Abstract
A family of NADPH-dependent flavin-containing monooxygenases (FMOs; EC 1.14.13.8) mediate the oxygenation of various N-containing molecules. Human FMO3 and FMO1 are the major hepatic and renal forms, respectively; monkey, dog, and pig FMO3 and FMO1 have also been identified. Triethylamine has been found in the urine of manufacturing workers. Although triethylamine was predicted to be metabolized by N-oxygenation by human FMO3 because of its classic base dissociation constant, enzymatic role of FMOs in triethylamine N-oxygenation remains to be investigated. Consequently, triethylamine N-oxygenation was investigated with FMO1/3 enzyme sources. The rate of triethylamine N-oxygenation mediated by human liver microsomes was similar to that mediated by human kidney microsomes. In contrast, the rates of triethylamine N-oxygenation by monkey, dog, and minipig liver microsomes were higher than those by kidney microsomes. Human liver microsomal triethylamine N-oxygenation was extensively suppressed by preheating liver microsomes at 45 °C for 5 minutes, suggesting a major role of FMOs in triethylamine N-oxygenation. Recombinant human, monkey, and dog FMO3 efficiently mediated triethylamine N-oxygenation, whereas only pig FMO1 exhibited this activity. Human liver microsomes and recombinant FMO3 showed similar K(m) values for triethylamine N-oxygenation. For the 4 species tested, positive correlations were observed between the rates of FMO3-mediated triethylamine N-oxygenation and its activities toward additional probe substrates benzydamine and trimethylamine. These results suggest that human FMO3 is responsible for triethylamine N-oxygenation, with some species differences in terms of FMO-mediated triethylamine N-oxygenation in humans and animal models, eg, monkeys, dogs, and minipigs. SIGNIFICANCE STATEMENT: Human flavin-containing monooxygenase (FMO) 3 mediates the N-oxygenation of triethylamine, a basic compound of interest in human biomonitoring survey results for manufacturing workers. The substrate specificity of FMO3 for triethylamine N-oxygenation in humans, monkeys, and dogs was evident. In contrast, pig FMO1 was the predominant FMO form for the N-oxygenation of triethylamine. Human hepatic FMO3 is responsible for triethylamine N-oxygenation; however, the rate of triethylamine N-oxygenation may be slower than that of typical FMO3 probe substrate trimethylamine.