Wax-tear and meibum protein, wax–β-carotene interactions in vitro using infrared spectroscopy

Abstract

Protein–meibum and terpenoids–meibum lipid interactions could be important in the etiology of meibomian gland dysfunction (MGD) and dry eye symptoms. In the current model studies, attenuated total reflectance (ATR) infrared (IR) spectroscopy was used to determine if the terpenoid β-carotene and the major proteins in tears and meibum affect the hydrocarbon chain conformation and carbonyl environment of wax, an abundant component of meibum. The main finding of these studies is that mucin binding to wax disordered slightly the conformation of the hydrocarbon chains of wax and caused the wax carbonyls to become hydrogen bonded or experience a more hydrophilic environment. Lysozyme and lactoglobulin, two proteins shown to bind to monolayers of meibum, did not have such an effect. Keratin and β-carotene did not affect the fluidity (viscosity) or environment of the carbonyl moieties of wax. Based on these results, tetraterpenoids are not likely to influence the structure of meibum in the meibomian glands. In addition, these findings suggest that it is unlikely that keratin blocks meibomian glands by causing the meibum to become more viscous. Among the tear fluid proteins studied, mucin is the most likely to influence the conformation and carbonyl environment of meibum at the tear film surface.

Lipidomic analysis of human tear fluid reveals structure-specific lipid alterations in dry eye syndrome

“…Consistent with our fi nding, Dougherty, Osgood, and McCulley ( 10 ) reported lower levels of saturated FAs in meibum WEs from chronic blepharitis patients, with corresponding increases in species containing monounsaturated FAs. The saturation of the fatty acyl component of WEs, instead of the alcohol moiety, is shown to be critical MGD in such patients ( 28 ). In agreement with this hypothesis, we found appreciable compositional changes in tear lipids as the absolute amounts of lipids in tears varied ( Fig.…”

The real reason for having a meibomian lipid layer covering the outer surface of the tear film – A review

“…Whereas b-lactoglobulin, and lysozyme did not appear to interact with components of meibum (Faheem et al, 2012), these and other proteins e.g. lactoferrin, found in the aqueous component of tears at concentrations 100 times less than that found in vivo are capable of adsorbing to and integrating with meibomian lipid layers Tragoulias et al, 2005).…”

“…As stated above, the proteins including mucins of the aqueous part of the tear film and possibly proteins of meibum are also likely to form integral parts of the lipid layer of the tear film and contribute to lowering the surface tension (Holly and Lemp, 1977;Miano et al, 2005;Nagyov a and Tiffany, 1999;Millar et al, 2006). An interesting finding in this context was the demonstration that mucins can cause a disordering of WEs (Faheem et al, 2012). However, this was a very artificial situation with buffer covering a bulk mixture of two WEs spread over a crystal, and therefore the types of molecular interactions between mucins and the TFLL still remain to be demonstrated.…”

“…the meibum is "stiffer", it has been argued that keratin would not make meibum in the ducts stiffer. This was on the basis that keratin did not affect the phase transition profile of WEs and therefore the decrease in meibum secretion in MGD would be more likely to be due to cellular debris blocking the orifices (Faheem et al, 2012). Once secreted, it is not known if the increased keratin influences the performance of the TFLL.…”

Tear film lipids

“…A lesser-known class of lipids that was studied in the relation to meibum is carotenoids (Faheem et al, 2012; Foulks et al, 2013; Oshima et al, 2009). The most common members of carotenoids that are found in human bodies are β-carotene, lycopene, lutein, and zeaxanthin.…”