12] Assessing metabolism of β-[13C]carotene using high-precision isotope ratio mass spectrometry

Abstract

This chapter describes a stable isotope tracer method based on the use of highly enriched β-[U-13C]carotene and high-precision isotope ratio mass spectrometry (IRMS). Biological applications of IRMS are reviewed in the chapter. In gas chromatography combustion (GCC)–IRMS, combusted analytes are detected as the CO2+ produced in a high-sensitivity tight ion source. Each carbon atom of the analyte molecule has an independent chance of ionization and therefore detection limits are related to moles of analyte carbon, rather than to moles of analyte as in gas chromatography/mass chromatography (GC/MS). The ionization probability and molecular ion stability of CO2 is constant for all analytes. For these reasons, compounds of low ionization efficiency, low stability, and high molecular weight tend to be detected with high sensitivity by GCC–IRMS compared to conventional organic GC/MS. GCC–IRMS permits use of low doses typical of daily β-carotene intake, which do not perturb endogenous pool sizes of β-carotene or retinol. β-[13C]carotene is clearly evident in plasma by 3 hr after a dose of 2 mg β-[U-13C]carotene. In contrast, no detectable increase in concentration of labeled βC could be observed prior to 5 hr postdose with 40 mg β-carotene-d8 uring organic mass spectrometry.