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Igor A. Butovich; Amphiphilic Lipids Of Human Meibum: Which, How Much, And What For?. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3743.
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© ARVO (1962-2015); The Authors (2016-present)
Polar lipids (or, more correctly, amphiphilic lipids, AL) are considered to be critical components of the human tear film (TF). They are thought to be a part of the TF lipid layer (TFLL) that separates the underlying aqueous layer of TF from the top lipid layer formed from nonpolar lipids that comprise the bulk of human meibomian gland secretions (MGS). The goal of this presentation is to discuss the newest information on the presence and nature of the major AL classes in MGS, and assess their effects on TFLL.
In this report, we will provide a detailed description of the major analytical techniques used in the presenter’s laboratory (emphasizing the role of modern chromatographic and mass spectrometric techniques), and those implemented in independent laboratories, to study MGS and TFLL. The strengths and weaknesses of current, and previously used, experimental techniques will be discussed.
The only major AL class detected in normal MGS was found to be extremely long chain (O-acyl)-omega-hydroxy fatty acids (OAHFA) of the Ck:l-C(O)O-Cm:n-COOH family (k=15 and 17; l=1 and 2; m = 29, 31, 33; n=0, 1, 2; only major isomers shown). OAHFA comprised more than 90% of all detected AL. These anionogenic compounds were consistently present in all tested MGS samples. Their molecular pattern was shown to be fairly reproducible from sample to sample. Other types of AL were also observed in MGS. Free fatty acids (FFA) of C15:1-COOH to C31:n-COOH (where n=0, 1, 2) were found in MGS, but their quantities (<0.1% of MGS, w/w) were two orders of magnitude lower than those reported earlier. The FFA pool closely resembled the FA content of cholesteryl esters of MGS. Only trace amounts of various ceramides (Cer), phospholipids (PL) and sphingomyelins (SM) were observed. The overall presence of these AL in MGS was less than 0.01% (w/w) each. Traces of FA amides observed in some samples were confirmed to be contaminations originated from organic solvents and lab ware. Cholesterol (Chl) was typically present in amounts not exceeding 0.7% of MGS. The effects of selected exogenously added AL on TFLL were evaluated in Langmuir trough experiments in vitro. A computational approach was developed to analyze the dynamic compression isotherms quantitatively. We found that these AL could negatively impact lipid layers formed from MGS through various mechanisms, but only if present in concentrations considerably exceeding normal ones.
While AL are apparently needed for TFLL stabilization, certain AL could exert damaging effects on its stability. A revised model of the TFLL that includes OAHFA as major AL is proposed.
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