Determination of isotopic enrichments of [1-13C]homocysteine, [1-13C]methionine and [2H3-methyl-1-13C]methionine in human plasma by gas chromatography–negative chemical ionization mass spectrometry

We describe a reliable method for the simultaneous determination of isotopic enrichments of [1-13C]homocysteine, [1-13C]methionine and [2H3-methyl-1-13C]methionine in human plasma. Accurate [1-13C]homocysteine calibration standards were prepared by chemical conversion via thiolactonisation of [1-13C]methionine standards. Based upon anion-exchange chromatography, (di)acetyl-3,5-bis–trifluoromethylbenzyl derivatives, preparation of accurate calibration curves and gas chromatography–negative chemical ionization mass spectrometry, isotopic enrichments in human plasma could be determined with TTR (%) <±0.2% (N=3) for [1-13C]homocysteine (enrichment range 0–8%), [1-13C]methionine (enrichment range 0–3%) and [2H3-methyl-1-13C]methionine (enrichment range 0–12%). The method was applied in a [2H3-methyl-1-13C]methionine tracer infusion study in a biological model.