1. MetaLink, St. Louis, MO 63130, USA.
2. Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130, USA.
3. Department of Medicine, Washington University in St. Louis, St. Louis, MO 63130, USA.
4. Center for Metabolomics and Isotope Tracing, Washington University in St. Louis, St. Louis, MO 63130, USA.
5. Siteman Cancer Center, Washington University in St. Louis, St. Louis, MO 63130, USA.