Protein oxidation and proteolysis by the nonradical oxidants singlet oxygen or peroxynitrite

Exposure of proteins to oxidants leads to increased oxidation followed by p referential degradation by the proteasomal system. The role of the biologic ally occurring oxidants singlet oxygen and peroxynitrite in oxidation of pr oteins in living cells and enhanced degradation of these proteins was exami ned in this study. Subsequent to treatment of an isolated model protein, fe rritin, with singlet oxygen or peroxynitrite, there was enhanced degradatio n by the isolated 20S proteasome. Treatment of clone 9 liver cells (normal liver epithelia) with two different singlet oxygen-generating systems or pe roxynitrite leads to a concentration-dependent increase in cellular protein turnover. At high concentrations of these oxidants, the protein turnover d ecreases without significant loss of cell viability and proteasome activity . To compare the increase of intracellular protein turnover with that obtai ned with other oxidants, cells were exposed to hydrogen peroxide or xanthin e/xanthine oxidase. The maximal increase in protein turnover was similar wi th the various oxidants. The oxidized protein moieties were removed by enha nced protein turnover. Removal of singlet oxygen- or peroxynitrite-damaged proteins is dependent on the proteasomal system, as suggested by the sensit ivity to lactacystin. Our results provide evidence that the proteasomal sys tem is able to selectively recognize and degrade proteins modified by singl et oxygen or peroxynitrite in vitro as well as in living cells. (C) 2001 El sevier Science Inc.