Tyrosine Hydroxylase: ProductsTyrosine Hydroxylase (TH) is the rate limiting enzyme involved in the biosynthesis of catecholamines. It catalyzes the conversion of the amino acid L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA), which is the precursor for dopamine. Dopamine, in turn, can be converted into norepinephrine and epinephrine. TH is expressed in dopaminergic and noradrenergic neurons, and also in neuroendocrine cells of the adrenal medulla. There are four potential isoforms of human TH that result from alternative splicing of the N-terminus.