Comprehensive Chemical Standards for LC Analysis

For sample extraction and LC analysis we used Fisher Optima^TM grade solvents (Fisher Scientific, Hanover Park, IL). The chemical standards were purchased from Cayman Chemical Company (stachydrine hydrochloride [i.e., proline betaine]) (Ann Arbor, MI) or Sigma-Aldrich (debrisoquine, chlorpropamide, 4-nitrobenzoic acid, Nε,Nε,Nε-trimethyllysine hydrochloride [TML], L-carnitine, cis-aconitic acid, citric acid, N1-acetylspermidine dihydrochloride, N-acetyl-L-arginine, creatine, niacinamide, 3-methyl-L-histidine, betaine hydrochloride, L-pipecolic acid, indole-3-acetic acid, and indoxyl sulfate) (St. Louis, MO). The hexosamine-valine-isoleucine-OH (Hex-V-I) was synthesized by Expert Synthesis Solutions (London, ON, Canada) with details previously provided [27 (link)]. Quality control reference materials were obtained from the National Institute of Standards and Technology (NIST) (NIST Standard Reference Material 3667 [creatinine in frozen human urine] and NIST Standard Reference Material 1950 [metabolites in frozen human plasma]).