The Identification of a Specific Fragment of Dog Thyroglobulin Responsible for Immunoreactivity to Parafollicular Cells

This study provides new information related to our earlier finding of a thyroglobulin-like antigen in parafollicular cells. Dog thyroglobulin was saltedout with ammonium sulfate, then chromatographed on a Bio-Gel A-5m column. Four distinct 280-nm-absorbing peaks (I–IV) were observed. Antisera to each of these fractions were studied by immunodiffusion and immunoelectrophoresis and showed that the main antibody produced was against 19S thyroglobulin. Each of the antisera gave distinct immunoreactions with colloid droplets and follicular cells when stained with the immunoperoxidase bridge technique. Only the antiserum to peak I, however, gave a strong positive reaction to the parafollicular cells. Absorption of this antiserum with adequate amounts of the peak II fraction removed the reaction to colloid droplets and follicular cells, but left the reaction with parafollicular cells intact, while absorption with the peak I fraction eliminated the reaction with the parafollicular cells. The results indicate that the thyroglobulin-like immunoreactivity in parafollicular cells is due to protein in the peak I fraction (known as the 27S iodoprotein fraction). Polyacrylamide gradient gel electrophoresis of the peak I fraction, however, demonstrated the presence of two or three additional components with molecular weights larger than 27S. Further study is necessary to determine which of the components of the peak I fraction is responsible for the parafollicular cell reaction. (Endocrinology102: 1702, 1978)