Catalyzes the oxidative metabolism of numerous xenobiotics, including mainly therapeutic drugs and insecticides that contain a soft nucleophile, most commonly nitrogen and sulfur and participates to their bioactivation (PubMed:18930751). Catalyzes the S-oxygenation of the prodrug ethionamide (ETA) to the S-oxide (ETASO), the first step in its bioactivation following by the second oxygenation to the sulfinic acid but to a lesser extend (PubMed:18930751). Optimum pH is 10.5. Belongs to the FMO family.