Digestion in the duodenum

Properties of pancreatic juice

Pancreatic juice is a colorless fluid produced by acinar pancreocytes. Its most important physicochemical property is a pronounced alkaline reaction (pH 7.8–8.4).

Alkalinity is provided by an extremely high concentration of bicarbonates (up to 150 mmol/L). Their main physiological task is the rapid and effective neutralization of acidic gastric chyme. Without this process, pancreatic enzymes could not function under optimal conditions. Between meals, on an empty stomach, the juice is secreted periodically, strictly in synchrony with the interdigestive migrating motor complex of the gastrointestinal tract.

Enzyme activation cascade

The pancreas synthesizes enzymes for digesting carbohydrates, fats, and nucleic acids directly in an active state (amylase, lipase, nucleases). However, proteolytic enzymes that break down proteins are secreted as safe proenzymes (zymogens) to prevent auto-digestion of the organ.

The initiation of digestion begins with the activation of trypsin.

1. Inactive trypsinogen enters the intestinal lumen, where it encounters enteropeptidase (enterokinase). This activator enzyme, discovered in the laboratory of I.P. Pavlov, is secreted by the intestinal mucosa and is itself preliminarily activated by duodenase.
2. Enteropeptidase cleaves a hexapeptide from trypsinogen, converting it into active trypsin (the process is accelerated in the presence of calcium ions).
3. An autocatalysis reaction occurs: the formed trypsin begins to independently activate the remaining trypsinogen.
4. Next, trypsin activates all other proteases: chymotrypsinogen is converted into chymotrypsin, procarboxypeptidases into carboxypeptidases A and B, and proelastase into elastase. Trypsin is also necessary for the activation of prophospholipase.

Protein breakdown: endo- and exopeptidases

Enzymatic hydrolysis of proteins proceeds in two major stages involving different classes of proteases.

Endopeptidases act in the stomach and duodenum. They break internal peptide bonds of polypeptide chains, splitting them into oligopeptides. These include:

• Pepsin: acts in the stomach, prefers bonds adjacent to aromatic and hydrophobic amino acids (e.g., phenylalanine and leucine).
• Trypsin: acts in the intestine, specific for basic amino acids (arginine and lysine).
• Chymotrypsin: cleaves bonds adjacent to aromatic amino acids (tyrosine and phenylalanine).
• Elastase (pancreatopeptidase).

Exopeptidases complete digestion in the small intestine by sequentially cleaving single amino acids from the ends of molecules:

• Aminopeptidases work from the N-terminus (amino group).
• Carboxypeptidases work from the C-terminus (carboxyl group).
• Dipeptidases hydrolyze the remaining bond in dipeptides, converting them into two free amino acids.

Digestion of carbohydrates, fats, and nucleic acids

In addition to proteins, pancreatic juice effectively breaks down other macronutrients:

• Carbohydrates: active $\alpha$-amylase breaks down polysaccharides (starch and glycogen) into mono- and disaccharides. The enzyme maltase breaks down the disaccharide maltose into glucose.
• Fats: pancreatic lipase hydrolyzes fats into monoglycerides and fatty acids. Colipase, calcium ions, and bile (bile acid salts) are critical for enhancing fat hydrolysis. Esterase and phospholipase A also participate in the process.
• Nucleic acids: ribonuclease breaks down RNA, and deoxyribonuclease destroys DNA.

Regulation and phases of secretion

After eating, juice begins to be secreted after a short latent period — 3–5 minutes. Secretion is finely adapted to the type of food consumed. For example, with a carbohydrate load (bread), the peak of secretion is recorded in the 1st hour. With a protein load (meat) — in the 2nd hour, and when consuming milk — in the 3rd hour.

The process is controlled by neural and humoral mechanisms that either directly affect pancreocytes or work through regulatory peptides and autonomic nervous system centers. There are four phases of regulation:

1. Complex reflex (cephalic).
2. Gastric.
3. Intestinal.
4. Circulatory (an additional phase of humoral correction of pancreatic function due to nutrients already absorbed into the blood).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of monosaccharide and amino acid absorption in the small intestine
• Physiological role of bile salts in fat emulsification
• Humoral regulation of the intestinal phase of pancreatic secretion
• Synchronization of GI tract function between meals (migrating motor complex)
• Molecular structure and function of small intestinal villi

Other topics in the "Digestion" section

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