Tyrosine requirements in children with classical PKU determined by indicator amino acid oxidation

Tyrosine (Tyr) is an essential amino acid in phenylketonuria (PKU) because of the limited hydroxylation of phenylalanine (Phe) to Tyr. The recommended intakes for Tyr in PKU are at least five times the recommended phenylalani ne intakes. This suggests that Phe and Tyr contribute similar to 20 and 80% , respectively, of the aromatic amino acid (AAA) requirement (REQ). In anim als and normal humans, dietary Tyr was shown to spare 40-50% of the Phe req uirement, proportions that reflect dietary and tissue protein composition. We tested the hypothesis that the Tyr REQ in PKU would account for 45% of t he total AAA REQ by indicator amino acid oxidation (IAAO). Tyr REQ was dete rmined in five children with PKU by examining the effect of varying dietary Tyr intake on lysine oxidation and the appearance of (CO2)-C-13 in, breath ((FCO2)-C-13) under dietary conditions of adequate energy, protein (1.5 g. kg(-1).day(-1)), and phenylalanine (25 mg.kg(-1).day(-1)). Lysine oxidation and (FCO2)-C-13 were determined using a primed 4-h oral equal-dose infusio n of L-[1-C-13]lysine. Lysine oxidation and (FCO2)-C-13 decreased linearly as Tyr intake increased, to a break point that was interpreted as the mean dietary Tyr requirement (16.3 and 19.2 mg.kg(-1).day(-1), respectively). At Tyr intakes of >16.3 and 19.2 mg.kg(-1).day(-1), lysine oxidation and (FCO 2)-C-13, respectively, were low and constant. This represents 40.4 and 44.4 %, respectively, of the total AAA intake. The current recommendations for T yr intake in PKU patients appear to be overestimated by a factor of similar to 5. This study is the first application of the IAAO technique in a pedia tric population and in humans with an inborn error of metabolism.