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ThePahenu1/enu2(ENU1/2) mouse is a heteroallelic orthologous model displaying blood phenylalanine (Phe) concentrations characteristic of mild hyperphenylalaninemia. ENU1/2 mice also have reduced liver phenylalanine hydroxylase (PAH) protein content (˜20% normal) and activity (˜2.5% normal). The mutant PAH protein is highly ubiquitinated, which is likely associated with its increased misfolding and instability. The administration of a single subcutaneous injection of L-Phe (1.1 mg L-Phe/g body weight) leads to an approximately twofold to threefold increase of blood Phe and phenylalanine/tyrosine (Phe/Tyr) ratio, and a 1.6-fold increase of both nonubiquitinated PAH protein content and PAH activity. It also results in elevated concentrations of liver 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4), potentially through the influence of Phe on GTP cyclohydrolase I and its feedback regulatory protein. The increased BH4 content seems to stabilize PAH. Supplementing ENU1/2 mice with BH4 (50 mg/kg/day for 10 days) reduces the blood Phe/Tyr ratio within the mild hyperphenylalaninemic range; however, PAH content and activity were not elevated. It therefore appears that BH4 supplementation of ENU1/2 mice increases Phe hydroxylation levels through a kinetic rather than a chaperone stabilizing effect. By boosting blood Phe concentrations, and by BH4 supplementation, we have revealed novel insights into the processing and regulation of the ENU1/2-mutant PAH.