Effect of phospholipids on alpha-1,2-mannosidase activity.

An alpha-1,2-mannosidase has been solubilized and partially purified from rabbit liver microsomes (Forsee, W. T., and Schutzbach, J. S. (1981) J. Biol. Chem. 256, 6577-6582). The partially purified enzyme was activated by the addition of zwitterionic phospholipids but maximal activity was found to be relatively independent of acyl chain length or degree of unsaturation. Titration of the enzyme with increasing concentrations of water-soluble and long acyl chain phospholipids demonstrated that an ordered lipid structure of either micelles or bilayers was required for alpha-mannosidase activity. Mixed micelles of Triton X-100 and zwitterionic phospholipids also activated the enzyme. The concentration of phospholipid in the mixed micelles required for activation was dependent upon acyl chain length, but maximal activity was unaffected by this parameter. The addition of negatively charged phospholipids not only failed to activate the enzyme but also inhibited alpha-mannosidase activity in the presence of zwitterionic phospholipids. Inhibition by negatively charged phospholipids was pH dependent with maximal inhibition at pH values of 6.0 or lower. These results suggest that the activity of the alpha-1,2-mannosidase could be subject to modulation by the composition and structure of the microsomal membranes.