COX activity was determined photometrically in lysated sperm cells, essentially as in Miró et al., 1998, following Cytochrome C oxidation. Briefly, by using a JASCO V-560 and V-520 dual-beam dual-wavelength system, the oxidation of externally added reduced Cytochrome C was measured as an absorbance decrease at 550 (absorbance peak of Cytochrome C reduced form) minus 540 (isosbestic point) nm. To measure total COX activity, spermatozoa (1 mg protein) were completely lysated before addition to reaction medium consisting of 1 mL of Tris–HCl 50 mM pH 7.2. The reaction was started by the addition of saturating concentration of reduced cytochrome C (usually 100 μM) and for each sample we checked that cyanide completely inhibited the reaction to rule out any other mechanism for COX oxidation. The COX activity was obtained as tangent at the initial part of experimental curve. COX activities of irradiated samples are expressed as percentage of control sample COX activity.