Measurement of epsilon-N-trimethyllysine in human blood plasma and urine

Individual variability in human blood metabolites identifies age-related differences

Metabolites present in human blood document individual physiological states influenced by genetic, epigenetic, and lifestyle factors. Using high-resolution liquid chromatography-mass spectrometry (LC-MS), we performed nontargeted, quantitative metabolomics analysis in blood of 15 young (29 ± 4 y of age) and 15 elderly (81 ± 7 y of age) individuals. Coefficients of variation (CV = SD/mean) were obtained for 126 blood metabolites of all 30 donors. Fifty-five RBC-enriched metabolites, for which metabolomics studies have been scarce, are highlighted here. We found 14 blood compounds that show remarkable age-related increases or decreases; they include 1,5-anhydroglucitol, dimethyl-guanosine, acetyl-carnosine, carnosine, ophthalmic acid, UDP-acetyl-glucosamine,N-acetyl-arginine,N6-acetyl-lysine, pantothenate, citrulline, leucine, isoleucine, NAD(+), and NADP(+) Six of them are RBC-enriched, suggesting that RBC metabolomics is highly valuable for human aging research. Age differences are partly explained by a decrease in antioxidant production or increasing inefficiency of urea metabolism among the elderly. Pearson's coefficients demonstrated that some age-related compounds are correlated, suggesting that aging affects them concomitantly. Although our CV values are mostly consistent with those CVs previously published, we here report previously unidentified CVs of 51 blood compounds. Compounds having moderate to high CV values (0.4-2.5) are often modified. Compounds having low CV values, such as ATP and glutathione, may be related to various diseases because their concentrations are strictly controlled, and changes in them would compromise health. Thus, human blood is a rich source of information about individual metabolic differences.

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.

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

Methods using tandem mass spectrometry for measurement of epsilon-trimethyllysine and gamma-butyrobetaine in human serum are described. Precursor ion scan analysis of a methylated sample was applied for gamma-butyrobetaine measurement. However, for epsilon-trimethyllysine measurement, homoarginine interfered with the methylated sample during precursor ion scan analysis. To overcome this interference, the sample was propylated and acetylated prior to precursor ion scan analysis. The obtained values resembled those obtained by enzymatic or HPLC measurement. Using tandem mass spectrometry, all members of the carnitine family, free carnitine, acylcarnitines, gamma-butyrobetaine, epsilon-trimethyllysine can be analyzed in 0.1 ml of serum. Thus, the proposed method appears to be suitable for clinical application, especially in the pediatric field.

Decreased plasma carnitine and trimethyl-L-lysine levels associated with lysosomal accumulation of a trimethyl-L-lysine containing protein in Batten disease

Batten disease, or juvenile neuronal ceroid-lipofuscinosis, is an autosomal-recessive hereditary disorder that leads to blindness, severe neurological degeneration, and premature death. The disease is characterized by massive accumulation of lysosomal storage bodies in most tissues. A significant constituent of the storage material is a protein that appears to be almost identical to a small hydrophobic inner mitochondrial membrane protein, subunit c of ATP synthase. The protein isolated from the storage bodies contains an epsilon-N-trimethyl-L-lysine (TML) residue at amino acid position 43. The presence of TML in the stored protein suggests that one of the lysine residues in subunit c is normally trimethylated, and this trimethylation may act as a signal to initiate degradation of the protein. Free TML produced by the degradation of TML-containing proteins is the first intermediate in the carnitine biosynthetic pathway. It is possible that trimethylated subunit c is a major source of the free TML used in carnitine biosynthesis. If this is the case, one would predict that the genetic defect resulting in the accumulation of TML containing subunit c would also reduce systemic levels of free TML and carnitine. To evaluate this possibility, plasma TML and carnitine levels were measured in affected human subjects, heterozygous carriers, and normal controls. Both TML and carnitine levels were significantly depressed in the affected individuals. This suggests that subunit c is normally a major source of TML for carnitine biosynthesis. In Batten disease, failure to degrade the TML-containing form of subunit c is probably responsible for the reduction in plasma TML and carnitine levels.

An ion-exchange chromatography procedure for the isolation and concentration of basic amino acids and polyamines from complex biological samples prior to high-performance liquid chromatography

The original objective of this study was to develop a selective and sensitive method for the analysis and quantification of basic amino acids from biological samples via reversed-phase high-performance liquid chromatography. Using various previously described techniques for the separation of amino acids, we were unsuccessful in measuring levels of histidine, arginine, ornithine, and lysine in biological samples due to the presence of interfering compounds. A "cleanup" procedure for the isolation of the basic amino acids using a weakly acidic cation exchange resin, Biorex-70 (Bio-Rad), is described in detail. Upon separation from the bulk of the neutral and acidic amino acids, the basic amino acids were subjected to precolumn fluorescence derivatization using 9-fluorenylmethyl chloroformate (FMOC) and the fluorescent derivatives were separated by RP-HPLC. The advantages of this method over previously described amino acid analysis techniques are (i) isolation and stable recovery (greater than 95%) of the desired basic amino acids, (ii) sensitivity of detection (low pmol range), (iii) complete resolution of derivatized amino acids via HPLC, (iv) limited amount of sample required for analysis, and (v) samples readily concentrated by lyophilization or rotoevaporating. This ion-exchange cleanup procedure was also adapted for the analysis of polyamines in concentrated culture media samples and proved additionally advantageous by eliminating the use of costly C-18 extraction columns required by previously described techniques.