Mitochondrial permeability transition (PT) involves the formation of proteaceous, regulated pores, probably by apposition of inner and outer mitochondrial membrane proteins which cooperate to form the mitochondrial megachannel (=mitochondrial PT pore). PT has important metabolic consequences, namely the collapse of the mitochondrial transmembrane potential, uncoupling of the respiratory chain, hyperproduction of superoxide anions, disruption of mitochondrial biogenesis, outflow of matrix calcium and glutathione, and release of soluble intermembrane proteins. Recent evidence suggests that PT is a critical, rate limiting event of apoptosis (programmed cell death): (i) induction of PT suffices to cause apoptosis; (ii) one of the immediate consequences of PT, disruption of the mitochondrial transmembrane potential (ΔΨm), is a constant feature of early apoptosis; (iii) prevention of PT impedes the ΔΨm collapse as well as all other features of apoptosis at the levels of the cytoplasma, the nucleus, and the plasma membrane; (iv) PT is modulated by members of the apoptosis-regulatory bcl-2 gene family. Recent data suggest that the acquisition of the apoptotic phenotype, including characteristic changes in nuclear morphology and biochemistry (chromatin condensation and DNA fragmentation), depends on the action of apoptogenic proteins released from the mitochondrial intermembrane space.

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