PAH and Hyperphenylalaninemia: As a cofactor of the phenylalanine hydroxylase (PAH) enzyme that catalyzes the transfer of L-tyrosine (Tyr) from L-phenylalanine (Phe), BH4D causes a build-up of Phe in the blood, which leads to hyperphenylalaninemia (HPA) (Tendi et al., 2022).