Extraction and immunochemical characterization of cholecystokinin-like peptides from pig and rat brain.

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Abstract
Two major classes of immunoreactive cholecystokinin peptides (iCCK) were identified in rat and pig brains: large basic peptides (big iCCK) resembling the 33-amino acid porcine cholecystokinin (pCCK33) in size and charge; and small acidic peptides (small iCCK) resembling the COOH-terminal fragments of CCK. Boiling 0.1 M HCl maximally extract big iCCK; boiling 0.1 M NaOH maximally extracts small iCCK. The differences in hormonal forms removed by these extractants are not likely to be due to enzymatic conversion during the extraction procedures. Fractionation on Sephadex G-50 and starch gel electrophoresis combined with radioimmunoassay using 3 antisera of different specificities (directed towards the NH2 terminus of pCCK33, produced by immunization with COOH-terminal fragment CCK8, produced by immunization with COOH-terminal fragment CCK4) are consistent with the hypothesis that a major fraction of big iCCK may represent intact cholecystokinin with a COOH-terminal extension, as recently suggested for gastrin, a molecule having a COOH-terminal pentapeptide identical with that of cholecystokinin.