Superoxide dismutase isoenzymes in human seminal plasma and spermatozoa

Superoxide radicals may exert both toxic and physiological regulating actions on spermatozoa. The objective of the present study was to examine the occurrence and distribution of the three superoxide dismutase (SOD) isoenzymes in human seminal plasma and spermatozoa. Human seminal plasma has previously been reported to possess high SOD activity. Here we show that the normally cytosolic CuZn-SOD remarkably accounts for 75% of the activity while the secretory extracellular SOD (EC-SOD) accounts for 25%. Studies of split ejaculates suggest that both these SOD isoenzymes are of primarily prostatic origin. The Mn-SOD activity was negligible. The total SOD activity of seminal plasma was 20 times higher than that of human blood plasma. While native EC-SOD shows high affinity for heparin and heparan sulphate, 90% of the EC-SOD in seminal plasma lacks the high affinity at ejaculation. Thus only a minor part of the seminal plasma EC-SOD has the potential to bind to cell surfaces. Human spermatozoa were found to contain exceptionally large amounts of CuZn-SOD. There was little Mn-SOD activity and the amount of EC-SOD was negligible. We conclude that spermatozoa in semen are exceptionally well protected against superoxide radicals both internally and externally. This should be of importance for both their survival and the integrity of DNA, and may also have physiological effects such as influencing capacitation.