Evaluation of a radioimmunoassay for cholecystokinin in human plasma

Abstract

A sensitive and specific radioimmunoassay for cholecystokinin (CCK) in human plasma was developed using an antiserum specific for sequence 26-29 of CCK-33 and 125I-Bolton-Hunter labelled sulphated CCK-8 as tracer. Plasma was extracted in 96% ethanol before assay. The detection limit of the assay was 0.3 pmol/1. CCK-33 and CCK-8 were stable in plasma at 0 °C for at least 3 h, but CCK-8 was degraded at 21 °C. The trypsin inhibitor, aprotinin, did not affect the degradation of CCK-8, while the aminopeptidase inhibitor, bestatin, had a significant inhibitory effect. The basal plasma concentration of CCK in 44 normal subjects was 1.6±0.2 pmol/1, ranging from undetectable (>0.3 pmol/1) to 4.4 pmol/1. After the ingestion of a mixed meal in seven normal subjects, concentrations of plasma CCK rose from 2.0±0.2 to 7.4±0.7 pmol/1. Diurnal registration in nine people showed similar increments after each meal. The validity of the assay was further substantiated by a strong correlation between CCK measurements of identical samples with other CCK specific antisera.