title: Hydrolysis of exogenous substrates by mitochondrial phospholipase A2.
abstract: Evidence is provided in this paper to indicate that hydrolysis of exogenously added phosphatidylethanolamine and phosphatidylcholine by the membrane-bound phospholipase A2 from rat-liver mitochondria is preceded by association of the substrates with the membranes. Hydrolysis of phosphatidylethanolamine after preincubation of mitochondria and substrate is nearly independent of incubation volume, indicating that substrate and mitochondria are not independently diluted. The association is greatly enhanced in the presence of Ca2+, especially for phosphatidylethanolamine. Association can be measured after sucrose-gradient centrifugation of mitochondria preincubated with phosphatidylethanolamine and can be visualized by freeze-fracture electronmicroscopy, showing substrate clusters fused with mitochondria. The association provides an explanation for the hydrolysis of exogenous substrates by a membrane-associated phospholipase A2 as well as for the high preference for phosphatidylethanolamine degradation often observed in studies on membrane-bound phospholipases A. This preference is likely to result in part from the tendency of unsaturated phosphatidylethanolamines to adopt non-bilayer lipid phases allowing a more extensive association with biomembranes in the presence of Ca2+, and does not reflect enzyme specificity per se. This phenomenon should be kept in mind when determining the substrate specificity of membrane-bound phospholipases A by the use of exogenous substrates.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology; D005613:Freeze Etching; D006868:Hydrolysis; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008854:Microscopy, Electron; D008930:Mitochondria, Liver / Q000201:enzymology*; D008968:Molecular Conformation; D010713:Phosphatidylcholines / Q000378:metabolism; D010714:Phosphatidylethanolamines / Q000378:metabolism; D010740:Phospholipases / Q000378:metabolism*; D010741:Phospholipases A / Q000378:metabolism*; D054467:Phospholipases A2; D051381:Rats; D013379:Substrate Specificity
chemical_list: D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D010740:Phospholipases; D010741:Phospholipases A; D054467:Phospholipases A2; D002118:Calcium