Monooxygenase able to convert a wide range of ketones to the corresponding esters or lactones via a Baeyer-Villiger oxidation reaction. Can act on long-chain aliphatic ketones (2-hexanone to 2-dodecanone) and on aromatic ketones (phenylacetone and benzylacetone). Is also able to catalyze enantioselective sulfoxidation of methyl-p-tolylsulfide. In vivo, likely functions as a BVMO, but the exact nature of the physiological substrate(s) remains to be established. Is responsible for the activation of several thiocarbamide-containing pro-drugs into cytotoxic species. Thus, catalyzes the oxidation of the antitubercular pro-drug ethionamide (ETH) to the corresponding sulfoxide, which is further oxidized by EthA to 2-ethyl-4-amidopyridine, presumably via the unstable doubly oxidized sulfinic acid intermediate; the final metabolite 2-ethyl-4-amidopyridine has no antitubercular activity, so the cytotoxic species is a metabolite intermediate formed by EthA. Also oxidizes thiacetazone (TAC), thiobenzamide, and isothionicotinamide and therefore is probably responsible, as suggested by the observation of crossover resistance, for the oxidative activation of these other thioamide antitubercular drugs. ethionamide + H(+) + NADPH + O2 = ethionamide S-oxide + H2O + NADP(+) Binds 1 FAD per subunit. Activity can be increased by one order of magnitude by adding bovine serum albumin in vitro. This result suggests that the enzyme is activated in vivo by protein-protein or interactions with other cellular components. kcat is 0.027 sec(-1) with ethionamide as substrate. kcat is 0.017 sec(-1) with phenylacetone as substrate. kcat is 0.021 sec(-1) with benzylacetone as substrate. kcat is 0.023 sec(-1) with 2-dodecanone as substrate. Exists as a mixture of relatively large homooligomers ranging from 200 to 600 kDa. Is most likely membrane-associated. Repressed by the transcriptional regulator EthR. Inactivation of this gene leads to a strong resistance to ETH. Belongs to the FAD-binding monooxygenase family.