The P12-binding sites are on the anterior region of the acrosome of mouse sperm. Corresponding to the histological localization of the cells developed during spermatogenesis and spermiogenesis in the testis, they are present in the cell layers closely restricted to the lumen of seminiferous tubules. Their absence in the spermatogenic cells such as spermatogonia and spermatocytes indicates that the P12-binding sites are synthesized predominantly in the postmeiotic cells such as spermatids and spermatozoa. Thus, the biosynthesis of P12-binding sites is likely to start in the early haploid spermatids during sperm generation.
The study of Boettger-Tong et al. shows the inhibitory effect of a PI (P12) on the binding of ZP3 to sperm. The recent study of Thaler and Cardullo indicates the presence of 3.0 X 104 ZP3-binding sites/sperm cell and the complexity of ZP3-sperm binding, which is attributed to the cooperative interaction between a low-affinity ZP3-binding site with a Kd value of 50 nM and a high-affinity ZP3-binding site with a Kd value of 0.72 nM. They suggest the involvement of multiple receptors on the sperm surface and/or multiple ligand moieties. This work illustrates a single type of P12-binding site ([1.49 ± 0.06] X 106 sites/ cell) on the sperm surface with a Kd value of 70.1 ± 5.3 nM. Apparently, the ZP3-binding sites cannot account for the capacity and characteristics of the P12-binding sites, even though the binding strength of P12-sperm binding sites is as strong as that of the low-affinity ZP3-binding sites. Cheap Diskus Advair
Trypsin shows a strong affinity to P12, with a Kd value of 0.15 nM. Since acrosin is a serine protease of the trypsin superfamily, this raises the question of whether ac-rosin or/and its zymogen form (proacrosin) is involved in the P12-binding sites in the acrosomal region. No active acrosin is present in intact spermatozoa, and it is released from the cell once proacrosin is activated during acrosomal exocytosis at the fertilization sites in the oviduct.