Digestive enzymes are released by the gastrointestinal system to aid in the breakdown, digestion and absorption of food. Most digestive enzymes come from specialized cells along the digestive tract, including the mouth, stomach, pancreas and small intestine. They are aided by several factors, including hydrochloric acid, bile and sodium bicarbonate. Since most digestion takes place in the small intestine, the large intestine does not secrete enzymes for the purposes of digestion.
Digestive Enzyme Stimulation
The release of digestive enzymes is stimulated by a number of factors. Sight, sound, smell or even thought of food can stimulate the secretion of enzymes, hormones and acid in the stomach. The presence of food in the mouth or stomach can also accomplish this. The perception of hunger from the gastrointestinal tract and signals from the brain's hypothalamus can also send signals to the body to release enzymes for the digestion of food.
Mouth and Stomach
The only digestive enzyme secreted from the mouth is through the salivary glands and called salivary amylase, or ptyalin. Salivary amylase initiates the process of starch breakdown by hydrolysis. Enzymes secreted by the stomach include pepsin and gastric lipase. Gastric lipase aids to break down smaller fats to fatty acids. Pepsin is responsible for converting protein to amino acids and peptides in the presence of hydrochloric acid.
Pancreas
Digestive enzymes secreted from the pancreas include lipase, cholesterol esterase, amylase, trypsin, chymotrypsin, carboxypeptidase, elastase, ribonuclease and deoxyribonuclease. Lipase, like gastric lipase from the stomach, helps to break down fat, but in this case with the help of bile, versus stomach acid. Cholesterol esterase breaks down cholesterol into free fatty acids and cholesterol esters, which are then packaged into fat transporters for the blood, called micelles. Amylase breaks down starches, and the enzymes trypsin, chymoptrypsin, and carboxypeptidase break down proteins and polypeptides.
Small Intestine
The small intestine secretes digestive enzymes through highly absorptive finger like projections, called villi. Enzymes secreted include, carboxypeptidase, enterokinase, sucrase, dextrinase, maltase, lactase, and nucleotidases. Sucrase, maltase, lactase and dextrinase each break down carbohydrates to yield glucose and other simple sugars. Proteolytic enzymes, some of which are also secreted by the pancreas, include carboxypeptidases, and break down proteins.
Natural Food Sources
Several foods are also sources of digestive enzymes. Such foods include, pineapple, papaya, sprouts, and other raw fruits, vegetables and seeds. Pineapple is a source of bromelain, a protease which can be found in the stem and fruit. Papain is the digestive enzyme found in papaya, and like bromelain, papain also breaks down protein in food. Other digestive enzymes from foods include actinidin from kiwi, ficin in fig and amylase in sweet potatoes.
References
- "Krause's Food, Nutrition and Diet Therapy"; L.K.Mahan, S. Escott-Stump; 2004
- "Food Enzymes: Structure and Mechanaism"; Dominic W.S. Wong; 1995
