Abstract
Blood phenylalanine-to-tyrosine (Phe/Tyr) ratio is an important indicator of metabolic control in phenylketonuria patients. We present the data that highlights the role of Phe/Tyr-ratio in the evaluation of tetrahydrobiopterin (BH(4))-responsiveness in patients with hyperphenylalaninemia. Our data complements the results from the original research article by Tansek et al., 2012 [1]. We performed a BH(4)-loading test in 32 patients after four days of increased protein intake (2000 mg/kg body weight). Blood sampling was performed 96, 72, 48, 24, 16 h, and moments before oral administration of BH(4) in a dose of 20 mg/kg body weight. Additional blood samples were collected 8 and 24 h after its administration. Phenylalanine (Phe) and Tyrosine (Tyr) levels were determined from dried blood spots by tandem mass spectrometry. Phe/Tyr-ratio reached a plateau after three days of increased dietary protein intake. Fifteen patients (47%) responded to BH(4), defined as a decrease of Phe-of at least 30% after 24 h of BH(4) administration. Phe/Tyr-ratios were significantly higher in non-responders compared to responders. In the responder group, Phe/Tyr-ratios decreased in average of 67% (p = 0.001) and 45% (p = 0.001) after 8 and 24 h of BH(4) administration, respectively. Phe/Tyr-ratio decreased after 8 h of drug administration also in the non-responder group, but not 24 h after administration.