toskes, 1986). The secretory response of the exocrine pancreas to protease inhibitors can be rapid (< 10 min), does not involve parallel increases in the secretion of all enzymes (Holm et al., 1992), and is probably mediated by a signaling pathway (see below). 

G. M. Green, Role of Gastric Juice in Feedback Regulation of Rat Pancreatic Secretion by Luminal Proteases , Pancreas 1990, 5, 445-451. 

The incretin hormone glucagon-like peptide-1 (GLP-1) plays a crucial role in the regulation of insulin by acting on the pancreas to potentiate glucose-induced insulin secretion. GLP-1 is rapidly cleaved by serine protease dipeptidylpeptidase IV. The inhibitors of DDP-IV are new drugs for the oral treatment of type 2 diabetes. 

Exocrine pancreatic insufficiency is most commonly caused by cystic fibrosis, chronic pancreatitis, or pancreatic resection. When secretion of pancreatic enzymes falls below 10% of normal, fat and protein digestion is impaired and can lead to steatorrhea, azotorrhea, vitamin malabsorption, and weight loss. Pancreatic enzyme supplements, which contain a mixture of amylase, lipase, and proteases, are the mainstay of treatment for pancreatic enzyme insufficiency. Two major types of preparations in use are pancreatin and pancrelipase. Pancreatin is an alcohol-derived extract of hog pancreas with relatively low concentrations of lipase and proteolytic enzymes, whereas pancrelipase is an enriched preparation. On a per-weight basis, pancrelipase has approximately 12 times the lipolytic activity and more than 4 times the proteolytic activity of pancreatin. Consequently, pancreatin is no longer in common clinical use. Only pancrelipase is discussed here. 

Otsuki, M., Ohki, A., Okabayashi, Y., Suehiro, I., and Baba, S. (1987). Effect of synthetic protease inhibitor camostate on pancreatic exocrine function in rats. Pancreas, 2, 164-169. 

For some enzymes, an inactive precursor called a zymogen is cleaved to form the active enzyme. Many proteolytic enzymes (proteases) of the stomach and pancreas are regulated in this way. Chymotrypsin and trypsin are initially synthesized as chymotrypsinogen and trypsinogen (Fig. 6-33). Specific cleavage causes conformational changes that expose the enzyme active site. Because this type of activation is irreversible, other... 

Minkowski tried unsuccessfully to prepare an extract of dog pancreas that would reverse the effect of removing the pancreas—that is, would lower the urinary or blood glucose levels. We now know that insulin is a protein, and that the pancreas is very rich in proteases (trypsin and chymotrypsin), normally released directly into the small intestine to aid in digestion. These proteases doubtless degraded the insulin in the pancreatic extracts in Minkowski s experiments. 

Cleavage of dietary protein by proteases from the pancreas. The peptide bonds susceptible to hydrolysis are shown for each of the five major pancreatic proteases. [Note Enteropeptidase is synthesized in the intestine.]... 

In the small intestine, proteases released by the pancreas as zymogens become active. Each has a different specificity for the amino acid R-groups adjacent to the susceptible peptide bond. Examples of these enzymes are trypsin, chymotrypsin, elastase, and car-boxypeptidase A and B. The resulting oligopeptides are cleaved by aminopeptidase found on the luminal surface of the intestine. Free amino acids and dipeptides are then absorbed by the intestinal epithelial cells. 

The digestive enzymes trypsin, chymotrypsin, elastase, and proteinase E are related serine proteases. All three are synthesized in the pancreas which secretes 5-10 g per day of proteins, mostly the inactive proenzymes (zymogens) of digestive enzymes.191,192... 

Some of the serine proteases are stored in the pancreas as inactive precursors that may be activated by proteolysis. Trypsinogen, for example, is converted to trypsin by the removal of the N-terminal hexapeptide on the cleavage of the bond between Lys-6 and Ile-7 by enterokinase. Chymotrypsinogen is activated by the tryptic cleavage of the bond between Arg-15 and He-16. (In this case, further proteolysis by the chymotrypsin that is released during the activation leads to the different forms of the enzyme—Figure 16.5.)... 

The serine proteases are a large family of proteolytic ( enzymes that use the reaction mechanism for nucleophilic catalysis outlined in equations (3) and (4), with a serine residue as the reactive nucleophile. The best known members of the family are three closely related digestive enzymes trypsin, chymotrypsin, and elastase. These enzymes are synthesized in the mammalian pancreas as inactive precursors termed zymogens. They are secreted into the small intestine, where they are activated by proteolytic cleavage in a manner discussed in chapter 9. 

In chapter 8, we mentioned that the pancreas secretes trypsin, chymotrypsin, and elastase as inactive zymogens, which are activated by extracellular proteases. Trypsin is activated when the intestinal enzyme enter-opeptidase cuts off an N-terminal hexapeptide. Trypsin in turn activates chymotrypsin by cutting it at the N-terminal end between Arg 15 and lie 16. This type of change in the covalent structure of an enzyme is termed partial proteoly-sis. Delaying the activation prevents the digestive enzymes from destroying the pancreatic cells in which they are synthesized. 

Protein catabolism begins with hydrolysis of the covalent peptide bonds that link successive amino acid residues in a polypeptide chain (fig. 22.3). This process is termed proteolysis, and the enzymes responsible for the action are called proteases. In humans and many other animals, proteolysis occurs in the gastrointestinal tract this type of proteolysis results from proteases secreted by the stomach, pancreas, and small intestine. 

However, it seems that empirical trials are still necessary in order to achieve a successful synthesis several enzymes from different natural sources should be tested, and even enzymes having different specificities. For laboratory-scale preparations, the cost of such enzymes as the lipases from porcine pancreas (PPL), Candida cylindracea (CCL), and Chromobacterium viscosum (CVL), and Protease N (Amano) is negligible. Subtilisin, a protease, is much more expensive. 

Hydrolytic enzymes Pancreatin Hydrolysis of starch (amylase), fat (lipase), and protein (protease) Porcine pancreas Digestive aid... 

Bovine lung or mucous QAE Sephadex A50 Proteases of Bacillus subtilis Proteolytic enzymes from pig pancreas... 

The iotai proteolytic activity of pancreas powder is determined by comparing the quantity of peptides nonprecipitabie by a 556 m/V solution of trichloroacetic acid R released per minute from a substrate of casein solution with the quantity of such peptides released by pancreas powder (protease) UK from the same substrate in the same conditions. For the test suspension and the reference suspen-sion, prepare the suspension and carry out tiie dilution at (W-°C. 

Trypsin is typically considered an enzyme found in the pancreas and small intestine. However, trypsin, thrombin, and plasmin are also widely expressed in endothelium, including epithelial immune cells as well as neurons. Upregulated expression and release occurs during both acute and chronic inflammation [60], Autocrine release of trypsin and thrombin causes activation of protease-activated receptors (PARs) reaction leading to cellular proliferation and inflammation [4], This response includes release of proteins by all cells during chronic inflammation. Bik prevents PAR activation on cell surfaces. 

---