1. School of Pharmaceutical Sciences, Sun Yat-sen University, 132 Waihuandong Road, University City, Guangzhou 510006, China
2. Laboratory of Metabolism, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA
3. Center for Metabolomics and Mass Spectrometry, The Scripps Research Institute, La Jolla, CA 92037, USA