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Tyrosine hydroxylase and Parkinson's disease
A consistent neurochemical abnormality in Parkinson's disease is degeneration of dopaminergic neurons in substantia nigra, leading to a reduction of stratial dopamine levels. As tyrosine hydroxylase catalyzes the formation of L-DOPA, the rate-limiting step in the biosynthesis of dopamine, the disease can be considered as a tyrosine hydroxylase-deficiency syndrome of the striatum.

A direct pathogenetic role of tyrosine hydroxylase has also been suggested, as the enzyme is a source of H2O2 and other reactive oxygen species (ROS), and a target for radical-mediated injury. It has been demonstrated that L-DOPA is effectively oxidized by mammalian tyrosine hydroxylase, possibly contributing to the cytotoxic effects of DOPA.

Like other cellular proteins, tyrosine hydroxylase is also a possible target for damaging alterations induced by ROS. This suggests that some of the oxidative damage to tyrosine hydroxylase could be generated by the tyrosine hydroxylase system itself.