We describe a simple stable isotope dilution method for accurate determination of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) in plasma as a clinical diagnostic test. Determination of SAM/SAH in plasma (20 μL) was performed by high-performance liquid chromatography coupled with electrospray positive ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Calibrators (SAM and SAH) and internal standards (2H3-SAM and 2H4-SAH) were included in each analytical run for calibration. Sample preparation involved combining 20 μL sample with 180 μL of internal standard solution consisting of heavy-isotope-labeled internal standards in mobile phase A and filtering by ultracentrifugation through a 10 kd MW cutoff membrane. Sample filtrate (3 μL) was injected by a Shimadzu Nexera LC System interfaced with a 5500 QTRAP® (Sciex). Chromatographic separation was achieved on a 250 mm × 2.0 mm EZ-faast column from Phenomenex. Samples were eluted at a flow rate of 0.20 mL/min with a binary gradient with a total run time of 10 min. The source operated in positive ion mode at an ion spray voltage of +5000 V. SAM and SAH resolved by a gradient to 100% methanol with retention times of 5.8 and 5.5 min, respectively. HPLC chromatographic conditions did not produce complete separation of SAM and SAH, but they were completely discerned by their different fragmentation pattern in the mass spectrometer working in the MS-MS mode. The observed m/z values of the fragment ions were m/z 399→250 for SAM, m/z 385→136 for SAH, m/z 402→250 for 2H3-SAM, and m/z 203→46. The calibration curve was linear over the range of 12.5-5000 nmol/L for SAM and SAH.
Keywords: Mass spectrometry; Methylation; S-adenosylhomocysteine; S-adenosylmethionine.
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