Role of superoxide dismutase in protecting rabbit spermatozoa from O2 toxicity due to lipid peroxidation.

Abstract

Cyanide-sensitive superoxide dismutase (SOD) activity in rabbit epididymal spermatozoa decreases with time of aerobic incubation of a washed cell suspension. The inactivation of SOD activity correlates linearly with the increasing percent immotile sperm observed under these con- ditions. We had previously shown a linear correlation between percent immotile sperm and spon- taneous lipid peroxidation in both high Naand high i��' media, the peroxidation rate being eightfold higher in the high Kmedium as compared to the high Ni' medium (Alvarez and Storey, 1982). These two linear correlations connect SOD inactivation and lipid peroxidation. In this study, the reaction of 03 with the sperm cells was characterized by a second order rate constant k5. The value of k5 for fresh cells is 1.2 X io� (cells/ml�'min'' in high Na4' medium and 12.5 X 10� (cel!s/m!F' minin high K' medium. This difference is reflected in the lipid peroxidation rate in the two media. As SOD is inactivated during aerobic incubation in the high Na+ medium, the value of k5 increases to 18.3 X 10-8 (cells/mli'mincorresponding to 100% inactivation of SOD. This value is close to that of 19.9 X 10 cells/mIT' mirf' obtained in the high Na4' medium and 22.9 X 10' (cells/mW' min' obtained in the high K' medium with fresh cells in the pres- ence of CN to inactivate the SOD. We suggest that attack of 02�' to induce lipid peroxidation in fresh cells occurs at the plasma membrane and is sensitive to the ionic composition of the medium, but that as SOD becomes inactivated during aerobic incubation, more intracellular lipid peroxidation occurs, which is not affected by the ionic composition of the medium. These results imply that SOD activity is the primary enzymic defense against oxidative degradation available to rabbit spermatozoa.