Isolation and Characterization of Seminal Fluid Proteins that Bind Heparin

Abstract

The molecular mechanism by which sperm are capacitated is poorly understood. Heparin has been shown to facilitate the acrosome reaction in bull cauda epididymal (Handrow et al., 1982) and ejaculated (Miller and Hunter, 1986; Parrish et al., 1985) sperm as well as rabbit sperm (Lenz et al., 1983). Heparin is structurally similar to heparan sulfate (Gallagher and Walker, 1985) and heparan-like material is found in follicular fluid (Ax et al., 1985) and oviductal fluid (Lee et al., 1986). Heparin binds to bull, rabbit and monkey sperm in a saturable, reversible, temperature-, pH- and Ca²⁺-dependent fashion (Handrow et al., 1984). A heparin-binding component was isolated by affinity chromatography of hypotonic MgCl₂ extracted ejaculated bull sperm. The primary protein isolated was a small 15–18 kilodalton (kD), acidic (pl from 4.2–5.0) protein (Lavin et al., 1986). Rabbit ejaculated, but not epididymal, sperm contain a 40 kD component that reacts with anti-fibronectin affinity-purified anti-sera (Koehler et al., 1980). Plasmin and chymotryspin digestion of fibronectin demonstrates 2 heparin-binding domains that are 29 and 31 kD (Woods et al., 1986). These fragments may be present in sperm or seminal plasma and may regulate capacitation.