isoMETLIN: A Database for Isotope-Based Metabolomics

The METLIN metabolite database has become one of the most widely used resources in metabolomics for making metabolite identifications. However, METLIN is not labeled. As a result, unbiasedly tracking the transformation of labeled metabolites with isotope-based metabolomics is a challenge. Here we introduce a new database, called isoMETLIN (http://isometlin.scripps.edu/), that has been developed specifically to identify metabolites incorporating isotopic labels. isoMETLIN enables users to search all computed isotopologues derived form METLIN on the basis of mass-to-charge values and sepcified isotopes of interes, such as C-13 or N-15. Additionally, isoMETLIN contains experimental MS/MS data on hundreds of isotopmers. These data assist in localizing the position of isotopic labels within a metabolite. From these experimental MS/MS isotopomer spedctra, precursor atoms can be mapped to fragments. The MS/MS spectra of additional isotopomers can then be computationally generated and included within isoMETLIN. Given that iosbric isotomers cannont be separated chromatographically or by mass bur are likely to occur simulataneously in a biological system, we have also implemented spectal-mixing function in isoMETLIN. This functionality allows users to combine MS/MS spectra form various isotopomers in different ratios to obtain a theoretical MS/MS experimental data, isoMETLIN facilitates the identification of isotopologues as well as isotopomers from biological samples and provieds a platform to drive the next generation of isoptoe based metabolomic studies.