Reactive oxygen species (ROS) which are chemically reactive molecules containing oxygen are
formed as a natural byproduct of the normal metabolism of oxygen and have important roles
in cell signaling and homeostasis [1]. However, high levels of ROS cause oxidative stress and
cell injury, including at least three reactions, lipid peroxidation of membranes, intracellular oxidative
modification of proteins, and oxidative damage to DNA [2–5]. These oxidative reactions
form adducts with lipids, protein and DNA, and lead to decreased functions of
intracellular organelles and further damage [6,7]. Excessive ROS causes oxidative stress, which
reflects an imbalance between the systemic manifestation of ROS and a biological system's ability
to readily detoxify the reactive intermediates or to repair the resulting damage. Oxidative
stress excites oxidative damage and has been involved in pathogenesis of many disease, such as
cancer, Parkinson's disease, Alzheimer's disease, heart failure, lung and skin diseases [8–11].
Oxidative stress has also been reported to play an important role in ocular diseases including
ocular surface inflammation and dry eye disease