Synthesis and superoxide dismutase activity of novel iron complexes

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 one in which Fe(II) is coordinated with five nitrogens of TPEN and one oxygen of sulfate anion as the counter anion [Chem. Pharm. Bull. 48 (2000) 223–230]. To investigate the relationship between these two structures and SOD activity, we synthesized novel Fe(II) complexes of TPEN analogues and measured their SOD activity by the cyt. c method. The Fe(II) tetrakis-N,N,N′,N′-(2-pyridylmethyl)trimethylenediamine complex (Fe(II)TPTN) and the Fe(II) tris(2-pyridylmethyl)triazacyclononane complex (Fe(II)TPTCN) had no SOD activity (IC50=>100 μM), probably because these two complexes have undistorted steric structure with no easily substituted ligand. On the other hand, other Fe(II) complexes with unsaturated coordination or an easily substituted ligand had high SOD activity (IC50=0.4–20 μM). The results indicate that the substitution reaction of a ligand with superoxide or the coordination of superoxide is essential for Fe(II)TPEN analogue complexes to have SOD activity. Moreover, we examined the effect of steric hindrance of the ligands on the SOD activity and the stability of the iron complexes to oxygen.