Specificity of apoptosis for cytochrome c. (a) Other redox proteins do not induce apoptosis. Bo cells were electroporated with cytochrome c (80 μg/mL), human hemoglobin, or cytochrome bc₁ complex at molar equivalents to cytochrome c. Only cytochrome c induces apoptosis. Pre-G1 apoptotic fractions: control, 5%; + cyt c, 31%; + Hb, 4.8%; + bc₁ 4.4%. (b) Only holo-but not apo-cytochrome c induces apoptosis. Bo (upper 3 panels) and B15 cells (lower 3 panels) were electroporated with 80 μg/mL of holo- or apo-cytochrome as in Fig 1 and recultured with IL-3 and samples were analyzed by flow cytometry after 3 hours. Pre-G1 apoptotic fractions were as follows: Bo: control <1%; with holocytochrome c 37%; with apocytochrome c <1%. B15: control 5%; with holocytochrome c 11%; with apocytochrome c 2%. Similar results were recorded for A2 and A15 cells (not shown).