Characterization of the Major Forms of Human Calcitonin in Tissue and Serum *

Publisher Website
Google Scholar
Abstract
We have characterized the major forms of immunoreactive calcitonin (i–hCT) in extracts of bothcancerous (thyroid and metastatic lymph node) and normal tissue (thyroid) and in serum from patients with medullary carcinoma of the thyroid (MCT). Samples were examined by isoelectric focusing with and without urea pretreatment. Affinity of the major calcitonin forms for both mannose– and N-acetylneuraminic acidspecific lectins was examined. Molecular weight of the major calcitonin forms was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Results of our investigation indicate that 1) the major forms of i–hCT in tissue extracts have apparent isoelectric points (pI) at pH ≤ 3.0, 5.0–5.5, 6.4–6.7, and 7.5–7.8; 2) the pI ≤ 3.0, 5.0–5.5, and 6.4–6.7 forms of calcitonin predominate in MCT serum; 3) pretreatment of both tissue extracts and MCT serum with urea, to reduceaggregation and protein binding, produced a 3– to 10– fold increase in detectable calcitonin; 4) when frozen at their pi, the major forms of calcitonin exhibit marked aggregation accompanied by a lowering of the pi; 5) the pI ≤ 3.0 form of i–hCT may be the only form that is a glycoprotein; and 6) the apparent molecular weights of the pI ≤ 3.0, 5.0–5.5, 6.4–6.7, and 7.5–7.8 forms of calcitonin are 63,000, 30,000, 20,000 and 4,000, respectively. The pI ≤ 7.5–7.8 form of calcitonin with a molecular weight of 4000 exhibited the same biochemical properties as synthetic human calcitonin and thus was assumedto represent the calcitonin monomer. While the calcitonin monomer was the predominant form of i–hCT in the normal thyroid, it was only a minor portion of the total i–hCT in both the MCT extract and MCT sera. Therefore, radioimmunoassays specific to the larger molecular weight acidic protein forms of i–hCT must be developed to accurately quantify i–hCT in cancer sera.

This publication has 4 references indexed in Scilit: