INDUCTION OF CAPACITATION IN HUMAN SPERMATOZOA IN-VITRO BY AN ENDOMETRIAL SIALIC ACID-BINDING PROTEIN

A Ca2+-dependent sialic acid-binding protein (SABP) of human endometri um, which specifically bound to human sperm head plasma membrane in vi tro, was found to increase the percentage motility and acrosome-reacte d pattern of uncapacitated spermatozoa, The protein was synthesized in the endometrium and secreted into the uterine fluid, This intra-uteri ne factor, which is apparently advantageous in vitro in inducing human sperm capacitation, may play a significant role in promoting the post release maturation of ejaculated spermatozoa by enhancing Ca-45 uptake into spermatozoa by a pathway which is insensitive to calcium-channel blockers. However, the Ca-45 uptake could be enhanced on exposure to the divalent cation ionophore A23187 and inhibited in the presence of the calmodulin inhibitor trifluoperazine, The SABP also induces an inc rease in intracellular Ca2+ in spermatozoa, as seen by FURA-2 AM studi es, Furthermore, overlay studies show human SABP to be a Ca2+-binding protein. The data presented here suggest that SABP induces in-vitro sp erm capacitation and the subsequent acrosome reaction by increasing in tracellular Ca2+ concentration.