Glutathione peroxidase has been found in most animals but not in plants (15-17). A low level of the enzyme has been demonstrated in some insects (18), and GPX activity has been detected in yeast, Hansenula mraki (19). The enzyme is found chiefly in
Table 1 Substrates and Their Structures of Glutathione Peroxidase
Glutathione Hydroperoxides g-Glu-Cys-Gly R-O-O-H
Organic hydroperoxidase: ethyl, cumene, tert-butyl, 5-phenyl-4-pentenyl, linoleic, linoleic acid hydroperoxides, and their methyl esters: 13-hydroperoxy-9, 11-octacadienoic acid, cholesterol 7b-hydroperoxide; allopregnanolone 17a-hydroperoxide; thymine hydroperoxide.
Lipid hydroperoxide: phosphatidylethanolamine, phosphatidyl-choline, phosphatidylserine, cardiolipin and phosphatidic acid hydroperoxides: mono-and dihydroperoxydilinoleoylphosphatidyl cholines respiratory tissues (blood vessels, lung, heart), digestive tissues (liver, kidneys), and brain tissues where oxygen is required. In rats, a high level of the enzyme is found in the liver with lower concentrations detected in red cells, heart, lung, and kidney. In hepatocytes 70% of the GPX activity is from the cytosol and 30% is localized within the mitochondrial matrix (20).
Classical GPX is present abundantly in erythrocytes, kidney, and liver (16). The lens of different animal species contain remarkably high GPX activity (21, 22). PHGPX has been identified in all tissues in which it was searched for, namely, rat kidney, heart, lung, muscle, and brain; bull spermatozoa; and fish liver (23). A high level of GPX activity has been observed in rat testis (24). pGPX is detected in plasma, milk, and lung (6, 7). giGPX has been found in the cytosol of human and rat gastrointestinal tracts (9). It was detected in human liver and colon, but not other human tissues including kidney, heart, lung, placenta, and uterus. In rodent tissues, giGPX is only detected in the gastrointestinal tract, not in other tissues. In rat, the highest activity of the enzyme is detected in stomach followed by decreasing activities in esophagus, colon, and intestine.
The study by Godin and Ganett (25) showed that the distribution of GPX in mammalian tissues is strictly functionally related to other antioxidant enzymes such as superoxide dismutase (SOD; EC 126.96.36.199) and catalase (EC 188.8.131.52). It should be kept in mind that the level of GPX is subject to numerous influences, including dietary selenium, age and sex, and estrous cycle, and other environmental factors (26-28).It has been documented that GPX activity is subject to large species differences in all organs (20).
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