Peroxynitrite inactivation of tyrosine hydroxylase: mediation by sulfhydryl oxidation, not tyrosine nitration.

Tyrosine hydroxylase (TH) is the initial and rate-limiting enzyme in the biosynthesis of dopamine (DA). TH activity is significantly diminished in Parkinson's disease (PD) and by the neurotoxic amphetamines, thereby accentuating the reductions in DA associated with these conditions. Reactive oxygen and nitrogen species have been implicated in the damage to DA neurons seen in PD and in reaction to amphetamine drugs of abuse, so we investigated the hypothesis that peroxynitrite (ONOO(-)) could interfere with TH catalytic function. ONOO(-) caused a concentration-dependent inactivation of TH. The inactivation was associated with tyrosine nitration (maximum of four tyrosine residues nitrated per TH monomer) and extensive sulfhydryl oxidation. Tetranitromethane, which causes sulfhydryl oxidation at pH 6 and 8 but which nitrates tyrosines only at pH 8, inactivated TH equally at either pH. Bicarbonate protected TH from ONOO(-)-induced inactivation and sulfhydryl oxidation but increased significantly tyrosine nitration. PNU-101033 blocked ONOO(-)-induced tyrosine nitration in TH but could not prevent enzyme inactivation or sulfhydryl oxidation. Together, these results indicate that the inactivation of TH by ONOO(-) is mediated by sulfhydryl oxidation. The coincident nitration of tyrosine residues appears to exert little influence over TH catalytic function.