Alternate pathway to ascorbate induced inhibition of Mycobacterium tuberculosis

Ascorbate has been demonstrated to interfere with the growth of Mycobacterium tuberculosis. It scavenges oxygen in the culture medium to induce dormancy of M. tuberculosis. It kills the mycobacteria by generating reactive oxygen intermediates via iron mediated Fenton reactions. In this study, we observed that ascorbate can inhibit M. tuberculosis isocitrate lyase (MtbICL) with an IC₅₀ of 2.15 mΜ. MtbICL is an essential enzyme for the survival of M. tuberculosis under dormancy. We studied the effect of ascorbate on the growth of M. tuberculosis H37Rv metabolizing through citric acid cycle or glyoxylate cycle with glucose or acetate respectively as the sole carbon source. It was observed that 4 mM ascorbate inhibited ∼89% of the growth in glucose medium, which was confirmed to be mediated by Fenton reaction, as the inhibition was significantly lesser (61%) under low iron condition. On the other hand, in acetate medium, ∼97% of the growth was inhibited and the inhibition was uninfluenced by the iron levels. 3-nitropropionate, a known inhibitor of MtbICL, was seen to cause significantly higher inhibition in the acetate medium than in the glucose medium; however it was indifferent to iron levels in either medium. Molecular docking and dynamic simulation studies confirmed stable binding of ascorbate to MtbICL leading to its inhibition. These observations suggest an additional pathway for ascorbate induced inhibition of M. tuberculosis through inhibition of glyoxylate cycle. Since human immune cells can accumulate ascorbate in millimolar concentrations, the in vitro activity range (1-4 mM) of ascorbate against M. tuberculosis could be extrapolated in vivo. Our result supports the possible benefits of adding high vitamin C diet in TB-treated patients.