Recent studies show that release of cytochrome c from mitochondria into the cytoplasm is an early event in signaling apoptotic cell death. Cytochrome c is a required factor for activating cytoplasmic CPP32, an ICE-like protease that has been linked to cleavage of poly (ADP-ribose) polymerase and other proteins that execute the apoptotic cell death program. Diverse chemical toxicants affect mitochondrial function and induce apoptosis. Our preliminary results show that some chemical toxicants (e.g., t-butylhydroperoxide, acetaldehyde) activate mitochondrial cytochrome c release at concentrations relevant to their induction of apoptosis. The purpose of this proposal is to determine whether mitochondrial cytochrome c release is an early, specific event in the mechanism of chemical-induced toxicity. In the first specific aim, known inducers of apoptosis (staurosporine and tert-butylhydroperoxide) will be used to determine the time courses of cytochrome c release and activation of CPP32 relative to the loss in mitochondrial O2 consumption, loss of mitochondrial membrane potential and loss of cellular ATP. These studies will allow us to distinguish between cell death due to selective release of cytochrome c that precedes loss of mitochondrial respiratory function and release as a consequence of mitochondrial failure, e.g., the mitochondrial permeability transition. Because mitochondrial failure also causes necrosis, we will determine whether stimulation of glycolysis during apoptosis can allow cells to preserve ATP and avoid necrosis despite loss of O2 consumption and membrane potential when cytochrome c is released. The second specific aim is to determine whether inhibition of normal mitochondrial electron transport due to loss of cytochrome c results in enhanced superoxide anion generation and whether this causes the observed oxidation of thiols that occurs during apoptosis. The third specific aim is to examine representative chemicals known to affect mitochondria to determine whether they stimulate cytochrome c release and activate CPP32 as early events of apoptosis. These studies will clarify the role of mitochondria in chemical-induced cell death and provide important new information concerning the mechanistic basis for apoptotic cell death induced by mitochondrial toxicants.