Lipidomics of equine sperm and seminal plasma: Identification of amphiphilic (O-acyl)-ω-hydroxy-fatty acids.

Using a nontargeted lipidomics analysis of equine sperm and seminal plasma, we were able to characterize a diverse array of individual lipids including ethanolamine and choline ether lipids and seminolipids essential to membrane raft function. We also detected, for the first time in sperm, the presence of (O-acyl)-ω-hydroxy-fatty acids (OAHFA) with up to 52 carbon chain lengths, which were localized to the head and not the tail of sperm. The only previous identification of OAHFAs has been in meibomian glands and their sebaceous secretions. The identities of these lipid amphiphiles were validated both by high-resolution mass spectrometry and by tandem mass spectrometry (<1 ppm mass error), which identified the fatty acid (FA) and hydroxy-FA components of individual OAHFAs. The amphiphilic and surfactant properties of these unique lipids could provide an interface between the complex lipid layers of the acrosome and the aqueous environment of the suspending seminal plasma. The potential roles of OAHFAs in orientation of critical proteins in the acrosomal membrane also remain to be explored with these new findings. Another unique finding of our lipidomics study was that phosphatidylethanolamines with mono- or di-unsaturated FA substitutions are present in seminal plasma but not in sperm suggesting a potential role of these glycerophospholipids in sperm capacitation and protecting sperm cells in the female reproductive tract. In summary, we have identified for the first time, the presence of OAHFAs in sperm and several phosphatidylethanolamines in seminal plasma, suggesting that these complex lipids may play critical roles in sperm function.