The digestion of Proteins

Enzymes which hydrolyze Proteins belong to Peptide hydrolases (it is subclass of Hydrolases). Peptide hydrolases are divided into 2 subsubclasses – endopeptidases (Pepsin, Trypsin, Chymotrypsin, Elastase, Collagenase) and exopeptidases (Aminopeptidases, Carboxypeptidases and Dipeptidases). Endopeptidases hydrolyze Proteins to Peptides. Exopeptidases hydrolyze Peptides to free amino acids.

The digestion of Proteins begins in gaster, then in small intestine. Pepsin (enzyme of gaster) acts on proteins and hydrolyzes them to peptides.

Proteins of food stuffs are mainly hydrolyzed in small intestine under action of pancreatic enzymes such as Trypsin, Chymotrypsin, Elastase, Collagenase and Carboxypeptidase. This process also occurs under action of some intestinal peptidases such as aminopeptidases and dipeptidases.

Thus, the digestion of proteins results in formation of mixture of free amino acids in the lumen of small intestine.

The rotting of Proteins in large intestine and detoxification of products of rotting in liver

Beside of digestion food proteins undergo the rotting. There are some differences between digestion and rotting of proteins:

Digestion:

1. Gaster, small intestine

2. peptide hydrolases of gaster, pancreas, small intestine

3. digestion results in mixture of free amino acids

4. amino acids are absorbed in portal vein and used in the body

Rotting:

1. large intestine

2. enzymes of microbs of large intestine

3. rotting is a cleavage of amino acids

4. products of rotting are detoxified in liver and excreted through kidneys

There are 2 pathways of rotting: 1) decarboxylation; 2) oxidation of side chain

Diaminomonocarboxylic amino acids undergo the decarboxylation, for example, Ornithine and Lysine:

Ornithineàputrescine+CO2

Lysineàcadaverine+CO2

Putrescine and Cadaverine are also formed in dead body therefore they are named as ptomaines. Afterwards it turned out, that these substances aren’t toxic and can be excreted without detoxification through the kidneys.

Cyclic amino acids mainly undergo the oxidation of side chain:

Tryptophan+[O]àScatoleàIndole

Tyrosine +[O]àcresolàphenol

Products of oxidation of side chain are called intestinal toxins. They enter the portal vein and undergo detoxification in liver by conjugation with sulfuric or glucuronic acids. Indole and scatole previously undergo the hydroxylation to Indoxyl and Scatoxyl, respectively:

Indoxyl+H2SO4àIndoxylsulfuric acid

Scatoxyl+H2SO4à Scatoxylsulfuric acid

Indoxyl+glucuronic acidà Indoxylglucuronic acid

Scatoxyl+glucuronic acidà Scatoxylglucuronic acid

Thus, the detoxification of Indoxyl, Scatoxyl, Phenol and Cresol results in formation of coupled esterosulfuric and esteroglucuronic acids which are eliminated through the kidneys.

“Indican” is a Potassium or sodium salt of Indoxylsulfuric acid.

This index is equal 0.04g/l in urine. Increase of its is observed in enhance of rotting processes, for example intestinal obstruction, cleavage of intestinal tumor. Decrease of this index is observed in diseases of liver (decreased antitoxic function of liver).