Search

The present knowledge of glutathione (GSH) peroxidase is briefly reviewed: GSH peroxidase has a molecular weight of about 85,000, consists of four apparently-identical subunits and contains four g atom of selenium/mol. The enzyme-bound selenium can undergo a substrate-induced redox change and is obviously essential for activity. In accordance with the assumption that a selenol group is reversibly oxidized during catalysis, ping-pong kinetics are observed. Limiting maximum velocities and Michaelis constants, indicating the formation of an enzyme-substrate complex, are not detectable. The enzyme is highly specific for GSH but reacts with many hydroperoxides. It can be deduced from the kinetic analysis of GSH peroxidase that in physiological conditions removal of hydroperoxide is largely independent of fluctuations in the cellular concentration of GSH. However, the system will abruptly collapse if the rate of hydroperoxide formation exceeds that of regeneration of GSH. By these considerations, the pathophysiological manifestation of disorders in GSH metabolism and pentose-phosphate shunt may be explained. With regard to its low specificity for hydroperoxides, GSH peroxidase could be involved in various metabolic events such as H2O2 removal in compartments low in catalase, hydroperoxide-mediated mutagenesis, protection of unsaturated lipids in biomembranes, prostaglandin biosynthesis, and regulation of prostacyclin formation.

Out of the growing number of so far known mammalian selenoproteins four are peroxidases. Their common catalytic mechanism involves redox shuttling of a selenocysteine residue in the active site, where it forms a characteristic catalytic triad with hydrogen-bonded tryptophan and glutamine residues. These peroxidases differ in tissue distribution, substrate specificity, regulation, responsiveness to selenium restriction, and likely in their biological role. Cytosolic glutathione peroxidase, which predominates in balancing hydroperoxide toxicity, rapidly declines in selenium deficiency. Prophylactic selenium supplementation is considered reasonable also in asymptomatic moderate selenium deficiency whenever a clinical condition known to cause oxidative stress has to be anticipated.