The selectivity of peptidases

Enzyme	Cleavage of peptide bonds
Pepsin	Phenylalanine, tyrosine, glutamic acid
Trypsin	Lysine, arginine
Chymotrypsin	Tryptophan, phenylalanine, tyrosine
Elastase	Glycine, alanine and serine
Carboxypeptidase A	Tryptophan, phenylalanine, tyrosine
Carboxypeptidase B	Lysine, arginine

The hydrolysis products of proteins are absorbed in the digestive tract mainly in the form of free amino acids, with the assistance of carrier proteins. Amino acids, like glucose, are absorbed by ions with Na ^+.

The absorption of small peptides is proved. Oligopeptides are hydrolyzed after absorption. In some cases, large peptides, such as botulism toxin, cholera and diphtheria are absorbed. Diphtheria toxin consists of two polypeptides. Transport of the two polypeptides, or a toxin through the lipid bilayer of biomembranes hitherto is considered unique and mysterious process.

Absorbed amino acids enter the portal vein to the liver. In the liver, the amino acids are involved in the synthesis of proteins, nucleotides, creatine, etc. Much of the amino acid is carried by the blood throughout the body and used for physiological purposes.

The parenteral protein nutrition leads to the development of sensitization, and the re-introduction of proteins can cause anaphylaxis. Hydrolysates of proteins (a mixture of amino acids) are used for protein nutrition. It does not cause allergic reactions. However, side effects are possible, such as disruption of mental (psychological) activity.

2.2. Amino acids metabolism

Conversion of amino acids under the action of intestinal microflora

All the conversions of amino acids caused by the activity of intestinal microorganisms were called "putrefaction of proteins in the intestine". Toxic metabolic products as hydrogen sulfide H₂S, methyl mercaptane CH₃SH, amines, cresol, phenol, skatole and indole are formed in the large intestine.

In the liver they undergo neutralization by chemical binding with sulfuric or glucuronic acid and are excreted in the urine.

The reactions of amino groups

1. Deamination of amino acids is the removal of amino group in the form of ammonia. nation

Hydrolytic deamination

Intramolecular deamination.

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urocanic acid

Oxidative deamination is predominant.

It takes place mostly in the liver and kidney. Glutamic acid is deaminated with the highest rate. Glutamate dehydrogenase containes coenzymes NAD⁺ or NADP⁺. At first, glutamate is oxidized to iminoglutarate, and then it is hydrolyzed with the formation of α-ketoglutarate and ammonia.

2. Transamination of amino acids is the reaction of intermolecular transfer of amino groups from amino acids to α-keto acids. Most of the L-amino acids are deaminated in the body by transamination with α-ketoglutaric acid:

Enzymes are aminotransferases or transaminases. They are contained in almost all organs, but transamination reactions occur most actively in the liver. Coenzyme of transaminases is pyridoxal phosphate (an active form of vitamin B₆). The formed glutamic acid is subjected to oxidative deamination. Some authors use the term transdeamination for such an indirect way of deamination of amino acids.

Aspartate transaminase (AST) and alanine transaminase (ALT) are the most important for clinical purposes. They catalyze the transfer of amino groups from aspartate and alanine to the - ketoglutarate:

Aspartate + α-ketoglutarate oxaloacetate + glutamate

Alanine + α-ketoglutarate pyruvate + glutamate

In the blood serum of healthy people the activity of these enzymes is thousands of times lower than in the organs. Therefore organic lesions in acute and chronic diseases accompanied by destruction of cells lead to release of the enzymes in the blood. Within 3-5 h after myocardial infarction AST level in serum is increased sharply (by 20-30 times). The level of ALT in the blood is increased in liver diseases.