Monoamine oxidase activity and triiodothyronine biosynthesis in human cultured thyroid cells

1 The proposal that monoamine oxidase (MAO) is a source of peroxide in thyroid hormone biosynthesis has been examined by use of isolated cultured human thyroid cells which retain the ability to secrete triiodothyronine (T3) in response to thyroid stimulating hormone (TSH). 2 The results demonstrated the presence of MAO A and B in human thyroid cells which oxidized 5‐hydroxytryptamine and 2‐phenylethylamine, respectively, and were selectively inhibited by the MAO inhibitors clorgyline and (−)‐deprenyl. 3 Addition of propylthiouracil to the culture system induced a 61% reduction in TSH‐stimulated T3 secretion, indicating that the bulk of such secretion apparently derives from de novo iodothyronine synthesis. 4 The MAO A and B substrate, tyramine, was ineffective in stimulating T3 secretion. 5 The selective MAO inhibitors, clorgyline and (−)‐deprenyl, alone and in combination, and in the presence and absence of tyramine, failed to inhibit basal as well as TSH‐stimulated T3 secretion in cultured human thyrocytes. 6 It is therefore apparent that even though thyroid MAO A and B enzyme reactions result in the generation of H202, this H202 does not seem to play a significant role in T3 biosynthesis.