Measurement of carnitine precursors, epsilon-trimethyllysine and gamma-butyrobetaine in human serum by tandem mass spectrometry

Determination of carnitine, its short chain acyl esters and metabolic precursors trimethyllysine and gamma-butyrobetaine by quasi-solid phase extraction and MS/MS detection

For the investigation of the metabolism and biosynthesis of carnitine, sensitive determination of carnitine and its metabolic precursors, trimethyllysine and gamma-butyrobetaine, is required. We present here a new simplified method for the analysis of carnitine, its acetyl- and propyl esters, as well as trimethyllysine and gamma-butyrobetaine without need for derivatization reactions by means of normal-phase LC and electrospray ionization tandem mass spectrometry. The limits of quantification were between 5 nM for acetyl carnitine and 70 nM for carnitine. Relative standard deviations in a fivefold determination of standard solutions were between <2% for carnitine and <10% for trimethyllysine. Quantifying the formation of deuterated carnitine from deuterated gamma-butyrobetaine, this method is also suitable for the determination of the activity of gamma-butyrobetaine dioxygenase in tissues.

Analysis of Carnitine Biosynthesis Metabolites in Urine by HPLC–Electrospray Tandem Mass Spectrometry

Background: We developed a method to determine the urinary concentrations of metabolites in the synthetic pathway for carnitine from N6-trimethyllysine and applied this method to determine their excretion in control individuals. In addition, we investigated whether newborns are capable of carnitine synthesis from deuterium-labeled N6-trimethyllysine.

Methods: Urine samples were first derivatized with methyl chloroformate. Subsequently, the analytes were separated by ion-pair, reversed-phase HPLC and detected online by electrospray tandem mass spectrometry. Stable-isotope-labeled reference compounds were used as internal standards.

Results: The method quantified all carnitine biosynthesis metabolites except 4-N-trimethylaminobutyraldehyde. Detection limits were 0.05–0.1 μmol/L. The interassay imprecision (CV) for urine samples with added compounds was 6–12%. The intraassay imprecision (CV) was 1–5% (3–10 μmol/L). Recoveries were 94–106% at 10–20 μmol/L and 98–103% at 100–200 μmol/L. The mean (SD) excretions of N6-trimethyllysine and 3-hydroxy-N6-trimethyllysine were 2.8 (0.8) and 0.45 (0.15) mmol/mol creatinine, respectively. γ-Butyrobetaine and carnitine excretions were more variable with values of 0.27 (0.21) and 15 (12) mmol/mol creatinine, respectively. After oral administration of deuterium-labeled N6-trimethyllysine, all urines of newborns contained deuterium-labeled N6-trimethyllysine, 3-hydroxy-N6-trimethyllysine, γ-butyrobetaine, and carnitine.

Conclusions: HPLC in combination with electrospray ionization tandem mass spectrometry allows rapid determination of urinary carnitine biosynthesis metabolites. Newborns can synthesize carnitine from exogenous N6-trimethyllysine, albeit at a low rate.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.