Tyrosine hydroxylase activity in caudate nucleus from Parkinson's disease: effects of iron and phosphorylating agents

Tyrosine hydroxylase (TH) activity of human postmortem brain tissues from controls and patients with Parkinson's disease (PD) was examined in the presence of Fe2+ and phosphorylation agents, such as cyclic AMP, exogenous protein kinase, calcium plus calmodulin (Ca2+-CaM), and ATP. TH activity from parkinsonian tissue was increased by 48% with statistical significance in the presence of exogenous protein kinase. Cyclic AMP alone had no effect, whereas Ca2+-CaM increased the activity by only 10%. The presence of acetylcholine resulted in a slight decrease in enzyme activity. Human TH was stimulated 13.17-fold in the presence of 1 mM Fe2+. For iron dependence, no significant differences could be shown for the Km values of TH in striata of PD, while the activity of TH was half of that of controls. Here stimulation with 1 mM Fe2+ raised the activity of TH 11-fold. Stimulation of rat, gerbil, pig, and human caudate nucleus TH with Fe2+ shows remarkable species differences. In particular, the sensitivity of human TH to stimulating processes is noteworthy. H2O2 decreases TH activity only at high concentrations. Species differences are noted for the combined incubation of Fe2+ and H2O2. In the gerbil caudate nucleus, H2O2 does not prevent the stimulating properties of Fe2+, while the pig shows a dose-dependent decline of TH activity. In conclusion, there are no significant changes in the stimulating properties of human caudate nucleus TH activity with Fe2+ in PD, while such differences are noted by using exogenous protein kinase. Furthermore, experimental evidence shows that TH activity declines at high concentrations of H2O2 only. Potentiation of this effect by Fe2+ seems to be species-dependent.