IDENTIFICATION OF IODINATED PROTEINS IN CULTURED THYROCYTES AND THEIRPOSSIBLE SIGNIFICANCE FOR THYROID-HORMONE FORMATION

The thyroid gland is known to generate the iodinated hormones T-3 and T-4 from the prohormone thyroglobulin. In this report we examined whet her polypeptides other than thyroglobulin are iodinated and hormonogen ic in thyrocytes and the prerequisites for their iodination. In primar y cultures of porcine thyrocytes, a substantial portion of organified radioiodine was incorporated into cellular proteins other than thyrogl obulin. Some of these were identified by immunoprecipitation. They inc luded proteins of the extracellular matrix, plasma membrane proteins, and lysosomal enzymes, which follow in part a secretion and recapture pathway. All of these proteins come into contact with the iodinating s ystem of thyrocytes located on the apical plasma membrane and possess iodination consensus sequences. Immunoprecipitation with T-3- or T-4-s pecific antibodies showed that thyroid hormones were detectable only w ithin thyroglobulin. This was confirmed by an analysis of the iodoamin o acids of thyroglobulin, cathepsin-D (representing a secretory protei n), and aminopeptidase-N (a membrane-integrated protein) by two-dimens ional TLC, which revealed the presence of T-3 and T-4 only within the polypeptide chain of thyroglobulin. These results indicate that iodopr oteins other than thyroglobulin do not participate in the generation o f thyroid hormones in situ.