Lipases are a minor but significant enzyme secretion. Partial digestion of fat occurs in the adult stomach and preliminary digestion of triacylglycerols aids the action of pancreatic lipases. These enzymes are particularly important in infants with respect to breakdown of milk fat, 30-60% of which is lipolyzed in the gastric lumen. Gastric lipases are glycoproteins with molecular weights of about 45 000. Lipase secretion is highest in the body and fundus and very low in the antrum. Secretory granules containing lipases are located in peptic cells, particularly in the fundic mucosa. Gastric lipases have a broad pH optimum (2.5-7.0); they are stable down to pH 1.5 and therefore survive in the stomach's acidic environment and will be active during feeding when the gastric pH rises to around 5.0. Secretion of gastric lipase is coupled with pepsin secretion by peptic cells in response to pentagastrin (a functional analoge of gastrin). Lipase secretion from isolated human
Table 2 Composition of gastric juice
Mucus (salivary and gastric) Lipases (E.C.220.127.116.11) Pepsins 1-6 (E.C.18.104.22.168) Urea
Intrinsic factor and haptocorrin H2O
Salivary amylase aOnly present when secreted into the mucous layer; once out in the lumen will react with H+ to form H2CO3 which can decompose to H2O + CO2.
gastric glands is stimulated by cholecystokinin (CCK) and carbachol but histamine has no effect. In the dog secretin and prostaglandin E2 stimulate lipase secretion. There is also a feedback loop in gastric lipase secretion: release of long-chain fatty acids from triacylglycerols by lipases stimulates CCK secretion, which in turn stimulates the secretion of pancreatic and gastric lipases.
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