Department of Genetics, Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110, USA, Scripps Center for Metabolomics and Mass Spectrometry, Departments of Chemistry, Molecular and Computational Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA and Department of Chemistry, Washington University, St. Louis, MO 63130, USA.