Tyrosine Hydroxylase

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of the catecholamines dopamine, epinephrine and norepinephrine. Therefore the regulation of the TH enzyme represents the central means for controlling the synthesis of these important catecholamines. TH has a large molecular diversity, resulting from differential splicing of its mRNA, which is tissue specific and might result in long-term changes in activity of the enzyme and the availability of neurotransmitter at various synapses. 

The presence of different DNA sequences at the TH locus confers susceptibility to various disorders of the brain including manic-depression and schizophrenia. Parkinson's disease is also considered a TH deficiency, as low dopamine levels are a consistent neurochemical abnormality.