Comparative study of serine-plasmalogens in human retina and optic nerve: identification of atypical species with odd carbon chains

Abstract

The objective of this work was to detect and identify phosphatidylserine plasmalogen species in human ocular neurons represented by the retina and the optic nerve. Plasmalogens (vinyl-ether bearing phospholipids) are commonly found in the forms of phosphatidylcholine and phosphatidylethanolamine in numerous mammalian cell types, including the retina. Although their biological functions are unclear, the alteration of cellular plasmalogen content has been associated with several human disorders such as rhizomelic chondrodysplasia punctata Type 2 and primary open-angle glaucoma. By using liquid chromatography coupled to high-resolution and tandem mass spectrometry, we have identified for the first time several species of phosphatidylserine plasmalogens, including atypical forms having moieties with odd numbers of carbons and unsaturation in sn-2 position. Structural elucidation of the potential phosphatidylserine ether linked species was pursued by performing MS(3) experiments, and three fragments are proposed as marker ions to deduce which fatty acid is linked as ether or ester on the glycerol backbone. Interpretation of the fragmentation patterns based on this scheme enabled the assignment of structures to the m/z values, thereby identifying the phosphatidylserine plasmalogens.

Fig. 1.

Ion chromatograms of PS plasmalogens in pooled retina sample. The numeric labels correspond to the PS plasmalogens as given in the tables.

Fig. 2.

Ion chromatograms of PS plasmalogens in pooled optic nerve sample. The numeric labels correspond to the PS plasmalogens as given in the tables.

Fig. 3.

Proposed collision-induced dissociation fragmentation pathway of PS plasmalogens. After the loss of the 87 Da unit reflecting the presence of serine moiety, three marker ions are postulated. These ions enable us to deduce which fatty acid is linked as ether or ester to the glycerol backbone.

Fig. 4.

MS³ fragmentation pattern of PS (P-16:0/18:1). Note the presence of the three marker ions and their accordance with Figure 1.

Fig. 5.

Relative abundance of PS plasmalogens in retina and optic nerve samples. The bars represent averaged results of the individual analyses. For deviation among the samples, see Tables 3 and 4.