PHOSPHOMANNOSYL RECEPTORS ON THE SURFACE OF SPERMATOZOA FROM THE CAUDA EPIDIDYMIS OF THE RAT

This study demonstrates that beta-glucuronidase from rat preputial gla nds binds with high affinity to spermatozoa from the cauda epididymis. The binding was calcium-independent and was inhibited by mannose-6-ph osphate, but not by other phosphorylated or non-phosphorylated sugars. Binding was also inhibited by alpha-mannosidase from Dictyostelium di scoideum, an enzyme known to have mannose-6-phosphate as the ligand. F rom solubilized sperm membranes, a protein of >200 kDa and one of 45 k Da, were adsorbed to a column of D. discoideum enzyme and to a phospho mannan column respectively, and eluted with mannose-6-phosphate. Accor ding to histochemical observations at the light and the electron micro scopic level, gold particles coated with the enzyme became bound to th e external surface of the plasmalemma in the acrosomal region of cauda l spermatozoa. Similar labelling was observed using gold particles coa ted with antibodies against the rat 300 kDa phosphomannosyl receptor. The existence of phosphomannosyl receptors on the sperm plasma membran e, and our previous demonstration of the presence of affinity sites fo r epididymal beta-galactosidase on these gametes which is inhibited by phosphofructosyl derivatives, suggest strongly that maturing spermato zoa could be a target for glycosidases secreted into the lumen of the cauda epididymis, which then become bound to these cells via different ligand-receptor systems.