Tryptophan

All reagents were purchased from Sigma Aldrich (St. Louis, MI, USA) if not stated otherwise. A standard mixture of 17 amino acids, namely histidine, serine, arginine, glycine, aspartate, glutamate, alanine, threonine, proline, cysteine, lysine, tyrosine, methionine, valine, isoleucine, leucine, and phenylalanine, was acquired from Waters (Milford, MA, USA). Asparagine, glutamine, taurine, histamine, ethanolamine, methylamine, citrulline, ethylamine, ornithine, putrescine, serotonin, cadaverine, tyramine, tryptamine, and 2-phenylethyl-amine standards were obtained from Sigma-Aldrich (St. Louis, MI, USA), and tryptophan was purchased from Cambridge Isotope Laboratories (Tewksbury, MA, USA).

Acetonitrile, methanol, formic acid and ammonium formate, all LC-MS grade were purchased from Fisher Scientific (Loughborough, UK). Ammonium hydroxide (25%, LC-MS grade) was bought from Merck (Darmstadt, Germany) and chloroform (HPLC-grade) was bought from BDH chemicals (Poole, UK). The isotopically labeled standards, guanine (¹³C, 2*¹⁵N, 98%), hypoxanthine (3*D, 98%), inosine (4*¹⁵N, >95%), tryptophan (5*D, 98%) and glutamine (5*D, 97%) were purchased from Cambridge Isotope laboratories (Andover, MA, USA) while valine (8* D, 98%) was obtained from Aldrich (St Louis, MO, USA). The standard compounds butyrylcarnitine (≥97%), hypoxanthine (≥99%), choline (≥99%), adenine (≥99%), L-tryptophan (≥99.5%), acetylcarnitine (≥99%), L-proline (≥99.5%), L-carnitine (≥98%) and L-arginine (≥98%) were obtained from Sigma (Steinheim, Germany). Phenylalanine was purchased from Merck. Creatine (99.5%) was bought from Chem Service (West Chester, PA, USA) and NADH (>95%) was bought from Appli Chem (Darmstadt, Germany). Betaine (>99%), taurine (>99%) and thiamine (99.9%) were purchased from Fluka (Steinheim, Germany). All water was purified using a Milli-Q purification unit (MilliporeMerck, Darmstadt, Germany).