The Effects of Low Temperature on the Conformation of Thyroglobulin

Abstract

Abstract Analyses of l-valine[14C] incorporation into protein by guinea pig thyroid tissue slices show different numbers of sedimenting species depending on the temperatures used for sucrose gradient centrifugation. When analyzed at 23° the radioactive peaks which are observed correspond to native 19 S thyroglobulin and species sedimenting in the 3 to 8 S range. At 2°, the temperature generally used for sucrose gradient analysis, a new sedimenting component is observed with a rate near 15 S. This species is produced from 19 S as the temperature is reduced and appears to represent an unfolded form of 19 S. A rapid, reversible equilibrium exists between the 19 and 15 S species. Low iodine thyroglobulin, obtained from goitrogen-fed guinea pigs, showed temperature-dependent changes in sedimentation rates similar to those of newly synthesized protein. Thyroglobulin from normal animals, however, did not show any changes at low temperatures. Evidently 19 S becomes more refractory to low temperature unfolding with increasing levels of iodine. With standing at 2°, the 15 S species dissociates into 12 S subunits. When the temperature is raised to 23°, 19 S is reformed from the 12 S subunits. In contrast to the rates of the 19 S ⇌ 15 S reaction, which are faster than the time required for sucrose gradient analysis, the rates of the 15 S ⇌ 12 S reaction are slower. In addition to iodine, it was observed that ammonium sulfate affects the stability at low temperatures of newly synthesized or low iodine thyroglobulin. Ammonium sulfate precipitation was found to enhance the formation of 12 S.