Methimazole (MMI), an antithyroid drug, is associated with idiosyncratic toxicity. Reactive metabolites resulting from bioactivation of the drug have been implicated in these adverse drug reactions. Mimicry of enzymatic oxidation of MMI was carried out by electrochemically oxidizing MMI using a coulometric flow-through cell equipped with a porous graphite working electrode. The cell was coupled on-line to electrospray ionization mass spectrometry (EC/ESI-MS. ESI spectra were acquired in both negative and positive modes. In acidic medium, ESI spectral analysis showed that the dimer was the main product, while in neutral and basic media, methimazole sulfenic acid, methimazole sulfinic acid and methimazole sulfonic acid were observed as the major electrochemical oxidation products. Oxidation of MMI and subsequent trapping with nucleophiles resulted in formation of adducts with N-acetylcysteine. Some of the electrochemically generated species observed in these experiments were similar to metabolites that have been observed from in vitro and in vivo studies. Trapping studies also showed that bioactivation of MMI proceeds predominantly through the S-oxide and not through formation of thyil radicals. These results show that electrochemistry coupled to mass spectrometry can be used in mimicry of oxidative metabolism and subsequent high throughput screening of metabolites.

The reaction between tetramethylthiourea (TTTU) and slightly acidic chlorite has been studied. The reaction is much faster than comparable oxidations of the parent thiourea compound as well as other substituted thioureas. The stoichiometry of the reaction in excess oxidant showed a complete desulfurization of the thiocarbamide to yield the corresponding urea and sulfate: 2ClO2(-) + (Me2N)2C ═ S + H2O → (Me2N)2C ═ O + SO4(2-) + 2Cl(-) + 2H(+). The reaction mechanism is unique in that the most stable metabolite before formation of the corresponding urea is the S-oxide. This is one of the rare occasions in which a low-molecular-weight S-oxide has been stabilized without the aid of large steric groups. ESI-MS data show almost quantitative formation of the S-oxide and negligible formation of the sulfinic and sulfonic acids. TTTU, in contrast to other substituted thioureas, can only stabilize intermediate oxoacids, before formation of sulfate, in the form of zwitterions. With a stoichiometric excess of TTTU over oxidant, the TTTU dimer is the predominant product. Chlorine dioxide, which is formed from the reaction of excess chlorite and HOCl, is a very important reactant in the overall mechanism. It reacts rapidly with TTTU to reform ClO2(-). Oxidation of TTTU by chlorite has a complex dependence on acid as a result of chlorous acid dissociation and protonation of the thiol group on TTTU in high-acid conditions, which renders the thiol center a less effective nucleophile.

The kinetics and mechanism of the oxidation of the important antitubercular agent, ethionamide, ETA (2-ethylthioisonicotinamide), by peracetic acid (PAA) have been studied. It is effectively a biphasic reaction with an initial rapid first phase of the reaction which is over in about 5 s and a second slower phase of the reaction which can run up to an hour. The first phase involves the addition of a single oxygen atom to ethionamide to form the S-oxide. The second phase involves further oxidation of the S-oxide to desulfurization of ETA to give 2-ethylisonicotinamide. In contrast to the stability of most organosulfur compounds, the S-oxide of ETA is relatively stable and can be isolated. In conditions of excess ETA, the stoichiometry of the reaction was strictly 1:1: CHCOH + Et(CH)C(═S)NH → CHCOH + Et(CH)C(═NH)SOH. In this oxidation, it was apparent that only the sulfur center was the reactive site. Though ETA was ultimately desulfurized, only the S-oxide was stable. Electrospray ionization (ESI) spectral analysis did not detect any substantial formation of the sulfinic and sulfonic acids. This suggests that cleavage of the carbon-sulfur bond occurs at the sulfenic acid stage, resulting in the formation of an unstable sulfur species that can react further to form more stable sulfur species. In this oxidation, no sulfate formation was observed. ESI spectral analysis data showed a final sulfur species in the form of a dimeric sulfur monoxide species, HSO. We derived a bimolecular rate constant for the formation of the S-oxide of (3.08 ± 0.72) × 10 M s. Oxidation of the S-oxide further to give 2-ethylisonicotinamide gave zero order kinetics.

The kinetics and mechanism of the oxidation of methimazole (1-methyl-3H-imidazole), MMI, by chlorite in mildly acidic environments were studied. It is a complex reaction that gives oligo-oscillations in chlorine dioxide concentrations in excess chlorite conditions. The stoichiometry is strictly 2:1, with the sulfur center being oxidized to sulfate and the organic moiety being hydrolyzed to several indeterminate species. In excess MMI conditions over chlorite, the sulfinic acid and sulfonic acid were observed as major intermediates. The sulfenic acid, which was observed in the electrochemical oxidation of MMI, was not observed with chlorite oxidations. Initial reduction of chlorite produced HOCl, an autocatalytic species in chlorite oxidations. HOCl rapidly reacts with chlorite to produce chlorine dioxide, which, in turn, reacts rapidly with MMI to produce more chlorite. The reaction of chlorine dioxide with MMI is competitive, in rate, with the chlorite-MMI and HOCl-ClO2(-) reactions. This explains the oligo-oscillations in ClO2 concentrations.

The use of N-acetyl-l-methionine (NAM) as a bio-available source for methionine supplementation as well as its ability to reduce the toxicity of acetaminophen poisoning has been reported. Its interaction with the complex physiological matrix, however, has not been thoroughly investigated. This manuscript reports on the kinetics and mechanism of oxidation of NAM by acidic iodate and aqueous iodine. Oxidation of NAM proceeds by a two electron transfer process resulting in formation of a sole product: N-acetyl-l-methionine sulfoxide (NAMS=O). Data from electrospray ionization mass spectrometry confirmed the product of oxidation as NAMS=O. The stoichiometry of the reaction was deduced to be IO3– + 3NAM → I– + 3NAMS=O. In excess iodate, the stoichiometry was deduced to be 2IO3– + 5NAM + 2H+ → I2 + 5NAMS=O + H2O. The reaction between aqueous iodine and NAM gave a 1 : 1 stoichiometric ratio: NAM + I2 + H2O → NAMS=O + 2I– + H+. This reaction was relatively rapid when compared with that between NAM and iodate. It did, however, exhibit some auto-inhibitory effects through the formation of triiodide (I3–) which is a relatively inert electrophile when compared with aqueous iodine. A simple mechanism containing 11 reactions gave a reasonably good fit to the experimental data.