Inhibition of Thyroidal Deiodination of Diiodotyrosine by Compounds Which Enhance NADPH Oxidation1

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Abstract
The deiodination of L-DIT-131I[3,5,diiodo-L-tyrosineJodine-131] in calf thyroid slices was inhibited by menadione, methylene blue and phenazine methosul-fate and in NADPH [reduced nicotinamide adenine dinucleotide phos -phate] -supplemented rat thyroid homogenates by these substances and also 2,6 -dichlorophenolindophenol (DCPIP), cytochrome c, potassium fer -ricyanide, serotonin, acetylcholine and epinephrine. In both slices and homogenates, NADPH and NADP enhanced deiodination and hemogenates menadione competitively inhibited the stimulating effect of NADPH on deiodination. Menadione, DCPIP, acetylcholine, serotonin and epinephrine enhanced NADPH disappearance in rat thyroid homogenates. The accelerated oxidation of NADPH in the presence of menadione and DCPIP was only partially blocked by Dicumarol, suggesting mediation by enzymatic pathways in addition to DT diaphorase. Dicumarol did not overcome the inhibition of deiodination produced by menadione. TSH [thyroid-stimulating hormone] in vitro, under experimental conditions claimed to increase NADP synthesis, failed to stimulate deiodination in dog or calf thyroid slices.