Glutathione peroxidase-like activity of simple selenium compounds. Peroxides and the heterocyclic N-oxide resazurin acting as O-atom donors.
Selenite and selenocystamine [(CyaSe)2] efficiently activate the decomposition of H2O2 by GSH and by other thiols, as demonstrated using a leuco crystal violet POD-based H2O2 assay which is applicable (unlike other assays) also in presence of thiols. The GPx-like activities were estimated to be 3.6 and 2.7 mumol H2O2/min per mumol SeO3(2-) and (CyaSe)2, respectively. Both selenium compounds also activate reduction of the heterocyclic N-oxide resazurin (RN-->O) to resorufin (RN) by GSH; H2O2 competes with reduction of this dye. GSSeH and CyaSeH, formed by interaction of GSH with SeO3(2-) and (CyaSe)2, respectively, are likely to be the active reductants. CyaSeH, generated gamma-radiolytically from (CyaSe)2, exhibits an absorption peak at 243 nm and is removed by H2O2 with a rate constant of 9.7 x 10(2) M-1 s-1, and slightly slower by hydroperoxides. We have no evidence for one-electron interactions between GSSeH or CyaSeH and H2O2, with formation of free radical intermediates, as previously proposed in the case of selenium-activated reduction of cytochrome c by GSH (Levander et al., Biochemistry 23, 4591-4595 (1973)). Our results can be explained by O-atom transfer from the substrate to the active selenol group, RSeH + H2O2 (RN-->O)-->RSeOH + H2O (RN), and recycling of RSeOH to RSeH (+ H2O) by GSH, analogous to the selenenic acid pathway of GPx. The substrate specificity appears to be different, however, in that GPx is unable to catalyse RN-->O reduction, and GSSeH hardly catalyses the decomposition of cumene- or t-butyl-hydroperoxide; CyaSeH, on the other hand, is active also with the hydroperoxides. RN-->O is reduced to RN also by certain oxidizing free radicals, e.g. by the thiyl CyaS.., O-atom transfer may in this case lead to the generation of reactive oxyl radicals.
Prütz
W A
WA
Universität Freiburg, Institut für Biophysik und Strahlenbiologie, Bundesrepublik Deutschland.
D002384:Catalysis; D003300:Copper; D003538:Cystamine / Q000031:analogs & derivatives*; D004247:DNA; D005609:Free Radicals; D005720:Gamma Rays; D005978:Glutathione / Q000031:analogs & derivatives; D019803:Glutathione Disulfide; D005979:Glutathione Peroxidase / Q000378:metabolism*; D006861:Hydrogen Peroxide; D007202:Indicators and Reagents; D007700:Kinetics; D009243:NAD; D016566:Organoselenium Compounds; D010078:Oxazines; D010084:Oxidation-Reduction; D012710:Serum Albumin, Bovine; D018038:Sodium Selenite; D013053:Spectrophotometry; D013438:Sulfhydryl Compounds; D013997:Time Factors; D014966:Xanthenes
D005609:Free Radicals; D007202:Indicators and Reagents; D016566:Organoselenium Compounds; D010078:Oxazines; D013438:Sulfhydryl Compounds; D014966:Xanthenes; D009243:NAD; C005843:resazurin; C014807:selenocystamine; D012710:Serum Albumin, Bovine; D003300:Copper; D004247:DNA; D006861:Hydrogen Peroxide; D005979:Glutathione Peroxidase; D005978:Glutathione; D018038:Sodium Selenite; D003538:Cystamine; D019803:Glutathione Disulfide