Mycolic Acid-like Lipids Act as Substrates for <i>Mycobacterium marinum</i> <i>melH</i>

<i>Mycobacterium tuberculosis</i> (<i>M. tuberculosis</i>), the pathogenic bacterium that causes tuberculosis, has developed its own mechanism to evade defense mechanisms to counteract the lethal effects of reactive oxygen species (ROS) generated within host macrophages during infection. The <i>melH</i> gene present in <i>Mycobacterium tuberculosis</i> and <i>Mycobacterium marinum</i> plays an important role in reducing ROS generated during infection. The <i>melH</i> gene encodes an epoxide hydrolase. Bioinformatics data suggests that the encoded enzyme utilizes lipid substrates for its function. To identify potential physiological substrates of MelH in <i>Mycobacterium marinum</i> (<i>M. marinum</i>), we employed a lipid fractionation approach combined with liquid chromatography-mass spectrometry and treatment using the active MelH enzyme. We found classes of mycolic acids (MA), predominantly epoxy MA, accumulate in the <i>melH</i> mutant and upon treatment with MelH are reduced in the lipid fraction. These results provide insight into how MelH, encoded in the <i>mel2</i> operon, contributes to <i>M. marinum</i> and <i>M. tuberculosis</i> persistence by converting epoxides to diols within the host, thereby alleviating toxicity and stress responses. Furthermore, these findings offer additional evidence supporting the potential mechanisms of action if MelH is targeted for antitubercular drug discovery.