Endocrine Abstracts

3-iodothyronamine (3-T₁AM) and thyronamine (T₀AM) are novel endogenous signaling molecules that exhibit great structural similarity to thyroid hormones but apparently antagonize classical thyroid hormone (T₃) 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. Since 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 (rT₃AM), 3′,5′-diiodothyronamine (3′,5′-T₂AM), 3,3′-diiodothyronamine (3,3′-T₂AM) as well as tyrosyl ring deiodinations of 3,5,3′-triiodothyronamine (T₃AM) and 3,5-diiodothyronamine (3,5-T₂AM) were observed with Dio1. These reactions were completely inhibited by the Dio1 specific inhibitor 6n-propyl-2-thiouracil (PTU). Dio2 containing preparations also deiodinated rT₃AM and 3′,5′-T₂AM 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 which 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-T₁AM and T₀AM.