Gastrointestinal physiology
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2015
Proteins found in the intestines are derived from endogenous sources (secretory proteins and desquamated cells) and exogenous proteins (dietary protein). Although 10% −15% of protein in the gastrointestinal tract is digested by gastric pepsin, protein digestion products in the stomach are important because they stimulate the secretion of proteases by the pancreas. Most protein digestion results from the actions of pancreatic proteolytic enzymes. Trypsinogen and chymotrypsinogen are activated by enterokinase or by autocatalysis with trypsin. Tryspin and chymotrypsin, which are endopeptidases, cleave internal peptide linkages to produce dipeptides, tripeptides and other small peptide chains that can be absorbed by intestinal cells. Carboxypeptidase (exopeptidase) is also produced by the pancreas and intestinal epithelial cells. Carboxypeptidases cleave the ends of a peptide chain producing free amino acids, which are readily absorbed by the intestine. The last step in the digestion of protein is achieved by enterocytes that line the villi. At the brush border, amino-peptidases split larger polypeptides into tripep-tides, dipeptides and some amino acids. These are transported into the enterocytes where multiple peptidases digest the dipeptides and tripeptides to amino acids, which then enter the blood.
The accessory organs: Pancreas, liver and gallbladder
Paul Ong, Rachel Skittrall in Gastrointestinal Nursing, 2017
A number of proteases are produced by acini cells in the pancreas. Two important ones are trypsin and chymotrypsin. As with pepsin in the stomach, these enzymes are manufactured in the inactive forms of trypsinogen and chymotrypsinogen to prevent autodigestion of pancreatic tissue. Trypsinogen is converted to the active trypsin by the enzyme enterokinase which is secreted by intestinal enterocyte epithelial cells. Once formed trypsin also has the effect of activating trypsinogen and chymotrypsinogen. Both trypsin and chymotrypsin destroy the peptide bonds linking amino acids together. This serves to break down polypeptides to peptides, therefore making peptide chains shorter, but these proteases cannot break down the peptides to the single molecule amino acids. This role is largely performed by peptidases. This is an enzyme released from the brush border membrane of the intestinal enterocyte epithelial cells.
The Modification of Tyrosine
Roger L. Lundblad, Claudia M. Noyes in Chemical Reagents for Protein Modification, 1984
The reaction of chymotrypsinogen A with diazotized arsanilic acid has been investigated.6 Diazotization of arsanilic acid is accomplished by treatment of p-arsanilic acid with nitrous acid (0.55 mM sodium nitrite in 0.15 M HCl at 0°C). After adjustment of the pH to 5.5 with NaOH the reagent is diluted to a final concentration of 0.02 M. Reaction with chymotrypsinogen is accomplished in 0.5 M sodium bicarbonate buffer, pH 8.5 with a 20-fold excess of reagent at 0°C. The reaction is terminated by the addition of a sufficient quantity of aqueous phenol (0.1 M) to react with excess reagent. The extent of the formation of monoazotyrosyl and monoazohistidyl derivatives is determined by spectral analysis.4,5 The extent of reagent incorporation is determined by atomic absorption analysis for arsenic. Tyrosine (~ 1.0 mol/mol) and lysine (~ 4 mol/mol) were the only amino acid residues modified to any significant extent under these reaction conditions. The arsaniloazo functional group provides a spectral probe that can be used to study conformational change in proteins. In this particular study, there was a substantial change in the circular dichroism spectrum during the activation of the modified chymotrypsinogen preparation by trypsin. It is of interest that the modification of chymotrypsinogen by diazotized arsanilic acid does not apparently affect either the rate of activation or amount of potential catalytic activity as judged by the hydrolysis of N-benzoyl-l-tyrosine ethyl ester.
Trypsinogen and chymotrypsinogen: potent anti-tumor agents
Published in Expert Opinion on Biological Therapy, 2021
Aitor González-Titos, Pablo Hernández-Camarero, Shivan Barungi, Juan Antonio Marchal, Julian Kenyon, Macarena Perán
Additionally, the human pancreas secretes different isoforms of Trypsinogen and Chymotrypsinogen. Three different isoforms of Trypsinogen: cationic isoform, anionic isoform and Mesotrypsinogen have been described. The prevalent form is the cationic isoform followed by the anionic isoform and finally the Mesotrypsinogen that represents less than 5% [7,23]. Cationic and anionic Trypsinogen have similar characteristics with respect to their molecular weight, amino acid composition, and optimal pH [24]. The differences between anionic and cationic isoforms involve the ability of cationic isoforms to autoactivate at an acidic pH and the higher stability of cationic Trypsinogen. On the other hand, the anionic isoform autolyzes itself faster at neutral or alkaline pH [25]. In addition, calcium ions are unable to stabilize the anionic isoform against autolysis [24]. The enzyme Mesotrypsin is characterized by its resistance to trypsin inhibitors and for promoting their degradation [26]. Specifically, it has been reported that this resistance is due to the presence of an arginine instead of glycine at position 198 [27]. Regarding Chymotrypsinogen, four different isoforms have been described: Chymotrypsinogen/Chymotrypsin B1, Chymotrypsinogen/Chymotrypsin B2, Chymotrypsinogen/Chymotrypsin C and Chymotrypsin-like protease. Chymotrypsin B1 has a preference for amino acids like Tryptophan and Tyrosine, while Chymotrypsin B2 has a preference for amino acids such as Phenylalanine and Tyrosine [28]. Chymotrypsin C preferentially cleaves the peptide bonds located in the C terminal of tyrosine, methionine and leucine and it can activate Trypsinogen by cleaving at the activation peptide between Phe-18 and Asp-19 residues [15]. Conversely, it can also cause the degradation of Trypsin by cutting into the calcium-bindingloop between the Leu-81 and Glu-82 residues which bind to Ca2+ to stabilize the protein resulting in a rapid degradation and inactivation of cationic trypsinogen. This may be a regulatory mechanism when trypsinogen is overexpressed or activated too early [29]. The specific cleavage of the Leu81-Glu82 peptide bond in human cationic trypsinogen by CTRC is primarily determined by its distinctively high activity on leucyl peptide bonds, with the P1ʹ Glu82, P3ʹ Asn84 and P4ʹ Glu85 residues serving as additional specificity determinants [28]