Determination of amino acid isotope enrichment using liquid chromatography-mass spectrometry.

The change in amino acid enrichment, an indicator of a change in protein synthesis and/or degradation, is usually measured using gas chromatography-mass spectrometry and/or (GC-combustion) isotope ratio mass spectrometry. Unfortunately, often a complex and sensitive derivatization procedure and/or a large amount of sample is required. Also, these techniques are less suited to study intermediary metabolism, in which the simultaneous application (and thus measurement) of multiple amino acid tracers is preferred. Alternatively, in this study the possibilities of the coupling of liquid chromatography and mass spectrometry were explored, resulting in the measurement of both the concentration and isotope enrichment of o-phthaldialdehyde (OPA)-derivatizated plasma amino acids in one run. This was achieved by the injection of OPA-derivatizated amino acids into an automated HPLC system. After the elution of buffer salts and reagent excess to drain using column switching, the column effluent was directed via a fluorescence detector into a Thermoquest Model LCQ benchtop LC-MS. Mass spectrometric measurements were performed in "zoom-scan" mode, employing multiple scan events if the target components were not baseline separated. Best signal-to-noise ratio's were obtained using the LCQ's electrospray probe in the negative mode. Still, when working under standard conditions the total ion current of OPA-amino acid derivatives eluting at the beginning of the chromatogram (e.g., citrulline, arginine and glycine) was by a factor of 5 lower, compared to components eluting in the last part of the chromatogram (leucine, valine, and ornithine). These differences could be minimized by increasing the temperature of the heated capillary to 260 degrees C and by applying 5% collision energy (between the skimmer and the first octapole) to the first eluting components. A further improvement could not be obtained by the addition of makeup liquids like ammonia, acetic acid, methanol, or acetonitrile (up to 25% of column effluent flow). Considering these results and the fact that the first eluting amino acid derivatives are the most polar ones, we hypothesized that hydration of these components interferes with the ionization process. A linear calibration curve was obtained for both fluorescent response and total ion current (TIC) for all amino acids in the range from 5 to 1000 pmol per injection. The coefficient of variation of the fluorescent response was typically on the order of 1-4%, for the TIC this was between 4 and 9%. However, measurement of isotope ratios requires not only the determination of the area of the base peak, but also of the area of the (enriched) isotopomeric peak(s), having a much lower abundance. Therefore, isotope ratio measurements require the injection of at least 25 pmol of the amino acid derivative of interest (except for ARG 50 pmol) to obtain true ratio's. The accuracy of the isotope enrichment measurement was determined by the injection of a standard containing all major physiological amino acids (400 pmol each) and a standard at physiological concentrations (ranging from 50 pmol (CIT) to 350 pmol (VAL). Standard deviation of the isotopic ratios ranged from 0.1 to 0. 5% for the high (400 pmol) standards and from 0.2 to 0.8% for the low (physiological) standard, which is comparable with GC-MS. A plot of the results against the theoretical values gave a linear curve for all isotopes studied (R2 ranged from 0.9984 to 0.9997). However, the [1-13C]-enriched amino acids measured (LEU, GLY, and VAL) gave a closer agreement to the expected values as was found for [ureido-13C-5,5-2H2]-enriched citrulline and [guanidino-15N2]-enriched arginine. We could not determine whether this was due to the measurement procedure itself or resulting from an instability of the tracers in solution. Nevertheless, the results were reproducible and the theoretical value could be calculated using the tangent of the enrichment curves. (ABSTRACT TRUNCATED)