The thyroid plasma membrane contains a Ca2+-regulated NADPH-dependent H2O2-generating system which provides H2O2 for the peroxidase-catalysed biosynthesis of thyroid hormones. The electron transfer from NADPH to O-2, catalysed by this system was studied by using diphenylene iodonium (DPI), an inhibitor of flavo- and haemo-proteins. The prosthetic group of the H2O2 generator was removed by incubation with 5 mM CHAPS at 40 degrees C, and an active holoenzyme was reconstituted with FAD, but not with FMN. The H2O2-generating system also had an intrinsic Ca2+-dependent NADPH:ferricyanide reductase activity which is probably linked to its flavodehydrogenase component (or domain). Both activities, H2O2 production and ferricyanide reductase activity, were inhibited by DPI, with similar K-1/2 (2.5 nmol/mg of protein). DPI only inhibited a system reduced with NADPH in the presence of Ca2+. NADPH could not be replaced by NADP(+), NADH or sodium dithionite, suggesting the need for specific mild reduction of a redox centre in a particular conformation. Ferricyanide protected both activities against inhibition by DPI; the NADPH:ferricyanide reductase activity was completely protected at a ferricyanide concentration 20 times lower than that needed to protect the H2O2 formation, implying at least two target sites for DPI. One might be the flavodehydrogenase component; the other was beyond, on the entity which transfers the electrons to O-2. This second site has not been identified.
