The human eye, the highly specialized organ of vision, is greatly influenced by oxidants of endogenous and exogenous origin. Oxidative stress affects all structures of the human eye with special emphasis on the ocular surface, the lens, the retina and its retinal pigment epithelium, which are considered natural barriers of antioxidant protection, contributing to the onset and/or progression of eye diseases. These ocular structures contain a complex antioxidant defense system slightly different along the eye depending on cell tissue. In addition to widely studied enzymatic antioxidants, including superoxide dismutase, glutathione peroxidase, catalase, peroxiredoxins and selenoproteins, inter alia, metallothioneins (MTs) are considered antioxidant proteins of growing interest with further cell-mediated functions. This family of cysteine rich and low molecular mass proteins captures and neutralizes free radicals in a redox-dependent mechanism involving zinc binding and release. The state of the art of MTs, including the isoforms classification, the main functions described to date, the Zn-MT redox cycle as antioxidant defense system, and the antioxidant activity of Zn-MTs in the ocular surface, lens, retina and its retinal pigment epithelium, dependent on the number of occupied zinc-binding sites, will be comprehensively reviewed.

Oxidative stress affects all the structures of the human eye, particularly the retina and its retinal pigment epithelium (RPE). The RPE limits oxidative damage by several protective mechanisms, including the non-enzymatic antioxidant system zinc-metallothionein (Zn-MT). This work aimed to investigate the role of Zn-MT in the protection of RPE from the oxidative damage of reactive oxygen intermediates by analytical and biochemical-based techniques. The Zn-MT system was induced in an in vitro model of RPE cells and determined by elemental mass spectrometry with enriched isotopes and mathematical calculations. Induced-oxidative stress was quantified using fluorescent probes. We observed that 25, 50 or 100 μM of zinc induced Zn-MT synthesis (1.6-, 3.6- and 11.9-fold, respectively), while pre-treated cells with zinc (25, 50, and 100 μM) and subsequent 2,2′-Azobis(2-methylpropionamidine) dihydrochloride (AAPH) treatment increased Zn-MT levels in a lesser extent (0.8-, 2.1-, 6.1-fold, respectively), exerting a stoichiometric transition in the Zn-MT complex. Moreover, AAPH treatment decreased MT levels (0.4-fold), while the stoichiometry remained constant or slightly higher when compared to non-treated cells. Convincingly, induction of Zn-MT significantly attenuated oxidative stress produced by free radicals’ generators. We conclude that the stoichiometry of Zn-MT plays an important role in oxidative stress response, related with cellular metal homeostasis.

Metallothioneins (MTs) are zinc-ion-binding proteins with a wide range of functions, among which are neuroprotection, maintenance of cellular zinc hom