Glutathione reductase, reactions involving

Enzymes involved I, ascorbate peroxidase and 2, glutathione reductase. Reactions 3 and 4 are nonenzymatic. Reaction 3 proceeds rapidly at pH 8, the pH of the stroma in illuminated chloroplasts. 

In a seminal study, Fisher and Walsh indirectly demonstrated that the glutathione reductase reaction likely involves direct hydrogen transfer from bound NADPH to N-5 of bound FAD, on the basis of the finding that [5- H]dihydro-5-deazaflavin (3H) results from incubating [4- H]NADPH with glutathione reductase reconstituted with S-deazaflavin (3) (82) [Eq. (10)] ... 

How the stereospecificity arises (a) Reactions involving flavin coenzymes (i) Glutathione reductase (EC 1.6.4.2)... 

A number of enzymes have been characterized that catalyze reactions involving DHA. In addition, other aspects of DHA biochemistry can be deduced from metabolic studies of ascorbic acid. Experiments demonstrating the biological oxidation of AA and reduction of DHA were first made in 1928 (10) and during the next decade several groups studied these reactions. By 1941 Crook (62) was able to separate the ascorbic acid oxidase and DHA reductase activities and to show that glutathione was used in the reductase reaction. 

In brief, the method involves the glutathione reductase-catalyzed reduction of GSSG by NADPH, followed by the reaction of GSH with Ellman s reagent, (5,5 -dithiobis-2-nitrobenzoic acid DTNB). The chromophoric product, TNB, has an absorbance between 405 -12 nm. The reaction can therefore be followed spectrophotometrically (Reaction 3). 

The normal catalytic process involves the sequence I- -II->-III->-IV- -I, while the steps IV- V- -VI- -III may also be part of the overall reaction. Thelander (131) has pointed out that the reaction mechanism of thioredoxin reductase differs from those of glutathione reductase and lipoyldehydrogenase because during the anaerobic titration of thioredoxin reductase no flavin semiquinone intermediate could be detected. Although thioredoxin reductase contains two subunits only one octapeptide... 

Moreover, the catalytically essential protein sulfhydryl groups, arising from Cys-58 and Cys-63, are located near the si face of the isoalloxazine ring. Thus, catalysis by glutathione reductase appears to involve hydrogen transfer reactions at both diastereotopic faces of the isoalloxazine ring system. 

The induction of DNA strand breaks and chromosomal aberrations by cadmium in mammalian cells was suppressed by antioxidants and antioxidant enzymes, indicating the involvement of ROS [34—36], Since the extent of ROS and damage to cellular macromolecules depends on the equilibrium between their generation and detoxification or repair, respectively, the occurrence of oxidative DNA damage is assumed to be due to an inhibition of the antioxidant defense by cadmium, such as the antioxidant enzymes catalase, superoxide dismutase, glutathione reductase, and glutathione peroxidase. One other mechanism proposed consists in the displacements of redox active metal ions, e.g., Fe ", for example in metallothionein, giving rise to Fenton reactions [35-37]. 

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