Thyronamines are isozyme-specific substrates of deiodinases

3-Iodothyronamine (3-T(1)AM) and thyronamine (T(0)AM) are novel endogenous signaling molecules that exhibit great structural similarity to thyroid hormones but apparently antagonize classical thyroid hormone (T-3) actions. Their proposed biosynthesis from thyroid hormones would require decarboxylation and more or less extensive deiodination. Deiodinases (Dio1, Dio2, and Dio3) catalyze the removal of iodine from their substrates. Because a role of deiodinases in thyronamine biosynthesis requires their ability to accept thyronamines as substrates, we investigated whether thyronamines are converted by deiodinases. Thyronamines were incubated with isozyme-specific deiodinase preparations. Deiodination products were analyzed using a newly established method applying liquid chromatography and tandem mass spectrometry (LC-MS/MS). Phenolic ring deiodinations of 3,3',5'-triiodothyronamine (rT(3)AM), 3',5'-diiodothyronamine (3', 5'-T(2)AM), and 3,3'-diiodothyronamine (3,3'-T(2)AM) as well as tyrosyl ring deiodinations of 3,5,3'-triiodothyronamine T(3)AM) and 3,5-diiodothyronamine (3,5-T(2)AM) were observed with Dio1. These reactions were completely inhibited by the Dio1-specific inhibitor 6n-propyl-2-thiouracil (PTU). Dio2 containing preparations also deiodinated rT(3)AM and 3',5'-T(2)AM at the phenolic rings but in a PTU-insensitive fashion. All thyronamines with tyrosyl ring iodine atoms were 5(3)-deiodinated by Dio3-containing preparations. In functional competition assays, the newly identified thyronamine substrates inhibited an established iodothyronine deiodination reaction. By contrast, thyronamines that had been excluded as deiodinase substrates in LC-MS/MS experiments failed to show any effect in the competition assays, thus verifying the former results. These data support a role for deiodinases in thyronamine biosynthesis and contribute to confining the biosynthetic pathways for 3-T(1)AM and T(0)AM.