Studies of electron spin resonance on bilirubin free radicals

The nature of ESR signals derived from bilirubin-IX alpha has been studied by focusing on the samples treated with free radical generating and inhibiting systems, i.e. X-XOD, Fe/EDTA, SOD, mannitol/ascorbate, CO, KCN, etc. In all the cases, the stable signals comprise those originated from semiquinone radical (g = 2.0012) and superoxide free radical (g parallel = 2.041, g perpendicular = 2.0040). The superoxide is shown binding with certain metal ions chelated by bilirubin. The free radical scavengers are able to destroy these radicals. The kinetic curve of the regeneration of bilirubin radicals has been determined and the reaction follows a zero order mechanism. It is likely that both the physiological and toxic actions of bilirubin are related to the characters of its free radicals. Bilirubin is discussed as "active oxygen sink" in mammalians.