Binding of the 35-s of 35-s-propylthiouracil by follicular thyroglobulin in vivo and in vitro.

Radioactivity was found bound to follicular thyroglobulin after administration of 35-S-propylthiouracil (PTU) to rats. Denaturation of the thyroglobulin using various procedures could not separate the 35-S from the protein; it was concluded that the 35-S is bound to thyroglobulin covalently. Fractionation of saline-soluble thyroid proteins was performed by ultracentrifugation on sucrose gradients. The PTU-sulphur/thyroglobulin (S/Tg) molar ratio was calculated in all fractions. One hour after the injection of PTU the S/Tg molar ratio was the same for 19S thyroglobulin from rats on stock diet and 18S thyroglobulin from rats on low iodine diet. Injection of KI to the animals before administration of 35-S-PTU significantly reduced the ratio. The highest S/Tg ratio 1.10 was noted at 19S thyroglobulin, 17 h after a single injection of PTU. Daily injection of PTU for six days increased the S/Tg ratio to 3.3. Inverse relationship between dose of PTU and S/Tg ratio was noted at one hour. In animals injected with large dose of 35-S-PTU and sacrificed several hours later the S/Tg was higher at the 12S subunits than at the 19S protein. The amount of PTU bound to 3-8S subunits was minimal. Sulphite liberated 64 percent of the 35-S bound to thyroglobulin which appeared as four compounds on thin layer chromatography plates. The main 35-S compound liberated by sulphite was sulphate.