Reversible inhibition of mammalian glutamine synthetase by tyrosine nitration

The effect of tyrosine nitration on mammalian GS activity and stability was studied in vitro. Peroxynitrite at a concentration of 5 μmol/l produced tyrosine nitration and inactivation of GS, whereas 50 μmol/l peroxynitrite additionally increased S-nitrosylation and carbonylation and degradation of GS by the 20S proteasome. (−)Epicatechin completely prevented both, tyrosine nitration and inactivation of GS by peroxynitrite (5 μmol/l). Further, a putative “denitrase” activity restored the activity of peroxynitrite (5 μmol/l)-treated GS. The data point to a potential regulation of GS activity by a reversible tyrosine nitration. High levels of oxidative stress may irreversibly damage and predispose the enzyme to proteasomal degradation.