Enzyme-mediated protein haptenation of dapsone and sulfamethoxazole in human keratinocytes: II. Expression and role of flavin-containing monooxygenases and peroxidases

Oregon State University, Corvallis, Oregon, United States
Journal of Pharmacology and Experimental Therapeutics (Impact Factor: 3.97). 11/2006; 319(1):497-505. DOI: 10.1124/jpet.106.105874
Source: PubMed


Arylamine compounds, such as sulfamethoxazole (SMX) and dapsone (DDS), are metabolized in epidermal keratinocytes to arylhydroxylamine metabolites that auto-oxidize to arylnitroso derivatives, which in turn bind to cellular proteins and can act as antigens/immunogens. Previous studies have demonstrated that neither cytochromes P450 nor cyclooxygenases mediate this bioactivation in normal human epidermal keratinocytes (NHEKs). In this investigation, we demonstrated that methimazole (MMZ), a prototypical substrate of the flavin-containing monooxygenases (FMOs), attenuated the protein haptenation observed in NHEKs exposed to SMX or DDS. In addition, recombinant FMO1 and FMO3 were able to bioactivate both SMX and DDS, resulting in covalent adduct formation. Western blot analysis confirmed the presence of FMO3 in NHEKs, whereas FMO1 was not detectable. In addition to MMZ, 4-aminobenzoic acid hydrazide (ABH) also attenuated SMX- and DDS-dependent protein haptenation in NHEKs. ABH did not alter the bioactivation of these drugs by recombinant FMO3, suggesting its inhibitory effect in NHEKs was due to its known ability to inhibit peroxidases. Studies confirmed the presence of peroxidase activity in NHEKs; however, immunoblot analysis and reverse transcription-polymerase chain reaction indicated that myeloperoxidase, lactoperoxidase, and thyroid peroxidase were absent. Thus, our results suggest an important role for FMO3 and yet-to-be identified peroxidases in the bioactivation of sulfonamides in NHEKs.

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Available from: Ronald Hines, Sep 13, 2015
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    • "Neither CYPs nor cyclooxygenases mediate this bioactivation of sulfamethoxazole and dapsone in NHECs. Methimazole, a prototypical substrate of FMOs, inhibited the bioactivation of sulfamethoxazole and dapsone in the NHECs (Vyas et al. 2006). Diphenylthiourea, a catalytic enhancer used during neoprene synthesis, was bioactivated by FMO-catalyzed S-oxygenation (Samuelsson et al. 2011). "
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    • "Contradictory results were obtained for the distribution of the dermal FMO isoform (Hu et al., 2010; Jackh et al., 2011). Vyas et al. (2006b) and Hu et al. (2010) did not identify FMO in the EpiDerm skin model and in HaCaT cells. "
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    • "SMX is predominantly inactivated through N-acetylation by two polymorphic enzymes, N-acetyltransferase 1 (NAT1) and NAT2 [6,7](Figure 1). Alternatively, SMX can be activated by cytochrome P450s (mainly CYP2C9) in the liver, or by peroxidases (MPO) [8], flavin-containing monooxygenases (FMOs) [9], and prostaglandin-endoperoxide synthase (PTGSs) [10] in liver or target tissues, producing toxic N4-hydroxylamine-SMX (HA-SMX). HA-SMX can auto-oxidize via nitroxide-SMX to nitroso-SMX [11]. "
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