Studies on a Soluble Thyroid Iodotyrosine Deiodinase: Activation by NADPH and Electron Carriers

Abstract

Unlike the microsomal iodotyrosine deiodinase activity, which was responsive to both NADPH and dithionite, the enzymatic activity solubilized and purified from bovine thyroid microsomes could no longer utilize NADPH for deiodination. The purified enzyme could be activated by dithionite, and by methyl viologen (MV), reduced by NADPH with thyroidal NADPH-cytochrome c reductase, or by ferredoxin (Fd), in presence of NADPH and Fd-NADP reductase. The purified enzyme could not be activated by photoreduced FMN or by reducing agents having more positive oxidation-reduction potentials than the Fd-NADP system (Eo'' = -420 mV). N-ethylmaleimide inhibited microsomal deiodination and the activation of the purified enzyme by reduced MV or Fd, but had no effect on activation by dithionite, possibly because the latter can release NEM bound to thiol groups. The iron-chelating agents, 2,2''-dipyridyl and o-phenanthroline, inhibited microsomal deiodination but did not affect the dithionite-, MV- or Fd-activation of the microsomes or of the purified enzyme. The data suggest that microsomal deiodination of diiodotyrosine involves reduction of a flavoprotein deiodinase whose catalytic activity requires unblocked sulfur and that the electron flow from NADPH to the substrate is via an intermediate of highly negative redox potential which probably contains Fe.