Reactivity of ACTH and synthetic ACTH peptides with antisera to human calcitonin

We studied reactivity of highly purified pituitary hormones in our human calcitonin (hCT) radioimmunoassay (RIA) which can detect 1 pg of hCT. ACTH at doses of greater than 1 microgram of peptide per RIA tube reacted in the hCT assay, as did beta-endorphin (beta EPH) at a dose of 10 micrograms per tube. No reactivity was observed with comparable concentrations of all other known pituitary hormones. ACTH also reacted at doses greater than 1 microgram per tube with 7 other hCT antisera which recognized differing antigenic determinants in the calcitonin molecule but it was not reactive with 2 antisera against porcine calcitonin or 2 antisera against salmon calcitonin. This slight degree of cross-reactivity of hACTH and beta EPH in the hCT RIA cannot account for the presence of immunoreactive CT in pituitary glands. Nevertheless, antisera used for the localization of peptides must be rigorously tested for the existence of cross-reactivities with other possible substances, especially if such antisera detect the peptide in unexpected tissues.