Circular dichroism studies of glutathione reductase

A high-throughput target-based screening approach for the identification and assessment of Mycobacterium tuberculosis mycothione reductase inhibitors

The World Health Organization's goal to combat tuberculosis (TB) is hindered by the emergence of anti-microbial resistance, therefore necessitating the exploration of new drug targets. Multidrug regimens are indispensable in TB therapy as they provide synergetic bactericidal effects, shorten treatment duration, and reduce the risk of resistance development. The research within our European RespiriTB consortium explores Mycobacterium tuberculosis energy metabolism to identify new drug candidates that synergize with bedaquiline, with the aim of discovering more efficient combination drug regimens. In this study, we describe the development and validation of a luminescence-coupled, target-based assay for the identification of novel compounds inhibiting Mycobacterium tuberculosis mycothione reductase (MtrMtb), an enzyme with a role in the protection against oxidative stress. Recombinant MtrMtb was employed for the development of a highly sensitive, robust high-throughput screening (HTS) assay by coupling enzyme activity to a bioluminescent readout. Its application in a semi-automated setting resulted in the screening of a diverse library of ~130,000 compounds, from which 19 hits were retained after an assessment of their potency, selectivity, and specificity. The selected hits formed two clusters and four fragment molecules, which were further evaluated in whole-cell and intracellular infection assays. The established HTS discovery pipeline offers an opportunity to deliver novel MtrMtb inhibitors and lays the foundation for future efforts in developing robust biochemical assays for the identification and triaging of inhibitors from high-throughput library screens.

IMPORTANCE

The growing anti-microbial resistance poses a global public health threat, impeding progress toward eradicating tuberculosis. Despite decades of active research, there is still a dire need for the discovery of drugs with novel modes of action and exploration of combination drug regimens. Within the European RespiriTB consortium, we explore Mycobacterium tuberculosis energy metabolism to identify new drug candidates that synergize with bedaquiline, with the aim of discovering more efficient combination drug regimens. In this study, we present the development of a high-throughput screening pipeline that led to the identification of M. tuberculosis mycothione reductase inhibitors.

Essential arginine residues in the pyridine nucleotide binding sites of glutathione reductase

Glutathione reductase (NAD(P)H: oxidized-glutathione oxidoreductase, EC 1.6.4.2) from human erythrocytes as well as from other sources was inactivated irreversibly by the arginine modifying reagents 2,3-butanedione, 1,2-cyclohexanedione and phenylglyoxal. The inactivation caused by these reagents increased with time and was biphasic in nature. Inactivation by 2,3-butanedione in borate buffer could be partially reversed by dialysis against phosphate buffer or by incubation with hydroxylamine. The dicarbonyl reagents simultaneously inactivated the transhydrogenase activity of glutathione reductase that is expressed by the pyridine nucleotide sites. Amino acid analysis of enzyme modified with 2,3-butanedione showed a decrease by about 5 in the number of arginine residues. Analysis of enzyme inactivated with phenylglyoxal indicated modification of two arginine residues. NADPH, NADP/ and other adenosine 2'-phosphate nucleotides partially protected the enzyme against inactivation. GSSG did not protect the enzyme significantly. Modified glutathione reductase displayed a diminished affinity for the nucleotides as shown by affinity chromatography on 2',5'-ADP-Sepharose and by stopped-flow kinetics of the reduction of the enzyme. Modification did not cause any gross structural changes of enzyme molecule. It is concluded that some of the arginine residues modified are located at the pyridine-nucleotide-binding sites of the enzyme.