Molecular Pharmacology, Vol 18, 267-273, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Lipid-Protein Interactions in the Multiple Forms of Monoamine Oxidases: Lipases as Probes Using Purified Intact Rat Brain Mitochondria

¹ Department of Biochemistry, College of Medicine, University of South Alabama, Mobile, Alabama 36688

It has been suggested that the functional states of the multiple forms of monoamine oxidases (MAO) are regulated by distinctly different lipid-protein interactions in situ in an intact rat brain mitochondrial preparation. Treatment of the enzymes with phospholipase A (L.A.) from porcine pancreas hydrolyzed almost all phospholipids and disintegrated the outer mitochondrial membrane with concomitant loss of both MAO-A- and MAO-B-type activities. Digestion with phospholipase C (L.C.) from Clostridium welchii hydrolyzed 90% of the phosphatidyicholine, one-third of the phosphatidylethanolamine, and one-half of the sphingomyelin. The specific action of L.C. on the polar zwitterionic head groups of one-half of the total membrane phospholipids resulted in a great reduction of the polar and ionic interactions at the hydrophilic surface which in turn increased the fluidity of the hydrophobic core of the membrane as probed with spin-labeled stearic acid I(12,3) with electron spin resonance (ESR). Treatment with phospholipase D (L.D.) removed one-half of the positively charged choline moiety of phosphatidyicholine. The surface charge of the membrane became more negative, the polarity was reduced, and the microviscosity of the hydrophobic core as probed by I(12,3) was altered. After L.C. or L.D. treatment, the bilayer structure of the outer mitochondrial membrane was retained. In the case of L.D. treatment, MAO-B enzyme was inactivated to a much greater extent (75%) than MAO-A enzyme (25%). In contrast, after L.C. treatment MAO-B enzyme was intact or activated whereas MAO-A was similarly inactivated as in the case of L.D. digestion. Surface charge and polar interactions provided by liposomes made of phosphatidylcholine, phosphatidylethanolamine, cardiolipin, or phosphatidic acid preferentially inhibited MAO-B activity. It was concluded that for MAO-A enzyme the active site was buried in the hydrocarbon core and the functional state was intimately modulated by the fluidity of the hydrophobic region proximal to the polar surface. For MAO-B enzyme the active site was situated closer to or partly in the peripheral hydrophiic region and its functional state was strongly dependent upon the ionic and polar characteristics of the surface layer of the membrane.