Footprints of neurotoxicity: Posttranslational modifications of tyrosine hydroxylase

Parkinson's disease (PD) is a common neurodegerative disease, in which dopaminergic neurons in the substantia nigra degenerate, leading to striatal dopamine deficiency. As tyrosine hydroxylase (TH) is the initial and rate-limiting enzyme in the biosynthesis of the dopamine, the disease can be related to decreased TH activity. TH is inhibited in vivo by the neurotoxic amphetamines and by the selective DA neuronal toxin MPTP (Gibb et al., 1990; Ara et al., 1998), through the production of reactive oxygen and nitrogen species (ROS/RNS). Peroxynitrite (ONOO-), as a reactive species, is best known to oxidize cysteine residues and nitrate tyrosine residues in proteins in vitro (Radi et al., 1991). Therefore the ONOO- -induced inhibition of TH may be mediated by either posttranslational modification (Kuhn et al., 1999). The role of ONOO- as an in vivo nitrating species has become an area of debate. Some investigators have reported that 3-nitrotyrosine can be formed via myeloperoxidase-catalyzed H2O2 dependent oxidation of NO2 and tyrosine without the intermediate formation of ONOO- (Eiserich et al., 1998). NO2 is a strong oxidant that is capable of nitrating tyrosines and oxidizing sulfhydryl residues of proteins. The objective of this study was to characterize the posttranslational modifications of TH that are caused by reactive nitrogen species (RNS) and assess their effects on enzyme activity. Intracellular antioxidants, including low molecular weight thiols such as glutathione and cysteine are scavengers of reactive species. Oxidation of low molecular weight thiols such as cysteine and glutathione by reactive oxygen and nitrogen species can lead to posttranslational modification of reactive protein thiols, processes referred to as S-thiolation or S-glutathiolation. TH is S-thiolated and inhibited by ONOO- and NO 2 in the presence of cysteine. S-thiolation of TH prevented enzyme nitration. Regulation of protein function by ROS/RNS via S-glutathiolation or S-thiolation may be a potential ROS/RNS sensing mechanism, which protects proteins from irreversible modification and loss of function.