Pig brain contains cholecystokinin octapeptide and several cholecystokinin desoctapeptides

A sequential method employing methanol extraction of the COOH-terminal fragment of cholecystokinin (CCK) from pig brain followed by HCl extraction of the more basic CCK peptides was used as the first step in purification of these peptides. Recovery was monitored with two different assays, one directed to the COOH terminus of CCK and the other to the NH2 terminus. The amino acid content and sequence were determined for each of five peptides after purification. The only peptide containing COOH-terminal immunoreactivity was CCK-octapeptide (CCK8). The other four peptides did not contain CCK8 and had lost one or two additional amino acids, perhaps as a consequence of the action of carboxypeptidases. These peptides were shown to be CCK33-desnonapeptide, CCK39-desnonapeptide and -desdecapeptide, and a large molecular weight precursor, CCK58-desnonapeptide, containing 19 amino acids (Ala-Val-Gln-Lys-Val-Asp-Gly-Glu-Ser-Arg-Ala-His-Leu-Gly-Ala-Leu-Leu-Ala-Arg) NH2-terminal to CCK39. The three NH2-terminal fragments of CCK58, CCK39, and CCK33 were about equally prominent. The brain, unlike the gut, appears to cleave CCK8 rapidly from a precursor peptide but to process the NH2-terminal portions of the molecule more slowly and incompletely.