Radioimmunoassay of cholecystokinin pancreozymin

Abstract

A sensitive and specific radioimmunoassay for cholecystokinin pancreozymin (CCK PZ) has been developed, using rabbit antisera to crude porcine hormone. Highly purified porcine CCK PZ, labelled with 131I, and repurified by column chromatography on Sephadex G15, was used as tracer. Separation of free from antibody bound labelled CCK PZ was carried out using charcoal, ion exchange resin, or a double antibody procedure. Non specific interference with the assay system by serum factors was abolished (as judged by in vitro and in vivo recovery studies) by boiling and diluting the serum samples before assay. Ninety nine per cent purine porcine CCK PZ (standard), commercial CCK PZ preparations, cerulein, the C terminal 8 and 12 amino acid fragments of the CCK PZ molecule, and endogenous human CCK PZ all cross reacted in the assay system and showed parallel inhibition curves. No significant cross reaction was found with gastrin, secretin, glucagon or insulin. The sensitivity of the assay is approximately 5 pg per ml of test solution, which proved adequate for measuring physiological levels of CCK PZ in peripheral blood in man. The mean immunoreactive CCK PZ concentration in 50 fasting normal subjects was 60.4 pg per ml. The distribution of individual values was skewed, however, so that the median was much lower (30 pg per ml). Older subjects had higher fasting levels of CCK PZ than were found in young adults.