A series of protease inhibitors were tested on the motility of human, rat, bull, and rabbit demembranated reactivated spermatozoa. Some inhibitors, including aprotinin, boc-gln-leu-lys-H, and D-phe-pro-arg-H, could inhibit motility as well as prevent initiation of motility. In general, with the exception of aprotinin, protease inhibitors were more potent in preventing the initiation of movement than in blocking motility of demembranated spermatozoa. Protease substrates could also block sperm motility. Of the substrates tested only those with arg or lys ester bonds were active. The inhibition of motility by protease substrates was reversible, as once spermatozoa hydrolyzed the added exogenous protease substrates, motility reappeared. The importance of ester bond in the inhibitory action of protease substrates was confirmed by experiments that showed the lack of effect of pre-hydrolyzed protease substrates. The results suggest that a serine protease with lys and arg ester bond specificity is involved in the control of sperm motility. The fact that protease substrates also block motility of intact spermatozoa further emphasizes the physiological relevance of this new regulatory system.

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