Propionylcarnitine

Reverse-osmosed ultrapure water was obtained from a Milli-Q Plus185 system (Millipore, Billerica, MA, USA). A MS grade methanol (MeOH), used for standards preparation, and acetonitrile (ACN) were obtained from Fisher Scientific (Hampton, NH, USA). Analytical grade ammonia solution (28%, GPR RECTAPUR^®) and acetic acid glacial (AnalaR^® NORMAPUR^®) were obtained from VWR Chemicals (Radnor, PA, USA).
Arachidonic acid, LPC 14:0, LPC 16:0, LPC 17:0, LPC 17:1, LPC 18:0, LPC 18:1, LPC 19:0, LPE 18:0, LPI 16:0, LPI 20:4, palmitoleic acid, oleamide, SPA-1P, sphinganine-C17, sphingosine, S1P, sphingosine d7, LPC 18:1 d7 were purchased from Avanti lipids (Birmingham, AL, USA). Bilirubin, lactic acid, lauric acid, oleic acid, adenosine, arginine, betaine, carnitine, cortisol, creatine, creatinine, hexanoylcarnitine, hippuric acid, hypoxanthine, leucine/isoleucine, phenylalanine, proline, propionylcarnitine, urea, carnitine d3, isoleucine d7, and palmitic acid d31 were purchased from Sigma-Aldrich (Darmstadt, Germany). phenylalanine d5 and valine d8 were purchased from Cambridge Isotope Laboratories Inc. (Andover, MA, USA).

One hundred micro-litres of each thawed 24 h urine sample were diluted with 900 μL of milliQ water/acetonitrile (90/10, v/v). For plasma deproteinization, 200 µL of fasting samples were mixed with 200 µL of cold methanol and stored at −20 °C overnight. The samples were then centrifuged and 100 µL of the supernatant was diluted with 900 μL of milliQ water/acetonitrile (90/10, v/v). All plasma and urine samples were then filtered using 0.2 nm Phenex-RC Syringe Filters with a cellulose membrane (Phenomenex, Le Pecq, France), transferred to a LC vial and either kept at −20 °C or stored at 4 °C if used within 1–2 days until analysis. Blank and quality control (QC) samples were injected every 10 analytical samples in order to control the possible instrument deviation. The blank samples contained acetonitrile and water (10/90, v/v) and QC samples contained the aliquots of all analyzed samples. All solvents used were liquid chromatography mass spectrometry (LC-MS) grade (Sigma-Aldrich; Saint Quentin Fallavier, France) and the aqueous solutions were prepared using purified Milli-Q water. The authentic standards of carnitine, acetyl-carnitine, propionyl-carnitine, butyryl-carnitine, 2-methylbutyroyl-carnitine, uric acid and 1-methylhistidine were purchased from Sigma-Aldrich.

Gradient grade water and LC-MS grade methanol for liquid chromatography plus the non LC-MS grade rinsing solvents 2-propanol and acetonitrile as well as formic acid (puriss.) and ammonia fluoride (puriss.) were purchased from Carl Roth (Carl Roth, Rothenfels, Germany). A collection of all l-amino acids and carnitine, acetylcarnitine, propionylcarnitine, butyrylcarnitine, valerylcarnitine, hexanoylcarnitine, octanoylcarnitine, decanoylcarnitine, lauroylcarnitine, myristoylcarnitine, palmitoylcarnitine and stearoylcarnitine, were purchased from Sigma-Aldrich (St. Louis, MO, USA).
The deuterated internal standards (IS) Cortisol ²H₄, Progesterone ²H₉, 17-Hydroxyprogesterone ²H₈, 11-Deoxycortisol ²H₅, 21-Deoxycortisol ²H₈, Androstenedione ¹³C₃, Corticosterone ²H₄, 11-Deoxycorticosterone ¹³C₃ were also products of Sigma-Aldrich (St. Louis, MO, USA). The deuterated amino acids and acylcarnitines were obtained from the MassChrom^® Amino Acids and Acylcarnitines from Dried Blood kit from Chromsystems (Munich, Germany). Dried blood spot cards (TFN filter paper) were provided by CAMAG (Muttenz, Switzerland). Fresh whole blood was obtained from the local blood donation center (Basel, Switzerland).