The mammalian aromatic amino acid hydroxylases (phenylalanine, tyrosine, and tryptophan hydroxylase (PheOH, TyrOH, TrpOH, respectively)) are functionally and structurally closely related enzymes (eg 65% sequence identity and 80% sequence homology in the catalytic domains) 1 (Figure 1). This family of enzymes shares a common catalytic mechanism, in which dioxygen is used by an active site containing a single, reduced iron atom to hydroxylate an unactivated aromatic substrate (ie, monooxygenase activity), concomitant with a two-electron oxidation of tetrahydropterin cofactor to its quinonoid dihydropterin form (Figure 2). 2 The enzymes play an important role in mammalian metabolism: PheOH initiates the catabolism/detoxification of high levels of phenylalanine, while TyrOH and TrpOH catalyze the rate-limiting steps in the biosynthesis of the neurotransmitters/hormones dopamine/noradrenaline/adrenaline and serotonin, respectively. In 1959, Kaufman3 related a deficiency in hPheOH to the genetic disease phenylketonuria (PKU), and more recently a deficiency in hTyrOH has been related to the genetic disease L-DOPA responsive dystonia4 and juvenile parkinsonism. 5 Current research is close to linking TyrOH with bipolar affective disorder6 and studying the relationship between DNA polymorphism of the TyrOH gene and schizophrenia7 (see section 4). Reduced biosynthesis of L-DOPA by TyrOH as a result of progressive degeneration of dopaminergic neurons in the nigrostriatal pathway is characteristic of idiopathic parkinsonism (Parkinson’s disease), and evidence has recently been presented that the TyrOH enzyme system itself may contribute to the oxidative stress considered to be responsible for the neurodegeneration. 8 While the enzymes operate on different amino acid substrates, the similarity of substrate structure is apparent, and recent data have provided important sequence and mechanistic links between the different hydroxylases.

In solution, the aromatic amino acid hydroxylases form homotetramers with molecular mass ranging from 204 to 217 kDa. Sequence homologies, sitedirected mutagenesis, and partial proteolysis experiments have demonstrated that these enzymes are composed of an N-terminal region (PheOH 1-142; TyrOH 1-155; TrpOH 1-177) with regulatory properties and a C-terminal region (PheOH 143-452; TyrOH 156-498; TrpOH 178-445) known to be