Determination of free methionine in human blood plasma by species-specific isotope dilution HPLC-ICP-MS using 34 S-labelled methionine

• Published in: Journal of analytical atomic spectrometry 2016, Vol.31 (9), p.1885-1894
• Main Authors: Galilea San Blas, Oscar, Moreno Sanz, F., Herrero Espílez, P., Prieto García, Belen, Álvarez Menéndez, Francisco V., Marchante-Gayón, Juan Manuel, García Alonso, José Ignacio
• Format: Article
• Language: English
• ISSN: 0267-9477; 1364-5544
• DOI: 10.1039/C6JA00125D

A species-specific Isotope Dilution (ID) method is described for the determination of free methionine in human blood plasma by High Performance Liquid Chromatography - Inductively Coupled Plasma Mass Spectrometry (HPLC-ICP-MS) using 34 S-labelled methionine as species-specific spike. The 34 S-labelled methionine was obtained from yeasts grown in a medium enriched with 34 S (>91%) in the form of sulphate. Methionine was extracted from yeasts using enzymatic digestion with protease XIV followed by isolation using preparative reverse phase HPLC. Two separate batches of the labelled methionine standard were obtained. The 34 S-labelled methionine standard solutions obtained were characterised both in terms of 34 S isotope enrichment (82.7 ± 0.6%) and total sulfur concentration (396 ± 6 μg g −1 ) by reverse Isotope Dilution HPLC-ICP-MS using a natural abundance methionine standard solution and a multicollector instrument working in the pseudo-high resolution mode to avoid spectral interference. Additionally, the identity of the 34 S-labelled methionine isolated by preparative HPLC and its isotope enrichment was confirmed by Gas Chromatography-Mass Spectrometry (GC-MS). Human blood plasma samples were spiked with the 34 S-labelled methionine spike. Then, plasma proteins were precipitated with trifluoroacetic acid and separated by centrifugation. Methionine was separated from the rest of sulfur containing compounds by reversed phase chromatography in isocratic mode using a mobile phase of 75 mM ammonium acetate (pH 7.4) containing 2% of methanol. The retention time of methionine (8.3 minutes) was confirmed by fortifying the samples with natural abundance methionine and also by collecting the methionine peak and further analysis by GC-MS. Finally, methionine in human blood plasma samples was determined by measuring the signals for 32 S and 34 S in a double focusing ICP-MS instrument working at medium resolution ( R = 4000). Concentrations were calculated by integrating the methionine peak for both masses and applying the isotope dilution equation after mass bias correction. The recoveries for samples fortified at different concentration levels ranged between 98.4 and 100.5%. Additionally, good agreement was obtained between the results found with this method and those reported by the clinical laboratory using a validated routine method.