Spasojevic Ivan, Batinic-Haberle Ines, Reboucas Julio S, Idemori Ynara Marina, Fridovich Irwin
Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA.
J Biol Chem. 2003 Feb 28;278(9):6831-7. doi: 10.1074/jbc.M211346200. Epub 2002 Dec 9.
The Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (Mn(III)TE-2-PyP(5+)) is a potent superoxide dismutase (SOD) mimic in vitro and was beneficial in rodent models of oxidative stress pathologies. Its high activity has been ascribed to both the favorable redox potential of its metal center and to the electrostatic facilitation assured by the four positive charges encircling the metal center. Its comparison with the non-alkylated, singly charged analogue Mn(III) beta-octabromo meso-tetrakis(2-pyridyl)porphyrin (Mn(III)Br(8)T-2-PyP(+)) enabled us to evaluate the electrostatic contribution to the catalysis of O(2)() dismutation. Both compounds exhibit nearly identical metal-centered redox potential for Mn(III)/Mn(II) redox couple: +228 mV for Mn(III)TE-2-PyP(5+) and +219 mV versus NHE for Mn(III)Br(8)T-2-PyP(+). The eight electron-withdrawing beta pyrrolic bromines contribute equally to the redox properties of the parent Mn(III)T-2-PyP(+) as do four quaternized cationic meso ortho pyridyl nitrogens. However, the SOD-like activity of the highly charged Mn(III)TE-2-PyP(5+) is >100-fold higher (log k(cat) = 7.76) than that of the singly charged Mn(III)Br(8)T-2-PyP(+) (log k(cat) = 5.63). The kinetic salt effect showed that the catalytic rate constants of the Mn(III)TE-2-PyP(5+) and of its methyl analogue, Mn(III)TM-2-PyP(5+), are exactly 5-fold more sensitive to ionic strength than is the k(cat) of Mn(III)Br(8)T-2-PyP(+), which parallels the charge ratio of these compounds. Interestingly, only a small effect of ionic strength on the rate constant was found in the case of penta-charged para (Mn(III)TM-4-PyP(5+)) and meta isomers (Mn(III)TM-3-PyP(5+)), indicating that the placement of the positive charges in the close proximity of the metal center (ortho position) is essential for the electrostatic facilitation of O(2)() dismutation.
中-四（N-乙基吡啶-2-基）卟啉锰（III）（Mn(III)TE-2-PyP(5+)）在体外是一种有效的超氧化物歧化酶（SOD）模拟物，对氧化应激病理的啮齿动物模型有益。其高活性归因于其金属中心有利的氧化还原电位以及金属中心周围四个正电荷所确保的静电促进作用。将其与未烷基化的单电荷类似物中-β-八溴中-四（2-吡啶基）卟啉锰（III）（Mn(III)Br(8)T-2-PyP(+)）进行比较，使我们能够评估静电对O₂⁻歧化催化的贡献。两种化合物对于Mn(III)/Mn(II)氧化还原对均表现出几乎相同的以金属为中心的氧化还原电位：Mn(III)TE-2-PyP(5+)为 +228 mV，Mn(III)Br(8)T-2-PyP(+)相对于标准氢电极（NHE）为 +219 mV。八个吸电子的β-吡咯溴对母体Mn(III)T-2-PyP(+)的氧化还原性质的贡献与四个季铵化的阳离子中邻位吡啶氮的贡献相同。然而，高电荷的Mn(III)TE-2-PyP(5+)的类SOD活性比单电荷的Mn(III)Br(8)T-2-PyP(+)（log k(cat) = 5.63）高100倍以上（log k(cat) = 7.76）。动力学盐效应表明，Mn(III)TE-2-PyP(5+)及其甲基类似物Mn(III)TM-2-PyP(5+)的催化速率常数对离子强度的敏感度比对Mn(III)Br(8)T-2-PyP(+)的k(cat)恰好高5倍，这与这些化合物的电荷比平行。有趣的是，对于五电荷的对位异构体（Mn(III)TM-4-PyP(5+)）和间位异构体（Mn(III)TM-3-PyP(5+)），仅发现离子强度对速率常数有很小的影响，这表明正电荷在金属中心（邻位）附近的位置对于O₂⁻歧化的静电促进作用至关重要。
