Thyroxine (T4) release from thyroglobulin and its T4-containing peptide by thyroid thiol proteases

Area of Science:

• Biochemistry
• Enzymology

Background:

• Thyroid hormones, including thyroxine (T4), are crucial for metabolic regulation.
• Thyroglobulin serves as the storage protein for thyroid hormones within the thyroid gland.
• Lysosomal thiol proteases play a role in the degradation of thyroglobulin and release of thyroid hormones.

Purpose of the Study:

• To investigate the T4-releasing activities of two purified hog thyroid lysosomal thiol proteases, TP-1 and TP-2.
• To determine the efficiency of these proteases in releasing T4 from intact thyroglobulin, a large fragment, and a smaller peptide.

Main Methods:

• Purification of two thiol proteases (TP-1 and TP-2) from hog thyroid lysosomal extracts.
• Assay of T4 release from hog thyroglobulin, a fragment, and a T4-containing peptide using reversed-phase high-performance liquid chromatography (HPLC).

Main Results:

• TP-1 released 8% of T4 from thyroglobulin, 55% from a fragment, and 95% from a peptide.
• TP-2 showed minimal T4 release (2%) from thyroglobulin and no release from the fragment or peptide.
• TP-2 addition synergistically increased TP-1's T4 release from thyroglobulin approximately twofold.

Conclusions:

• TP-1 is an effective T4-releasing enzyme, with activity increasing as the substrate size decreases.
• TP-2 can facilitate TP-1's T4-releasing activity, suggesting a role in initial thyroglobulin degradation.
• TP-1 likely cleaves T4 from the N-terminal region of thyroglobulin, based on peptide analysis.