Thyroglobulin biosynthesis in the rat thyroid

Abstract

The formation of thyroglobulin was studied by following the in vivo incorporation of leucine-3H into the soluble proteins of rat thyroid microsomal and supernatant cell fractions. The proteins were separated by centrifugation in a sucrose density gradient. Their concentrations were estimated spectrophotometrically at 280 mμ, and their radioactivity was determined in a liquid scintillation spectrometer. Analyses of both supernatant and microsomal proteins showed two optical density peaks corresponding to sedimentation constants of 19 S (thyroglobulin) and 3–8 S. No peak was observed in the 12 S region. Radioactivity determinations showed a clear labeling of both supernatant and microsomal 3–8 S proteins already 10 minutes after the leucine-3H injection. In the supernatant fraction there was at this time a high radioactivity peak also in the 12 S region. At 30 minutes the supernatant 19 S thyroglobulin showed a definite labeling but the specific activity of the 12 S component was much higher. Both radioactivities increased markedly during the following hours but the 12 S labeling reached a maximum at 3 hours while the 19 S labeling continued to increase to 6 hours. The microsomal proteins showed throughout a rather low and fairly constant labeling in the 12 S region. The specific activity of the microsomal 19 S thyroglobulin was higher than that of the supernatant 19 S at 1 hour and 3 hours but decreased rapidly between 3 and 6 hours, simultaneously with the increase of supernatant 19 S radioactivity. Considering the results from similar studies on guinea pigs (6) the following interpretations of the findings are suggested: the 12 S component has a higher turnover rate in the rat than in the guinea pig thyroid; 12 S is a precursor of 19 S; microsomal 19 S is a precursor of supernatant 19 S.