Ethylenethiourea (ETU) is a thyroid carcinogen present in foods formed by degradation and metabolism of ethylenebis[dithiocarbamate] fungicides. ETU inhibits thyroid peroxidase (TPX), the enzyme that catalyzes synthesis of thyroid hormones. Inhibition of TPX-catalyzed reactions by ETU occurs only in the presence of iodide ion with concomitant oxidative metabolism to imidazoline and bisulfite ion. Inhibition ceases upon consumption of ETU with no loss of enzymatic activity and negligible covalent binding of ETU to TPX. TPX inhibition by ETU is unlike that for derivatives of imidazoline-2-thione, which cause suicide inactivation via covalent binding to the prosthetic heme group. These results demonstrate a metabolic route for detoxication of ETU in the thyroid and suggest that low-level or intermittent exposure to ETU would have minimal effects on thyroid hormone production.

Derivatives of imidazole-2-thiones block reactions catalyzed by thyroid peroxidase (TPX) and the closely related lactoperoxidase (LPX), and this property is used therapeutically to treat hyperthyroidism. The reactions of a series of benzimidazoline-2-thiones with chemical and enzymatic oxidants were investigated to probe systematically the mechanism of inhibition. Oxidation of benzimidazoline-2-thione (I) and 1-methylbenzimidazoline-2-thione (II) with 3-chloroperbenzoic acid (PBA) yielded reaction products and stoichiometry consistent with benzimidazole-2-sulfenic acids as reactive intermediates. The N,N'-disubstituted nature of 1,3-dimethylbenzimidazoline-2-thione (III) precludes sulfenic acid formation by tautomerization, and the oxidation of III with PBA yielded products and stoichiometry that were consistent with a benzimidazole-2-sulfonyl ylide as the reactive intermediate. I and II are suicide inhibitors of LPX and TPX, but III was found to inhibit only peroxidase-catalyzed iodination reactions by an alternate substrate mechanism. These results provide support for the hypothesis that imidazole-2-sulfenic acids are important reactive intermediates in the suicide inactivation of TPX and LPX and relate the chemical reactivity of the inhibitor with both the potency and mechanism of inhibition. These results suggest that 1,3-disubstituted thiourea derivatives represent a new class of potential antihyperthyroid drugs that block TPX-catalyzed tyrosine iodination but do not cause irreversible enzyme inactivation.

Hexachlorobenzene (HCB) elicits concentration-dependent and saturable type 1 binding spectra when added to oxidized (Fe3+) cytochrome P-450 (CYT P-450) in control, phenobarbital- (PB) induced, and beta-naphthoflavone- (BNF) induced male Sprague-Dawley rat liver microsomes. The spectral binding constants (Ks) for HCB in control and PB-induced microsomes are 180 microM and 83 microM, respectively, and correlate inversely with the specific content of CYT P-450 (0.9 and 2.1 nmol/mg) in the two microsomal preparations. BNF-induced microsomes show type 1 interaction only at low HCB concentration. Overall biotransformation of HCB, monitored by loss of [14C]HCB from the reaction medium, is dependent on NADPH and intact microsomes. Dimethyl sulfoxide (Me2SO), a potent hydroxyl radical scavenger and the solvent used for HCB dissolution, does not affect the biotransformation of HCB in aerobic reactions. Pentachlorobenzene (PCB) appears to be the initial and major isolatable CYT P-450 mediated dechlorination product of HCB with NADPH-fortified rat liver microsomes. Trace levels of pentachlorophenol (PCP) and an unidentified metabolite are also observed. PCB formation is enhanced under anaerobic conditions but is inhibited by metyrapone and carbon monoxide. PCB formation is also inhibited with aerobic reaction conditions, while PCP formation is observed. The data indicate that CYT P-450 in hepatic microsomes supports the reductive dechlorination of HCB to PCB.

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