Euphrasia spp. as a natural stabilizer of hyaluronic acid – A step closer to physiological human tear fluids?

Natural tears contain water, sodium chloride and various other components like proteins, lipids, mucins, antioxidants and buffer substances. Tear substitutes which are used for therapy of the dry eye syndrome mostly lack natural tear components and ingredients with antioxidative potential. Thus, the aim of our study was to identify substances with protective function against ultraviolet (UV) light and ozone, which could be used in artificial tear substitutes. Viscosity measurements were perfomed using a 0.25% hyaluronic acid solution with a viscosity of 2.12 centi Stokes (cSt) [range 2.09 – 2.15], as well as extracts and substances with possible antioxidative effects after irradiation with ultraviolet light and ozone. UV light and ozone lead to a depolymerisation of hyaluronic acid associated with an decrease of viscosity to 1.76 cSt [range 1.56 – 1.87] and 1.46 cSt [range 1.41 – 1.63], respectively. Eleven substances with possible hyaluronic acid stabilizing effects were investigated. Eyebright (Euphrasia spp.) and mannitol could significantly protect hyaluronic acid from a degradation (p < 0.01). The viscosity was 2.05 cSt [range 2.04 – 2.05] and 2.07 cSt [range 2.05 – 2.08] through influence of UV light and ozone, respectively, with the protective components. Uric acid (1.76 cSt) and melatonine (1.75 cSt) were able to achieve a significant stabilizing effect of hyaluronic acid against ozone (p < 0.01), but not against UV light. Arginine, curcumin, fructose, urea, lysine, spermidine and taurine did not stabilize hyaluronic acid after UV and ozone stress.

For maintaining the antioxidative function of tears Euphrasia spp. extracts and mannitol could be added to artificial tears. Both substances are capable of stabilizing hyaluronic acid in tears as well as in artificial tears by preventing UV light and ozone induced degradation of hyaluronic acid. This stabilizing effect extends the retention time and lubricating effect on the ocular surface.