Phenylalanine conversion to tyrosine: Comparative determination with l-[ring-²H₅]phenylalanine and l-[1-¹³C]phenylalanine as tracers in man

The in vivo rate of conversion of phenylalanine to tyrosine (PheOH) can be estimated using combinations of stable isotope-labeled phenylalanine and tyrosine. We have compared in four healthy adult men the rates of phenylalanine conversion to tyrosine based on the following pairs of primed, continuous tracer infusions administered simultaneously: (1) l-[ring-²H₅]phenylalanine and ²H₂-tyrosine with a ²H₄-tyrosine prime, and (2) l-[1-¹³C]phenylalanine and ²H₂-tyrosine with a 1-¹³C-tyrosine prime. Phenylalanine oxidation was determined from measurement of ¹³CO₂ excretion in expired air. Tracers were given for 8 hours, with subjects being in the postabsorptive state during the first 3 hours and in the fed state during the remaining 5 hours. Mean (±SD) rates (μmol · kg^-1 · h^-1) of phenylalanine conversion to tyrosine for fasted and fed states, respectively, were 5.1 ± 2.9 and 6.8 ± 3.4 with ²H₅-phenylalanine and significantly higher (P < .05) at 11.1 ± 5.6 and 12.7 ± 7.7 with ¹³C-phenylalanine as tracer. Phenylalanine oxidation was 9.9 ± 2.0 and 13.5 ± 2.6, respectively, for fasted and fed states, and these mean values did not differ (P > .1) from the rate of phenylalanine conversion to tyrosine determined using ¹³C-phenylalanine. These results indicate the need for caution in interpreting kinetic aspects of phenylalanine metabolism when based on isotopic data from multideuterated phenylalanine. It is suggested that when determinations of rates of phenylalanine conversion to tyrosine based on l-[ring-²H₅]phenylalanine are corrected by factors of about 2.2 and 1.8 for fasted and fed states, respectively, they would provide a better estimated of the in vivo rate of conversion of phenylalanine to tyrosine.