A rapid method to determine plasma homocysteine concentration and enrichment by gas chromatography/mass spectrometry

Homocysteine is an independent risk factor for cardio- and/or cerebrovascular diseases. Many methods are used to measure plasma homocysteine levels in physiological fluids. Current gas chromatographic/mass spectrometric (GC/MS) methods allow determination not only of plasma homocysteine concentration, but also of its turnover. However, they have some methodological limitations due to the reduction of disulfide bonds between homocysteine and other thiols or proteins often requiring the use of several very toxic compounds or multi-step procedures that are particularly time-consuming, and/or utilize expensive instruments. Herein is described a rapid and precise GS/MS method to determine homocysteine turnover from a relatively low volume of plasma (200 microL). First disulfide bonds were reduced by 2-mercaptoethanol, which allows the maintenance of the reduced status preventing the rebuilding of the disulfide bond. Then the sample was derivatized to form the bis-tert-butyldimethylsilyl derivative. A deuterated internal standard, DL-[3,3,3',3',4,4,4',4'-2H8]-homocystine, was employed to account for losses associated with each analytical step. To evaluate the 'in vivo' homocysteine metabolic turnover, [1-13C]-methionine was infused and the derived [1-13C]-homocysteine quantitated. So a standard curve of [1-13C]-homocysteine was prepared by the decomposition of the [1-13C] methionine. The ions at m/z 325 and 326 were monitored, corresponding to the unlabeled [12C]-homocysteine and to labeled [13C]-homocysteine, respectively. The ion at m/z 325 ([M-114)]+) probably resulted from the loss of one derivatizing group to regenerate a free amino group. The intra-assay coefficient of variation (CV-intra%) was consistently less than 1.06%, the inter-assay (CV-inter%) less than 1.05%. The method described here seems to be simpler, more rapid, and less toxic than those published so far. In particular, its main strength appears to be the degree of precision obtained. We suggest applying this method to the measurement of the 'in vivo' rate of production of homocysteine (by the plasma 13C-homocysteine enrichment) from its precursor (13C-methionine).