Purification and characterization of a phospholipid-dependent alpha-mannosidase from rabbit liver.

An alpha-mannosidase specific for the hydrolysis of alpha-1,2-mannosyl-mannose linkages has been solubilized and partially purified from rabbit liver microsomes. The enzyme is inhibited by EDTA and has optimal activity in the presence of calcium ions. The purified enzyme has a requirement for nonionic detergents or for specific phospholipids. At detergent concentrations appreciably below the critical micelle concentration, the enzyme is active in the presence of phosphatidylcholine or phosphatidylethanolamine but not with phosphatidylinositol, phosphatidylglycerol, or phosphatidic acid. At concentrations of phosphatidylcholine which provide optimal activity, the enzyme is strongly inhibited by phosphatidylinositol or phosphatidylglycerol. The substrate specificity of the alpha-mannosidase toward oligosaccharide substrates suggests that the enzyme may be involved in the processing of the oligosaccharide chains of mammalian glycoproteins.