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Tyrosine metabolism in health and disease

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Phenylalanine (Phe) hydroxylase (PAH) deficiency leads to hyperphenylalaninemia (HPA) and tyrosine (Tyr) depletion. The study investigates Tyr homeostasis in patients with PAH deficiency and the effect of a slow-release amino acids therapy in phenylketonuria (PKU).

Four complementary investigations were performed:

(1) Tyr concentrations were monitored in 114 patients (10.6 ± 11.9 years) with PKU on dietary treatment supplemented with traditional amino acid formulations (n=52, 1175 samples) or non-PKU HPA on a free diet (n=62, 430 samples); (2) Tyr metabolism in PKU was quantitatively

evaluated in three patients by a simple Tyr oral loading test (100 mg/kg); (3) diurnal and (4) long-term Tyr concentrations were evaluated in 5 and 13 patients with PKU, respectively, who switched from a traditional formula to Afenil® micro 3H (a slow-release amino acids mixture). In all patients, the switch was due to unsatisfactory palatability of the traditional mixtures.

The results of the study are: (1) Tyr concentrations in the PKU population were subnormal and significantly lower than in non-PKU HPA (p<0.01); (2) the response to a Tyr loading test in PKU was normal, with basal Tyr concentrations reached within 12 h; (3) the diurnal metabolic profile in patients on slow-release amino acids therapy revealed higher morning fasting and nocturnal Tyr concentrations with respect to traditional therapy (p<0.01); (4) this picture was confirmed at followup, with normalization of morning fasting Tyr concentrations in patients on slow-release amino acids therapy (p<0.01) and unchanged Phe control (p=0.19).

Therefore, slow-release amino acids therapy can improve Tyr homeostasis in PKU. If associated with optimized Phe control, such a metabolic goal may allow long-term clinical benefits in patients with PKU.