[Studies on the activation of molecular oxygen and the biological defence mechanism against active oxygen species]

Abstract

One of the active oxygen species, superoxide (O2-), was generated by the electrolytic reduction of molecular oxygen in acetonitrile. O2- was determined by the ultraviolet (UV) (lambda max/nm = 255, epsilon = 1.48 x 10(3) M-1 cm-3) and the electron spin resonance (ESR) (g parallel = 2.083, g perpendicular = 2.008) spectrum. O2- could easily react with tocopherols (vitamin E and its derivatives) to give the corresponding chromanoxyl radicals of which structures were determined by ESR. ESR studies of the reactions of O2- with tocopherols or their model compounds indicate that the radical concentrations from tocopherol models correlate with the physiological activities of the tocopherols. O2- could also react with some biologically active quinones such as vitamin K3 and vitamin E quinone to give the corresponding semiquinone radicals. The fact that vitamin E quinone, an irreversible metabolite of vitamin E, was reduced by O2- to the semiquinone radical suggests that, like vitamin E, vitamin E quinone may also scavenge O2- and protect living cells from the effects of O2- in a hydrophobic environment. Further, O2- could react with some metalloporphyrins. In this case, non-redox metalloporphyrins such as Zn(II)TPP (TPP: tetraphenylporphine), Cd(II)TPP, Mg(II)TPP generated the superoxide adduct by the reaction with O2-. On the other hand, redox-active metalloporphyrins such as Cr(III)TPP.Cl, Mn(III)TPP.Cl, Co(II)TTP TTP: tetra-p-tolylporphine) and Co(III)TTP.Cl underwent the addition and/or redox reactions with O2-. Another active oxygen species, hydroxyl radical (OH.), was first detected from some copper (II) coplexes such as Cu(en)2 (en: ethylenediamine) with hydrogen peroxide (H2O2) by ESR spin trapping and thiobarbituric acid (TBA) methods. Further, by using Cu(en)2-H2O2 system the most active OH. scavenger was determined. This Cu(en)2-H2O2 system will be useful for determing the antioxidant ability against OH..