Gastric juice: its composition and secretion

The gastric juice is a clear, pale, yellow fluid of 0,2-0,5% HCl with a pH of about 1,0-2,5. It contains 97-99% water. The remainder consists of mucin and inorganic salts, the digestive enzymes (pepsin and rennin), and lipase (in a small quantity; its concentration is high in the age about 1-3, only).

The main component of gastric juice is hydrochloric acid. It kills microorganisms, denaturates proteins, provides the cleavage of pepsinogen and makes acid environment for the action of pepsin. There are acidic phosphates and carbonates, too.

Pepsin

Pepsinogen is secreted by stomach cells and then converted to the active form – Pepsin due to hydrochloric acid. The first molecules of formed pepsin can cleave the pepsinogen molecule, too. And this process is called autocatalysis of pepsinogen:

Substrate specificity of pepsin: it is endopeptidase; it cleaves internal peptide bonds between aromatic amino acid residues; pH optimum is about 1,5-2,5.

Rennin

It is important in digestive processes of infants because it prevents the rapid passage of milk from the stomach. In the presence of calcium, rennin changes the casein of milk irreversibly to a procasein, which is then acted on by pepsin. Rennin is reported to be absent from the stomach of adults. It is used in the making of cheese (rennet).

Gastriksin

It is endopeptidase; it cleaves internal peptide bonds between dicarboxylic amino acid residues; pH optimum is about 3,3.

Regulation of gastric juice secretion

1) Castrin family control the gastric acid and pepsin secretion:

Gastrin stimulates secretion of gastric juice.

Cholecystokinin inhibits secretion of gastric juice, stimulates contraction of stomach and secretion of pancreatic juice.

2) Histamine stimulates secretion of gastric juice.

3) Bombesin-like immunoreactive substance stimulates release of gastrin and cholecystokinin.

4) Vasoactive intestinal peptide (produced by mucosa cells) inhibits secretion of hydrochloric acid.

5) Enterogastron (produced by duodenum mucosa cells) inhibits secretion of hydrochloric acid and pepsinogen.

The major products of peptic hydrolysis of proteins in stomach are large peptides and some free amino acids. The liberated peptides act as stimulants for secretion of cholecystokinin in the duodenum, setting the stage of pancreatic proteolysis.

Pancreatic proteolysis

Endopeptidases and carboxypeptidases in pancreatic juice are secreted as proenzymes from pancreatic acinar cells under the influence of secretin and cholecystokinin (pancreozymin).

Activation of trypsinogen

Activation of trypsinogen occurs by the action of enterokinase, secreted by the duodenal epithelial cells and then we can consider its autocatalysis.

Trypsin is factor for activation of many pancreatic proenzymes in the small intestine:

Chymotrypsin is endopeptidase, its specificity is similar to pepsin but it can cleave another substances such as esters, hydroxyamides, and another acylated derivatives; pH optimum is about 8,0.

Carboxypeptidases A (Zn²⁺- containing) and B are exopeptidases, they cleave external peptide bonds, but their specificity is not similar. Both of them cleave C-terminal peptide bond, but Carboxypeptidase A cleaves the bond between aromatic acid residues and Carboxypeptidase B cleaves the bond between Lys and Arg residues.

Elastase (endopeptidase) has its special target – substrate elastin (the main protein component of connective tissue; it contains a large number of glycine and serine residues. Elastase attacks preferably the sites of polypeptide where amino acids with small hydrophobic radicals are (Ala, Ser, Gly).

Regulation of pancreatic proteolysis:

1) Secretin and cholecystokinin stimulate pancreatic juice secretion.

2) Chymodenin stimulates secretion of pancreatic juice with high concentration of chymotrypsinogen.

3) Enterokinin stimulates secretion of epithelial intestine cells (secretion of enterokinase and intestinal proteases).

Intestinal proteases

Aminopeptidases and dipeptidases

On the intestinal cell surface the digestion of protein to free amino acids continues. Dipeptides and tripeptides are usually absorbed as such and are digested to free amino acids by Dipeptidases and Tripeptidases accordingly within intestinal epithelial cells. Aminopeptidases remove the N-terminal amino acid residue from short polypeptides.

Absorption of free amino acids takes place in the small intestine

All amino acids are divided into five separate systems for transport mechanism across membranes.

Groups 1 (Small neutral amino acids) and 2 (Large neutral and aromatic amino acids) are transported across membrane of any cell by γ-Glutamyl transferase system.

And there are special transport systems for: Acidic amino acids (3), Basic amino acids and Cystine (4), Proline and Glycine (5).

Determination of some indexes to estimate the protein digestion in patients (for gastric juice, only)

The main tests for gastric juice (indexes for adults) are:

1) Determination of its volume:

a) on an empty stomach – not higher then 50 ml;

b) the intensity of basal secretion – 50-100 ml/per 1 hour;

c) secretion after stimulation (N.P. Leporsky method) – 50-110 ml/per 1 hour;

d) submaximal stimulation by histamine – 100-140 ml/per 1 hour;

e) maximal stimulation by histamine - 180-200 ml/per 1 hour.

2) The total acidity of gastric juice must be in a region 40-60 mmol/lit.

3) The content of free hydrochloric acid must be in region 20-40 mmol/lit.

4) The content of conjugated hydrochloric acid must be in region 10-20 mmol/lit.

The increase of total acidity of gastric juice and content of free hydrochloric acid (Hyperchlorhydria) may be during Stomach Ulcer and Superacidity.

The decrease of total acidity of gastric juice and content of free hydrochloric acid (Hypochlorhydria) is observed during Subacidic gastritis or gastric carcinoma.

A complete absence of hydrochloric acid in gastric juice and considerable lowering of total acidity (Achlorhydria) may be also during gastric carcinoma.

Achlorhydria may be called also Achylia. Achlorhydria after stimulation by histamine is a sign of atrophy of epithelial cells of stomach, the sign of gastritis with blocked secretion.

5) Lactate is appeared in gastric juice during the intensity of fermentation processes in stomach. There is the suggestion that lactate is a product of cancer cells in stomach.

6) Concentration of Pepsin (after stimulation) on empty stomach must be in region 0,2-0,4 mg/ml or 20-40 units/ml (Pyatnitsky et all, method).

Doctors consider:

1) Anacidic state if pH (after maximal stimulation by histamine) of gastric juice is about 6,0 or higher;

2) Achlorhydria state if pH (after maximal stimulation by histamine) of gastric juice is in a region 3,5-6,0;

3) Hypochlorhydria state if pH (after maximal stimulation by histamine) of gastric juice is in a region 2,5-3,5.

Дата добавления: 2015-11-14