MYCOBACTERIUM-SMEGMATIS MALATE-DEHYDROGENASE - ACTIVATION OF THE LIPID-DEPLETED ENZYME BY ANIONIC PHOSPHOLIPIDS AND PHOSPHATIDYLETHANOLAMINE

Phospholipid-protein interactions have been investigated in a phosphol ipid-requiring enzyme, FAD-dependent malate dehydrogenase isolated fro m Mycobacterium smegmatis membranes, to correlate these interactions w ith enzyme function. The ability of several natural and synthetic phos pholipids including CL and PE, which are major phospholipids in M. sme gmatis membranes, to activate purified, lipid-depleted, enzymatically inactive malate dehydrogenase was examined. Anionic phospholipids and PE activated the enzyme, while zwitterionic phospholipids did not. A P E/PC mixture activated the enzyme in the form of both bilayer and non- bilayer structure. CL/PE mixtures activated malate dehydrogenase much more than each single phospholipid species. All anionic phospholipids used stabilized the enzyme, while PE and zwitterionic phospholipids di d not. CL and a CL/PE mixture protected malate dehydrogenase from prot einase digestion, while PE did not. Ah phospholipids and phospholipid mixtures tested caused little secondary structural change in malate de hydrogenase. The results obtained in this study suggest that CL and CL /PE mixtures could form stable, enzymatically active complexes with ma late dehydrogenase which might be similar to the native complex in M. smegmatis membranes. Although PE could activate malate dehydrogenase i n both bilayer and non-bilayer form, it formed a complex with malate d ehydrogenase which was inferior in terms of stability and susceptibili ty to proteinases, indicating that PE alone poorly reconstitutes the a ctive enzyme-phospholipid complex.