Pepsin action

Pepsin, action on polypeptides, 46. 1-Phenylalanyl glycine, 27. Phenylcarbamino-glycine, 13. 

Table 2. Kinetics of pepsin action on synthetic substrates of the type Carbobenzoxy-His-Phe-X-OMe at pH 4.0 and 37 C...

Fruton JS, Bergmann M. The specificity of pepsin action. Science 87 557, 1938 idem. The specificity of pepsin. J Biol Chem 127 627-641, 1939 Fruton JS, Fujii S, Knap-penberger MH. The mechanism of pepsin actions. Proc Natl Acad Sci USA 47 759-761, 1961 Fruton JS. Enzymic hydrolysis and synthesis of peptide bonds. Harvey Lectures 51 64-87, 1957. 

SPECIFICITY AND MECHANISM OF PEPSIN ACTION ON SYNTHETIC SUBSTRATES... 

The available data on the primary specificity of pepsin may be summarized in terms of a strong preference for two aromatic L-amino acid residues forming the sensitive X-Y bond, the best substrates being those in which X= Phe and Y = Trp, Tyr, or Phe. The introduction of a Phe(4N02) residue in the X-position did not alter markedly the kinetic parameters of pepsin action this permitted the development of a spectrophotometric method for following the hydrolysis of the Phe(N02)-Phe bond (6). In contrast to the widely used analytical procedures for estimating the rate of formation of the amine product (e.g., Phe-OMe) by means of its reaction with ninhydrin or fluorescamine, this method measures the rate of formation of the acidic product e.g., Z-His-Phe(4N02). 

In addition to the nature of the amino acid chains and the configuration of the dipeptidyl unit providing the sensitive bond, the primary specificity of pepsin appears to involve a requirement for both the N-terminal NH group and the C-terminal CO group of the dipeptidyl unit. This inference is drawn from the finding that 0-acetyl-3-phenyl-L-lactyl-L-Phenylalanine (12) and Z-His-Phe-L-phenylalaninol (6) are resistant to pepsin action under conditions where Ac-Phe-Phe and Z-His-Phe-Phe-OMe are readily cleaved by the enzyme. 

In addition to the types of synthetic substrate for pepsin mentioned above, others include depsipeptides such as Z-His-Phe(4N02)-Pla-OMe, whose rapid hydrolysis showed that pepsin can act as an esterase provided the primary specificity requirements are met (30), sulfite esters such as bis-p-nitrophenyl sulfite (31,32) which are also cleaved rapidly, and various more resistant compounds such as Tfa-Phe (33) and Leu-Tyr-Leu or Leu-Tyr-NH2 (34), whose mode of cleavage by pepsin has also suggested various hypotheses about the mechanism of pepsin action. 

Concerning the mechanism of pepsin action itself, it is expedient to focus on three problems (1) what is the cause of the known pH-dependence of pepsin catalysis (2) are there amino-enzyme and acyl-enzyme intermediates in the pathway of pepsin action and (3) what is the chemical nature of the intermediates. 

Fruton, J., 1976. The mechanism of die catalytic action of pepsin and related acid proteina.ses. Advances in Enzymology 44 1-36. 

These drug inhibit die action of histamine at histamine H2 receptor cells of die stomach, which then reduces die secretion of gastric acid and reduces total pepsin output. The decrease in acid allows the ulcerated areas to heal. Examples of histamine H2 antagonists include cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid Pulvules), ranitidine (Zantac). 

Mucus is produced by the mucus neck cells and by the surface epithelial cells of the stomach wall. A thick layer of mucus adheres to the wall of the stomach, forming the gastric mucosal barrier. The function of this barrier is to protect the gastric mucosa from injury — specifically, from the corrosive actions of HCl and pepsin. Together with bicarbonate ion released into the lumen of the stomach, mucus neutralizes the acid and maintains the mucosal surface at a nearly neutral pH. 

The problem to be solved with respect to the chemical reactions that constitute metabolism and sustain life is that, without the action of catalysts, they are far too slow. Let s consider the digestion of the proteins themselves, an important constituent of our diet. In an enviromnent similar to that of our digestive system, several tens of thousand years would be required to digest half of the protein content of a typical meal in the absence of a catalyst. Clearly, this will not do. In reality, the stomach secretes one protein catalyst, the enzyme pepsin, and the pancreas secretes several enzymes that catalyze the digestion of proteins. In the presence of these enzymes, dietary proteins are fully digested and reduced to their basic constituents, the amino acids, in a matter of hours. Obviously, these enzymes are enormously potent catalysts." ... 

This aspartic proteinase [EC 3.4.23.22], from the ascomy-cete Endothia parasitica, catalyzes the hydrolysis of proteins with broad specificity similar to that of pepsin A, with preferential action on substrates containing hydrophobic residues at PI and PI. ... 

The barrier formed at the ulcer site protects the ulcer from the potential ulcerogenic properties of pepsin, acid, and bile, thus allowing the ulcer to heal. Pharmacokinetics Sucralfate is minimally absorbed from the Gl tract following an oral dose. The duration of action depends on the time that the drug is in contact with this site. Binding to the ulcer site has been shown for up to 6 hours. Approximately 95% of the dose remains in the Gl tract. 

The question at once arose whether this a-pyrrolidine carboxylic acid, or a-proline as Fischer termed it in 1904, was a primary product or a secondary product formed by the action of mineral acids upon other products, but its formation by hydrolysis by alkali and by the action of pepsin followed by trypsin decided that it was a primary product and therefore one of the units of the protein molecule. Sorensen, in 1905, suggested that it might arise from an a-amino-S-oxyvalerianic acid which he synthesised, but the fact that this amino acid has not yet been obtained by hydrolysis of protein and the above facts seem to exclude this possibility. 

We have approached this problem by studying the interactions between pepsin and ketones with structures based on that of pepstatin. Our strategy was to design ketones which would serve as pseudosubstrates, that is, be subject to the catalytic action of the enzyme, but only to the point of formation of a tetrahedral intermediate which, because of the increased stability of a C-C vs a C-N bond, would not break down to products. Such a stable tetrahedral intermediate would then, in principle, be amenable to study by the appropriate physical methods. appeared to be an ideal method since changes... 

Mechanism of Action An antiulcer agent that forms an ulcer-adherent complex with proteinaceous exudate, such as albumin, at ulcer site. Also forms a viscous, adhesive barrier on the surface of intact mucosa of the stomach or duodenum. Therapeutic Effect Protects damaged mucosa from further destruction by absorbing gastric acid, pepsin, and bile salts. 

The use of enzymes for the manufacture of leather played a major role for the industrial scale production of enzymes. For the preparation of hides and skins for tanning, the early tanners kept the dehaired skins in a warm suspension of the dungs of dogs of birds. Wood was the first in 1898 to show that the bating action of the unpleasant dungs was caused by the enzymes (pepsin, trypsin, lipase) which they contained. In the context... 

It precipitates surface proteins at ulcer base and act as a physical barrier, preventing acid, pepsin and bile from coming in contact with ulcer base. It also augments gastric mucosal PG synthesis thereby enhancing protective action. It has no acid neutralizing action. 

Each kinin is formed from a kininogen by the action of a different enzyme. Bradykinin is released by plasma kallikrein, lysylbradykinin by tissue kallikrein, and methionyllysylbradykinin by pepsin and pepsin-like enzymes. The three kinins have been found in plasma and urine. Bradykinin is the predominant kinin in plasma, whereas lysylbradykinin is the major urinary form. 

The chemical action of these substances includes neutralization of gastric HCI, which in turn affects the action of pepsin in the stomach and the binding of intestinal phosphate. Laxatives also stimulate the muscles of the lower bowel as well as absorb water themselves. All of the drugs listed above decrease the Gl absorption of other drugs and therefore should not be taken with other drugs. 

The conditions necessary Tor the plastein reaction have been reviewed by Fujimaki et al. ( ), and compared to those necessary for proteolysis by Arai et al. ( ). The substrate for the synthetic reaction must consist of low molecular weight peptides, preferably in the tetramer to hexamer range. These are usually produced from proteins by protease action. A number of proteolytic enzymes and protein substrates have been investigated for producing plastein reaction substrates. The most often used proteases are pepsin JJ), and papain (12,13), but others... 

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