Flavin-containing monooxygenase S-oxygenation of a series of thioureas and thiones

Abstract

Mammalian flavin-containing monooxygenase (FMO) is active towards many drugs with a heteroatom having the properties of a soft nucleophile. Thiocarbamides and thiones are S-oxygenated to the sulfenic acid which can either react with glutathione and initiate a redox-cycle or be oxygenated a second time to the unstable sulfinic acid. In this study, we utilized LC-MS/MS to demonstrate that the oxygenation by hFMO of the thioureas under test terminated at the sulfenic acid. With thiones, hFMO catalyzed the second reaction and the sulfinic acid rapidly lost sulfite to form the corresponding imidazole. Thioureas are often pulmonary toxicants in mammals and, as previously reported by our laboratory, are excellent substrates for hFMO2. This isoform is expressed at high levels in the lung of most mammals, including non-human primates. Genotyping to date indicates that individuals of African (up to 49%) or Hispanic (2–7%) ancestry have at least one allele for functional hFMO2 in lung, but not Caucasians nor Asians. In this study the major metabolite formed by hFMO2 with thioureas from Allergan, Inc. was the sulfenic acid that reacted with glutathione. The majority of thiones were poor substrates for hFMO3, the major form in adult human liver. However, hFMO1, the major isoform expressed in infant and neonatal liver and adult kidney and intestine, readily S-oxygenated thiones under test, with Kms ranging from 7–160 μM and turnover numbers of 30–40 min−1. The product formed was identified by LC-MS/MS as the imidazole. The activities of the mouse and human FMO1 and FMO3 orthologs were in good agreement with the exception of some thiones for which activity was much greater with hFMO1 than mFMO1.

Regulated methionine oxidation by monooxygenases

“…How this MetO is produced and/or excreted requires further studies, but the available literature suggests that mammalian FMOs may indeed be involved in this process. S-oxidation by FMO has been described for several organic compounds, including thioanisoles and thioureas 58,66,76 . A recent paper identified an FMO as the protein responsible for the inactivation of the transcription factor NirA in Aspergilus nidulans via the catalytic oxidation of a Met residue in its nuclear export signal 77 , suggesting that this mechanism may be more common than originally thought.…”

Human FMO2-based microbial whole-cell catalysts for drug metabolite synthesis

“…The substrates typically described for human FMO2*1 are sulfur derived compounds such as thioureas [ 6 , 7 ], thioetherorganophosphates [ 6 ], thiazetazone [ 8 ] and ethionamide [ 8 , 9 ]. FMO2 homologues from other mammalian species were shown to catalyze also the N -oxidation, e.g.…”

Morphological and Mechanistic Aspects of Thiourea-Induced Acute Lung Injury and Tolerance in the Rat

“…3 Sulfenic acids, which can be further oxidized to sulphinic and sulfonic acids, are extremely reactive electrophiles and are thought to react promptly with the parent drug or with nucleophiles such as glutathione (GSH) or other sulfhydryls to produce disulfides which disrupt the redox balance and induce oxidative stress, eventually resulting in irreversible modification of cellular proteins. 4,5 The expression of the FMO genes varies considerably between different animal species, 6 despite the relatively low number of genes (5 functional members, FMO1 to FMO5) and allelic variants of this protein family. In the wild rat lung, FMO1 and FMO2 seem to be the only FMO members expressed at significant levels.…”

“…3 Sulfenic acids, which can be further oxidized to sulphinic and sulfonic acids, are extremely reactive electrophiles and are thought to react promptly with the parent drug or with nucleophiles such as glutathione (GSH) or other sulfhydryls to produce disulfides which disrupt the redox balance and induce oxidative stress, eventually resulting in irreversible modification of cellular proteins. 4,5…”