Novel α-D-mannosidase of rat sperm plasma membranes: Characterization and potential role in sperm-egg interactions

Abstract

During the course of a study of glycoprotein processing mannosidases in the rat epididymis, we have made an intriguing discovery regarding the presence of a novel α-D-mannosidase on the rat sperm plasma membranes. Unlike the sperm acrosomal 'acid' mannosidase which has a pH optimum of 4.4, the newly discovered α-D-mannosidase has a pH optimum of 6.2, and 6.5 when assayed in sperm plasma membranes and intact spermatozoa, respectively. In addition, the two enzymes show different substrate specificity. The acrosomal α-D-mannosidase is active mainly towards synthetic substrate, p-nitrophenyl α-D-mannopyranoside, whereas the sperm plasma membrane α-D-mannosidase shows activity mainly towards mannose-containing oligosaccharides. Evidence is presented which suggest that the sperm plasma membrane α-D-mannosidase is different from several processing mannosidases previously characterized from the rat liver. The newly discovered α-D-mannosidase appears to be an intrinsic plasma membrane component, since washing of the purified membranes with buffered 0.4 M NaCl did not release to enzyme in soluble form. The enzyme requires nonionic detergent (Triton X-100) for complete solubilization. The enzyme is activated by Co2+ and Mn2+. However, Cu2+ and Zn2+ are potent inhibitors of the sperm plasma membrane α-D-mannosidase. At a concentration of 0.1 mM, these divalent cations caused nearly complete inactivation of the sperm enzyme. In addition methyl-α-D-mannoside, methyl-α-D-glucoside, mannose, 2-deoxy-D-glucose, and D-mannosamine are inhibitors of the sperm surface α-D-mannosidase. The physiological role of the newly discovered enzyme is not yet known. Several published reports in three species, including the rat, suggest that the sperm surface α-D-mannosidase may have a role in binding to mannose-containing saccharides presumably present on the zona pellucida.