Eosinophil peroxidase oxidizes isoniazid to form the active metabolite against M. tuberculosis, isoniazid-NAD

The formation of isonicotinyl-nicotinamide adenine dinucleotide (INH-NAD⁺) by the mycobacterial catalase-peroxidase enzyme, KatG, was known to be the major component of the mode of action of isoniazid (INH), an anti-tuberculosis drug. However, there are other enzymes that may catalyze this reaction. We have previously reported that neutrophil myeloperoxidase (MPO) is capable of metabolizing INH through the formation of INH-NAD⁺ adduct, which could be attributed to being a possible mode of action of INH. However, eosinophilic infiltration of the lungs is more pronounced and characteristic of granulomas in Mycobacterium tuberculosis-infected patients. Thus, the aim of the present study is to investigate the role of eosinophil peroxidase (EPO), a key eosinophil enzyme, during INH metabolism and the formation of its active metabolite, INH-NAD⁺ using purified EPO and eosinophils isolated from asthmatic donors. UV-Vis spectroscopy revealed INH oxidation by EPO led to a new product (λ_max = 326 nm) in the presence of NAD⁺. This adduct was confirmed to be INH-NAD⁺ using LC-MS analysis where the intact adduct was detected (m/z = 769). Furthermore, EPO catalyzed the oxidation of INH and formed several free radical intermediates as assessed by electron paramagnetic resonance (EPR) spin-trapping; a carbon-centred radical, which is considered to be the reactive metabolite that binds with NAD⁺, was found when superoxide dismutase was included in the reaction. Our findings suggest that eosinophilic EPO may also play a role in the pharmacological activity of INH through the formation of INH-NAD⁺ adduct, and supports further evidence that human cells and enzymes are capable of producing the active metabolite involved in tuberculosis treatment.