Oral versus intravenous L-phenylalanine loading compared by simultaneous application of L-[2H5] and...

Oral loading with 1.5 g of L-[15N]phenylalanine was performed simultaneously with an intravenous infusion of 1.5 g of L-[2H5]phenylalanine in two healthy volunteers with normal phenylalanine-hydroxylase activity. For both volunteers peak levels of oral L-[15N]phenylalanine were about 20 micrograms/ml compared to peak levels of around 50 micrograms/ml for intravenous L-[2H5]phenylalanine. Throughout the four hours following application, the plasma levels of the intravenously administered phenylalanine were higher than the plasma levels of the phenylalanine administered orally. In contrast, similar plasma levels of L-[15N]tyrosine and of L-[2H4]tyrosine formed in vivo by hydroxylation of the corresponding stable isotope labelled L-phenylalanine precursors were observed during the test, indicating that about equal fractions of both the oral and of the intravenous L-phenylalanine are converted into L-tyrosine.