RADIOIMMUNOASSAY OF PLASMA CHOLECYSTOKININ (CCK), DUODENAL RELEASE OF CCK, DIURNAL-VARIATION OF PLASMA CCK, AND IMMUNOREACTIVE PLASMA CCK COMPONENTS IN MAN

Abstract

A precise and specific radioimmunoassay method for measuring plasma cholecystokin (CCK) is described. The present assay system using a stable tracer iodinated by means of a modified Chloramine-T method followed by purification on a Sephadex G-15 and a SP Sephadex C-25 column, as well as careful corrections for non-specific plasma effects, allows measurements of fasting plasma CCK in the low pmol/l range, the significant rise in plasma CCK following duodenal infusion of fat and the significant diurnal variation of plasma CCK. Apparent immunoreactive meal-stimulated plasma CCK was eluted from a Sephadex G-50 superfine column in 4 fractions. The 1st and largest peak probably represents plasma CCK bound to plasma proteins and non-specific plasma effects, the 2nd and smaller peak big CCK with MW between 5000 and 30,000, the shoulders following the 2nd peak ordinary CCK33 and CCK39 variant, and the final, and by far the smallest peak, may possibly represent COOH-terminal tetra- or octapeptides of CCK.