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Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tracts of animals (including humans) and in the traps of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival. Digestive enzymes are diverse and are found in the saliva secreted by the salivary glands, in the stomach secreted by cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the intestinal (small and large) secretions, or as part of the lining of the gastrointestinal tract. 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. In the human digestive system, the main sites of digestion are the oral cavity, the stomach, and the small intestine. Digestive enzymes are secreted by different exocrine glands including: *Salivary glands *Secretory cells in the stomach *Secretory cells in the pancreas *Secretory glands in the small intestine ==Mouth== Complex food substances that are taken by animals and humans must be broken down into simple, soluble, and diffusible substances before they can be absorbed. In the oral cavity, salivary glands secrete an array of enzymes and substances that aid in digestion and also disinfection. They include the following:〔Brown, Thomas A. "Rapid Review Physiology." Mosby Elsevier, 1st Ed. p. 235〕 *lingual lipase: Lipid digestion initiates in the mouth. Lingual lipase starts the digestion of the lipids/fats. *Salivary amylase: Carbohydrate digestion also initiates in the mouth. Amylase, produced by the salivary glands, breaks complex carbohydrates to smaller chains, or even simple sugars. It is sometimes referred to as ptyalin. *lysozyme: Considering that food contains more than just essential nutrients, e.g. bacteria or viruses, the lysozome offers a limited and non-specific, yet beneficial antiseptic function in digestion. *Haptocorrin (also known as R-factor): Helps with the absorption of Vitamin B12. After Vitamin B12 is released from its original carrier protein in the stomach, it gets bound to Haptocorrin. Haptocorrin protects it from acidic conditions of the stomach but is cleaved in the duodenum by pancreatic proteases. Vitamin B12 can then bind to intrinsic factor (IF) that has been produced by parietal cells. Finally, the IF-Vitamin B12 complex is taken up by in ileum via the cubam receptor. Of note is the diversity of the salivary glands. There are two types of salivary glands: *serous glands: These glands produce a secretion rich in water, electrolytes, and enzymes. A great example of a serous oral gland is the parotid gland. *Mixed glands: These glands have both serous cells and mucous cells, and include sublingual and submandibular glands. Their secretion is mucinous and high in viscosity. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Digestive enzyme」の詳細全文を読む スポンサード リンク
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