Development of a novel method for analysing N-acetyl-DL-leucine enantiomers in human fingernail by UPLC-ESI-MS/MS and the evaluation in diabetes mellitus

Background Recent research has been reported that N -acetyl-leucine content is significantly reduced in the saliva of diabetic patients, but no reports of detection in human nails have been found. This study aims to develop a novel method for the chiral separation of N -acetyl-DL-leucine (Ac-DL-Leu) in human fingernails to investigate the differences between healthy volunteers (HVs), prediabetes (PDs) and diabetic patients (DPs), and to verify its effectiveness in early warning of diabetes. Method Chiral resolution was performed using DBD-Apy pre-column derivatization on a C18 column (2.1 × 150 mm, 1.9 μm) at 40 °C, and detected by UPLC-ESI-MS/MS. Results The resolution and the limit of detection (LOD) of Ac-DL-Leu were 1.75 and 1.50 fmol, respectively. The linear range of Ac-DL-Leu was 10–2000 fmol and the determination coefficient (R 2 ) was above 0.9997. The recovery of Ac-DL-Leu in human nails was 96.92–105.69 %. The contents of Ac-D-Leu and Ac-L-Leu were analyzed in 18 HVs, 13 PDs and 16 DPs fingernails. The results showed that their contents were significantly lower in DPs than in PDs and HVs ( p  < 0.0001). Conclusions A method for evaluating the effectiveness of Ac-DL-Leu enantiomers in human fingernails as a biomarker for diabetes was firstly developed.