Differential Coordination Demands in Fe versus Mn Water-Soluble Cationic Metalloporphyrins Translate into Remarkably Different Aqueous Redox Chemistry and Biology

Abstract

The different biological behavior of cationic Fe and Mn pyridylporphyrins in Escherichia coli and mouse studies prompted us to revisit and compare their chemistry. For that purpose the series of ortho and meta isomers of Fe(III) meso-tetrakis-N-alkylpyridylporphyrins, alkyl being methyl to n-octyl, were synthesized and characterized by elemental analysis, UV/vis spectroscopy, mass spectrometry, lipophilicity, protonation equilibria of axial waters, metal-centered reduction potential, E1/2 for MIIIP/MIIP redox couple (M = Fe, Mn, P=porphyrin), kcat for the catalysis of O2•− dismutation, stability towards peroxide-driven porphyrin oxidative degradation (produced in the catalysis of ascorbate oxidation by MP), ability to affect growth of SOD-deficient E. coli and toxicity to mice. Electron-deficiency of the metal site is modulated by the porphyrin ligand, which renders Fe(III) porphyrins ≥ 5 orders of magnitude more acidic than the analogous Mn(III) porphyrins, as revealed by the pKa1 of axially coordinated waters. The 5 log units difference in the acidity between the Mn and Fe sites in porphyrin translates into the predominance of tetracationic (OH)(H2O)FeP complexes relative to pentacationic (H2O)2MnP species at pH ~7.8. This is evidenced in large differences in the thermodynamic parameters - pKa of axial waters and E1/2 of MIII/MII redox couple. The presence of hydroxo ligand labilizes trans-axial water which results in higher reactivity of Fe- relative to Mn center. The differences in the catalysis of O2•− dismutation (log kcat) between Fe and Mn porphyrins is modest, 2.5-5-fold, due to predominantly outer-sphere, with partial inner-sphere character of two reaction steps. However, the rate constant for the inner-sphere H2O2-based porphyrin oxidative degradation is 18-fold larger for (OH)(H2O)FeP than for (H2O)2MnP. The in vivo consequences of the differences between the Fe- and Mn porphyrins were best demonstrated in SOD-deficient E. coli growth. Based on fairly similar log kcat(O2.− values, very similar effect on the growth of SOD-deficient E. coli was anticipated by both metalloporphyrins. Yet, while MnTE-2-PyP5+ was fully efficacious at ≥20 μM, the Fe analog, FeTE-2-PyP5+ supported SOD-deficient E. coli growth at 200-fold lower doses in the range of 0.1 to 1 μM. Moreover the pattern of SOD-deficient E. coli growth was different with Mn- and Fe porphyrins. Such results suggested different mode of action of these metalloporphyrins. Further exploration demonstrated that: (1) 0.1 μM FeTE-2-PyP5+ provided similar growth stimulation as 0.1 μM Fe salt, while 20 μM Mn salt provides no protection to E. coli; and (2) 1 μM Fe porphyrin is fully degraded by 12 hours in E. coli cytosol and growth medium; while Mn porphyrin is not. Stimulation of the aerobic growth of SOD-deficient E. coli by the Fe porphyrin is therefore due to iron acquisition. Our data suggest that in vivo, redox-driven degradation of Fe porphyrins resulting in Fe release plays a major role in their biological action. Possibly, iron reconstitutes enzym...

Simple Biological Systems for Assessing the Activity of Superoxide Dismutase Mimics

“…(ii) The growth curve of sodA sodB + FeP should be similar to the growth curve of parental E. coli. The data (122) indicate that at such low concentrations, FePs act in an identical way as simple Fe salts, thus supporting metabolism by providing Fe as an Fe source.…”

“…For example, the k cat (O 2 -) of the meta isomers (alkyl side chain varying from methyl to octyl) is an order of magnitude lower than k cat (O 2 -) of the corresponding ortho isomers, and their redox potentials differ significantly. E 1/2 of the meta MnP compounds falls in the region of 52-74 mV versus NHE, while for the ortho analogs, it is in the range from 220 to 367 mV versus NHE, depending on the nature of the alkyl side chains (11,122). Nevertheless, both ortho and meta analogs efficiently substituted for SOD when tested on sodA sodB E. coli (Fig.…”

“…The meta ethyl has similar R f as meta methoxyethyl, and the meta hexyl has R f similar to meta methoxyhexyl (inset) (123). (19,122), the in vivo FeP-based protection could have ensued from both peroxynitrite decomposition and superoxide dismutation (17,120 (19) (Fig. 14).…”

SOD Therapeutics: Latest Insights into Their Structure-Activity Relationships and Impact on the Cellular Redox-Based Signaling Pathways

“…With E 1/2 of + 228 mV versus NHE (* + 300 mV vs. NHE for SOD enzyme), MnTE-2-PyP 5 + reduces and oxidizes O 2 $ -with nearly identical rate constants as does the SOD (31,32). Such a design later led to imidazolyl analog, MnTDE-2-ImP 5 + (26-28, 131, 211, 225, 239), and to a series of Fe porphyrins (FePs) (30,146,172,173,193,207,214,215,244,259,273) (see also under Drug Design section). Further, the increased bioavailability and reduced toxicity was achieved through modifications of the substituents in ortho positions (216,274).…”

“…Recently, we synthesized a series of ortho and meta Fe complexes to understand the intriguing differential in vivo behavior of Fe and MnPs (273). FePs have also been often used in cellular and animal models of oxidative stress (146,172,173,193,207,214,215,244,259 …”