Effect of BH4 on blood phenylalanine and tyrosine variations in patients with phenylketonuria

In patients with phenylketonuria, stability of blood phenylalanine and tyrosine concentrations might influence brain chemistry therefore patient outcome. This study prospectively investigated the effects tetrahydrobiopterin (BH4), as a chaperone hydroxylase on diurnal day-to-day variations concentrations. Blood were measured in dried spots (DBS) four times daily for 2 days (fasting, before lunch, dinner, evening) once (fasting) 6 randomized cross-over design period BH4 without BH4. The sequence was randomized. Eleven proven responsive PKU participated, 5 them used protein substitutes during treatment. Natural intake substitute dosing adjusted order to keep DBS levels within target range. Patients filled out 3-day food diary both periods. Variations Tyr expressed standard deviations (SD) coefficient variation (CV). treatment did not significantly nor variations, but decreased (median SD 17.6 ?mol/l, median CV 21.3%, p = 0.01) compared diet only 34.2 43.2%). Consequently, phenylalanine/tyrosine ratio smaller, while fasting tended be higher. impact possibly explained by less use increased synthesis.