Structural profiling and quantification of sphingomyelin in human breast milk by HPLC-MS/MS

The sphingolipid composition of food as well as of physiological samples has received considerable interest due to their positive biological activities. This study quantified the total amount of sphingomyelin (SM) in 20 human breast milk samples from healthy volunteers and determined the structures of SM by detailed mass spectrometric studies in combination with enzymatic cleavage. The quantification of SM was performed by hydrophilic interaction liquid chromatography coupled to electrospray ionization-tandem mass spectrometry (HILIC-HPLC-ESI-MS/MS) measuring the characteristic fragment ion of the phosphorylcholine group at m/z 184.2 and by using hexanoylsphingomyelin (C6-SM) and heptadecanoylsphingomyelin (C17-SM) as internal standards. The structures of SM species were identified after enzymatic cleavage with alkaline sphingomyelinase (SMase) to the corresponding ceramides. Structure elucidation of the sphingoid base and fatty acid backbone was performed by reversed-phase HPLC-ESI-MS/MS. The method includes the sphingoid bases dihydrosphingosine (d18:0), sphingosine (d18:1(Δ4)), 4,8-sphingadienine (d18:2(Δ4,8)), 4-hydroxysphinganine (phytosphingosine (t18:0)), and 4-hydroxy-8-sphingenine (t18:1(Δ8)) and fatty acids with even-numbered carbon atoms (C12-C26) as well as their (poly)unsaturated and monohydroxylated analogues. The total amount of SM in human breast milk varied from 3.87 to 9.07 mg/100 g fresh weight. Sphingosine (d18:1) was the predominant sphingoid base, with 83.6 ± 3.5% in human breast milk, followed by 4,8-sphingadienine (d18:2) (7.2 ± 1.9%) and 4-hydroxysphinganine (t18:0) (5.7 ± 0.7%). The main SM species contained sphingosine and palmitic acid (14.9 ± 2.2%), stearic acid (12.7 ± 1.5%), docosanoic acid (16.2 ± 3.6%), and tetracosenoic acid (15.0 ± 3.1%). Interestingly, the fatty acid composition of SM species in this study differs from the total fatty acids in human breast milk, and the fatty acids are not consistently distributed among the different sphingoid bases.