Cytochrome c release is an apoptosis-specific process.

Cytochrome c is not released during necrotic cell death in mouse L929 fibroblasts. L929 cells (3 × 10⁶) were incubated with medium or with TNF-α (10 ng/mL) plus actinomycin D (Act D, 1 μg/mL) in the absence or presence of the caspase inhibitor zVAD-fmk (100 μM) for 8 hours. (B) Cytochrome c is not released during necrotic cell death in human Jurkat cells. Cells (1 × 10⁶) were cultured in normal growth medium (Control), or in the presence of staurosporine (Stauro, 2.5 μM) or the indicated concentrations of H₂O₂ for 18 hours. The release of cytochrome c was determined by immunoprecipitation of culture medium (m) and corresponding cellular extracts (c) and subsequent immunoblotting. Heavy and light chains of the anticytochrome c antibody and cytochrome c are indicated by open and closed arrowheads, respectively. Cell death was measured by propidium iodide uptake and apoptosis by flow cytometric detection of hypodiploid nuclei. The SDs were less than 9%.