Novel alpha-D-mannosidase of rat sperm plasma membranes: characterization and potential role in sperm-egg interactions.
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 alpha-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 alpha-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 alpha-D-mannosidase is active mainly towards synthetic substrate, p-nitrophenyl alpha-D-mannopyranoside, whereas the sperm plasma membrane alpha-D-mannosidase shows activity mainly towards mannose-containing oligosaccharides. Evidence is presented which suggest that the sperm plasma membrane alpha-D-mannosidase is different from several processing mannosidases previously characterized from the rat liver. The newly discovered alpha-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 the 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 alpha-D-mannosidase. At a concentration of 0.1 mM, these divalent cations caused nearly complete inactivation of the sperm enzyme. In addition methyl-alpha-D-mannoside, methyl-alpha-D-glucoside, mannose, 2-deoxy-D-glucose, and D-mannosamine are inhibitors of the sperm surface alpha-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 alpha-D-mannosidase may have a role in binding to mannose-containing saccharides presumably present on the zona pellucida.
Department of Obstetrics and Gynecology, Vanderbilt School of Medicine, Nashville, Tennessee 37232-2633.
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Tulsiani
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Skudlarek
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Orgebin-Crist
D000818:Animals; D002458:Cell Fractionation; D002462:Cell Membrane / Q000201:enzymology / Q000648:ultrastructure; D002499:Centrifugation, Density Gradient; D005260:Female; D007700:Kinetics; D008297:Male; D008361:Mannosidases / Q000302:isolation & purification / Q000378:metabolism*; D008854:Microscopy, Electron; D009919:Orchiectomy; D051381:Rats; D012016:Reference Values; D013084:Sperm-Ovum Interactions; D013094:Spermatozoa / Q000201:enzymology*; D013379:Substrate Specificity; D013737:Testis / Q000502:physiology; D043323:alpha-Mannosidase
D008361:Mannosidases; D043323:alpha-Mannosidase