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

Two major classes of immunoreactive cholecystokinin peptides (iCCK) have been identified in rat and pig brains: (i) large basic peptides (big iCCK) resembling the 33-amino acid porcine cholecystokinin (pCCK33) in size and charge; (ii) small acidic peptides (small iCCK) resembling the COOH-terminal fragments of CCK. Boiling 0.1 M HCl maximally extracts 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 three antisera of different specificities--(i) directed towards the NH2 terminus of pCCK33, (ii) produced by immunization with COOH-terminal fragment CCK8, (iii) 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 has recently been suggested for gastrin, a molecule having a COOH-terminal pentapeptide identical with that of cholecystokinin.