Thiol-dependent peroxidase

Other thiol-dependent peroxidase and reductase activities have been suggested to be present in G. intestinalis but the nature of the thiol cofactors is not known (46). 

Smith, N. C., Bryant, C. and Boreham, P. F. L. (1988) Possible roles for pyruvate ferredoxin oxidoreductase and thiol-dependent peroxidase and reductase activities in resistance to nitroheterocyclic drugs in Giardia intestinalis. Int. J. Parasitol. 18 991-997. 

As well as its role as a general thiol redox buffer, GSH acts as a co-factor for a variety of enzymes, including GSH-dependent peroxidases, glutathione-S-transferases (GST), glutaredoxins and glyoxalase. 

The obtained electrochemical responses of the CySH biosensor for the PBS and cysteine in the presence and absence of bicarbonate (HCO3 ) are shown in Figure 3.12. There is no significant change in the electrochemical response for CySH (curves c and d) in 0.1 M PBS. But in the presence of HCOs (curves a and b), the electrode responds with a remarkable increase in current at the cathodic peak potential at —0.37 V. From this result, it is clearly revealed that the bienzymatic electrode exhibited a synergistic electrochemical response with cysteine due to the bicarbonate-dependent peroxidase activity stimulated by thiol oxidase activity of SOD (Karunakaran et al., 2005). 

Karunakaran, C., Zhang, H., Joseph, J., Antholine, W.E., Kalyanaraman, B., 2005. Thiol oxidase activity of copper, zinc superoxide dismutase stimulates bicarbonate-dependent peroxidase activity via formation of a carbonate radical. Chem. Res. Toxicol. 18, 494—500. 

Thiols are also important protection against lipid peroxidation. Glutathione (7-Glu-Cys-Gly) is used by several glutathione-dependent enzymes such as free-radical reductase (converts vitamin E radical to vitamin E), glutathione peroxidase (reduces hydrogen peroxide and lipid hydroperoxides to water and to the lipid alcohol, respectively), and others. In addition, the thiol group of many proteins is essential for function. Oxidation of the thiol of calcium ATPases impairs function and leads to increased intracellular calcium. Thiol derivatives such as the ovothiols (l-methyl-4-mercaptohistidines) (Shapiro, 1991) have been explored as therapeutics. 

Many peroxovanadates have potent insulin-mimetic properties [1,2]. Apparently, this functionality derives from the ability of these compounds to rapidly oxidize the active site thiols found in the group of protein tyrosine phosphatases that are involved in regulating the insulin receptor function [3], The discovery of vanadium-dependent haloperoxidases in marine algae and terrestrial lichens provided an additional stimulus in research toward obtaining functional models of peroxidase activity, and there is great interest in duplicating the function of these enzymes (see Section 10.4.2). 

---