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

Abstract

BH 4 administration results in the reduction of blood phenylalanine level in patients with tetrahydrobiopterin (BH 4)-responsive phenylalanine hydroxylase (PAH) deficiency. The mechanism underlying BH 4 response remains unknown. Here, we studied the effects of BH 4 and phenylalanine on in vivo PAH activity of normal controls using the phenylalanine breath test (PBT) by converting l-[1- 13C] phenylalanine to 13CO 2. Phenylalanine oxidation rates were expressed as Δ 13C ( 13CO 2/ 12+13CO 2, ‰) and cumulative recovery rates over 120 min (CRR 120, %; total amount of 13CO 2/the administered dose of 13C-phenylalanine). Under physiological conditions of blood phenylalanine, BH 4 administration reduced the Δ 13C peak from 40.8‰ to 21.6‰ and CRR 120 from 16.9% to 10.2%. Under high blood phenylalanine conditions, administration of BH 4 increased the Δ 13C peak from 30.7‰ to 46.0‰, while the CRR 120 was similar between phenylalanine (19.9%) and phenylalanine + BH 4 (21.1%) groups. Corrected Δ 13C and CRR 120 were calculated against serum phenylalanine levels to remove the effects of phenylalanine loading. After BH 4 administration, the corrected Δ 13C peak increased from 82.7‰ to 112.6‰, while the corrected CRR 120 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 BH 4 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 BH 4 supplementation must be reviewed in patients with BH 4-responsive PAH deficiency.