The preparation of a stable fully immunoreactive 125I-labeled CCK39 using a modified Iodo-gen method with high specific radioactivity; the production of an avid and specific cholecystokinin (CCK) antiserum, and a sensitive, precise and specific radioimmunoassay method allowing measurements of fasting plasma CCK in the low picomole per liter range together with the significant rises in plasma CCK following a test meal and duodenal infusion of fat are described. Apparent immunoreactive fasting plasma CCK was eluted from a Sephadex G-50 Fine column in one peak probably representing plasma CCK bound to plasma proteins and nonspecific plasma effects. Apparent immunoreactive postprandial plasma CCK was eluted from a Sephadex G-50 Fine column in four peaks. The first peak probably represents plasma CCK bound to plasma proteins and nonspecific plasma effects; the second peak probably represents component I with a molecular weight between some 5,000 and 30,000; the third peak probably represents component II or CCK33, and the fourth peak probably represents component IV or CCK8.
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Burhol et al. (2009) studied this question.