CCK has an identical five amino-acid carboxy terminal sequence to gastrin. Its specificity is conferred by the adjacent three amino acids, which include a sulphated tyrosine residue at position 7. CCK is found in the gut in predominantly thirty-three, thirty-nine, or fifty-eight amino-acid molecular forms, in contrast to the brain in which it is found at high concentrations in an eight amino-acid form. It is produced by the I cells of the duodenal and jejunal mucosa, which open onto the lumen. A small amount of CCK is found in specific enteric neurones of the upper gastrointestinal tract.
