An LC-MS/MS method to quantify acylcarnitine species including isomeric and odd-numbered forms in plasma and tissues

Acylcarnitines are intermediates of fatty acid and amino acid oxidation found in tissues and body fluids. They are important diagnostic markers for inherited diseases of peroxisomal and mitochondrial oxidation processes and were recently described as biomarkers of complex diseases like the metabolic syndrome. Quantification of acylcarnitine species can become challenging because various species occur as isomers and/or have very low concentrations. Here we describe a new LC-MS/MS method for quantification of 56 acylcarnitine species with acyl-chain lengths from C2 to C18. Our method includes amino acid-derived positional isomers, like methacrylyl-carnitine (2-M-C3:1-CN) and crotonyl-carnitine (C4:1-CN), and odd-numbered carbon species, like pentadecanoyl-carnitine (C15:0-CN) and heptadecanoyl-carnitine (C17:0-CN), occurring at very low concentrations in plasma and tissues. Method validation in plasma and liver samples showed high sensitivity and excellent accuracy and precision. In an application to samples from streptozotocin-treated diabetic mice, we identified significantly increased concentrations of acylcarnitines derived from branched-chain amino acid degradation and of odd-numbered straight-chain species, recently proposed as potential biomarkers for the metabolic syndrome. In conclusion, the LC-MS/MS method presented here allows robust quantification of isomeric acylcarnitine species and extends the palette of acylcarnitines with diagnostic potential derived from fatty acid and amino acid metabolism.