Effect of Thyrotropin on Iodoprotein of Thyroid Cell Fractions

Abstract

Fractions of dog thyroid homogenate were prepared by differential centrifugation after prelabeling the animals in vivo with radioactive iodine. The fraction obtained at 15,000 x g contained protein with the electrophoretic properties and the iodoamino acid content of thyro-globulin. This fraction showed colloid droplets by electron-microscopy. When thyrotropin (TSH) was administered to prelabeled dogs treated with methimazole, a marked increase in labeled protein sedimenting at 15,000 x g was noted. A smaller increase was detected in the 800 x g fraction, while there was little effect on the 100,000 x g fraction or the supernatant of this fraction. The increase in labeled protein in the 15,000 x g fraction was noted as early as 10 min after TSH administration. These studies support the theory that TSH enhances the migration of protein from the follicular lumen into intracellular particulate droplets as the first step in the stimulated release of thyroid hormones, and they should provide the basis for a more detailed investigation of the succeeding steps in this process.