Abstract
Mycothiol ligase (MshC) is a key enzyme in the biosynthesis of mycothiol, a small molecular weight thiol that is unique to actinomycetes and whose primary role is to maintain intracellular redox balance and remove toxins. MshC catalyzes the adenosine triphosphate (ATP)—dependent condensation of cysteine and glucosamine-inositol (GI) to produce cysteine-glucosamine-inositol (CGI). MshC is essential to Mycobacterium tuberculosis and therefore represents an attractive target for chemotherapeutic intervention. A screening protocol was developed to identify MshC inhibitors based on quantification of residual ATP using a coupled luminescent assay. The protocol was used to screen a library of 3100 compounds in a 384-well plate format (Z′ ≥ 0.65). Fifteen hits (0.48%) were identified from the screen, and 2 hits were confirmed in a secondary assay that measures production of CGI. The structures of both hits contain N-substituted quinolinium moieties, and the more potent of the 2—namely, dequalinium chloride—inhibits MshC with an IC50 value of 24 ± 1 µM. Further studies showed dequalinium to be an ATP-competitive inhibitor of MshC, to bind MshC with a KD of 0.22 µM, and to inhibit the growth of M. tuberculosis under aerobic and anaerobic conditions with minimum inhibitory and anaerobic bactericidal concentrations of 1.2 and 0.3 µg/mL, respectively. The screening protocol described is robust and has enabled the identification of new MshC inhibitors. (Journal of Biomolecular Screening 2009:643-652)