Stereospecificity of phenylalanine plasma kinetics and hydroxylation in man following oral application of a stable isotope-labelled pseudo-racemic mixture of L- and D-phenylalanine.
L-[15N]Phenylalanine and D-[2H5]phenylalanine have been administered orally to two healthy adult volunteers as a pseudo-racemic mixture at a dose of 25 mg/kg each. After oral application, the plasma kinetics of phenylalanine and tyrosine have been followed by the combined use of high pressure liquid chromatography and field desorption mass spectrometry. Additional incubation with D-amino acid oxidase was used to determine the enantiomeric composition of the differently labelled species of phenylalanine and tyrosine. D-Phenylalanine plasma levels show a faster rise to higher maximum values compared to L-phenylalanine (D/L ratio at maximum 3.19, 3.26). L-Phenylalanine is efficiently hydroxylated to L-tyrosine. In contrast, conversion of D-phenylalanine to the L-form with subsequent hydroxylation to L-tyrosine was observed. From the plasma kinetics it is estimated that about 1/3 of the applied dose of 25 mg/kg of D-phenylalanine is converted to the L-isomer. Of the administered dose of L-phenylalanine only very small amounts are excreted into urine as such (0.25%, 0.8%), whereas a substantial amount of the D-phenylalanine dose is found in urine (27.4%, 38.0%).
D000328:Adult; D002851:Chromatography, High Pressure Liquid; D003903:Deuterium; D006801:Humans; D006900:Hydroxylation; D007700:Kinetics; D013058:Mass Spectrometry; D009587:Nitrogen Isotopes; D010649:Phenylalanine / Q000097:blood* / Q000652:urine; D013237:Stereoisomerism; D014443:Tyrosine / Q000097:blood
D009587:Nitrogen Isotopes; D014443:Tyrosine; D010649:Phenylalanine; D003903:Deuterium