Determination of the isotope enrichment of one or a mixture of two stable labelled tracers of the same compound using the complete isotopomer distribution of an ion fragment; theory and application to in vivo human tracer studies.

Calculations of flux rates for stable isotope tracer studies are based upon enrichment values of an infused tracer. We propose the determination of enrichment values by gas chromatography/mass spectrometry, which is based on tracer mole fraction and mass spectrometer signals, normalized over the total signal of an ion fragment isotopomer distribution. The method accounts for overlap of the signals of one or two tracers and the tracee, high tracer mole fraction and incomplete labelling of the (infused) tracer. For the single and multiple tracer case a linear relationship between tracer mole fraction (from zero to one) and all normalized mass spectrometer signals is derived. This linearity over the entire range is demonstrated with a single (1-13C)glucose tracer and for mixtures of (1-13C)- and (3,3-2H2)tyrosine tracers. The linearity allows determination of the tracer mole fraction for two tracers, using multiple linear regression. The corresponding calibration can rely on measurements of the pure tracer and tracee compound, without weighing or check for chemical purity. This is compared with a calibration based on tracer/tracee mixtures. Estimates for the tracer mole fraction are slightly better if based on a calibration, using standard mixtures. In all cases the tracer mole fraction can be determined with high precision (coefficient of variation smaller than 5%) and high accuracy. For tyrosine it is demonstrated that the measurement of seven channels rather than three, for the main isotopomers, does not reduce the precision in the prediction of the tracer mole fraction. Equations are also derived to use the tracer mole fraction to estimate the endogenous production of the tracee under study conditions, assuming a steady state of the host metabolism.