Gastric pepsin

Histamine has long been recognized as a powerful stimulant of gastric acid secretion and, to a lesser extent, of gastric pepsin and intrinsic factor production. The effect is caused by activation of H2 receptors on gastric parietal cells and is associated with increased adenylyl... 

Enzymes are protein catalysts of remarkable efficiency and specificity. Lipid, carbohydrate, nucleotide, or metal-containing prosthetic groups may be attached to these enzymes and serve as essential components of their catalyses by enhancing specificity and/or stability (8—13). Each enzyme has a specific temperature and pH range where it functions to its optimal capacity the optima for these proteins usually He between 37—47°C, and pH optima range from acidic, ie, 1.0 in the case of gastric pepsin, to alkaline, ie, 10.5 in the case of alkaline phosphatase. However, enzymes from extremely thermotolerant bacteria have become available these can function at or near the boiling point of water, and therapeutic use of these ultrastable proteins can be anticipated. 

Histamine receptors variously mediate the bronchoconstrictant, inflammatory, irritant, vasodilator, gastric pepsin secretion and immune suppression actions of histamine. Associated with the immune response, cytokines cause release of histamine from mast cells. Histamine acts via HI, H2, H3 and H4 GPCRs. HI and H2 receptors couple via both Gas (elevating cAMP) and Gaq (elevating Ca2+ in a pertussis toxin-insensitive fashion) and H3 couples via Gai (decreasing cAMP). 

HIS-R antagonist [inhibits HIS-induced guinea pig ileum contraction (0.3)] HIS-R agonist [bronchoconstrictant, inflammatory, irritant, vasodilator, promotes gastric pepsin secretion]... 

I) Proteases 1. Proteinases a) Gastric Pepsin A (alkali-labile) 34500 -Gly Try-Phe-, -Glu Phe Hydrolyses only gelatin -Tyr Ser-, -PhelSer 1.8... 

Days Gastric Pepsin (U 24 hr" ) Urine Hydroxycorticoids (mg 24 hr Urine Pepsinogen (Uhr- ) Plasma Pepsinogen (U%) Creatinine Clearance (L24 hr" ) Pepsinogen Clearance (L24 hr" )... 

Figure 5-2. Stimulation of acid and pepsin secretion by intravenous infusion of histamine in human subjects. (From Hirschowitz B, London J, Pollard HM. Histamine stimulation of gastric pepsin secretion in man. Gastroenterology 32 85-87, 1957.)...

Janowitz HD, Hollander F. Relation of uropepsinogen to gastric pepsin secretion in man. J Appl Physiol 4 53-56, 1951. 

Gilman A, Cowgill GR. The effect of histamine upon the secretion of gastric pepsin. 

Hirschowitz Bl, London J, Pollard HM. Histamine stimulation of gastric pepsin secretion in man. Gastroenterology 32 S5- 1, 1957. 

An acid protease with an optimum pH of 2.5 was first described in human seminal plasma as pepsin and pepsinogen (1), but had not been purified or characterized. Recently, we have purified the acid protease and its proenzyme from human seminal plasma (2,3). In many respects, the properties of seminal plasma acid protease are similar to those of gastric pepsin. Since the proenzyme is more stable than the active enzyme in alkaline solution and can be converted into its active form in acidic solution, the acid protease is likely to exist in seminal plasma, at the physiological pH around 7.5 (4), in proenzyme form. 

Frislid K, Berstad A (1986) Effect of ranitidine on meal-induced gastric pepsin and acid secretion and the influence of adding ethanol to the meal. Scand J Gastroenterol 21 123-128... 

FIGURE 2.16 pH versus enzymatic activity. The activity of enzymes is very sensitive to pH. The pH optimum of an enzyme is one of its most important characteristics. Pepsin is a protein-digesting enzyme active in the gastric fluid. Trypsin is also a proteolytic enzyme, but it acts in the more alkaline milieu of the small intestine. Lysozyme digests the cell walls of bacteria it is found in tears. 

FIGURE 14.11 The pH activity profiles of four different enzymes. Trypsin, an intestinal protease, has a slightly alkaline pH optimnm, whereas pepsin, a gastric protease, acts in the acidic confines of the stomach and has a pH optimmn near 2. Papain, a protease found in papaya, is relatively insensitive to pHs between 4 and 8. Cholinesterase activity is pH-sensitive below pH 7 but not between pH 7 and 10. The cholinesterase pH activity profile suggests that an ionizable group with a pK near 6 is essential to its activity. Might it be a histidine residue within the active site ... 

The second enzyme to be crystallized (byjohn Nordrrnp in 1930). Even more than nrease before it, pepsin. study by Northrnp established tirat enzyme activity comes from proteins. fAiso known as rennin, it is tire major pepsinlike enzyine in gastric Jnice of fetal and newborn animals. 

Gastrointestinal tract Inhibition of most gut hormones, gastric acid, pepsin, bile and colonic fluid secretion... 

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). 

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. 

The proteases are secreted as inactive zymogens the active site of the enzyme is masked by a small region of its peptide chain, which is removed by hydrolysis of a specific peptide bond. Pepsinogen is activated to pepsin by gastric acid and by activated pepsin (autocatalysis). In the small intestine, trypsinogen, the precursor of trypsin, is activated by enteropeptidase, which is secreted by the duodenal epithelial cells trypsin can then activate chymotrypsinogen to chymotrypsin, proelas-tase to elastase, procarboxypeptidase to carboxypepti-dase, and proaminopeptidase to aminopeptidase. 

Ulcer formation is the net result of a lack of homeostasis between factors within the gastrointestinal tract responsible for the breakdown of food (e.g., gastric acid and pepsin) and factors that promote epithelial defense and repair (e.g., bicarbonate, mucus secretion, and prostaglandins). 

Hydrochloric acid and pepsin are the primary substances that cause gastric mucosal damage in PUD. Three different stimuli... 

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. 

Rodriguez et al. [68] studied the stability of niclosamide in artificial gastric and intestinal juices. The gastric juice contained sodium chloride and hydrochloric acid with or without pepsin. The intestinal juice contained sodium phosphate with or without pancreatin. Niclosamide was incubated with the juices at 37°C for 6 h. The remaining intact drug and its degradation products (2-chloro-4-nitroaniline, 5-chlorosalicylic acid) were extracted with chloroform/methanol (5 1) and determined by TLC and HPLC. The drug was stable in these media for at least for 6 h. 

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