Monoiodotyrosine formation during thyroglobulin processing in Golgi vesicles.

Area of Science:

• Biochemistry
• Cell Biology
• Endocrinology

Background:

• Thyroglobulin (Tg) is a key protein in thyroid hormone synthesis.
• The precise timing and location of initial iodination are crucial for understanding thyroid hormone production.

Purpose of the Study:

• To investigate the in vitro iodination capacity of vesicles derived from porcine thyroid Golgi apparatus.
• To determine the stage of pre-thyroglobulin processing at which iodination initiates.

Main Methods:

• Differential centrifugation was used to isolate Golgi-enriched subfractions from porcine thyroid glands.
• Vesicles were incubated in a medium to assess their ability to concentrate iodide and bind it to proteins.
• The effects of methylmercaptoimidazole and an H2O2 generating system on iodination were evaluated.
• Protein content of vesicles was analyzed, focusing on thyroglobulin molecules and their iodination status.

Main Results:

• Golgi-derived vesicles concentrated iodide and bound it to proteins in vitro.
• The iodination process was inhibited by methylmercaptoimidazole and enhanced by H2O2.
• Analysis revealed 18 S and 12-13 S thyroglobulin molecules lacking mannose residues and containing monoiodotyrosine.
• Iodination was observed to occur in smooth-surfaced vesicles derived from the Golgi apparatus.

Conclusions:

• In vitro iodination of thyroglobulin can commence within smooth-surfaced vesicles derived from the Golgi apparatus.
• This iodination occurs before exocytosis, initiated upon N-acetylglucosamine incorporation onto the pre-thyroglobulin molecule.
• These findings provide insights into the early intracellular events of thyroid hormone synthesis.