Involvement of superoxide radicals in the mouse two-cell block

The effect of oxygen toxicity on the development of mammalian embryos was asssessed by the use of superoxide dismutase (SOD), a potent scavenger of superoxide radicals. Mouse pronuclear embryos recovered 17 h after human chorionic gonadotropin (hCG) were cultured in medium BWW at 37°C under an atmosphere of 5% CO2 in air. Culture of mouse pronuclear embryos in the presence of Cu ∣ Zn‐SOD (500 μg/ml) significantly increased the blastulation rate (44.6%) when compared with the control culture system (4.2%). Essentially the same effects were observed in SOD containing either Mn or Fe in the catalytic center. Heat treatment of the SOD preparation, and the addition of anti‐SOD antibodies to the culture medium, significantly reduced the attenuation of the two‐cell block by SOD, indicating that this effect is SOD dependent. SOD activity was detected in rabbit oviduct fluid (3,675 ± 3,084 mlU/mg protein) by electron spin resonance. These results suggest that active oxygen is involved in the two‐cell block phenomenon in mouse embryos exposed to air and that SOD in the oviduct may play an important role in the protection of embryos from superoxide radicals.