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the enzymes that break down food in the course of digestion. Digestive enzymes are secreted by the organs of digestion and belong to the class of enzymes called hydrolases, each of which splits a specific type of bond. Proteases hydrolyze certain peptide bonds of proteins in a step-by-step fashion until eventually individual amino acids remain. They comprise endopeptidases, including pepsin, trypsin, and chymo-trypsin, and exopeptidases, including aminopeptidase, carboxy-peptidase, dipeptidase, and tripeptidase. Glycosidases catalyze various stages in the hydrolysis of carbohydrates. Amylases split starch and glycogen, while a- and β-glycosidases hydrolyze oligosaccharides and disaccharides to form monosaccharides. Esterases hydrolyze various esters, for example, lipase splits fats to form glycerin and fatty acids, alkaline phosphatase hydrolyzes phosphoric esters, and nucleases hydrolyze nucleic acids.
Digestive enzymes participate in the many digestive reactions by acting outside the cell, inside the cell, or within the cell membrane. It is assumed that in some nondigestive organs, they are able to participate in intermediate metabolism and perform nondigestive functions. Digestive enzymes may vary considerably from species to species, depending on the animal’s diet and mode of life, with the greatest variety occurring among omnivores. Carnivores possess digestive enzymes with high proteolytic and low glycosidase activities, whereas the opposite case holds for herbivores.
Shipworms, carpenter moths, and many microorganisms are examples of species with a high degree of food specialization; they have the enzyme cellulase, which permits them to feed on wood and plant fibers and to split cellulose. Some species use the digestive enzymes of symbionts to digest food or the digestive enzymes in the food itself. Many parasites have lost most of their vital digestive enzymes and usually feed on the digestive products of their host.
REFERENCEProsser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. Chapter 5. (Translated from English.)
A. M. UGOLEV and V. V. EGOROVA