Submitted to: Archives Of Biochemistry and Biophysics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/1996
Publication Date: N/A
Citation: Interpretive Summary: Selenium is essential for humans and is used by the body to make selenoproteins. We need to understand the biochemical functions of selenium to be able to define the requirement for selenium and to be able to test for selenium deficiency. Glutathione peroxidase is an important selenoprotein that protects against oxidative damage by destroying compounds called hydroperoxides. A major selenoprotein in human blood plasma has been called "glutathione peroxidase", but it's hard to see how it could use glutathione, since there is parctically no glutathione in blood plasma. We found that plasma "glutathione peroxidase" is also very inefficient at destroying naturally occurring hydroperoxides, preferring to react only with small artificial molecules, unlike real glutathione peroxidase. We concluded that plasma "glutathione peroxidase" is probably not a glutathione peroxidase at all, but is more likely to react with other proteins rather than with small molecules like glutathione and hydroperoxides. This work adds to the mounting evidence that plasma "glutathione peroxidase", while related to real glutathione peroxidase, actually performs a completely different function. This should stimulate further research to find the real funciton of plasma "glutathione peroxidase". This knowledge will lead to an understanding of how this selenoprotein contributes to the health benefits of selenium.
Technical Abstract: To reveal clues to the funciton of human plasma glutathione peroxidase (GPx), we investigated its catalytic effectiveness with a variety of hydroperoxides. Plasma GPx was purified 1290-fold from human plasma to approximately 25% purity. Comparisons of hydroperoxides as substrates for plasma GPx based on the ratio of Vmax/Km were blocked by an inability to demonstrate Michaelis-Menten kinetics. The limited solubility of the organic hydroperoxides prevented kinetic saturation of the enzyme at the chosen glutathione concentration. Therefore, we compared the hydroperoxides by the relative increases in their apparent first-order rate constants due to addition of plasma GPx. The reductions of aromatic and small hydrophobic hydroperoxides (cumene hydroperoxide, t-amyl hydroperoxide, t-butyl hydroperoxide, paramenthane hydroperoxide) were better catalyzed by plasma GPx than were reductions of the more "physiological" substrates (linoleic acid hydroperoxide, hydrogen peroxide, peroxidized plasma lipids, and oxidized cholesterol). These results suggest that plasma GPx is not well adapted for catalyzing reduction of physiological hydroperoxides by glutathione and that the putative hydroperoxide substrate of plasma GPx may be small and/or very hydrophobic. When viewed in light of recent reports on the interactions of plasma GPx with dithiol enzymes, this study invites speculation that the natural oxidizing substrate may not be a hydroperoxide at all, but a protein disulfide.