Selectivity in tyrosyl iodination sites in human thyroglobulin

Area of Science:

• Biochemistry
• Endocrinology
• Molecular Biology

Background:

• Thyroid hormone synthesis involves the iodination of thyroglobulin (Tg) by thyroid peroxidase (TPO).
• Previous research suggests TPO preferentially iodinates hormonogenic tyrosyl residues on Tg.
• The relative contributions of Tg structure and TPO activity in site selection remain unclear.

Purpose of the Study:

• To investigate the roles of thyroglobulin (Tg) substrate and thyroid peroxidase (TPO) enzyme in selecting early iodination sites.
• To compare chemical and enzymatic iodination of Tg to elucidate site-specific iodination mechanisms.
• To determine if Tg's native structure dictates preferential iodination of hormonogenic sites.

Main Methods:

• Low-iodine human Tg was iodinated chemically and enzymatically with 125I to a matched low level.
• Iodinated Tg was reduced, alkylated, digested with trypsin, and fractionated using reverse-phase HPLC.
• Purified iodinated peptides were sequenced to identify the location of iodinated tyrosyl residues.

Main Results:

• Both chemical and enzymatic iodination yielded nearly identical 125I-peptide maps.
• Iodination occurred at the same three tyrosyl sites (Tyr 5, Tyr 2553, Tyr 2520) in both conditions.
• Enzymatic iodination showed a higher diiodotyrosine (DIT) content compared to chemical iodination, suggesting Tg-peroxidase interaction.

Conclusions:

• The native structure of thyroglobulin (Tg), not the enzyme thyroid peroxidase (TPO), primarily dictates the preferential iodination of hormonogenic tyrosyl sites.
• TPO plays a minimal role in selecting early iodination sites on Tg.
• Preferential diiodotyrosine (DIT) formation appears to result from an interaction between Tg and the iodoperoxidase enzyme.