Labeled Trimethyllysine Load Depletes Unlabeled Carnitine in Premature Infants without Evidence of Incorporation

Abstract

6-N-Trimethyl-[d₉]-L-lysine (dTML), the labeled form of a mammalian carnitine precursor, was administered to two groups of premature infants. Although the urinary output of dTML significantly increased in the low-dose-treated group (100 μmol/day), this amount did not affect the urinary output or plasma levels of carnitine and carnitine esters. In the second group of infants, after administration of 500 μmol dTML the plasma-free carnitine concentration increased (from 9.95 ± 0.63 to 12.9 ± 0.87 nmol/ml, p > 0.05) with a significant increase in the urinary excretion of free carnitine on the day of dTML administration and on the posttreatment day (from 4.79 ± 1.36 to 9.85 ± 1.18 and to 17.5 ± 2.31 μmol/day, respectively). Analysis of urine using fast atom bombardment mass spectrometry (FAB-MS) revealed only the presence of the dTML in the urine of the newborns; no change was detected in the relative abundance of any other carnitine precursor. Surprisingly, in the second group, which received the higher dose of dTML supplement, only the signal intensity of the unlabeled carnitine increased after dTML administration; no new peak appeared in the urine that would correspond to the de novo synthesized carnitine containing the stable isotope-labeled trimethyl group of dTML. Thus, the FAB-MS analysis clearly demonstrated that contrary to the likely prediction, the 270% extra free carnitine output was a consequence of a dose-dependent dTML-induced depletion of the free carnitine reserves from the newborns. The absence of the incorporation of the label from dTML into carnitine strongly suggests that circulating TML is not the precursor of carnitine in premature infants.

Carnitine biosynthesis in mammals

“…After loading, all the metabolites of carnitine biosynthesis could be detected in urine in deuterium-labelled form, except for TMABA. In addition, deuterium-labelled carnitine was also incorporated into acylcarnitines (F. M. Vaz ), however, was excreted unchanged in urine, which is in agreement with previous findings that humans do not use exogenous TML efficiently as a precursor for carnitine biosynthesis [117,134,135]. These results show that newborns have the capability to synthesize carnitine from exogenous TML, albeit at a low rate.…”

“…Another important observation made by Melegh and coworkers [134,135] is that administration of deuterium-labelled TML considerably increased unlabelled carnitine and butyrobetaine excretion. In previous studies in which unlabelled precursors were used [79,94], the carnitine excretion was used to calculate the rate of carnitine biosynthesis, assuming that this carnitine was a result of actual biosynthesis.…”

“…More recently, Melegh and co-workers [134,135] performed a single-day loading study in premature infants using orally administered deuterium-labelled TML. They could not detect incorporation of the deuterium label into urinary carnitine using fast-atom-bombardment MS. As an extension of this study, Vaz and co-workers performed a similar experiment with seven full-term newborns, who received deuterium-labelled TML for 5 days, and used our novel assay for the analysis of the carnitine biosynthesis metabolites in urine.…”