USE OF REGION SPECIFIC RADIOIMMUNOASSAYS FOR CHARACTERIZATION OF CIRCULATING CALCITONIN IN PATIENTS WITH MEDULLARY CARCINOMA OF THE THYROID-GLAND

Two antisera with known region specificities have been used to characterize calcitonin immunoreactivity (iCT) in serum of patients with medullary thyroid carcinoma (MCT). Antiserum I which was raised against the synthetic hormone (1-32 amino acid residues), contained heterogeneous populations of immunoglobulins directed predominantly against carboxyterminal sequences of the hormone, but the antiserum reacted also with the amino-terminal fragment (1-10 amino acid residues). Antiserum II, which was raised against the carboxy-terminal hormone fragment (11-32 amino acid residues) reached equally well with the intact hormone and the C-terminal fragment, but showed negligible binding of the amino terminal fragment. Antiserum I measured therefore both amino-terminal and carboxy-terminal sequences of calcitonin while antiserum II measured only carboxy-terminal amino acid sequences. In 40 patients with MCT, antiserum I measured usually the highest concentration of serum iCT suggesting the presence of non-uniform hormone immunoreactivity. The different molecular forms of circulating iCT in 7 MCT patients were explored by using antiserum I after gel filtration on Sephadex G-100. The patients who were selected on basis of iCT measurement in serum using antiserum I and II, could be divided into 3 groups which showed characteristic iCT profiles. Group 1, in which antiserum II measured a higher concentration of serum iCT, contained predominantly (60-70%) small fragments of calcitonin immunoreactivity. On the other hand, in the sera of group 3 in which antisera I measured an equal or the highest concentrations, the dominant form of the hormone consisted of molecular sequences equal to or larger than the intact hormone (90%). In group 2, the two antisera measured an equal amount of serum iCT and molecular forms consisting mostly of larger hormone fragments dominated (50%). All the patients were normocalcaemic in spite of frequently grossly elevated serum iCT, and 33 out of 36 patients had normal serum immunoreactive parathyroid hormone. In conclusion: Serum iCT is heterogeneous and represents peptides of quite different molecular size with nor or low biological activity. Most of the serum calcitonin immunoreactivity consists of peptides with carboxy-terminal amino acid sequences. Most, if not all, of the amino-terminal calcitonin immunoreactivity is due to monomeric and polymeric hormonal forms.