Spectroscopy and the Etiology of Cataracts and Dry Eye

Lipid hydrocarbon chain saturation was related to hydrocarbon chain order (stiffness) and phase transition temperature in a wide range of synthetic and natural membranes, including human tear film lipid layer (TFLL) and human lens membranes and was also related to resistance to oxidation. Lens membrane saturation was measured using NMR spectroscopy in species ranging in lifespans from 1 year to 200 years. The longer animals live, the longer they require clear lenses. Lens sphingolipid content and lifespan were correlated. Sphingolipids resist oxidation better than phospholipids because they are more saturated. Sphingolipids resist oxidative degradation so much so that they were the only biomolecules found in a mammoth buried in ice for 40,000 years. Thus, one may suggest that longer lived species have adapted so that their lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing their lenses to stay clear for a longer time relative to shorter lived species. It is interesting that bowhead whales can live over 200 years and do not get cataracts. Like humans, they likely have adapted to have the highest amount of sphingolipids in their lenses.
In a masked clinical trial, saturation levels were measured in participants without dry eye disease (DED) and with mild and severe DED. Lipid saturation levels correlated with the severity of DED suggesting saturation may contribute to DED. Infrared spectroscopy revealed that saturation of TFLL resulted in an increase of hydrocarbon chain order, phase transition temperature, cooperativity, and on an aqueous surface, the formation of stiffer, thicker, and more elastic films. At the same time, saturation altered the spreading and heterogeneous structure of the TFLL. Spectroscopy was instrumental in elucidating lipid compositional, structural and functional relationships contributing to our understanding of the etiology of dry eye and cataracts.