Comparative resistance of the 20S and 26S proteasome to oxidative stress

Oxidatively modified ferritin is selectively recognized and degraded by the 20S proteasome. Concentrations of hydrogen peroxide (H₂O₂) higher than 10 µmol/mg of protein are able to prevent proteolytic degradation. Exposure of the protease to high amounts of oxidants (H₂O₂, peroxynitrite and hypochlorite) inhibits the enzymic activity of the 20S proteasome towards the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-methylcoumarylamide (Suc-LLVY-MCA), as well as the proteolytic degradation of normal and oxidant-treated ferritin. Fifty per cent inhibition of the degradation of the protein substrates was achieved using 40 µmol of H₂O₂/mg of proteasome. No change in the composition of the enzyme was revealed by electrophoretic analysis up to concentrations of 120 µmol of H₂O₂/mg of proteasome. In further experiments, it was found that the 26S proteasome, the ATP- and ubiquitin-dependent form of the proteasomal system, is much more susceptible to oxidative stress. Whereas degradation of the fluorogenic peptide, Suc-LLVY-MCA, by the 20S proteasome was inhibited by 50% with 12 µmol of H₂O₂/mg, 3 µmol of H₂O₂/mg was enough to inhibit ATP-stimulated degradation by the 26S proteasome by 50%. This loss in activity could be followed by the loss of band intensity in the non-denaturing gel. Therefore we concluded that the 20S proteasome was more resistant to oxidative stress than the ATP- and ubiquitin-dependent 26S proteasome. Furthermore, we investigated the activity of both proteases in K562 cells after H₂O₂ treatment. Lysates from K562 cells are able to degrade oxidized ferritin at a higher rate than non-oxidized ferritin, in an ATP-independent manner. This effect could be followed even after treatment of the cells with H₂O₂ up to a concentration of 2 mM. The lactacystin-sensitive ATP-stimulated degradation of the fluorogenic peptide Suc-LLVY-MCA declined, after treatment of the cells with 1 mM H₂O₂, to the same level as that obtained without ATP stimulation. Therefore, we conclude that the regulation of the 20 S proteasome by various regulators takes place during oxidative stress. This provides further evidence for the role of the 20S proteasome in the secondary antioxidative defences of mammalian cells.