treat PKU. However, this very practice of excluding Phenylalanine from the diet was not working for other forms of HPA. This was attributed to a deficiency of tetrahydrobiopterin (BH4), a cofactor for PAH. The most common form of BH4 deficiency is due to PTPS. Later on PTPS was discovered and it was learned that PKU causes HPA but not all HPA is PKU. See also 6-pyruvoyltetrahydropterin synthase References External links Amino acid metabolism disorders Autosomal recessive disorders Vitamin, coenzyme, and cofactor metabolism disorders