Placental Content and Characterization of Human Chorionic Thyrotropin¹

Abstract

The human chorionic thyrotropin (HCT) content of 15 placentas was measured by bioassay and radioimmunoassay after extraction of each placenta individually. There was marked individual variation, with most placentas containing very little activity. Only 4 of 15 placentas approximated the range of HCT content found previously in processing of batches of placentas and 1 of the 15 greatly exceeded it. Incubation of HCT with a placental proteinase preparation failed to destroy biologic or immunologic activity, suggesting that proteolytic degradation of HCT within the placenta was not the explanation for the marked individual variation of placental HCT content. Gel filtration of the thyrotropic fraction (89 mU/mg protein) from the placenta rich in HCT yielded a material with activity of 1000 mU/mg protein. The molecular weight of this HCT, based on its mobility on Sephadex, was very similar to that of 125I-labeled bovine thyrotropin (B-TSH). The highly purified HCT was tagged with 125I and reacted with an antibody to B-TSH; 62% of the labeled HCT was immunoreactive. The 125I-labeled HCT formed precipitin bands corresponding to B-TSH on radioautographs after immunoelectrophoresis and immunodiffusion against anti-B-TSH. Disc gel electrophoresis showed that 125I-labeled HCT was much more electronegative than 125I-B-TSH. These data indicate that, although HCT and B-TSH share a major immunologic determinant and have a similar molecular weight, there is a significant molecular difference between them.