| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH |
|
|
|
|||||||
|
|||||||||
Electronic Letters to:
|
|
Electronic letters published:
![[Read eLetter]](/icons/misc/sectionDOWN.gif){kind=icon}
On the Presence of Oleamide in Human Meibum: Quantification by LC/MS
|
|
|||
|
Igor A. Butovich
Send letter to journal:
igor.butovich{at}utsouthwestern.edu Igor A. Butovich
|
ON THE PRESENCE OF OLEAMIDE IN HUMAN MEIBUM: QUANTIFICATION BY LIQUID CHROMATOGRAPHY-MASS SPECTROMETRY In a recent paper by Nichols et al.,1 the authors presented novel and potentially very important findings regarding the newly discovered presence of large amounts of a well-known lipid signaling factor, oleamide, in human meibum. The authors suggested that oleamide, besides being a lipid signaling factor, could have a structural role in formation and functioning of human tear film lipid layer (TFLL). In earlier pioneering publications, oleamide was shown to be present in brain2 and other tissues, but never in human meibum. Curiously, oleamide is also a ubiquitous and very stable compound which finds various uses in the chemical, food, and publishing industries, serving as a lubricant in making polyethylene, component of food packing materials, anti-fogging agent, water-proofer for paper, dispersant in printing and dying, and as a precursor in making emulsifiers, anti-static agents, fabric softeners, etc.3 Therefore, there is a chance that oleamide could be found in samples of many tissues and biological fluids, either as a compound of biological origin, or as an exogenous contaminant. To verify the findings of Nichols et al.,1 we conducted a high performance liquid chromatography-mass spectrometric (HPLC/MS) study of the human meibum collected from young healthy volunteers of the median age of ~32 years who, according to standard clinical tests, did not have any ocular diseases. Our procedures conformed to the tenets of the Declaration of Helsinki. It is important to note that in our experiments the collected meibum samples have never been in contact with any other material except for glass, stainless steel, platinum, and Teflon. Special care has been taken to avoid any contact of organic solvents and the meibum samples with plastic. The whole samples (a waxy off white-to-yellow material, melting point 33 ± 1°C, average size of dry sample 0.4 mg), were analyzed using HPLC with atmospheric pressure chemical ionization MS detection of the analytes (APCI MS). In preliminary experiments the method was shown to be adequate for analyzing various nonpolar compounds, including oleamide. A standard curve for authentic oleamide was used to quantify the oleamide in meibum. The experiments showed that as little as 1 nanogram of the compound per injection could be detected by this method using either oleamide itself, or its mixtures with human meibum. As the average size of the analyzed meibum samples ranged from 1000 to 2000 nanograms per injection, that was a level of sensitivity that would be more than sufficient to detect oleamide if it were a predominant component of human meibum, as suggested earlier.1 We corroborate the finding of Nichols et al.1 that oleamide is present in human meibum, but its overall presence was found to be 0.1% (w/w) of the dry meibum, or less. Therefore, oleamide is hardly a major constituent of human meibum, which is shown to have a much higher percentage of other nonpolar compounds, e.g., wax and sterol esters, di- and triglycerides, etc. (more than 60%), free fatty acids (2%), or polar lipids (up to 16%).4 This observation is in line with a general opinion that physiologically active compounds like oleamide are bound to be present in vivo in low concentrations; otherwise, their effects would overwhelm the signaling pathways thus creating metabolic havoc in the corresponding cells, tissues, and organs. Very low yields of meibum collected by Nichols et al.1 (around 5 nanoliters of wet sample per subject) should be partly blamed for an exaggerated presence of oleamide in those samples as, at the level of sensitivity required for the analysis of such a small sample, all impurities in the solvents, vials, labware, etc. might become critical. If present in meibum at less than 0.1% (w/w), it is unclear whether oleamide would be able to have any structural role in maintaining, or disrupting, for that matter, of TFLL, leaving its putative signaling role as the most, if not the only, viable option. Igor A. Butovich, Eduardo Uchiyama, and James P. McCulley Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas References 1. Nichols KK, Ham BM, Nichols JJ, Ziegler C, Green-Church KB. Identification of fatty acids and fatty acid amides in human meibomian gland secretions. Invest Ophthalmol Vis Sci. 2007;48:34-39. |
|||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH |
eLetters: