Redox modulation of oxidative stress by Mn porphyrin-based therapeutics: The effect of charge distribution

Abstract

We evaluate herein the impact of positive charge distribution on the in vitro and in vivo properties of Mn porphyrins as redox modulators possessing the same overall 5+ charge and of minimal stericity demand: Mn(III) meso-tetrakis(trimethylanilinium-4-yl)porphyrin (MnTTriMAP(5+)), Mn(III) meso-tetrakis(N,N'-dimethylpyrazolium-4-yl)porphyrin (MnTDM-4-PzP(5+)), Mn(III) meso-tetrakis(N,N'-dimethylimidazolium-2-yl)porphyrin (MnTDM-2-ImP(5+)), and the ortho and para methylpyridinium complexes Mn(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MnTM-4-PyP(5+)) and Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin (MnTM-2-PyP(5+)). Both Mn(III)/Mn(II) reduction potential and SOD activity within the series follow the order: MnTTriMAP(5+)<MnTDM-4-PzP(5+)<MnTM-4-PyP(5+)<MnTM-2-PyP(5+)<MnTDM-2-ImP(5+). The kinetic salt effect (KSE) on the catalytic rate constant for superoxide dismutation (k(cat)) indicates that the electrostatic contribution to the O(2)*(-) dismutation is the greatest with MnTM-2-PyP(5+) and follows the order: MnTM-4-PyP(5+)<MnTDM-4-PzP(5+) approximately MnTDM-2-ImP(5+)<MnTM-2-PyP(5+). The KSE observed on k(cat) suggests that the charges are relatively confined within specific regions of the aryl rings. Whereas the charges in imidazolium, pyrazolium, and MnTM-4-PyP(5+) compounds are distributed in-plane with the porphyrin ring, the charges of MnTM-2-PyP(5+) are either above or below the plane, which channels the negatively-charged superoxide toward the axial positions of the Mn porphyrin more efficiently, and leads to the highest KSE. This mimics the tunneling effect observed in the SOD enzymes themselves. The modulation of the reactivity of the Mn center by the electronic perturbations caused by the meso-aryl substituent could be explained by DFT calculation, whereby a correlation between the Mn(III)/Mn(II) reduction potential (and/or SOD activity) and meso-aryl fragment softness descriptors for nucleophilic (s(f)(+)) and radical (s(f)(o)) attacks was observed. MnTDM-4-PzP(5+) and MnTM-4-PyP(5+) did not protect SOD-deficient E. coli grown aerobically, which is in agreement with their low k(cat). MnTM-2-PyP(5+) and MnTDM-2-ImP(5+) have similar high k(cat), but MnTDM-2-ImP(5+) was significantly less protective to E. coli, probably due to its bulkier size, decreased cellular uptake, and/or observed toxicity. The placement of charges closer to the metal center and spatial charge localization increases both the in vitro and the in vivo SOD activity of the compound.

SOD-like activity of Mn(II) β-octabromo-meso-tetrakis(N-methylpyridinium-3-yl)porphyrin equals that of the enzyme itself

“…A. Imlay [33]. The experiments were carried out as described in detail by Rebouças et al [2]. Briefly, cultures were grown aerobically in a five-amino acid medium in 96-well flat-bottom microtiter plates [34] on a thermostatic shaker at 37°C and 200 rpm.…”

“…Thus, Mn III TE-2-PyP 5+ is 130-fold more potent SOD mimic than its singly charged Mn III Br 8 T-2-PyP + [3], and 400-fold more potent than its negatively charged analogue [Mn III Br 8 TSPP] 3− [4]. Charge distribution had also remarkable impact [2]. In summary both metal-centered redox potential and charges in close proximity to the metal site have a major impact on the antioxidant potency of the Mn porphyrin.…”

“…We have been developing Mn porphyrin-based SOD mimics for over a decade. We have established structure activity relationship (SAR) for both Mn and Fe porphyrins [1] between the metalcentered redox potential and the rate constant for O 2 •− dismutation and refined it recently to account for the differently charged/sized Mn porphyrins [2][3][4][5]. The introduction of both electron-withdrawing groups and positive charges onto the porphyrin core affords thermodynamic and kinetic/electrostatic facilitation for the approach of superoxide to the redox-cycling metal site [1][2][3][4].…”

“…In summary both metal-centered redox potential and charges in close proximity to the metal site have a major impact on the antioxidant potency of the Mn porphyrin. As our knowledge on porphyrins increases it became obvious that another major factor is their in vivo availability [2,4,24,25]. The optimal combination of thermodynamics, kinetics, bioavailability and low toxicity will determine the best drug candidate.…”

“…Para isomer was toxic due to its interactions with nucleic acids [26]. Guided by these collective in vitro, in vivo, and SAR results [1][2][3][4][5]12,[24][25][26], we resumed herein our studies on the meta Mn(III) Nalkylpyridylporphyrins by preparing a brominated derivative of Mn III TM-3-PyP 5+ . This is the first report on the synthesis and characterization of Mn II Br 8 TM-3-PyP 4+ (Fig.…”