Lipidomics of the chicken egg yolk: high-resolution mass spectrometric characterization of nutritional lipid families

Abstract

While previous studies have characterized the fatty acids and global lipid families of the chicken egg yolk, there have been no publications characterizing the individual lipids in these lipid families. Such an in-depth characterization of egg yolk lipids is essential to define the potential benefits of egg yolk consumption for the supply of structural and anti-inflammatory lipids. Historically, the major focus has been on the cholesterol content of eggs and the potential negative health benefits of this lipid, while ignoring the essential roles of cholesterol in membranes and as a precursor to other essential sterols. A detailed analysis of egg yolk lipids, using high-resolution mass spectrometric analyses and tandem mass spectrometry to characterize the fatty acid substituents of complex structural lipids, was used to generate the first in-depth characterization of individual lipids within lipid families. Egg yolks were isolated from commercial eggs (Full Circle Market) and lipids extracted with methyl-t-butylether before analyses via high-resolution mass spectrometry. This analytical platform demonstrates that chicken egg yolks provide a rich nutritional source of complex structural lipids required for lipid homeostasis. These include dominant glycerophosphocholines (GPC) (34:2 and 36:2), plasmalogen GPC (34:1, 36:1), glycerophosphoethanolamines (GPE) 38:4 and 36:2), plasmalogen GPE (36:2 and 34:1), glycerophosphoserines (36:2 and 38:4), glycerophosphoinositols (38:4), glycerophosphoglycerols (36:2), N-acylphosphatidylethanolamines (NAPE) (56:6), plasmalogen NAPE (54:4 and 56:6), sphingomyelins (16:0), ceramides (22:0 and 24:0), cyclic phosphatidic acids (16:0 and 18:0), monoacylglycerols (18:1 and 18:2), diacylglycerols (36:3 and 36:2), and triacylglycerols (52:3). Our data indicate that the egg yolk is a rich source of structural and energy-rich lipids. In addition, the structural lipids possess ω-3 and ω-6 fatty acids that are essential precursors of endogenous anti-inflammatory lipid mediators. These data indicate that eggs are a valuable nutritional addition to the diets of individuals that do not have cholesterol issues.

Keywords: egg yolk; sphingolipid; structural glycerophospholipid.

Figure 1

MS² of 844.5633, the chloride adduct [M+Cl]^- of egg yolk GPC 38:4. The product ions clearly indicate that the sn-2 fatty acid is 20:4 (303.2329; 0.32 ppm) and sn-1 is 18:0 (283.2642; 0.77 ppm), supporting the identity of GPC 38:4 as mainly GPC 18:0/20:4, with trace quantities of GPC 18:1/20:3 (281.2486/305.2486). Abbreviation: GPC, glycerophoscholines.

Figure 2

Relative levels of egg yolk major (upper graph) and minor (middle graph) glycerophoscholines (GPC), and GPC plasmalogens (GPCp; lower graph). The [M + H]⁺ cations GPC and GPCp were monitored in positive electrospray ionization. (N = 6 eggs; 2 replicates).

Figure 3

Relative levels of egg yolk glycerophosphoethanolamines (GPE), ethanolamine plasmalogens (GPEp), and alkyl-acyl GPE (GPEe). The [M-H]⁻ anions were monitored in negative ESI. (N = 6 eggs; 2 replicates).

Figure 4

Relative levels of egg yolk lyso-glycerophosphocholines (Lyso-GPC) and lysopho-glycerophosphoethanolamines (Lyso-GPE). The [M + H]⁺ cations of GPC were monitored in positive ESI and the [M-H]⁻ anions of lyso-GPE were monitored in negative ESI. (N = 6 eggs; 2 replicates).

Figure 5

Relative levels of egg yolk glycerophosphoserines (GPS), glycerophosphoinositols (GPI), glycerophospholglycerols (GPG) and lyso-GPG. The [M-H]⁻ anions were monitored in negative ESI. (N = 6 eggs; 2 replicates).

Figure 6

Relative levels of egg yolk N-acylphosphatidylethanolamines (NAPE) and plasmenyl NAPES (NAPEp). The [M-H]⁻ anions of NAPE and NAPEp were monitored in negative ESI. (N = 6 eggs; 2 replicates).

Figure 7

Relative levels of egg yolk sphingomyelins (SM) and hydroxy-sphingomyelins (SM[-OH]). The [M+Cl]⁻ adducts of SM were monitored in negative ESI while the [M + H]⁺ cations of SM(⁻OH) were monitored in positive ESI. (N = 6 eggs; 2 replicates).

Figure 8

Relative levels of egg yolk ceramides (CER), ceramide phosphoethanolamines (CER-PE), and hexosyl-ceramides (Hex-CER). The [M+Cl]⁻ adducts of CER, CER-PE, and Hex-CER were monitored in negative ESI. (N = 6 eggs; 2 replicates).

Figure 9

Relative levels of egg yolk cyclic phosphatidic acids (cPA). The [M-H]^- anions of cPA were monitored in negative ESI. (N = 6 eggs; 2 replicates).

Figure 10

Relative levels of egg yolk neutral lipids. Monoacylglycerols (MAG); Diacylglycerols (DAG); and Triacylglycerols (TAG). The [M+Cl]⁻ adducts of MAG and DAG were monitored in negative ESI while the [M + NH₄]⁺ cations of TAG were monitored in positive ESI. (N = 6 eggs; 2 replicates).