Fig. 1.

(a) Tyrosine metabolic pathway – highlighting the site of the enzyme defect observed in alkaptonuria and type 1 hereditary tyrosinaemia and the site of action of nitisinone. (b) Catecholamine metabolic pathway showing the formation of metadrenalines and (c) tryptophan metabolic pathway – highlighting the proposed site tyrosine inhibits tryptophan hydroxylase activity. TyH tyrosine hydroxylase, AADC aromatic acid decarboxylase, COMT catechol-O-methyl transferase, MAO monoamine oxidase, TrH tryptophan hydroxylase