The most abundant phospholipid in tears reported here is lysophosphatidylcholine. This finding is consistent with prior TLC and recent mass spectrometric studies,
23,58 but contrasts with other recent studies.
22,24 All of the major peaks reported in tears by Rantamäki et al.
23 were verified in this study and the relative intensity of each peak found is strikingly concordant (
Table 1). Apparent disparity to other mass spectrometric studies probably can be attributed to differences in the instrument response function, the sample sizes loaded, and the mass range scanned. Rantamäki et al.
23 also found peaks in tears that were beneath our limit of detection.
23 Campbell et al.
22 did not find phospholipids in tears but astutely pointed out that lysophosphatidylcholine is a presumed enzymatic degradation product of phospholipid that would be predicted from action of phospholipases that exist in tears. Both our results and those of Rantamäki et al.
23 are concord that the major lysophosphatidylcholine is m/z 496.3Da. This is the predicted mass for the intact SN-1 (C16:0) configuration product derived by action of phospholipase A2 on the major PC, 758.6 Da (
Fig. 7). The products from other phospholipases would not be detected by our methods. No lysophosphatidylcholines were found in extracts from any of the protein fractions of tears. Possible explanations include retention of molecules on the column resin, elution in a micellar or unbound state, and degradation on the column. We suspect interaction with the column resin as the most likely cause. Removal from the resin was obviated because the concentration of organic solvents needed for elution would likely cause permanent damage to the resin. One might expect lysophosphatidylcholines to bind less strongly to TL than intact parent PCs if the remaining chain is shorter, or contains alkenic bonds or polar functional groups. Lipid-binding affinity to TL varies as a function of alkyl chain length
51 up to 18 carbons.
69 The hydration free energy is reduced with increasing alkenic bonds.
70 As noted previously, the putative chain length of the major lysophosphatidylcholines would have a predicted alkyl chain length of C16. The 522-Da and 520-Da lysophosphatidylcholines could have a maximum chain length of C18 with 1 or 2 double bonds, respectively. Clarification of actual structures using ion trap mass spectrometry may clarify the apparent lower affinity of lysophosphatidylcholines for TL.