Iodination tyrosine

Amino acid-derived hormones include the catecholamines, epinephrine and norepinephrine (qv), and the thyroid hormones, thyroxine and triiodothyronine (see Thyroid AND ANTITHYROID PREPARATIONS). Catecholamines are synthesized from the amino acid tyrosine by a series of enzymatic reactions that include hydroxylations, decarboxylations, and methylations. Thyroid hormones also are derived from tyrosine iodination of the tyrosine residues on a large protein backbone results in the production of active hormone. 

Isoflavones have been implicated in goiter induction. Soybean extracts inhibit reactions catalyzed by thyroid peroxidase (TPO), essential to the synthesis of thyroid hormones (Divi et al., 1997). Genistein and daidzein (at about 1-10 p,M of IC50) may act as alternative substrates for tyrosine iodination (Divi et al., 1997). Furthermore, genistein and daidzein have also been shown to cause the irreversible inactivation of TPO in the presence of hydrogen peroxide. Genistein also inhibits thyroxine synthesis in the presence of iodinated... 

Lee PHK, Nguyen TM-D, Chung NN, Schiller PW, Chang KJ. Tyrosine-iodination converts the delta opioid peptide antagonist TIPP to an agonist. Eur J Pharmacol 1995 280 211-214. 

Only two amino acids, tyrosine and histidine, form stable derivatives as the result of peroxidase-catalyzed iodination. All the tyrosine and histidine residues in a protein are not identical with respect to their reactivity or their geographic position. The residue which will be iodinated by lactoperoxidase must have the proper geometric position, while other methods of halogenation are influenced only by reactivity. The reactivity depends upon the microenvironment of the residue. There is an inverse relationship between the extent of tyrosine iodination and the dielectric constant of the environment of the tyrosine. Tyrosine iodination increases with decreasing dielectric constant. Steric factors also influence iodination since the relatively large iodine atom may be blocked in either the production of monoiodotyrosine or the formation of diiodotyrosine. 

Thiram also leads to thyroid dysfunction. This effect is thought to be a result of metabolic release of sulfur in follicular cells, causing inhibition of tyrosine iodination and ultimately hormone synthesis. Thiram... 

L. P. Cellular responses to Pyrularia thionin are mediated by Ca2+ influx and phospholipase A2 activation and are inhibited by thionin tyrosine iodination. Proc. Nail. Acad. Sci. USA 1989, 86, 5849-5853... 

Antithyroid drug prototype inhibits tyrosine iodination and coupling reactions orally active. Tox rash, agranulocytosis (rare). Methimazole is similar but more lipid-soluble. 

Various factors are known to influence tyrosine iodination chronic thyroiditis, cretinism, antithyroid drugs, carcinogens, etc. It is often stated that propyl-... 

The enzymes involved in tyrosine iodination remain unknown. It has been proposed that in addition to the iodide peroxidase, a specific tyrosine iodinase is also required. Whether this iodinase catalyzes the iodination of both mono- and diiodotyrosine remains to be established. 

Inhibitors of tyrosine iodination have been found in supernatants obtained after differential centrifugation of thyroid homogenate. The role of these inhibitors in regulating the rate of iodination and thyroid activity is unknown. 

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