Abstract
A healthy human female was loaded with D- and L-phenylalanine (Phe)-d5 (8 mg/kg body weight), and free Phe-d5 and tyrosine (Tyr)-d4, and protein-bound Phe-d5 in the plasma were assayed by gas chromatography-mass spectrometry with selected ion monitoring. Free Phe-d5 appeared more rapidly and abundantly in the plasma and disappeared more slowly from the plasma after loading with the D-isomer than after loading with the L-isomer. The maximum concentrations were 1.5 mg/100 ml plasma at 30 min after loading with the D-isomer, and 0.1 mg/100 ml plasma at 2 hr after loading with the L-isomer. The half-lives were 3.3 and 0.5 hr for the D-isomer and the L-isomer, respectively. After loading with the D-isomer, as after loading with the L-isomer, free Tyr-d4 and protein-bound Phe-d5 were found in the plasma.
