Digestive enzymes break down polymeric macromolecules into their smaller building blocks.
Enzymes facilitate their absorption by the body.
Digestive enzymes are found in the saliva secreted by the salivary glands, in the cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the secretions of cells lining the small and large intestines.
Digestive enzymes are classified based on their target substrates;
Proteases and peptidases split proteins into small peptides and amino acids.
Lipases split fat into three fatty acids and a glycerol molecule.
Amylases split carbohydrates such as starch and sugars into simple sugars such as glucose.
Nucleases split nucleic acids into nucleotides.
The main sites of digestion are the oral cavity, the stomach, and the small intestine.
Digestive enzymes are secreted by different exocrine glands including:
Secretory cells in the stomach
Secretory cells in the pancreas
Secretory glands in the small intestine
Complex food substances must be broken down into simple, soluble, and diffusible substances before they can be absorbed.
Salivary glands of the oral cavity secrete an array of enzymes and substances that aid in digestion and also disinfection.
Enzymes of salivary origin include:
lingual lipase, which starts the digestion of the lipids/fats.
Salivary amylase, that initiates carbohydrate digestion, breaking down complex carbohydrates to smaller chains, or even simple sugars.
lysozyme: offers a limited and non-specific, yet beneficial antiseptic function in digestion.
There are two types of salivary glands:
Serous: glands produce a secretion rich in water, electrolytes, and enzymes.
An example of a serous oral gland is the parotid gland.
Mixed glands: have both serous cells and mucous cells, and include sublingual and submandibular glands. Their secretion is mucinous and high in viscosity.
The stomach plays a major role in digestion, by mixing and crushing the food, and by enzymes.
The following are enzymes produced by the stomach:
Pepsin is the main gastric enzyme produced by the chief cells of the stomach.
Pepsin’s inactive form is pepsinogen, which is a zymogen.
Pepsinogen is then activated by the stomach acid into its active form, pepsin.
Pepsin breaks down the protein in the food into peptide fragments and amino acids.
Protein digestion, therefore, first starts in the stomach, unlike carbohydrate and lipids, which start their digestion in the mouth.
Gastric lipase is an acidic lipase secreted by the gastric chief cells in the fundic mucosa in the stomach.
Gastric lipase, together with lingual lipase, comprise the two acidic lipases, and do not require bile acid for optimal enzymatic activity.
Acidic lipases make up 30% of lipid hydrolysis occurring during digestion, with gastric lipase contributing the most of the two acidic lipases.
Hydrochloric acid (HCl), stomach acid, is produced parietal cells, and functions to denature ingested ingested, destroys any bacteria or virus that remains in the food, and also activates pepsinogen into pepsin.
Intrinsic factor is also produced by the parietal cells of the stomach.
Vitamin B12 requires assistance for absorption in the terminal ileum.
In the saliva, haptocorrin secreted by salivary glands binds Vit. B, creating a
The Vitamin B12-Haptocorrin complex protects Vitamin B12 from hydrochloric acid produced in the stomach.
In the duodenum, haptocorrin is cleaved with pancreatic enzymes, releasing the intact vitamin B12, and Intrinsic factor (IF) produced by the parietal cells then binds Vitamin B12, creating a Vit. B12-IF complex.
This complex is then absorbed at the terminal portion of the ileum.
Gastrin is a hormone produced by the “G cells” of the stomach.
G cells produce gastrin in response to stomach stretching after food enters it, and also after stomach exposure to protein.
Gastrin is an endocrine hormone and therefore enters the bloodstream.
Gastrin returns to the stomach where it stimulates parietal cells to produce hydrochloric acid (HCl) and Intrinsic factor (IF).
There are four types of cells in the stomach:
Parietal cells: Produce hydrochloric acid and intrinsic factor.
Gastric chief cells: Produce pepsinogen.
Chief cells are mainly found in the body of stomach, which is the middle or superior anatomic portion of the stomach.
Mucous neck and pit cells: Produce mucin and bicarbonate to create to protect the stomach lining from the acid or irritants in the stomach chyme.
G cells: Produce the hormone gastrin in response to distention of the stomach mucosa or protein, and stimulate parietal cells production of their secretion.
G cells are located in the antrum of the stomach, which is the most inferior region of the stomach.
Secretion by the G cells is controlled by the enteric nervous system.
Distention in the stomach or innervation by the vagus nerve, via the parasympathetic division of the autonomic nervous system, activates the enteric nervous system, in turn leading to the release of acetylcholine.
Acetylcholine activates G cells and parietal cells.
Digestive or exocrine function of pancreas is as significant to the maintenance of health as is its endocrine function.
Two populations of cells in the pancreatic parenchyma make up its digestive enzymes:
Ductal cells: Mainly responsible for production of bicarbonate (HCO3), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus.
Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions.
Highly acidic stomach chyme entering the duodenum stimulates duodenal cells called “S cells” to produce the hormone secretin and release it to the bloodstream to affect pancreatic ductal cells to stimulate them to produce their bicarbonate-rich juice.
Secretin also inhibits production of gastrin by “G cells”, and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme.
Acinar cells are responsible for production of the inactive pancreatic enzymes known as zymogens.
Zymogens present in the small bowel, become activated and perform digestive functions by breaking down proteins, fat, and DNA/RNA.
Acinar cells are stimulated by cholecystokinin (CCK), which is a hormone/neurotransmitter produced by the intestinal cells in the duodenum.
CCK stimulates production of the pancreatic zymogens, as well.
Cholecystokinin (CCK) is a peptide released by the duodenal “I cells” in response to chyme containing high fat or protein content.
CCK works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content.
CCK increases gallbladder contraction, resulting in bile squeezed into the cystic duct, common bile duct and eventually the duodenum.
Pancreatic juice, composed of the secretions of both ductal and acinar cells, is made up of the following digestive enzymes:
Trypsinogen is an inactive protease.
Trypsinogen is activated in the duodenum into trypsin, and it breaks down proteins at the basic amino acids.
Trypsinogen is activated by the duodenal enzyme enterokinase into its active form trypsin.
Chymotrypsinogen, which is an inactive protease that is activated by duodenal enterokinase, turning it into chymotrypsin.
Chymotrypsin breaks down proteins to their aromatic amino acids.
Chymotrypsinogen can also be activated by trypsin.
Pancreatic lipase degrades triglycerides into fatty acids and glycerol.
Pancreatic amylase breaks down starch and glycogen which are alpha-linked glucose polymers.
Pancreatic bio-feedback mechanisms are required for the maintenance of pancreatic juice homeostasis.
Secretin, a hormone produced by the duodenal “S cells” in response to the stomach chyme containing high acidity.
Secretin is released into the blood stream.
Secretin decreases gastric emptying, increases secretion of the pancreatic ductal cells, and stimulates pancreatic acinar cells to release their zymogenic juice.
Secretin is an endocrine hormone,
Bile helps absorption of the fat by emulsifying it, increasing its absorptive surface.
Gastric inhibitory peptide (GIP) is produced by the mucosal duodenal cells.
Gastric inhibitory peptide (GIP) is a response to chyme containing high amounts of carbohydrate, proteins, and fatty acids.
Gastric inhibitory peptide’s main function is to decrease gastric emptying.
Somatostatin is produced by the mucosal cells of the duodenum and also the delta cells of the pancreas.
Somatostatin hormone inhibits pancreatic activity.
The duodenum produces: Secretin, Cholecystokinin, Gastric inhibitory peptide, Erepsin, Maltese, Lactase and Sucrase.
Throughout the lining of the small intestine there are numerous
Brush border enzymes throughout the lining of the small intestine function is to further break down the chyme released from the stomach into absorbable particles.
These enzymes include:
Erepsin: converts peptones and polypeptides into amino acids.
Maltase: converts maltose into glucose.
Lactase: converts lactose into glucose and galactose.
A majority of Middle-Eastern and Asian populations lack lactase.
Lactase decreases with age.
Lactose intolerance is often a common abdominal complaint in the Middle-Eastern, Asian, and older populations, manifesting with bloating, abdominal pain, and osmotic diarrhea.
Sucrase: converts sucrose into glucose and fructose.