Orientation of thyroid peroxidase in hog thyroid microsomes

Area of Science:

• Biochemistry
• Cell Biology
• Endocrinology

Background:

• Thyroid peroxidase is a key enzyme in thyroid hormone synthesis.
• Understanding its orientation within thyroid microsomes is crucial for elucidating hormone production mechanisms.

Purpose of the Study:

• To determine the orientation of thyroid peroxidase within hog thyroid microsomal vesicles.
• To investigate the localization of thyroid peroxidase relative to the microsomal membrane.

Main Methods:

• Trypsin digestion of microsomes and deoxycholate-treated microsomes.
• Gel filtration chromatography to assess molecular size changes.
• Binding assays using Concanavalin A Sepharose.
• Iodination studies of endogenous and exogenous thyroglobulin.

Main Results:

• Trypsin treatment solubilized NADPH-cytochrome c reductase but not completely thyroid peroxidase.
• Thyroid peroxidase molecular size was unaffected by trypsin on intact microsomes but reduced after deoxycholate treatment.
• Thyroid peroxidase did not bind to Concanavalin A Sepharose, indicating it is not exposed on the outer surface.
• Endogenous thyroglobulin was iodinated, but exogenous thyroglobulin was not, suggesting peroxidase acts from within the vesicle.

Conclusions:

• Thyroid peroxidase is oriented towards the luminal side of the microsomal vesicles.
• This luminal orientation is consistent with its role in iodinating thyroglobulin within the thyroid follicle.