The different forms in which proteins digestion are taken in diet are as follows: 1. Gastric Juice 2. Pancreatic Juice 3. Succus Entericus 4. Nucleoprotein 5. Caseinogen (Milk) 6. Collagen and Gelatin 7. Mucin.
1. Digestion in the Gastric Juice:
Pepsin is the proteolytic enzyme of gastric juice. It acts with the help of HCI and converts all digestible proteins up to the peptone stage. Proteolytic enzymes other than pepsin (e.g., cathepsin, parapepsins, gastricsin) with the pH optima from 1.8—4.5 are present in gastric juice.
Although gastric digestion of protein is dispensable (since atrophy of gastric mucosa does not interfere nitrogen equilibrium) but in normal functioning of stomach at best 10-15% of protein is broken down to amino acid in the stomach as evident by the study of the product of gastric digestion of protein of the diet delivered in the intestine which is a mixture of undigested bundles of muscle fibres, a native protein in solution and products of peptic digestion, i.e., from large polypeptide to a few amino acids.
Peculiarities of Pepsin and its Action:
i. Pepsin is protein in nature.
ii. It remains as pepsinogen in the peptic cells of the stomach. HCI of gastric juice converts it into active pepsin. About 15% of the nitrogen content of the pepsinogen molecule is lost during activation.
iii. Optimum pH is 2.0, i.e., highly acid. In alkaline or neutral medium it is inactive.
iv. Optimum temperature is about 40°C.
v. It attacks the peptide linkages of the protein molecules and breaks them at these links.
vi. It acts on proteins and digests them up to peptone. After complete peptic digestion about one-tenth of the total number of peptide linkages are broken down.
vii. Its action on the living gastric mucous membrane is prevented by the presence of anti-pepsin in the gastric mucosa.
Stages of Digestion:
Pepsin in presence of HCI digests proteins to peptone (mainly) through the following stages – Protein acid metaprotein → primary proteoses → secondary proteoses peptone → amino acid (Fig. 9.59).
The last component is allowed for absorption and consequently the major part of protein digestion must occur in the intestine by other proteolytic enzymes.
2. Digestion in the Pancreatic Juice:
Trypsin is the proteolytic enzyme of pancreatic juice.
Trypsin Nature and Conditions of Action:
i. Trypsin is secreted as inactive trypsinogen. It is activated by the action of enteropeptidase (enterokinase) present in the succus entericus. Consequently, until pancreatic juice comes in contact with succus entericus it will possess very little proteolytic activity. Inactive trypsinogen can act on protamines and peptones slightly. Activation can also be brought about by calcium salts.
ii. Trypsin is protein in nature.
iii. Optimum reaction is slightly on the alkaline side but it can act to a fair extent in a slightly acid medium. Optimum pH varies with the type of protein to be acted upon, viz., for caseinogen and peptone, 8.0; for gelatin, 9.7, etc.
iv. Optimum temperature is about 40°C.
v. It can act upon native proteins as well as upon the products of protein digestion—such as metaprotein, proteose, peptones, polypeptides, etc., and converts them all into lower peptides (tri- and dipeptides).
Trypsin attacks the peptide linkage. The products formed after the action of pepsin will be cleaved if arginyl or lysyl group. Protamines containing large amount of arginine increase tryptic activity. In prolonged tryptic digestion even some amino acids may be formed, viz., leucine and tyrosine. This wide range of action is its characteristic feature which is not found with pepsin.
Inactive pancreatic juice contains two zymogens:
Enterokinase converts trypsinogen into trypsin and trypsin activates chymotrypsinogen into chymotrypsin. So that when enterokinase is added to inactive pancreatic juice both the zymogens become activated. Trypsin itself can similarly activate trypsinogen into trypsin as a process of autocatalysis. Chymotrypsin coagulates milk like rennin. It also hydrolyses casein and gelatin, but its mode of action is different from trypsin. Both trypsin and chymotrypsin are endopeptiodases.
Further breakdown of proteins is carried out by the exopeptidases, e.g.:
(c) Tripeptidase, and
These enzymes act upon different fractions of the digested protein molecule. Amino-peptidase splits off those amino acids from the polypeptide molecule which possess a free amino group. Similarly, carboxypeptidase takes away those ones which possess a free carboxyl group. Tri-peptidase and di-peptidase split off tri- and dipeptides to amino acids. Pancreatic juice also contains two other protein- splitting enzymes namely elastase and collagenase.
Stages of Breakdown:
Stages of tryptic activity are as follows:
Proteins → alkali metaproteins → primary proteoses → secondary proteoses → peptones → polypeptides → lower peptides amino acids. 50-60 % of proteins is digested in the pancreatic juice. Here, tryptic digestion ends and erepsin action begins.
3. Digestion in the Succus Entericus:
Digestion of protein in the intestinal juice depends upon the tryptic activity as the proteolytic enzyme present in the intestinal juice is unable to hydrolyse the protein as such. Removal of protease results loss of 50% protein in the stool which may be checked by the introduction of pancreatic trypsin.
Consists of a mixture of enzymes (intracellular) including amino-peptidases and di-peptidases.
Erepsin Nature and Conditions of Action:
i. It acts best in slightly alkaline medium, optimum pH is about 8.0.
ii. It cannot act on native proteins. It acts only upon lower peptides and converts them completely into amino acids.
iii. It is not a pure entity. It consists of many different peptidases. The various peptidases act serially, one after the other, upon the smaller and smaller fragments of the peptide molecule, till they are completely broken down into amino acids. At each stage of digestion a different peptidase comes into action. The epithelium of the intestine is very rich in these peptidases. From this it is suggested that the digestion of lower peptides may take place partly inside the epithelial cells during absorption.
Thus the proteins are converted into amino acids, and are absorbed.
4. Digestion of Nucleoproteins:
The proteolytic enzymes, pepsin and trypsin, hydrolyse nucleoproteins into their protein and prosthetic part nucleic acid components, in the first step. The former is digested as outlined in protein digestion. The nucleic acids are depolymerized by the hydrolytic action of pancreatic ribonuclease and deoxyribonuclease with the production of oligonucleotides and certain pyrimidine mononucleotides. The former are converted to mononucleotides by intestinal phosphodiesterases. The mononucleotides are hydrolised by non-specific phosphatases yielding nucleosides and inorganic phosphate.
The different stages of digestion of nucleoproteins which takes place in the intestinal mucosal cells mainly are summarized in Fig. 9.60.
Digestion in the intestine takes place partly in the lumen by succus entericus, containing enzyme derived from desquamated cells and mainly in the epithelium of the gut, because, the enzymes are concentrated in the epithelial cells. It is believed that the two pyrimidine nucleosides are not hydrolysed further and are absorbed as such.
5. Digestion of Casein (Milk):
The chief protein of milk is casein, a complex phosphoprotein. The fourth stomach of ruminants contains a protein-splitting enzyme rennin which causes clotting of milk. It liberates paracasein from casein which is precipitated as calcium paracaseinate. Although rennin is absent in human stomach, but chymosin is present in infant’s stomach which does the same action of rennin of ruminants. Chymosin is distinct from pepsin and absent in adults. Other proteases present in the stomach can convert casein to paracasein. Tryptic and chymotryptic digestions convert it to phosphopeptones containing phosphoserine.
Digestion of Milk:
Three constituents of milk require digestion:
Its digestion has been described above.
b. Lactalbumin and Laciglobulin:
They are digested in the same way as protein—successively by pepsin, trypsin and erepsin.
They undergo the same process of digestion as other fats.
Digested by lactase to glucose and galactose.
6. Digestion of Collagen and Gelatin:
Collagen is present in white fibrous tissue, tendons, bones, etc. Gelatin is prepared from it by boiling with acid.
Its digestion proceeds as follows:
In the gastric juice:
Collagen → gelatin → gelatoses → gelatin peptone. Pancreatic juice digests this peptone up to polypeptide which in its turn is digested by succus entericus up to amino acids.
7. Digestion of Mucin:
It is a glucoprotein. Gastric juice breaks it into glucosamine and another peptone-like substance. The latter is further digested as other peptones. Glucosamine is probably absorbed as such.