Antioxidant defenses in the ocular surface

Abstract

The human eye is subjected constantly to oxidative stress due to daily exposure to sunlight, high metabolic activities, and oxygen tension. Reactive oxygen species generated from environmental insults and pathological conditions render the human eye particularly vulnerable to oxidative damage. The ocular surface composed of the tear film, the cornea, and the aqueous humor forms the first physical and biochemical barrier of the eye and plays a pivotal role in combating free radicals. These ocular compartments are enriched in certain antioxidants in the form of metabolic enzymes or small molecules. Such an antioxidant defense system in the ocular surface is essential for the maintenance of redox homeostasis in the eye and protection against oxidative damage. Herein, we review the properties and functions of key constituent antioxidants of the ocular surface.

Figure 1

Formation of reactive oxygen species and antioxidant defenses in the eye. Environmental exposures (such as solar ultraviolet radiation and high atmospheric oxygen) and certain pathological conditions (such as inflammation and prolonged contact lens wearing) induce the generation of reactive oxygen species (ROS) in ocular tissues. ROS are derived from diatomic oxygen (O₂), including superoxide anion radical (O₂^−•), hydrogen peroxide (H₂O₂), hydroxyl (OH^•), and peroxyl radicals (LOO^•). ROS have high potential to react with DNA, proteins and cellular membranes, resulting in modifications of these macromolecules and consequent cellular damage. The antioxidant defense systems in the ocular surface function to combat ROS and protect ocular tissues from oxidative damage. Superoxide dismutases (SODs), catalase (CAT), glutathione peroxidases (GPXs), glutathione reductase (GR) and aldehyde dehydrogenases (ALDH3A1 and ALDH1A1) represent enzymatic antioxidants. Glutathione (GSH), ascorbic acid (VitC), α-tocopherol (VitE), NADPH and ferritin represent nonenzymatic anti-oxidants. The properties and functions of each antioxidant are discussed in detail in the text.

Figure 2

The absorption of solar ultraviolet radiation by the eye. Solar ultraviolet radiation (UVR) consists of UVA at 315–400 nm, UVB at 280–315 nm, and UVC at 100–280 nm. The cornea absorbs all UVC and most UVB, whereas UVA is primarily absorbed by the lens. No UVC or UVB and very little UVA (<1%) reach the retina.