Thus, no acrosin but proacrosin exists in spermatozoa before their passage to the oviduct. Since proacrosin is not exposed on the sperm surface but is located in the acrosomal matrix of intact sperm, P12 is unlikely to interact with the zymogen unless P12 penetrates into the acrosomal region. In fact, there is one protein component, which is distinct from acrosin/proacrosin but is capable of binding a PI of seminal vesicle origin, in the supernatant of frozen-thawed murine epididymal sperm suspension. proventil inhaler
As stated by Barros, the fertile condition of spermatozoa is not a terminal condition but rather a transient one. It is well recognized that mammalian sperm from epididymis should undergo some Ca2+-dependent modifications before fertilization. In the reproductive tract, the Ca2+ concentration is sufficient to elevate intracellular Ca2+ in the induction of these cell modifications at any time earlier than the sperm-egg encounter. However, these modifications far from the oviduct would cause spermatozoa to become infertile. Thus, the calcium movement across the membrane of spermatozoa should be prohibited at ejaculation until the cells reach the oviduct. In this regard, caltrin is believed to play a most important role in the inhibition of Ca2+ uptake by sperm. This kind of protein was identified and purified originally from bovine seminal plasma and subsequently from seminal vesicle fluid of the guinea pig. Our results support the idea that P12 is a caltrin-like protease inhibitor. The P12-sperm binding leading to the suppression of Ca2+ movement across the cell membrane sheds some light into the function of the P12-binding sites. The primary structure of P12 shows a high degree of similarity to that of a caltrin containing 56 amino acid residues in rat seminal vesicle fluid. A caltrin consisting of 75 amino acid residues in mouse seminal vesicle fluid has been characterized by Coronel et al.. This caltrin and P12, though they coexist in seminal vesicle fluid, show no appreciable sequence similarity. It is possible that they exhibit different mechanisms in the regulation of calcium movement across the membranes of sperm. Future study is needed to elucidate how the dual roles of P12 as a protease inhibitor and as a caltrin interplay in the regulation of sperm activity.