Molecular & Biochemical Parasitology 172 (2010) 152–155 Contents lists available at ScienceDirect Molecular & Biochemical Parasitology Short communication Susceptibility of Plasmodium falciparum to glutamate dehydrogenase inhibitors—A possible new antimalarial target Isabela M. Aparicio a , Alejandro Marín-Menéndez b , Angus Bell b , Paul C. Engel a,∗ a Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin, Ireland b Department of Microbiology, School of Genetics & Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Dublin, Ireland article info Article history: Received 3 February 2010 Received in revised form 31 March 2010 Accepted 8 April 2010 Available online 23 April 2010 Keywords: Plasmodium falciparum Antimalarial Glutamate dehydrogenase Anti-oxidant defences Enzyme inhibition Isophthalic acid abstract With the rapid spread of drug-resistant strains of Plasmodium falciparum, the development of new antimalarials is an urgent need. As malaria parasites live in a highly pro-oxidant environment, their anti-oxidant defences have frequently been suggested as candidate drug targets. A key point in such defences is the production of NADPH e.g. for maintaining anti-oxidant glutathione in the reduced state. Some authors have attributed this function in P. falciparum to a glutamate dehydrogenase, therefore proposed as a potential drug target. Here we show that isophthalic acid inhibits both Plasmodium GDH and bovine GDH but showing marked discrimination (70-fold lower K i for the parasite GDH). Isophthalic acid impairs intra-erythrocytic growth of P. falciparum in vitro whilst o-phthalic acid, not a GDH inhibitor, shows no effect. This offers hope that with careful design or thorough screening it should be possible to find inhibitors with the necessary selectivity between parasite and human GDHs. © 2010 Elsevier B.V. All rights reserved. Every year over 1 million deaths are attributed to falciparum malaria [1], and available drugs are gradually becoming ineffective, as new resistant strains emerge. The development of effective new drugs is thus of utmost importance for public health. Spending most of their mammalian cycle within red blood cells, Plasmodium species are subject to dangerous oxidative stress. The parasite digests host haemoglobin, freeing the reactive haem group. In this environment, reactive oxygen species (ROS) are liable to be formed mainly through Fenton-like reactions [2]. The parasite’s anti-oxidant defences include various peroxidases and thiol-dependent reductases [3]. It seems, however, that these mechanisms are barely adequate and are supplemented by a reliance on parallel defences of the host red cell, as evidenced, for example, by the prevalence of glucose 6-phosphate dehydrogenase deficiency in malarial areas. This suggests that the anti-oxidant defences are a suitable target for antimalarial attack. A central ele- Abbreviations: DMIPA, dimethyl isophthalic acid; GDH, glutamate dehydroge- nase; INT, 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride; IPA, isophthalic acid; NAD(H), nicotinamide adenine dinucleotide oxidised/reduced; NADP(H), nicotinamide adenine dinucleotide phosphate oxidised/reduced; o-PA, orthophthalic acid; PES, phenazine ethosulfate; pf, Plasmodium falciparum; RBCs, red blood cells; ROS, reactive oxygen species. ∗ Corresponding author at: UCD School of Biomolecular and Biomedical Sciences, Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland. Tel.: +353 1 716 6764; fax: +353 1 283 7211. E-mail address: paul.engel@ucd.ie (P.C. Engel). ment of these defences is the supply of NADPH, e.g. for the reduction of glutathione, and it has been suggested that in Plasmodium falci- parum a glutamate dehydrogenase (GDH) is the main generator of NADPH [4]. This enzyme is abundantly expressed by the parasite, and, since it is absent from the host red blood cell, it is actually a marker for the presence of the parasite [5]. Glutamate dehydrogenases are widely distributed enzymes that catalyse the reversible oxidative deamination of l-glutamate to give 2-oxoglutarate and ammonia, using as a co-factor either NAD(H), NADP(H) or both: l-Glutamate+NAD(P) + ↔ 2-oxoglutarate + NH 4 + +NAD(P)H + H + P. falciparum NADP + -GDH has been cloned and expressed in Escherichia coli [6] making possible inhibitor studies. However, since GDH is also important for the host, and has a well-conserved active site, an initial question is whether sufficiently selective inhibition can be achieved. Encouraging initial proof-of-principle experiments [7] showed remarkable selectivity in inhibition of bovine and clostridial GDHs (used as surrogates) by various com- pounds. Solved structures for parasite [8] and human [9] GDHs offer a valuable framework for inhibitor design. A useful starting point is provided by a wide range of compounds screened by Caughey et al. [10] 30 years before the solution of the first GDH struc- ture [11,12]. The best inhibitors (of bovine GDH) were compounds incorporating an extended glutarate structure (e.g. isophthalic acid) or structurally similar molecules, e.g. bromofuroic acid, where 0166-6851/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molbiopara.2010.04.002