Disulphide reductase

The thioredoxins appear to have a highly specific relationship with the enzyme carrying out ttieir reduction. Yeast thioredoxin for example is not reduced by the thioredoxin reductase from E. coli. In contrast reduced thioredoxins may donate electrons to a variety of acceptors. Reduced thioredoxin is a good general disulphide reductant. In combination with its reductase a disulphide reductase system is formed which is capable of reducing lipoic acid, oxidized glutathione and other similar structures. In these cases the thioredoxin-disulphide redox system does not appear to require additional enzymatic components. 

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. 

Recently, active recombinant a-LTX has been generated using bacteria in which both thioredoxin reductase and glutathione reductase are inactivated to improve the formation of disulphide bonds in expressed proteins (Li et al. 2005). The toxin is expressed as a fusion with glutathione-S-transferase (GST), which is used for affinity purification of the recombinant toxin and can be subsequently removed by selective proteolysis. Considering the relative ease of generating recombinant proteins in bacteria, this approach will facilitate structure-function studies of a-LTX. 

NO-induced protein S-glutathionylation was proposed for the first time in 1988 by J.W. Park as a possible mechanism for the inactivation of yeast alcohol dehydrogenase by NO [32]. However, it took almost 10 years until the possibility that NO might be able to direct the incorporation of GSH to protein sulfhydryls was reconsidered. In 1997. it could be demonstrated that micromolar concentrations of GSNO inhibit aldose reductase through site-specific mixed disulphide formation at a conserved cysteine residue in its catalytic site... 

Aboagye-Kwarteng, T., Smith, K. and Fairlamb, A. H. (1992) Molecular characterization of the trypanothione reductase gene from Crithidia fasciculata and Trypanosoma brucei comparison with other flavoprotein disulphide oxidoreductases with respect to substrate specificity and catalytic mechanism. Mol. Microbiol. 6 3089-3099. 

Other measures for monitoring oxidative stress include superoxide dismutase, glutathione peroxidase, catalase, glutathione, glutathione disulphide, and glutathione reductase. The oxidation of LDL has been implicated in the development of cardiovascular diseases and diabetes. Oxidized LDL can be measured using... 

The sulphate reductase factor which has already been mentioned was the first of these polypeptide dithiol-disulphide cofactors to be recognized . In this case the reduction of PAPS to PAP and sulphite was shown possible with a dithiol reductant such as dihydrolipoate or with NADPH and two protein components. One of the protein factors was not inactivated by heating. Incubation of the two protein fractions with NADPH generated... 

Figure 1 A hypothetical scheme to illustrate the Tietze reaction. DTNB is represented as < >SS(t> and reacts with GSM to form the mixed disulphide GSS< >. This appears to be a substrate for glutathione reductase and is reduced to regenerate GSM, which then enters another round with DTNB, continuing to produce the yellow anion TNB at a rate that is dependent on the initial concentration of GSM.

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