Identification of the major binding protein of salmon calcitonin in the rat

The major serum binding protein of salmon calcitonin (sCT) in the rat was identified. High-molecular-weight (HMW) forms of sCT, produced by the incubation of radioactive sCT in rat serum, were isolated by gel filtration and analysed by chromatofocusing. The major radioactive peak was eluted at the region of albumin in gel filtration, and this peak had a slightly higher pI than albumin on chromatofocusing. Immunoreactivity of the radioactive peak fraction in chromatofocusing, examined by immunodiffusion, showed that the major protein in the fraction was albumin. Immunoreactivity of the radioactive HMW fraction isolated by gel filtration was studied by immunoprecipitation, indicating that a large portion of the fraction reacted with anti-rat albumin antiserum. These results suggested that the radioactive peak in chromatofocusing represents the complex of sCT and rat albumin. Further, HMW forms of sCT were analysed by immunoelectrophoresis and autoradiography, and most of the radioactivity was found on the precipitation line of albumin. These results demonstrate that the major binding protein of sCT in rat serum is albumin.