Suppression of the pro-apoptotic function of cytochrome c by singlet oxygen via a haem redox state-independent mechanism

• Published in: Biochemical journal Vol. 392; no. 2; pp. 399 - 406
• Main Authors: Suto, Daisuke, Sato, Kazuaki, Ohba, Yoshihiro, Yoshimura, Tetsuhiko, Fujii, Junichi
• Format: Journal Article
• Language: English
• Published: England Portland Press Ltd 01.12.2005
• Subjects: Animals; Apoptosis - drug effects; Caspases - metabolism; Cell Line, Tumor; Cysteine - pharmacology; Cytochromes c - metabolism; Enzyme Activation - drug effects; Glutathione - pharmacology; Heme - metabolism; Horses; Humans; Oxidation-Reduction - drug effects; Singlet Oxygen - metabolism; Singlet Oxygen - pharmacology
• ISSN: 0264-6021; 1470-8728; 1470-8728
• DOI: 10.1042/BJ20050580

Stimuli for apoptotic signalling typically induce release of cyt c (cytochrome c) from mitochondria. Cyt c then initiates the formation of the apoptosome, comprising Apaf-1 (apoptotic protease-activating factor 1), caspase-9 and other cofactors. The issue of whether the redox state of the haem in cyt c affects the initiation of the apoptotic pathway is currently a subject of debate. In a cell-free reconstitution system, we found that only oxidized cyt c was capable of activating the caspase cascade. Oxidized cyt c was reduced by the physiological reductants cysteine and glutathione, after which it was unable to activate the caspase cascade. It is thus likely that cyt c with oxidized haem is in a conformation capable of interaction with Apaf-1 and forming apoptosomes. When either oxidized or reduced cyt c was treated with submillimolar concentrations of endoperoxide, which affected less than 3% of the redox state of haem, the ability of the oxidized cyt c to activate the caspase cascade was abolished. Higher amounts of singlet oxygen were required to affect the optical spectral change of haem, suggesting that the suppressed pro-apoptotic function of oxidized cyt c is a mechanism that is separate from the redox state of haem. Oxidative protein modification of cyt c by singlet oxygen was evident, on the basis of elevated contents of carbonyl compounds. Our data suggest that singlet oxygen eliminates the pro-apoptotic ability of oxidized cyt c not via the reduction of haem, but via the modification of amino acid residues that are required for apoptosome formation.