Tyrosine phosphorylation blocks tyrosine free radical formation and, hence, the hormonogenic iodination reaction.

It is known that a tyrosine free radical is produced during the hormonogenic iodination reaction of tyrosine residues on thyroglobulin. In the hormonogenic region of thyroglobulin, phosphorylated tyrosine residues have been detected. Using an vitro tyrosine iodinating system we report that the hormonogenic reaction cannot go off if tyrosine becomes phosphorylated. Enzymatic dephosphorylation of the modified amino acid restored the ability of the molecule to become iodinated. Considering the mechanism of the tyrosine free radical formation, these observations are due to the inability of the phosphorylated amino acid to form a free radical. Our data may suggest a putative regulatory mechanism in thyroid hormone synthesis by phosphorylation of hormonogenic tyrosine residues on thyroglobulin.

Measurement of phosphotyrosine phosphatase activity using the Folin-Ciocalteu phenol reaction.

An assay for the estimation of phosphotyrosine phosphatase using the Folin-Ciocalteu phenol reaction to monitor enzyme activity is presented. The method is based on the property of the substrate phosphotyrosine not to react as a phenol until it is dephosphorylated. The method is sensitive, there is no interference from the use of detergents and it does not rely on special laboratory equipment to distinguish tyrosine from phosphotyrosine.