Partially 13C-labeled mouse tissue as reference for LC-MS based untargeted metabolomics

The inclusion of stable isotope-labeled reference standards in the sample is an established method for the detection and relative quantification of metabolic features in untargeted metabolomics. In order to quantify as many metabolites as possible, the reference should ideally include the same metabolites in their stable isotope-labeled form as the sample under investigation. We present here an attempt to use partially C-13-labeled mouse material as internal standard for relative metabolite quantification of mouse and human samples in untargeted metabolomics. We fed mice for 14 days with a C-13-labeled Ralstonia eutropha based diet. Tissue and blood amino acids from these mice showed C-13 enrichment levels that ranged from 6% to 75%. We used MetExtract II software to automatically detect native and labeled peak pairs in an untargeted manner. In a dilution series and with the implementation of a correction factor, partially C-13-labeled mouse plasma resulted in accurate relative quantification of human plasma amino acids using liquid chromatography coupled to mass spectrometry, The coefficient of variation for the relative quantification is reduced from 27% without internal standard to 10% with inclusion of partially C-13-labeled internal standard. We anticipate the method to be of general use for the relative metabolite quantification of human specimens.