Modification of rat liver iodothyronine 5'-deiodinase activity with diethylpyrocarbonate and rose bengal; evidence for an active site histidine residue.

Iodothyronine 5'-deiodinase activity of rat liver microsomes was rapidly and completely lost by treatment with diethylpyrocarbonate (DEP) and by photo-oxidation with Rose Bengal (RB). In both cases inactivation followed pseudo first order reaction kinetics. Inactivation by DEP was diminished in the presence of substrate or competitive inhibitors, and was reversed by hydroxylamine treatment. In addition to photo-oxidation, deiodinase activity was also inhibited by RB in the dark. This inhibition was reversible and competitive with substrate (Ki 60 nM). These results suggest the location of an essential histidine residue at or near the active site of rat liver iodothyronine deiodinase.