Rapid extracellular release of cytochrome c is specific for apoptosis and marks cell death in vivo.

Diverse death stimuli including anticancer drugs trigger apoptosis by inducing the translocation of cytochrome c from the outer mitochondrial compartment into the cytosol. Once released, cytochrome c cooperates with apoptotic protease-activating factor-1 and deoxyadenosine triphosphate in caspase-9 activation and initiation of the apoptotic protease cascade. The results of this study show that on death induction by chemotherapeutic drugs, staurosporine and triggering of the death receptor CD95, cytochrome c not only translocates into the cytosol, but furthermore can be abundantly detected in the extracellular medium. The cytochrome c release from the cell is a rapid and apoptosis-specific process that occurred within 1 hour after induction of apoptosis, but not during necrosis. Interestingly, elevated cytochrome c levels were observed in sera from patients with hematologic malignancies. In the course of cancer chemotherapy, the serum levels of cytochrome c in the majority of the patients grew rapidly as a result of increased cell death. These data suggest that monitoring of cytochrome c in the serum of patients with tumors might serve as a useful clinical marker for the detection of the onset of apoptosis and cell turnover in vivo.