ROLE OF QUINONE-IRON(III) INTERACTION IN NADPH-DEPENDENT ENZYMATIC GENERATION OF HYDROXYL RADICALS

To study the effect of chelation of iron ions by quinones on the generation of OH radicals in biological redox systems, we have synthesized quinones that can form complexes with Fe(III) ions: 2-phenyl-4-(butylamino)naphtho[2,3-h]quinoline-7,12-dione (Q(c)b) and 2-phenyl-4-(octylamino)naphtho[2,3-h]quinoline-7,12-dione (Q(c)o). A quinone with a similar structure without chelating group was synthesized as a control sample: 2-phenyl-5-nitronaphtho[2,3-g]indole-6,11-dione (Q(n)). Using optical spectroscopy, we determined the stability constant of Q(c)b with Fe(III) [K(s) = (7 +/- 1) x 10(18) M-3] and the stoichiometry of the complex Fe(Q(c)b)3 in chloroform solutions. One-electron reduction potentials of Q(c)b, Q(n), and adriamycin in dimethyl sulfoxide were measured by cyclic voltammetry. In the presence of Fe(III) the one-electron reduction potentials shifted toward positive values by 0.16 and 0.1 V for Q(c)b and adriamycin, respectively. Using the spin trap 5,5'-dimethyl-1-pyroline N-oxide (DMPO) and EPR, it was found that Q(c)b in the Fe(III) complex stimulated the formation of OH radicals in the enzymatic system consisting of NADPH and NADPH-cytochrome P-450 reductase more efficiently than adriamycin and quinone Q(n). This is indicated by the absence of a lag period in the spin adduct appearance for Q(c)b and by a significantly higher rate of the spin adduct production, as well as by a larger absolute concentration of the spin adduct obtained for Q(c)b in comparison with Q(n) in the presence of Fe(III). The results obtained allow one to explain the efficient stimulation of oxygen radicals by Q(c)b in the presence of Fe(III) in enzymatic systems by intramolecular electron transfer in the semiquinone of the Q(c)b - Fe(III) complex.