Determination of glycation biomarkers in human fingernails by isotope-dilution liquid chromatography tandem mass spectrometry (LC-MS/MS)

Glycation is a non-enzymatic, post-translational modification of proteins which is elevated in several pathologies, notably diabetes. An early-stage glycation product, glycated hemoglobin (HbA1c), is used in the clinical management of diabetes, and advanced glycation end-products (AGEs) are implicated in the etiology of diabetic complications. Fingernail clippings contain a time-integrated repository of several metabolic processes during the preceding 3-5 months, are easily sampled, and various elements and molecules have been shown to remain stable within them for long periods without refrigeration. Building upon a few underexploited studies, we investigated fingernails as a non-invasive matrix to assess glycation using liquid chromatography-mass spectrometry to quantify ungual biomarkers of early- and advanced glycation (respectively furosine, as a fructose-lysine derivative, and two AGEs (N^ε-carboxymethyllysine (CML) and N^ε-carboxyethyllysine (CEL)). The method was appropriately validated and provided accurate and precise measurements of two amino acids and the glycation biomarkers. Sample storage at ± 25 °C for 12 months had no effect upon these analytes, and the method was applied to fingernails from 87 people with diabetes. There was a moderate, linear correlation between ungual furosine concentrations and HbA1c at the time of nail sampling (r_s = 0.339, p = 0.0011). Among subjects for whom previous measurements were available, there was no correlation between ungual glycation and HbA1c measured > 3 months before nail sampling, indicating that ungual furosine reflects early-stage glycation over a similar period to HbA1c. This study provides further evidence, using modern analytical techniques, that fingernails offer the possibility to quantitatively and non-invasively assess glycation.