Selenenyl iodide: a new substrate for mammalian thioredoxin reductase.

Areneselenenyl iodide stabilised by internal chelation has been synthesized and evaluated as a substrate of thioredoxin reductase (TrxR). The reactivity of TrxR obtained from human placenta towards selenenyl iodide was found to be much higher than that of the E. coli enzyme, indicating the essential nature of a selenocysteine residue in the active site of the human enzyme. The addition of thioredoxin (Trx) significantly enhanced the TrxR-catalysed reduction of selenenyl iodide 1. These studies on the reduction of a selenenyl iodide by the thioredoxin system suggest that stable selenenyl iodides could be new substrates for human TrxR. The Trx system could act as a cofactor for iodothyronine deiodinase by reducing the selenenyl iodide intermediate in the second-half of the deiodinase catalytic cycle to regenerate the active site. The TrxR-catalysed reduction of 1 was not inhibited by the anti-thyroid drug, PTU, suggesting that the involvement of the Trx system in the deiodinase cycle may be responsible for the insensitivity of certain deiodinases towards clinically useful thiourea drugs.