ORIGINAL ARTICLE The measurement of urinary Δ1-piperideine-6-carboxylate, the alter ego of α-aminoadipic semialdehyde,

Abstract The assessment of urinary α-aminoadipic semial-dehyde (α-AASA) has become the diagnostic laboratory test for pyridoxine dependent seizures (PDS). α-AASA is in spontaneous equilibrium with its cyclic form Δ1-piperi-deine-6-carboxylate (P6C); a molecule with a heterocyclic ring structure. Ongoing diagnostic screening and monitoring revealed that in some individuals with milder ALDH7A1 variants, and patients co-treated with a lysine restricted diet, α-AASA was only modestly increased. This prompted us to investigate the diagnostic power and added value of the as-sessment of urinary P6C compared to α-AASA. Urine sam-ples were diluted to a creatinine content of 0.1 mmol/L, followed by the addition of 0.01 nmol [2H9]pipecolic acid as internal standard (IS) and 5 μLwas injected onto aWaters C18 T3 HPLC column. Chromatography was performed using water/methanol 97/3 (v/v) including 0.03 % formic acid by volume with a flow rate of 150 μL/min and detection was accomplished in the multiple reaction monitoring mode: P6C m/z 128.1>82.1; [2H9]pipecolic acid m/z 139.1>93.1. Due to the dualistic nature of α-AASA/P6C, and the lack of a proper internal standard, the method is semi quantitative. The intra-assay CVs (n010) for two urine samples of proven PDS patients with only modest P6C increases were 4.7 % and 8.1%, whereas their inter-assay CVs (n010) were 16 and 18 % respectively. In all 40 urine samples from 35 individuals with proven PDS, we detected increased levels of P6C. There-fore, we conclude that the diagnostic power of the assessments of urinary P6C and α-AASA is comparable