Figure 1.

Metabolism of tyrosine and block in HT‐1. The block in the enzyme fumarylacetoacetate hydrolase (FAH) causes buildup of fumarylacetoacetate and succinylacetone, which causes hepatic and renal toxicity, as well as inhibition of porphobilinogen synthase activity, resulting in accumulation of δ‐aminolevulinic acid. NTBC inhibits the enzyme 4‐hydroxyphenylpyruvate dioxygenase, prior to the accumulation of toxic compounds