Glutathione peroxidase

Reaction

The main reaction that glutathione peroxidase catalyzes is: : 2GSH + H2O2 → GS–SG + 2H2O where GSH represents reduced monomeric glutathione, and GS–SG represents glutathione disulfide. The mechanism involves oxidation of the selenol of a selenocysteine residue by hydrogen peroxide. This process gives the derivative with a selenenic acid (RSeOH) group. The selenenic acid is then converted back to the selenol by a two step process that begins with reaction with GSH to form the GS-SeR and water. A second GSH molecule reduces the GS-SeR intermediate back to the selenol, releasing GS-SG as the by-product. A simplified representation is shown below: : RSeH + H2O2 → RSeOH + H2O : RSeOH + GSH → GS-SeR + H2O : GS-SeR + GSH → GS-SG + RSeH

Glutathione reductase then reduces the oxidized glutathione to complete the cycle: : GS–SG + NADPH + H+ → 2 GSH + NADP+.

Human isozymes

Several isozymes are encoded by different genes, which vary in cellular location and substrate specificity. Glutathione peroxidase 1 (GPx1) is the most abundant version, found in the cytoplasm of nearly all mammalian tissues, whose preferred substrate is hydrogen peroxide. Glutathione peroxidase 4 (GPx4) has a high preference for lipid hydroperoxides; it is expressed in nearly every mammalian cell, though at much lower levels. Glutathione peroxidase 2 is an intestinal and extracellular enzyme, while glutathione peroxidase 3 is extracellular, especially abundant in plasma. So far, eight different isoforms of glutathione peroxidase (GPx1-8) have been identified in humans.

Structure of isozymes

Mammalian GPx1, GPx2, GPx3, and GPx4 have been shown to be selenium-containing enzymes, whereas GPx6 is a selenoprotein in humans with cysteine-containing homologues in rodents. GPx1, GPx2, and GPx3 are homotetrameric proteins, whereas GPx4 has a monomeric structure. As the integrity of the cellular and subcellular membranes depends heavily on glutathione peroxidase, its antioxidative protective system itself depends heavily on the presence of selenium.

Animal models

Mice genetically engineered to lack glutathione peroxidase 1 (Gpx1−/− mice) are grossly phenotypically normal and have normal lifespans, indicating this enzyme is not critical for life. However, Gpx1−/− mice develop cataracts at an early age and exhibit defects in muscle satellite cell proliferation. Gpx1 −/− mice showed up to 16 dB higher auditory brainstem response (ABR) thresholds than control mice. After 110 dB noise exposure for one hour, Gpx1 −/− mice had up to 15 dB greater noise-induced hearing loss compared with control mice."

Mice with knockouts for GPX3 (GPX3−/−) or GPX2 (GPX2−/−) also develop normally

However, glutathione peroxidase 4 knockout mice die during early embryonic development. Some evidence, though, indicates reduced levels of glutathione peroxidase 4 can increase life expectancy in mice.

Clinical significance

It has been shown that low levels of glutathione peroxidase as measured in the serum may be a contributing factor to vitiligo. Lower plasma glutathione peroxide levels were also observed in patients with type 2 diabetes with macroalbuminuria and this was correlated to the stage of diabetic nephropathy. In one study, the activity of glutathione peroxidase along with other antioxidant enzymes such as superoxide dismutase and catalase was not associated with coronary heart disease risk in women. Glutathione peroxidase activity was found to be much lower in patients with relapsing-remitting multiple sclerosis. One study has suggested that glutathione peroxidase and superoxide dismutase polymorphisms play a role in the development of celiac disease.

The activity of this enzyme has been reported to be decreased in case of copper deficiency in the liver and plasma.

Evolution

GPxs are a key part of animal (including human) antioxidant defenses. They are also find in bacteria, plants, and fungi. GPx was the first selenoprotein discovered, with a highly reactive Sec residue at the active site. Comparison of GPx sequences from all these types of life suggest that the ancestral GPx did not contain selenium; instead, acquision of Sec happened early in animal evolution, before the sponges diverged from other animals.

Humans have eight Gpx genes, but only five of them contain Sec (GPX1, GPX2, GPX3, GPX4, GPX6). The non-existence of Sec in GPX7 and GPX8 appears to be universal among animals.

Determination of activity

Activity of glutathione peroxidase is measured spectrophotometrically using several methods. A direct assay by linking the peroxidase reaction with glutathione reductase with measurement of the conversion of NADPH to NADP is widely used. The other approach is measuring residual GSH in the reaction with Ellman's reagent. Based on this, several procedures for measuring glutathione peroxidase activity were developed using various hydroperoxides as substrates for reduction, e.g. cumene hydroperoxide, tert-butyl hydroperoxide and hydrogen peroxide.

The other methods include the use of CUPRAC reagent with spectrophotometric detection of the reaction product or o-phtalaldehyde as a fluorescent reagent.

References

References

1. (June 1983). "The refined structure of the selenoenzyme glutathione peroxidase at 0.2-nm resolution". European Journal of Biochemistry.
2. (December 2010). "Cadmium-induced oxidative stress in Saccharomyces cerevisiae". Indian Journal of Biochemistry & Biophysics.
3. (May 2011). "Glutathione peroxidase3 of Saccharomyces cerevisiae protects phospholipids during cadmium-induced oxidative stress". Antonie van Leeuwenhoek.
4. (November 1957). "Hemoglobin catabolism. I. Glutathione peroxidase, an erythrocyte enzyme which protects hemoglobin from oxidative breakdown". The Journal of Biological Chemistry.
5. (November 2010). "Functional mimics of glutathione peroxidase: bioinspired synthetic antioxidants". Accounts of Chemical Research.
6. (August 2007). "Trends in oxidative aging theories". Free Radical Biology & Medicine.
7. (November 2000). "Targeted mutation of the gene for cellular glutathione peroxidase (Gpx1) increases noise-induced hearing loss in mice". Journal of the Association for Research in Otolaryngology.
8. (September 2001). "Mice with combined disruption of Gpx1 and Gpx2 genes have colitis". American Journal of Physiology. Gastrointestinal and Liver Physiology.
9. (May 2010). "Extracellular glutathione peroxidase (Gpx3) binds specifically to basement membranes of mouse renal cortex tubule cells". American Journal of Physiology. Renal Physiology.
10. (September 2007). "Reduction in glutathione peroxidase 4 increases life span through increased sensitivity to apoptosis". The Journals of Gerontology. Series A, Biological Sciences and Medical Sciences.
11. (Mar 2015). "Low glutathione peroxidase activity levels in patients with vitiligo". Journal of Cutaneous Medicine and Surgery.
12. (November 2014). "Erythrocyte superoxide dismutase, glutathione peroxidase, and catalase activities and risk of coronary heart disease in generally healthy women: a prospective study". American Journal of Epidemiology.
13. (June 2014). "Dietary habits and selenium, glutathione peroxidase and total antioxidant status in the serum of patients with relapsing-remitting multiple sclerosis". Nutrition Journal.
14. (February 2014). "Evaluation of glutathione peroxidase and superoxide dismutase enzyme polymorphisms in celiac disease patients". Genetics and Molecular Research.
15. (2014). "The many 'faces' of copper in medicine and treatment". Biometals.
16. (12 November 2021). "Going Forward and Back: The Complex Evolutionary History of the GPx.". Biology.
17. (2004). "Reconsidering the evolution of eukaryotic selenoproteins: a novel nonmammalian family with scattered phylogenetic distribution". EMBO Reports.
18. (July 1967). "Studies on the quantitative and qualitative characterization of erythrocyte glutathione peroxidase". The Journal of Laboratory and Clinical Medicine.
19. (September 1978). "A semiautomated system for measurement of glutathione in the assay of glutathione peroxidase". Analytical Biochemistry.
20. (1986). "[A simple and specific method for determining glutathione peroxidase activity in erythrocytes]". Laboratornoe Delo.
21. (August 2018). "Correction to: A Method of Measuring Glutathione Peroxidase Activity in Murine Brain: Application in Pharmacological Experiment". Bulletin of Experimental Biology and Medicine.
22. (2021). "A validated method to assess glutathione peroxidase enzyme activity". Chemical Papers.
23. (1979). "A sensitive fluorimetric microassay for the determination of glutathione peroxidase activity. Application to human blood platelets". Analytical Biochemistry.