The Thioredoxin System

Obviously the redox poise in biological systems is very important and the movement of selenium through this process has been investigated for denitrifiers such as Paracoccus denitrificans,159 a specialized selenate-respiring bacterium Thauera selenatis which used selenate as the sole electron acceptor,160,161 and phototrophic bacteria which produced different reduced forms of selenium when amended with either selenite or selenate and even added insoluble elemental Se.162 As noted above, Andreesen has commented on the importance of redox active selenocysteines135 and Jacob et al.136 note the importance of the thioredoxin system to redox poise. 

Nikitovic, D., Holmgren, A., S-Nitrosoglutathione is cleaved by the thioredoxin system with liberation of glutathione and redox regulating nitric oxide,/. Biol. Chem. 27 (1996),... 

Dietrichs D, Meyer M, Rieth M, Andreesen JR. 1991. Interaction of selenoprotein PA and the thioredoxin system, components of the NADPH-dependent reduction of glycine in Eubacterium acidaminophilum and Clostridium litorale. J Bacteriol 173 5983-91. 

Ribonucleotide reductase and the thioredoxin system. In some lactobacilli, vitamin B12 is involved in the reduction of ribonucleotide triphosphate to deoxyribonucleotide. 

In neural cells, the redox status is controlled by the thioredoxin (Trx) and glutathione (GSH) systems that scavenge harmful intracellular ROS. Thioredoxins are antioxidants that serve as a general protein disulphide oxidoreductase (Saitoh et al., 1998). They interact with a broad range of proteins by a redox mechanism based on the reversible oxidation of 2 cysteine thiol groups to a disulphide, accompanied by the transfer of 2 electrons and 2 protons. These proteins maintain their reduced state through the thioredoxin system, which consists of NADPH, thioredoxin reductase (TR), and thioredoxin (Trx) (Williams, Jr. et al., 2000 Saitoh et al., 1998). The thioredoxin system is a system inducible by oxidative stress that reduces the disulfide bond in proteins (Fig. 7.4). It is a major cellular redox system that maintains cysteine residues in the reduced state in numerous proteins. 

Fig. 7.4 Reactions associated with the thioredoxin system. Thioredoxin is a redox-regulating protein with a redox-active disulfide/dithiol within the conserved active site sequence -Cys-Gly-pro-Cys-. Thioredoxin reductase, a 55 kDa flavoprotein that catalyzes the NADPH-dependent reduction of thioredoxin (1) and thioredoxin oxidase (2), a flavin-dependent sulfhydry 1 oxidase that catalyzes the oxidative protein folding with the generation of disulfides...

Goswami and Rosenberg have suggested that liver and kidney microsomes contain in addition to the type I deiodinase multiple low-Mm enzymes for the ORD of T4 and rT3 that differ from the type II enzyme [60,64,65]. This was mainly based on different susceptibilities to iopanoic acid and PTU if reactions were carried out at low substrate concentrations in the presence of various cofactors, i.e., DTT, GSH, glutaredoxin and thioredoxin [60,64,65]. It was even reported that the deiodinase activity stimulated by the thioredoxin system accepted rT3 but not T4 as substrate [64]. The uncertainty in the estimation of the low conversion rates in the nM substrate range which are not accounted for by residual activity of the type I deiodinase, however, questions the validity of the above conclusions. The possible exist-... 

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

Kistner A, Habermann E (1992) Reductive cleavage of tetanus toxin and botulinum neurotoxin A by the thioredoxin system from brain. Evidence for two redox isomers of tetanus toxin. Naunyn. Schmiedebergs Arch. Pharmacol. 345 227-34... 

One possibihty for minimizing oxidized protein damage is the thiol repair (Fig. 3). This repair system requires either glutathione or the thioredoxin system. The thioredoxin/thioredoxin reductase repair system [10] is able to reduce disulfide bonds. It can dethiolate protein disulfides and thus is an extremely important regulator for redox homeostasis in the cells. Thioredoxin is a smaU ubiquitous protein that contains a pair of cysteines that undergo reversible oxidation and are re-reduced by the enzyme thioredoxine reductase. The thioredoxin reductase transfers electrons from NADPH to thioredoxin via a flavin. 

The reduction of ribonucleotides to deoxyribonucleotides is linked to NADPH by thioredoxin and thioredoxin reductase. Thioredoxin is a small protein which is oxidized from a dithiol to a disulfide form during ribonucleotide reduction. The dithiol form of thioredoxin is regenerated by NADPH and a specific flavoprotein, thioredoxin reductase. The thioredoxin system consisting of thioredoxin and thioredoxin reductase was first identified as the reducing system from E. coli by Reichard and co-workers (35, 36) and both proteins have since been purified to homogeneity (37, 38). 

The thioredoxin system, consisting of thioredoxin and thioredoxin reductase, was originally discovered as the hydrogen carrier system, which provides, with NADPH, the reducing potential for the reduction of ribonucleotides (5, 35). Since then considerable evidence has been accumulated to indicate that this or a closely related system also participates in a variety of other enzymatic reductions. For instance thioredoxin can function as an electron carrier between NADPH and several disulfides, such as insulin, lipoate and oxidized glutathione. Furthermore Porque et al. (114) have shown that thioredoxin and thioredoxin reductase from yeast can function as hydrogen carriers in the reduction of methionine sulfoxide and sulfate. 

Shahak (10) with the thylakoid ATP synthase tentatively activated by the thioredoxin system (16). This is also corroborated because of a higher activation by DTT of the thylakoid-bound FBPase (about 5.5 times) than that of the soluble enzyme (1.6 times). It must be probably due to the existence of membrane-bound thioredoxin (11), which acts as a link in the FBPase activation by DTT (Table I). 

When photosynthesis in leaves or intact chloroplasts is limited by light, most reaction centers of PSI and PSII are in the photochemi-cally active ( open ) form [4-6], while reductive enzyme activation (via the thioredoxin system) already saturates at very low light, far below light-saturation of electron transport [11]. 

In principle, the thioredoxin system competes for electrons with the ferredoxin-NADP-reductase (FNR) reaction, but also, to some extent, with other electron consuming reactions such as N02"-reduction and (pseudo)cyclic reactions. Efficient distribution of electrons to the thioredoxin-system may require regulation of... 

Figure 4.2 Thioredoxin pathways mediate the reduction of hydrogen peroxide. Mitochondrial respiratory substrates and the cytosolic pentose phosphate cycle reduce NADP to NADPH, which in turn feeds reducing equivalents to the thioredoxin system. In...

The thioredoxin system, composed of nicotinamide adenine dinucleotide phosphate (reduced form), thioredoxin and thioredoxin reductase, may be involved in the cellular sensitivity to cis-diamminedichloroplatinum (Sasada et al. 1999). HeLa cells cultured with cisplatin showed a time-and dose-dependent reduction of intracellular thioredoxin reductase activity, which was well correlated with the decrease in cell viability after exposure to cisplatin. In a cell-free system, cisplatin was found to directly inactivate the reduced form of purified human thioredoxin reductase. The cis-platin-resistant variants of HeLa cells, estabUshed by continuous exposure to cisplatin, exhibited an increased expression and activity of thioredoxin reductase as well as thioredoxin compared with the... 

Thioredoxin System. Thioredoxin and thioredoxin reductase are important constituent of the intracellular redox environment. Thioredoxin reductase is homologous to glutathione reductase (GR) and depends on NADPH as electron donor. The thioredoxin system can repair perox5mitrite induced disulfides in brain tubulin and therefore may be of high importance for brain cells under pathological conditions [20, 23]. 

Sun, Y. and Rigas, B. (2008). The thioredoxin system mediates redox-induced cell death in human colon cancer cells imphcations for the mechanism of action of anticancer agents. Cancer Res. 68, 8269-8277. 

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