Effects of tetrahydrobiopterin and phenylalanine on in vivo human phenylalanine hydroxylase by phenylalanine breath test

BH4 administration results in the reduction of blood phenylalanine level in patients with tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency. The mechanism underlying BH4 response remains unknown. Here, we studied the effects of BH4 and phenylalanine on in vivo PAH activity of normal controls using the phenylalanine breath test (PBT) by converting l-[1-13C] phenylalanine to 13CO2. Phenylalanine oxidation rates were expressed as Δ13C (13CO2/12+13CO2, ‰) and cumulative recovery rates over 120 min (CRR120, %; total amount of 13CO2/the administered dose of 13C-phenylalanine). Under physiological conditions of blood phenylalanine, BH4 administration reduced the Δ13C peak from 40.8‰ to 21.6‰ and CRR120 from 16.9% to 10.2%. Under high blood phenylalanine conditions, administration of BH4 increased the Δ13C peak from 30.7‰ to 46.0‰, while the CRR120 was similar between phenylalanine (19.9%) and phenylalanine + BH4 (21.1%) groups. Corrected Δ13C and CRR120 were calculated against serum phenylalanine levels to remove the effects of phenylalanine loading. After BH4 administration, the corrected Δ13C peak increased from 82.7‰ to 112.6‰, while the corrected CRR120 was similar (47.6% and 45.6%). These results indicate that phenylalanine worked as a regulator of in vivo PAH by serving as both a substrate and an activator for the enzyme. Excessive dosages of BH4 inhibited PAH under normal phenylalanine conditions and activated PAH under conditions of high phenylalanine. The regulation system is therefore designed to maintain phenylalanine levels in the human body. Appropriate BH4 supplementation must be reviewed in patients with BH4-responsive PAH deficiency.