Fig. 1. Structure of rat pre-pro-CCK. The vertical lines indicate the location of cleavage sites; the single amino acid code shows the amino acids flanking these sites. The boxes at the top indicate the location of the different carboxyamidated forms of CCK. The asterisks mark the location of the sulfated tyrosine residues. The lower portion of the figure shows a proposed model of the temporal order of cleavages in the processing of pre-pro-CCK to CCK 8 in endocrine tumor cells and the class of enzyme responsible for the cleavage is indicated on the right.