Fig. 2: Metabolism in tyrosinemia. 1. 4-hydroxylphenylpyruvate (4-HPP) is matabolized by 4-HPP dioxygenase (the enzyme inhibited by 2-(2-nitro-4 trifluoromethylbenzoyl)-1,3cyclohexadione [NTBC] during therapy). 2. Maleylacetoacetate is thought to be responsible for the renal abnormalities in tyrosinemia. 3. Fumarylacetoacetate hydrolase (FAH) is the enzyme that is deficient in tyrosinemia. 4. The presence of succinylacetone is used as the marker in screening children for the disorder; it inhibits porphobilinogen synthase in the porphyrin synthesis pathway.12