Deiodases

Deiodases are selenoenzymes that activate or inactivate thyroid hormones such as iodothyronines as well as thyronamines and iodothyroacetates by stereospecific deiodination.

A distinction is made between 3 deiodases:

• Type I deiodase (DIO1)
• Type II deiodase (DIO2)
• Type III deiodase (DIO3).

All 3 deiodinases are able to remove iodine atoms from thyroid hormones, thyronamines and iodothyroacetates.

The iodine atoms on the inner ring of iodothyronines are located at the 3- and 5-position, while the iodine atoms on the outer ring are called 3'- and 5'-iodine atoms. The removal of iodine atoms from the inner ring deactivates thyroid hormones.

DIO1 can catalyse both 5'-deiodination and 5-deiodination. DIO2 is a pure 5'-deiodase and can thus activate thyroxine to triiodothyronine, the active thyroid hormone. DIO3 is a 5-deiodase that inactivates thyroxine and triiodothyronine.

1. Marsili A et al (2011) Physiological role and regulation of iodothyronine deiodinases: a 2011 update-. J Endocrinol Invest. 2011 May; 34: 395-407.