Amino acid solutions for premature infants contain little tyrosine; Tyr must be made available by in vivo synthesis from phenylalanine. To study the regulation of Phe to Tyr conversion, we used the chronically catheterized fetal sheep at 120 days gestation (term = 150); Phe and Tyr kinetics were characterized using isotopic tracers. Each animal was studied twice: Study 1 - Basal Phe/Tyr kinetics, followed by fetal amino acid supplementation with Aminosyn PF™ (Phe, 3.9 μmols/min, Tyr, 0.38μmols/min). Study II - Basal Phe/Tyr kinetics, followed by fetal amino acid supplementation + glycyl-tyrosine (Phe, 3.9 μmols/min, Tyr, 6.0μmols/min).

Results: All data are reported as mean±SEM, μmols/min.Table

Table 1
Full size table

Phe Ra increases during infusion of either supplemental mixture. However, Tyr Ra increases substantially only during the second study, when it is specifically supplemented with glycyl-tyrosine. This is in spite of the fact that Phe to Tyr conversion increases to the same extent in both studies. Phe to Tyr was highly correlated to total Phe Ra (Phe to Tyr = 0. 114*PheRa - 0.29, p<.001, R2 =.68.) Conclusions: 1.) The mammalian fetus is capable of converting Phe to Tyr. 2.) Phe availability, reflected by Phe Ra, is the primary regulator of Phe to Tyr conversion. 3.) If no supplemental Tyr is present, Tyr Ra does not increase. In conditions where Tyr supply may be limited, its availability may become limiting for protein synthesis 4.) Tyrosine supplied as glycyl-tyrosine is bioavailable and does increase Tyr Ra.