Pure porcine thyrocalcitonin was labeled at high specific radioactivity (200–500 μCi/μg) with 131I.Thyrocalcitonin- 131I was purified by adsorption on, and elution from, microfine granules of precipitated silica. Addition of 2- mercaptoethanol to the buffers stabilized the labeled peptide during purification, storage and incubation. Antiserum was prepared by immunization of a rhesus monkey with thyrocalcitonin conjugated to a copolymer of L-lysine and L-glutamic acid. Unlabeled porcine thyrocalcitoninthyrocalcitonin competitively inhibited the binding of thyrocalcitonin-13lI to antiporcine thyrocalcitonin. The method detected 5 MRC μU of porcine thyrocalcitonin. Comparative bioassays and immunoassays were in good agreement over more than a 2000-fold range of specific biological activities. Calf and sheep thyrocalcitonins could also be detected immunologically, but the extent of the cross-reactions was small. Rat, pigeon and human thyrocalcitonins did not cross-react significantly. (Endocrinology84: 140. 1969)

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