**4. Non-enzymatic antioxidants**

Also known as synthetic antioxidants or dietary supplements, this group influences in an exogenous way the antioxidant defense system of the organism. The most common are: vitamins C and E, selenium, zinc, taurine, hypotaurine, glutathione, β-carotene, and carotene.

Vitamin E may block the initiation of lipid peroxidation as well as its propagation phase (Bornoden, 1994).

Glutathione is the major non-protein sulfhydryl component of mammalian cells and has an important role in cellular protection from oxidative stress (Meister, 1983). Glutathione synthesis increases throughout oocyte development and maturation until the periovulatory follicle stage (Perreault et al., 1988). After fertilization, glutathione participates in the sperm decondensation process, while the oocyte activation process occurs, and the sperm head turns into the male pronucleus (Calvin et al., 1986; Perreault et al., 1984, 1988; Yoshida, 1992, 1993). A study performed with bovine oocytes has shown the important role of COCs during the in vitro maturation process. Through gap junctions, cumulus oophorus cells (COCs) might mediate glutathione synthesis by the oocytes, a crucial enzyme for the cytoplasmic and nuclear maturation process. This intimate relation between COCs and oocytes apparently occurs due to the presence of gap junctions (De Matos et al., 1997).
