Pore formation and uncoupling initiate a Ca2+-independent degradation of mitochondrial phospholipids.
Mitochondria contain a type IIA secretory phospholipase A(2) that has been thought to hydrolyze phospholipids following Ca(2+) accumulation and induction of the permeability transition. These enzymes normally require millimolar Ca(2+) for optimal activity; however, no dependence of the mitochondrial activity on Ca(2+) can be demonstrated upon equilibrating the matrix space with extramitochondrial Ca(2+) buffers. Ca(2+)-independent activity is seen following protonophore-mediated uncoupling, when uncoupling arises through alamethicin-mediated pore formation, or upon opening the permeability transition pore. Under the latter conditions, activity continues in the presence of excess EGTA but is somewhat enhanced by exogenous Ca(2+). The Ca(2+)-independent activity is best seen in media of high ionic strength and displays a broad pH optimum located between pH 8 and pH 8.5. It is strongly inhibited by bromoenol lactone but not by arachidonyl trifluoromethyl ketone, dithiothreitol, and other inhibitors of particular phospholipase A(2) classes. Immunoanalysis of mitochondria and mitochondrial subfractions shows that a membrane-bound protein is present that is recognized by antibody against an authentic iPLA(2) that was first found in P388D(1) cells. It is concluded that mitochondria contain a distinct Ca(2+)-independent phospholipase A(2) that is regulated by bioenergetic parameters. It is proposed that this enzyme, rather than the Ca(2+)-dependent type IIA phospholipase A(2), initiates the removal of poorly functioning mitochondria by processes involving autolysis.
Broekemeier
Kimberly M
KM
Department of Chemistry, Ohio Northern University, Ada, Ohio 45018, USA. k-broekemeier@onu.edu
Iben
James R
JR
LeVan
Emily G
EG
Crouser
Elliott D
ED
Pfeiffer
Douglas R
DR
D000408:Alamethicin / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism*; D004229:Dithiothreitol / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism*; D008930:Mitochondria, Liver / Q000187:drug effects / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D000083162:Mitochondrial Permeability Transition Pore; D024101:Mitochondrial Proteins; D009281:Naphthalenes / Q000494:pharmacology; D009994:Osmolar Concentration; D010539:Permeability / Q000187:drug effects; D010741:Phospholipases A / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism*; D011189:Potassium Chloride / Q000378:metabolism; D011753:Pyrones / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014475:Uncoupling Agents / Q000378:metabolism* / Q000494:pharmacology; D000071256:Uncoupling Protein 1
D002352:Carrier Proteins; D007473:Ion Channels; D008565:Membrane Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000083162:Mitochondrial Permeability Transition Pore; D024101:Mitochondrial Proteins; D009281:Naphthalenes; D010743:Phospholipids; D011753:Pyrones; D014475:Uncoupling Agents; D000071256:Uncoupling Protein 1; D000408:Alamethicin; D004533:Egtazic Acid; D011189:Potassium Chloride; C068314:6-(bromomethylene)tetrahydro-3-(1-naphthaleneyl)-2H-pyran-2-one; D010741:Phospholipases A; D002118:Calcium; D004229:Dithiothreitol