N acetyl valine

NMR experiments were performed on glycine (identified as the alpha-glycine polymorph from X-ray diffraction) and N-acetyl-valine (NAV), both acquired from Sigma-Aldrich and used without further purification.

Rifampicin (RIF), verapamil (Ver), ritonavir (RTV), pravastatin (PV), 5-fluorouracil and fexofenadine (FEX) were purchased from Shanghai Macklin Co., Ltd (Shanghai, China), Tetradecanedioate (TDA), hexadecanedioate (HAD), 1-methylnicotinamide, pyridoxine, N-acetyl-L-phenylalanine, N-acetylvaline and azelaic acid (AZA) were purchased from Sigma-Aldrich (St Louis, MO, USA). L-serine-O-sulfate was purchased from Toronto Research Chemicals (Toronto, Canada). All primers for the real-time quantitative polymerase chain reaction (PCR) detecting system (qPCR) or quantitative real-time polymerase chain reaction (qRT-PCR) were synthesized by Sunbiotech Co., Ltd. (Beijing, China). Anti-P-gp was purchased from Abcam (ab170904, rat, USA). Anti-Oatp1b2 was purchased from Santa Cruz Biotechnology (sc-376904, rat, USA). Anti-β-actin was purchased from Affinity (AF7018, rat, China). Methanol and acetonitrile were high-performance liquid chromatography (HPLC)-grade (Fisher Scientific, NJ, USA). All other reagents and solvents were of analytical grade and were commercially available.