We have previously shown that the Fe (II) tetrakis-N, N, N′, N′-(2-pyridylmethyl) ethylenediamine complex (Fe (II) TPEN) has high superoxide dismutase (SOD) activity, using the xanthine–xanthine oxidase–cytochrome c method (cyt. c method)[J. Biol. Chem. 264 (1989) 9243–9249]. X-ray analysis showed that Fe (II) TPEN has two different coordination structures, one in which Fe (II) is coordinated with six nitrogens of TPEN and ...