Plasma concentrations of BH4 determined as biopterin (nmol/L) by reversed-phase HPLC. The BH4 concentration was significantly reduced in both +,- and hph mice compared to WT mice (###p < 0.001) (n = 6 for WT and +,- mice, and n = 7 for hph mice). Moreover, hph mice displayed significantly lower BH4 concentrations compared to +,- mice ($$$p < 0.001). One-way ANOVA with pair-wise comparisons using the Fisher’s LSD test. Data are presented as mean + SEM.