The rate-limiting enzyme in catecholamine synthesis is tyrosine hydroxylase. It is phosphorylated at serine (Ser) residues Ser8, Ser19, Ser31 and Ser40 in vitro, in situ and in vivo. A range of protein kinases and protein phosphatases are able to phosphorylate or dephosphorylate these sites in vitro. Some of these enzymes are able to regulate tyrosine hydroxylase phosphorylation in situ and in vivo but the identity of the kinases and phosphatases is incomplete, especially for physiologically relevant stimuli. The stoichiometry of tyrosine hydroxylase phosphorylation in situ and in vivo is low. The phosphorylation of tyrosine hydroxylase at Ser40 increases the enzyme's activity in vitro, in situ and in vivo. Phosphorylation at Ser31 also increases the activity but to a much lesser extent than for Ser40 phosphorylation. The phosphorylation of tyrosine hydroxylase at Ser19 or Ser8 has no direct effect on tyrosine hydroxylase activity. Hierarchical phosphorylation of tyrosine hydroxylase occurs both in vitro and in situ, whereby the phosphorylation at Ser19 increases the rate of Ser40 phosphorylation leading to an increase in enzyme activity. Hierarchical phosphorylation depends on the state of the substrate providing a novel form of control of tyrosine hydroxylase activation. Keywords: activity, catecholamine synthesis, phosphorylation, protein kinases, protein phosphatases, tyrosine hydroxylase. The catecholamines (CAs) dopamine (DA), noradrenaline and adrenaline are physiologically important neurotransmitters and hormones. It has been established that when the CAs are secreted there is generally no decrease in their levels within tissues (Zigmond et al. 1989). This is because there is a concomitant increase in the rate of CA synthesis that is closely coupled to secretion. Tyrosine hydroxylase (TH; tyrosine 3-monooxygenase; E.C. 1.14.16.2) is the first and rate-limiting enzyme in CA synthesis and it catalyses the hydroxylation of L-tyrosine to DOPA. The activity of TH can be modulated by two mechanisms: medium-to long-term regulation of gene expression (enzyme stability, transcriptional regulation, RNA stability, alternative RNA splicing and translational regulation) and short-term regulation of enzyme activity (feedback inhibition, allosteric regulation and phosphorylation) (Kumer and Vrana 1996). TH activation by phosphorylation is the primary mechanism responsible for the maintenance of CA levels in tissues after CA secretion. TH can be phosphorylated at serine residues (Ser) 8, 19, 31 and 40 by a variety of protein kinases. The regulation of the phosphorylation of these sites and the consequences in terms of TH activity in vitro have been extensively investigated. In situ and in vivo it is less clear which protein kinase(s), and/or phosphatase(s), modulates TH phosphorylation and which mechanism(s) leads to the activation of TH and CA synthesis. The major focus of this review is therefore the regulation and consequences of the (Kaufman 1995;Nagatsu 1995;Kappock and Caradonna 1996;Kumer and Vrana 1996;...

Class I MHC molecules acquire peptides from endogenously synthesized proteins, whereas class II antigens present peptides derived from extracellular compartment molecules. This dichotomy is due to the fact that the invariant chain associates with class II molecules in the endoplasmic reticulum, preventing binding of endogenous peptides. The mutually exclusive binding of peptide and invariant chain to class II molecules suggests that the invariant chain might play a part in autoimmune disease.

In this study, we investigated the involvement of glutathione peroxidase-GPx in methylmercury (MeHg)-induced toxicity using three models: (a) in mouse brain after treatment with MeHg (40 mg/L in drinking water), (b) in mouse brain mitochondrial-enriched fractions isolated from MeHg-treated animals, and (c) in cultured human neuroblastoma SH-SY5Y cells. First, adult male Swiss mice exposed to MeHg for 21 days showed a significant decrease in GPx activity in the brain and an increase in poly(ADP-ribose) polymerase cleavage, an index of apoptosis. Second, in mitochondrial-enriched fractions isolated from MeHg-treated mice, there was a significant reduction in GPx activity and a concomitant decrease in mitochondrial activity and increases in ROS formation and lipid peroxidation. Incubation of mitochondrial-enriched fractions with mercaptosuccinic acid, a GPx inhibitor, significantly augmented the toxic effects of MeHg administered in vivo. Incubation of mitochondrial-enriched fractions with exogenous GPx completely blocked MeHg-induced mitochondrial lipid peroxidation. Third, SH-SY5Y cells treated for 24 h with MeHg showed a significant reduction in GPx activity. There was a concomitant significant decrease in cell viability and increase in apoptosis. Inhibition of GPx substantially enhanced MeHg toxicity in the SH-SY5Y cells. These results suggest that GPx is an important target for MeHg-induced neurotoxicity, presumably because this enzyme is essential for counteracting the pro-oxidative effects of MeHg both in vitro and in vivo.