THE RELATIVE EFFECTIVENESS OF HUMAN PLASMA GLUTATHIONE-PEROXIDASE AS A CATALYST FOR THE REDUCTION OF HYDROPEROXIDES BY GLUTATHIONE

To reveal clues to the function of human plasma glutathione peroxidase (GPx), we investigated its catalytic effectiveness with a variety of hydroperoxides. Comparisons of hydroperoxides as substrates for plasma GPx based on the ratio of V-max/K-m were blocked by the limited solub ility of the organic hydroperoxides, which prevented kinetic saturatio n of the enzyme at the chosen glutathione concentration. Therefore, we compared the hydroperoxides by the fold increase in the apparent firs t-order rate constants of their reactions with glutathione owing to ca talysis by plasma GPx. The reductions of aromatic and small hydrophobi c hydroperoxides (cumene hydroperoxide, t-amyl hydroperoxide, t-butyl hydroperoxide, paramenthane hydroperoxide) were better catalyzed by pl asma GPx than were reductions of the more ''physiological'' substrates (linoleic acid hydroperoxide, hydrogen peroxide, peroxidized plasma l ipids, and oxidized cholesterol).