Sulphoxidation of ethyl methyl sulphide, 4-chlorophenyl methyl sulphide and diphenyl sulphide by purified pig liver flavin-containing monooxygenase

Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism

Flavin-containing monooxygenase (FMO) oxygenates drugs and xenobiotics containing a "soft-nucleophile", usually nitrogen or sulfur. FMO, like cytochrome P450 (CYP), is a monooxygenase, utilizing the reducing equivalents of NADPH to reduce 1 atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. FMO and CYP also exhibit similar tissue and cellular location, molecular weight, substrate specificity, and exist as multiple enzymes under developmental control. The human FMO functional gene family is much smaller (5 families each with a single member) than CYP. FMO does not require a reductase to transfer electrons from NADPH and the catalytic cycle of the 2 monooxygenases is strikingly different. Another distinction is the lack of induction of FMOs by xenobiotics. In general, CYP is the major contributor to oxidative xenobiotic metabolism. However, FMO activity may be of significance in a number of cases and should not be overlooked. FMO and CYP have overlapping substrate specificities, but often yield distinct metabolites with potentially significant toxicological/pharmacological consequences. The physiological function(s) of FMO are poorly understood. Three of the 5 expressed human FMO genes, FMO1, FMO2 and FMO3, exhibit genetic polymorphisms. The most studied of these is FMO3 (adult human liver) in which mutant alleles contribute to the disease known as trimethylaminuria. The consequences of these FMO genetic polymorphisms in drug metabolism and human health are areas of research requiring further exploration.

Elemental sulfur: toxicity in vivo and in vitro to bacterial luciferase, in vitro yeast alcohol dehydrogenase, and bovine liver catalase

The aim of this research was to analyze the effects and the modes of action of elemental sulfur (S(0)) in bioluminescence and respiration of Vibrio fischeri cells and the enzymes crude luciferase, pure catalase, and alcohol dehydrogenase (ADH). Metallic copper removed sulfur and reduced the toxicity of acetone extracts of sediment samples analyzed in the bioluminescence test. The sulfur inhibition of cell bioluminescence was noncompetitive with decanal, the luciferase substrate; reversible, with maximum toxicity after 15 min (EC(50) = 11.8 microg/L); and almost totally recovered after 2 h. In vitro preincubation of crude luciferase extract with sulfur (0.28 ppm) weakly inhibited bioluminescence at 5 min, but at 30 min the inhibition reached 60%. Increasing the concentration of sulfur in the parts per million concentration range in vitro decreased bioluminescence, which was not constant, but depended on exposure time, and no dead-end/total inhibition was observed. The redox state of enzymes in the in vitro system significantly affected inhibition. Hydrogen peroxide restored fully and the reducing agent dithiothreitol, itself toxic, restored only partially luciferase activity in the presence of sulfur. Sulfur (5.5 ppm) slightly inhibited ADH and catalase, and dithiothreitol enhanced sulfur inhibition. High sulfur concentrations (2.2 ppm) inhibited the bioluminescence and enhanced the respiration rate of V. fischeri cells. Elemental sulfur data were interpreted to show that sulfur acted on at least a few V. fischeri cell sites: reversibly modifying luciferase at sites sensitive to/protected by oxidative and reducing agents and by affecting electron transport processes, resulting in enhanced oxygen consumption. Sulfur together with an enzyme reducing agent inhibited the oxidoreductive enzymes ADH and catalase, which have --SH groups, metal ion cofactors, or heme, respectively, in their active centers.

Studies on the metabolism of the thiofurans furfuryl mercaptan and 2-methyl-3-furanthiol in rat liver

The metabolism of two thiofurans, namely furfuryl mercaptan (FM) and 2-methyl-3-furanthiol (MTF), to their corresponding methyl sulphide and methyl sulphoxide derivatives has been studied in male Sprague-Dawley rat hepatocytes and liver microsomes. Rat hepatocytes converted FM to furfuryl methyl sulphoxide (FMSO) and MTF to 2-methyl-3-(methylthio)furan sulphoxide (MMFSO). Liver microsomes catalysed the NADPH-dependent metabolism of furfuryl methyl sulphide (FMS) to FMSO and 2-methyl-3-(methylthio)furan sulphide (MMFS) to MMFSO. FMS and MMFS metabolism to their thiofuran methyl sulphoxide derivatives was induced by the treatment of rats with Aroclor 1254 and inhibited in liver microsomes treated with 1-aminobenzotriazole. The NADPH-dependent metabolism of FM to FMSO and MTF to MMFSO in liver microsomes was observed in the presence of S-adenosylmethionine. In summary, both thiofurans can be metabolised in rat liver to their thiofuran methyl sulphide derivatives which can be subsequently S-oxidised to form thiofuran methyl sulphoxides. FM and MTF appear to be substrates for rat hepatic microsomal thiol methyltransferase and the S-oxidation of FMS and MMFS appears to be primarily catalysed by cytochrome P450 forms.