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 Ca2+ accumulation and induction of the permeability transition. These enzymes normally require millimolar Ca2+ for optimal activity; however, no dependence of the mitochondrial activity on Ca2+ can be demonstrated upon equilibrating the matrix space with extramitochondrial Ca2+ buffers. Ca2+-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 Ca2+. The Ca2+-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? 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 Ca2+-independent phospholipase A, that is regulated by bioenergetic parameters. It is proposed that this enzyme, rather than the Ca2+-dependent type IIA phospholipase A(2), initiates the removal of poorly functioning mitochondria by processes involving autolysis.