The role of the regulatory enzymes of catecholamine synthesis in Parkinson's disease.

The major neuropathology of Parkinson's disease (PD) is the degeneration of nigrostriatal dopamine (DA), resulting in a deficiency of DA, and of the enzyme tyrosine hydroxylase (TH), which catalyzes the synthesis of L-dopa. The symptomatic treatment of PD consists of replenishing DA by administering L-dopa, which is enzymatically converted to DA in the striatum. The increase of TH activity by modification of the enzyme leads to an increased synthesis of striatal L-dopa, and thereby replenishes the missing DA more efficiently. The activity of TH is increased by protein kinase-dependent phosphorylation of the enzyme or by inhibition of dephosphorylation with specific phosphatase inhibitors. Thus, modification of TH results in an activated form of the enzyme, which might provide a basis for developing new strategies in the treatment of PD. The extraneuronal enzyme, catechol-O-methyl transferase (COMT), inactivates catecholamines by O-methylation, and its inhibition leads to increased levels of striatal DA. The availability of selective and nontoxic COMT inhibitors makes it possible to assess their therapeutic role in treatment of PD. The intraneuronal enzymes, monoamine oxidase (MAO)-A and MAO-B, inactivate catecholamines and other biogenic amines, such as serotonin, by deamination. Inhibition of these enzyme activities leads to increased levels of striatal DA. The irreversible MAO-B inhibitor selegiline was shown to exert antiparkinsonian activity, especially in the early stages of parkinsonism. Selegiline also prevents the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in MPTP-treated mice and monkeys. Its role in the prevention of the disease is under investigation in several clinical centers.