ESR spin trapping studies on the OH* free radical reactions of idebenone

Experiments have been performed using oxidized (ID-O) and reduced (ID-H) idebenone in various spin trapping systems. The following results were obtained. (1) ID-O does not have any detectable scavenger effect on the OH* free radicals when they are generated by means of Fenton reaction and trapped using DMPO, PBN or 4-POBN. (2) ID-H represents a serious competition for all spin traps used in capturing OH* free radicals. The mechanism of this competition is, however, not a direct reaction, but it is based on the rather quick autoxidation of the dissolved ID-H generating O2(-*) radicals. They interact with the OH* radicals (or with the OH-spin adducts) by means of electron donation to them. This statement has been proven by showing (i) that O2* radicals generated during the autoxidation of ID-H can directly be trapped on DMPO; (ii) the effect of ID-H on the OH* free radicals is abolished if SOD is added to the system; (iii) the O2(-*) radicals generated by ID-H autoxidation reduce directly the OH-spin adducts on various kinds of nitroxide type spin traps (e.g. TEMPO, TMPN). (3) The rate of autoxidation of ID-H in solution is about an order of magnitude faster than that of ubiquinone. Similar differences could be observed in the rate of reduction of the oxidized forms of both compounds by Na-borohydride. The results fully explain the in vitro protective effect of ID-H against lipid peroxidation of artificial membranes.