A microgel electrophoretic study of the stability of thyroid iodoproteins

Abstract

The stability of the large iodoproteins in the rat thyroid gland was studied by polyacrylamide gel electrophoresis. 19S Tyroglobulin was stable in phosphate-buffered saline (0.01 M phosphate, 0.15 M NaCl, pH 6.8; PBS) containing 20% (w/w) sucrose. Reduction of the sucrose concentration by dilution with PBS resulted in the appearance of two 12S fractions and a band migrating with the front. After the same treatment of 27S iodoprotein, also stable in PBS + 20% sucrose, no large dissociation products appeared, only a band moving with the front. Urea of 8 M, pH 6.8, and d2 M urea, pH 8.1, partially dissociated 19S thyroglobulin into three species and a band at the electrophoretic front. The main dissociation product probably represents a modified 12S protein. Both treatments further resulted in a reduced gel migration of undissociaetd tyroglobulin. After ürea treatment of 27S iodoprotein, larger amounts of front species were found than after the same treatment of 19S tyroglobulin, where only trace amounts of large dissociation products were observed. The main dissociation product in the front band of 19S tyroglobulin had a molecular weight of 82,000–86,000 (SDS gel electrophoresis). Four other distinct fractions were observed. It was concluded that sucrose has a stabilizing effect on the large thyroid iodoproteins. The different dissociation patterns of 19S thyroglobulin and 27S iodoprotein indicate that the subunits of 27S, to a larger extent than those of 19S, are linked together by non-covalent bonds.