Rapid transmethylation and stable isotope labeling for comparative analysis of fatty acids by mass spectrometry

Fatty acids covalently bonded with other molecules have been implicated in many important biological processes. We describe here a rapid approach termed isotope-coded fatty acid transmethylation (iFAT) that integrates extraction, transmethylation, and isotopic labeling into a single step with the aid of ultrasonic irradiation for comparative analysis of fatty acids by mass spectrometry. In this approach, samples without any prefractionation were mixed with a methanol solution of 0.5 M NaOH and an n-hexane solution. The intense wave shocks and cavitations generated by ultrasonic irradiation not only speed the alkaline-catalyzed transmethylation reaction but also facilitate the simultaneous mass transfer of fatty acid methyl esters into the top n-hexane extraction phase that was injected into a GC/MS system. By using commercially available d(3)-methanol, we were able to compare the intensity of labeled and unlabeled methyl esters and their corresponding fragment ions. The detection limit can be down to the picogram level. Major advantages of the iFAT strategy are summarized in the following: (1) Efficient heterogeneous reactions. Solid samples such as dried cell lysates or detergent-resistant fractions can be readily transformed and analyzed with the aid of ultrasound irradiation. (2) Accurate quantification of fatty acids. Evaluation of the completeness or losses of transformation reactions across lipid classes has been hampered due to a lack of suitable methods. Isotope labeling can be used as an internal standard for accurate comparison of the fatty acid composition in different cell states. (3) Reduced interferences from complex biological context. The iFAT strategy not only differentially labels fatty acids in different samples, but also volatilizes those molecules, and thus, they are isolated from the bulk background and analyzed by GC/MS. This proposed approach has been applied to quantitatively determine the fatty acid composition in plant oil and in budding yeast cell lysates and detergent-resistant fractions. It should provide a widely applicable means for quantitative comparison of the fatty acid composition in cells and tissues.