Specificity of cholecystokinin antibody may influence choice of tracer for radioimmunoassay.

When CCK33 was iodinated at His 20 with lactoperoxidase, the labelled hormone was less immunoreactive under radioimmunoassay conditions than CCK39 which is readily iodinated at Tyr 1. Since the difference in immunoreactivities could not be due to different degrees of oxidative damage, the regional specificity of the assay antibody (UT122, from the laboratory of J.C. Thompson, University of Texas) was re-examined. A synthetic parital sequence, CCK33 6-16, which had similar conformation to the same sequence in the intact peptide, as shown by CD analysis, was devoid of immunoreactivity. Results with sulphated and non-sulphated C-terminal octapeptides of the hormone, CCK8 (CCK33 27-33), indicate limited dependence of the C-terminal region for binding to antibody. Thus, the lower binding with 125I-CCK33 than with 125I-CCK39 indicates that His 20 is an important feature of the immunogenic site and that the iodine atom is large enough to cause steric hindrance. This explains why the method of iodination might have to be varied depending on the regional specificity of antibodies.