Re-evaluation of some factors affecting the stability of rat thyroglobulin

The stability of thyroglobulin obtained after in vivo or in vitro labeling of thyroids from rats kept chronically on an iodine-poor diet (about 30 micrograms I/kg) has been studied by sucrose gradient density centrifugation. The role played by factors such as the degree of iodination, the procedure of labeling, the temperature and the ionic strength, has been reevaluated. Under the experimental conditions used in this study, the stability of thyroglobulin from rats on iodine-poor diet decreases with increasing time of the diet. For comparable levels of iodination, in vivo labeling results in a more stable thyroglobulin than in vitro labeling of the glands: The sedimentation coefficient of in vivo labeled poorly iodinated thyroglobulin is slightly lower (17.5--18.5 S) than that of well iodinated thyroglobulin (19.0 S), independent of the temperature and/or ionic strength used, though at low temperature and/or low ionic strength there is some unfolding of thyroglobulin. The results obtained demonstrate that in order to compare data on thyroglobulin stability it is necessary to keep the experimental conditions constant.