Proteasomal activity in mammalian spermatozoa

The proteasome is a multicatalytic cellular complex, which possess three different enzymatic activities, trypsin-like, chymotrypsin-like, and peptidylglutamyl peptidase. Its function is to remove abnormal or aged proteins. Recently, it has been suggested the participation of the sperm proteasome during mammalian fertilization. In this study, we present evidence that indicates that sperm extracts from several mammalian species, including hamster, mice, rats, bovine, rabbits, and humans all possess proteasome activity. We characterized the three specific activities of the proteasome using specific synthetic substrates and specific proteasome inhibitors. The results indicates that the highest specific activity detected was in mouse sperm toward the trypsin substrates and it was 1,114% of the activity of human sperm toward the chymotrypsin substrate Suc-Leu-Leu-Val-Tyr-AMC (SLLVY-AMC, which was considered as 100%). In all cases, the lowest activity was toward substrates for the peptidylglutamyl peptidase hydrolyzing activity, and it was lowest for rabbit sperm (1.7% of the activity of human sperm toward the chymotrypsin substrate SLLVY-AMC). In addition, specific proteasome inhibitors were able to block all proteasome activities almost 100%, with the exception of clasto-Lactacystin beta-lactone upon rat sperm. All sperm extracts tested evidenced bands of about 29-32 kDa by Western blots using a monoclonal antibody against proteasome subunits alpha 1, 2, 3, 5, 6, and 7. In conclusion, sperm from several mammals possess enzymatic activities that correspond to the proteasome. The proteasome from the different species hold similar but distinctive enzymatic characteristics.