Abstract
Purpose: :
To investigate antioxidative and radical scavenging properties of different polymers modified by the covalent attachment of the thiol antioxidant cysteine, which naturally occurs in human tear fluid. Polymers were evaluated in vitro with regard to their potential use in artificial tear formulations to prevent oxidative stress on the ocular surface.
Methods: :
Poly(acrylic acid) (PAA), chitosan and hyaluronic acid (HA) were modified with thiol groups via covalent attachment of cysteine and cysteine derivatives N-acetylcysteine and cysteamine, respectively. Antioxidative properties of 0.1% solutions of thiolated polymers and corresponding unmodified polymers were investigated using an in vitro model of corneal lipid peroxidation. Hydroxyl radical scavenging properties of the polymer solutions were evaluated with the deoxyribose test method.
Results: :
Chitosan, HA and PAA were shown to inhibit in vitro lipid peroxidation of the cornea in the following rank order: PAA>Chitosan>HA. The unmodified polymers act as ferrous ion chelators, reduction agents and/or radical scavengers and therefore exhibit antioxidative properties. Modification with cysteine and cysteamine significantly increased the antioxidant properties of PAA and HA, respectively. Antioxidative properties of chitosan remained unchanged by thiolation and the protective effects of chitosan-N-acetylcysteine, chitosan and HA-cysteamine were in the same range. When using the deoxyribose test method PAA-cysteine, chitosan and chitosan-N-acetylcysteine were the most effective radical scavengers. A clear correlation between the degree of modification with thiol groups and radical scavenging properties of polymers could be demonstrated using the HA-cysteamine conjugate as example.
Conclusions: :
By the covalent attachment of cysteine and cysteine derivatives the antioxidant properties of HA and PAA could be further increased, while the excellent antioxidative properties of chitosan remained unchanged. The use of these polymers in artificial tear fluid might be beneficial in the treatment of the dry eye syndrome to protect the ocular surface from oxidative damage.
Keywords: cornea: tears/tear film/dry eye • antioxidants • oxidation/oxidative or free radical damage