INHIBITION OF L-THYROXINE MONODEIODINATION BY THYROXINE ANALOGS*

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Abstract
The inhibition by thyroxine analogs of the enzymatic monodeiodination of thyroxine to 3,5,3[image]-triiodo-L-thyronine was studied in vitro with kidney slices from hyperthyroid rats as the enzyme source. Of the several analogs tested, tetraiodothyropropionic acid and 3,3[image],5-triiodo-DL-thyronine proved to be the most effective inhibitors, followed by D-thyroxine and O-methyl-DL-thyroxine. Alpha-methyl thyroxamine and diiodotyrosine were essentially inactive. It is suggested that all three reactive groups are involved in varying degree in effecting thyroxine antagonism, and that this inhibition is competitive in nature. These data, together with the observations by others that one of these analogs (3,3[image],5-triiodo-DL-thyronine) blocks the metabolic effect of thyroxine in vivo, support the concept that thyroxine is converted to another compound before becoming active at the cellular level, and that one step in this conversion is the removal of one iodine atom from the terminal ring.