Cytochrome c, an ideal antioxidant

Generation of Δψ (membrane potential) by cytochrome oxidase proteoliposomes oxidizing superoxide-reduced cytochrome c has been demonstrated. XO+HX (xanthine oxidase and hypoxanthine) were used to produce superoxide. It was found that the generation of Δψ is completely abolished by cyanide (an uncoupler) or by superoxide dismutase, and is enhanced by nigericin. Addition of ascorbate after XO+HX causes a further increase in Δψ. On the other hand, XO+HX added after ascorbate do not affect Δψ, indicating that superoxide does not have measurable protonophorous activity. The half-maximal cytochrome c concentration for Δψ generation supported by XO+HX was found to be approx. 1 μM. These data and the results of some other researchers can be rationalized as follows: (1) O₂ accepts an electron to form superoxide; (2) cytochrome c oxidizes superoxide back to O₂; (3) an electron removed from the reduced cytochrome c is transferred to O₂ by cytochrome oxidase in a manner that generates Δμ_H⁺ (transmembrane difference in electrochemical H⁺ potential). Thus cytochrome c mediates a process of superoxide removal, resulting in regeneration of O₂ and utilization of the electron involved previously in the O₂ reduction. It is important that cytochrome c is not damaged during the antioxidant reaction, in contrast with many other antioxidants.