INFLUENCE OF IODINE ON THE FORMATION OF 27S AND LARGER IODOPROTEINS IN RAT THYROID FOLLICLES

Abstract

SUMMARY The importance of the presence of iodine for the formation of iodoproteins larger than 19S thyroglobulin, i.e. 27S and larger iodoproteins (large-TG), was studied. Samples of colloid were collected in vivo by micropuncture of single rat thyroid follicles after treatment with a low iodine diet or potassium perchlorate. The protein composition of the colloid was analysed by microgel electrophoresis on polyacrylamide gels. It was observed that the large-TG fractions were reduced by approximately 50% after treatment with a low iodine diet for 3 months. The large-TG fractions were almost absent after treatment with perchlorate for 3 weeks. Administration of 5 or 10 μg potassium iodide to the animals on a low iodine diet resulted in a significant increase of the relative amount of the large-TG fractions in the colloid within approximately 2 h, the higher dose of KI producing a more pronounced accumulation of the large-TG fractions. Inhibition of protein synthesis by cycloheximide did not prevent the increase of large-TG proteins in animals on a low iodine diet after the administration of 5 μg KI. The administration of iodide to rats treated with potassium perchlorate did not result in the appearance of large-TG fractions. It is concluded that the aggregation of thyroglobulin molecules occurs in the follicle lumen as a consequence of iodine administration. This formation of large-TG fractions does not require exocytosis of newly synthesized protein.