Thioredoxin reductase, TrxR

In the search for possible targets the inhibition activity of a range of RAPTA compounds was studied for two specific enzymes - namely thioredoxin reductases (TrxR) and cathepsin B (cat B), important targets in cancer chemotherapy [28]. TrxR was chosen as it is highly relevant with respect to gold-based drugs (see below) and cat B is implicated in various stages of metastasis which correlate to the observed in vitro effects of RAPTA-T [29], It turned out that while the selected compounds (Fig. 3) are not inhibitors of TrxR with the exception of carboRAPTA-C, they are active inhibitors of cat B [28],... 

Thioredoxin reductase (TrxR) acts in the reverse direction and shows a somewhat different mechanism, which is dependent on the protein source. Prokaryotes, plants, and lower eukaryotes contain a 35-kDa TrxR with one redox-active disulfide. Higher eukaryotes produce a 55-kDa TrxR that has either an additional redox-active disulfide or a selenenylsulfide in the flexible C-terminal part of the neighboring subunit (15). In low Mr TrxR, a large conformational change is required to move reducing equivalents from the apolar flavin site to the surface of the protein where the thioredoxin redox partner binds. In high Mr TrxR, this transfer is mediated by the second disulfide or selenylsulfide, and the conformational changes required are comparatively small (17). 

Although the GOGAT reaction consumes one NADPH it produces two molecules of glutamate that can yield, via GDH, two NADPH that can in turn serve as an electron source for the anti-oxidative enzymes glutathione reductase (GR) and thioredoxin reductase (TrxR see p. 264-266) glutamine could also be used in pyrimidine biosynthesis. (The glutamate metabolic pathway can be found online at http //sites.huji.ac. il/malaria/maps/glutamatemetpath.html last accessed 16 July 2008.)... 

P. falciparum has been shown to contain a thioredoxin redox system— thioredoxin reductase (TrxR), thioredoxin and thioredoxin peroxidases (Nickel et ah, 2006). P. falciparum thioredoxin reductase is similar to GR in that it is homodimeric, NADPH dependent and binds one FAD per subunit. TrxR appears to be essential to parasite survival as knockout of the gene is lethal (Krnajski et ah, 2002). In a screen of 350,000 compounds, several unsaturated Mannich bases were identified as potential inhibitors of TrxR. The screen, as well as structure-activity relationship studies, resulted in the development of the first specific PfTrxR inhibitors (Andricopulo et ah, 2006 Davioud-Charvet et ah, 2003). 

Evidence suggests that inhibition of thioredoxin reductase (TrxR), a seleno-enzyme critically involved in the regulation of the intracellular redox state and of mitochondrial functions [20], could be highly relevant. In fact, inhibition of TrxR by gold compounds results in the opening of the mitochondrial pore leading to the release of cytochrome c and ultimately inducing apoptosis [21]. 

Berners-Price also described NHC-Au compounds that targeted the mitochondria and exhibited thioredoxin reductase (TrxR) inhibition properties. Compounds 27 and 31-33 showed toxicity to breast cancer cell lines MDA-MB-231 and MDA-MB-468, but did not show toxicity on normal human mammary epithelia cells (Figure 4.8). Compound 28, an intermediate hpophihc complex, showed the best selectivity and cytotoxicity potential. This compound, while being inactive on glutathione reductase, was observed to... 

TrxRs are homodimeric flavoproteins [80] that catalyze the NADPH-dependent reduction of thioredoxin (Trx), a ubiquitous 12 kDa protein that is the major protein disulfide reductase in cells [81], and belongs to the pyridine nucleotide-disulfide oxidoreductase family [82]. Each monomer includes an FAD prosthetic group, a NADPH binding site and an active site containing a redox-active selenol group. Electrons are transferred from NADPH via FAD to the active-site selenol of TrxR, which then reduces the substrate Trx [83]. The crystal structure of TrxR is shown in Fig. 13 [84],... 

---