Position-specific isotope fragmentation differential quantification (PIF-DQ) of phenylalanine positional isotopomers:...

The indicator amino acid oxidation (IAAO) method is a minimally invasive approach for estimating amino acid and protein requirements using a ¹³C-labeled indicator amino acid. In IAAO studies using l-[1-¹³C]phenylalanine ([1-¹³C]Phe), accurate determination of tracer-derived enrichment relative to unlabeled phenylalanine is important for calculating the tracer-to-tracee ratio (TTR). A key analytical challenge is distinguishing the administered [1-¹³C]Phe tracer from naturally occurring M + 1 phenylalanine isotopologues. In this study, we developed and validated a novel analytical method, termed Position-specific Isotope Fragmentation Differential Quantification (PIF-DQ), using ultra-high performance liquid chromatography-tandem mass spectrometry for the simultaneous quantification of l-phenylalanine (Phe) and its isotopologues in human urine. By optimizing collision energy, we established specific multiple reaction monitoring transitions to differentially quantify the species: m/z 166.1 > 120.2 for Phe; m/z 167.1 > 78.1, selective for the phenyl-group labeled Natural-(M + 1)-Phe; and m/z 167.1 > 77.1 for Total-(M + 1)-Phe (the sum of natural and tracer isotopologues). The method demonstrated good linearity (R² > 0.995), sensitivity (LLOQ ≤ 0.0004 µg/mL), accuracy (recoveries of 94.0-111.0 %), and precision (RSD < 13.1 %) over a 4.5-minute run time. Application to urine samples from an IAAO study successfully distinguished the tracer-derived elevation in the Total-(M + 1)-Phe/Phe ratio from the stable natural abundance ratio, demonstrating the robustness of the method against physiological variations like urinary concentration.