Biosynthesis of the 27 S thyroid iodoprotein

Abstract

Biosynthesis of the 27 S thyroid iodoprotein was studied in rats in vivo, and evidence was obtained that 19 S, the major thyroid iodoprotein, is the immediate precursor of 27 S. In untreated rats, labeled 19 S is found within minutes after the administration of [ 3H]leucine, whereas the appearance of 27 S is preceded by a lag-phase of at least 1 hr. Inhibition of protein synthesis by cycloheximide administration resulted in a several fold increase in the proportion of [ 3H]leucine-labeled 27 S relative to 19 S. In contrast, inhibition of iodination with antithyroid agents prevented the formation of 27 S, but not of 19 S synthesis. Since formation of 27 S is dependent on iodination, but not on de novo protein synthesis, 27 S must be derived from the preexisting pool of 19 S in the follicular colloid. It is suggested that 27 S is formed by chemical alterations produced as a by-product of iodination of the tyrosyl residues, probably involving oxidative side-reactions. It appears that the quaternary structure of the 27 S iodoprotein is not encoded into its structural gene, but is a result of post-transcriptional events.