Tyrosine hydroxylase (TH) is the first enzyme in catecholamine biosynthesis, as well as being the rate-limiting enzyme in that process. TH requires tetrahydrobiopterin and uses iron as a cofactor in the 3,4-hydroxylation of tyrosine to produce dopa. Four isoforms of TH are expressed in the human brain and all have enzymatic activity (Nagatsu, T, 1989; Lewis, DA et al, 1993).
