Gastrointestinal input of proteins
About 70-100 g of dietary proteins are digested and absorbed daily.
Factors that influence on daily requirement:
1)The age of patient :
This factor is considered first of all for children:
Age g/ kg of mass ± 20%
0,88
0,81
0,77
Adults 0,59
2) Protein quality:
It is measured by comparing the proportions of essential amino acids in a food . The closer the proportions are the higher the protein quality with the proportions required for good nutrition. Egg and milk proteins are highly-quality proteins for human. Meat proteins are of high protein quality, whereas several proteins from plants used as major food sources are relatively deficient in certain essential amino acids, e.g. Trp and Lys in maize (corn), Lys in wheat and beans combined provides a satisfactory amino acid intake.
3) High energy intake:
The energy derived from carbohydrate and fat effects protein requirements because it spares the use of protein as an energy source. To use expensive (high-quality) dietary protein efficiently and to reduce requirements for it to a minimum; it is necessary to ensure adequate provision of energy from non-protein sources, some of which should be carbohydrate in order to spare protein from gluconeogenesis.
4) Physical activity:
It increases nitrogen retention from dietary protein.
Protein energy malnutrition:
It encompasses a range of disorders of starvation and malnutrition that involve deficiencies of vitamins and minerals in addition to protein.
Two extreme forms are recognized:
In marasmus, there is generalized wasting due to deficiency of both energy and protein.
In kwashiorkor, which is characterized by edema, while energy intake may be adequate, there is a deficiency in both the quantity and the quality of protein.
Digestion of proteins in gastrointestinal tract
Polypeptides and proteins are not absorbed intact but must first be hydrolyzed to free amino acids. Proteolytic enzymes are found in gastric juice, pancreatic juice and on the intestinal cell surface.
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
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.
Histamine stimulates secretion of gastric juice.
Bombesin-like immunoreactive substance stimulates release of gastrin and cholecystokinin.
Vasoactive intestinal peptide (produced by mucosa cells) inhibits secretion of hydrochloric acid.
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:
Secretin and cholecystokinin stimulate pancreatic juice secretion.
Chymodenin stimulates secretion of pancreatic juice with high concentration of chymotrypsinogen.
Enterokinin stimulates secretion of epithelial intestine cells (secretion of enterokinase and intestinal proteases).