Cholecystokinin (CCK) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. It is synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum, and causes the release of digestive enzymes from the pancreas and bile from the gallbladder. CCK is a family of hormones identified by number of amino acids depending on post-translational modification of preprocholecystokinin, including CCK58, CCK33 and CCK8. CCK is very similar in structure to another peptide hormone gastrin. They share five identical amino acids at their C-termini.CCK mediates a number of physiological processes, including digestion and satiety. Secretion of CCK by the duodenal and intestinal mucosa is stimulated by fat- or protein-rich chyme entering the duodenum. It then inhibits gastric emptying and gastric acid secretion and mediates digestion in the duodenum. It stimulates the acinar cells of the pancreas to release water and ions and stimulates the secretion of a juice rich in pancreatic digestive enzymes, hence the old name pancreozymin. Together these enzymes catalyze the digestion of fat, protein, and carbohydrates. Thus, as the levels of the substances that stimulated the release of CCK drop, the concentration of the hormone drops as well. The release of CCK is also inhibited by somatostatin.CCK also causes the increased production of hepatic bile, and stimulates the contraction of the gall bladder and the relaxation of the Sphincter of Oddi (Glisson"s sphincter), resulting in the delivery of bile into the duodenal part of the small intestine. Bile salts form amphipathic micelles that emulsify fats, aiding in their digestion and absorption.