A profile of sphingolipids and related compounds tentatively identified in yak milk

This work characterized a fraction of constituents in yak milk within the realm of approximately 1,000 to 3,000 Da using matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. Eleven samples of yak milk powder from the Sichuan province of China were received by the Department of Food Science, University of Wisconsin-Madison, and stored at room temperature until analysis. Sample preparation involved delipidation and deproteinization of yak milk samples and cold ethanol precipitation. Subsequently, MALDI time-of-flight mass spectrometry was performed in positive ion, reflector mode (AB Sciex TOF/TOF 4800 MALDI; AB Sciex, Foster City, CA). The instrument was first calibrated with the manufacturer's 6-peptide mixture, and each spectrum was internally calibrated using the accurate mass of ACTH Fragment 18-39 standard peptide (protonated mass at m/z 2464.199) present in each sample. Laser power was adjusted for the calibration standards and for each sample so that the signal obtained for the most-abundant ion in each spectrum could be maximized, or kept below ~2 × 104 to preserve spectral quality. Structure and name based on mass were matched using the Metlin metabolite database (https://metlin.scripps.edu/index.php). Results of the current work for yak milk powder showed a large variety of sphingolipid structures with clusters around 1,200, 1,600, and 2,000 Da. The profiling matched several glycosphingolipids, such as gangliosides GA1, GD1a, GD1b, GD3, GM1, GM2, GM3, and GT2 and several other unique moieties, including deaminated neuraminic acid (KDN) oligosaccharides, and fucose containing gangliosides. Matrix preparation and MALDI time-of-flight parameters were important factors established in this work to allow high resolution profiling of complex sphingolipids in yak powder milk.