Proteolytic degradation of tyrosine nitrated proteins

Tyrosine nitration is a covalent posttranslational protein modification tha t has been detected under several pathological conditions. This study repor ts that nitrated proteins are degraded by chymotrypsin and that protein nit ration enhances susceptibility to degradation by the proteasome. Chymotryps in cleaved the peptide bond between nitrated-tyrosine 108 and serine 109 in bovine Cu,Zn superoxide dismutase. However, the rate of chymotryptic cleav age of nitrated peptides was considerably slower than control. In contrast, nitrated bovine Cu,Zn superoxide dismutase was degraded at a rate 1.8-fold faster than that of control by a gradient-purified 20S/26S proteasome frac tion from bovine retina. Exposure of PC12 cells to a nitrating agent result ed in the nitration of tyrosine hydroxylase and a 58 +/- 12.5% decline in t he steady-state levels of the protein 4 h after nitration. The steady-state levels of tyrosine hydroxylase were restored by selective inhibition of th e proteasome activity with lactacystin. These data indicate that nitration of tyrosine residue(s) in proteins is sufficient to induce an accelerated d egradation of the modified proteins by the proteasome and that the proteaso me may be critical for the removal of nitrated proteins in vivo. (C) 2000 A cademic Press.