Radioimmunoassay of cholecystokinin in plasma.

A sensitive and specific radioimmunoassay for cholecystokinin (CCK) has been developed. Synthetic unsulfated carboxy-terminal fragment, CCK-8, was radioiodinated by the conventional Chloramine-T method. Antibodies were raised against sulfated CCK-8 covalently coupled to bovine thyroglobulin via 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. By purification, highly immunoreactive 125I-labeled CCK-8 was obtained. The antiserum was highly avid, and plasma could be assayed directly. The detection limit of the assay was 5 pmol of sulfated CCK-8 per liter. The assay measured fragments CCK-8, CCK-33, and CCK-39 with equimolar potency. CCK-4, gastrin, and vasoactive intestinal polypeptide were not detected, even at higher concentrations. The concentration of CCK, as the sum of these CCK peptides, in plasma during fasting was low (10.5 +/- 2.1 pmol/L, mean +/- SEM) but still detectable in all normal subjects examined (range, 6.4-20.1 pmol/L). After ingestion of a test meal, CCK in plasma increased rapidly, peaking at 41.3 (SEM 5.7) pmol/L at 40 min and remaining high for 3 h after the meal. This supports the concept that CCK has important roles in digestion and absorption.