The reactions of seven-coordinate [Fe-III(dapsox)(H2O)(2)]ClO4 center dot H2O (1), [Fe-II(H(2)dapsox) (H2O)(2)] (NO3)(2)center dot H2O (2), and [Mn-II(H(2)dapsox)(CH3OH)(H2O)](CIO4)(2)(H2O) (3) complexes of the acyclic and rigid pentadentate H(2)dapsox ligand [H(2)dapsox = 2,6-diacetylpyridinebis(semioxamazide)] with superoxide have been studied spectrophotometrically, electrochemically, and by a submillisecond mixing UV/vis stopped-flow in dimethyl sulfoxide (DMSO). The same studies were performed on the seven-coordinate [Mn-II(Me-2[15]pyridinaneN(5))(H2O)(2)]CI2 center dot H2O (4) complex with the flexible macrocyclic Me-2[15]pyridinaneN(5) ligand (Me-2[15]pyridinaneN(5) = trans-2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14 ,16-triene), which belongs to the class of proven superoxide dismutase (SOD) mimetics. The X-ray crystal structures of 2-4 were determined. All complexes possess pentagonal-bipyramidal geometry with the pentadentate ligand in the equatorial plane and solvent molecules in the axial positions. The stopped-flow experiments in DMSO (0.06% of water) reveal that all four metal complexes catalyze the fast disproportionation of superoxide under the applied experimental conditions, and the catalytic rate constants are found to be (3.7 +/-0.5) x 10(6), (3.9 +/- 0.5) x 10(6), (1.2 +/- 0.3) x 10(7), and (5.3 +/- 0.8) x 10(6) M-1 s(-1) for 1-4, respectively. The cytochrome c McCord-Fridovich (McCF) assay in an aqueous solution at pH = 7.8 resulted in the IC50 values (and corresponding k(mcCF) constants) for 3 and 4, 0.013 +/- 0.001 mu M (1.9 +/- 0.2 x 10(8) M-1 s(-1)) and 0.024 +/- 0.001 mu M (1.1 +/- 0.3 x 10(8) M-1 s(-1)), respectively. IC50 values from a nitroblue tetrazolium assay are found to be 6.45 +/- 0.02 and 1.36 +/- 0.03 mu M for 1 and 4, respectively. The data have been compared with those obtained by direct stopped-flow measurements and discussed in terms of the side reactions that occur under the conditions of indirect assays.