I. Spasojevic et I. Batinic-haberle, Manganese(III) complexes with porphyrins and related compounds as catalytic scavengers of superoxide, INORG CHIM, 317(1-2), 2001, pp. 230-242
Two groups of Mn-based catalytic antioxidants are described in terms of the
ir catalytic activities and electrochemical properties. In the first group,
manganese porphyrins, phthalocyanine, and porphyrazine employ the Mn(III)/
Mn(II) couple in the catalysis of O-2 radical anion dismutation (disproport
ionation). The catalytic rate constant is dependent upon the metal-centered
redox potential, as shown previously for water-soluble Mn porphyrins. The
limitation of this simple relation becomes obvious with compounds of high o
dor potential (+2 metal oxidation state is stabilized) which exhibit a weak
metal/ligand binding; although of high superoxide dismutase (SOD)-like act
ivity, the compounds are not stable under physiological conditions. The sec
ond generation of the potent O(2)radical anion scavengers are manganese com
plexes with biliverdin IX and its derivatives which have an RO- functionali
ty as a fifth coordination to the metal center in a dimeric structure. Such
a coordination pattern stabilizes the +4 oxidation state of the manganese
so that the Mn(III)/Mn(IV) redox (E-1/2 = similar to + 0.46 V vs. NHE) beco
mes coupled to the O(2)radical anion dismutation. More importantly, despite
operating at a high positive metal-centered redox potential and having the
+3 oxidation state as the resting state of the metal center, metallobilive
rdins still retain a high ligand affinity in solution. Independently of the
ir charge (two neutral and the other two negatively charged) metallobiliver
dins studied are of similar SOD-like activity comparable to the efficacy of
highly charged manganese(III) ortho N-alkylpyridylporphyrins. These most p
otent in vitro SOD-like Mn porphyrins are also reactive towards peroxynitri
te, nitric oxide, hydrogen peroxide and oxygen. Since the fifth coordinatio
n site of the metal center is occupied no reactivity of the manganese(III)
biliverdin IX dimethyl ester towards NO. and H2O2 is observed. Thus, mangan
ese(III) porphyrins and manganese(III) biliverdins are expected to differ w
ith regards to their tissue localization and to the type and the concentrat
ion of reactive oxygen species they would encounter in biological systems.
Comparative kinetic and thermodynamic studies of these catalytic antioxidan
ts would help us understand not only the prevalent mode of their in vivo bi
ological action but the mechanism of oxidative stress injuries as well. (C)
2001 Elsevier Science B.V. All rights reserved.