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.

Abstract We describe a reliable method for the simultaneous determination of isotopic enrichments of [1- 13 C]homocysteine, [1- 13 C]methionine and [ 2 H 3 -methyl-1- 13 C]methionine in human plasma. Accurate [1- 13 C]homocysteine calibration standards were prepared by chemical conversion via thiolactonisation of [1- 13 C]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 (%) N =3) for [1- 13 C]homocysteine (enrichment range 0–8%), [1- 13 C]methionine (enrichment range 0–3%) and [ 2 H 3 -methyl-1- 13 C]methionine (enrichment range 0–12%). The method was applied in a [ 2 H 3 -methyl-1- 13 C]methionine tracer infusion study in a biological model.