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Mn porphyrins redox-based inactivation of NF-κB transcription factor, a major regulator of inflammatory and immune responses

The most efficacious Mn(III) porhyrins (MnPs) have the ability to both eliminate reactive oxygen and nitrogen species and impact the progression of oxidative stress-dependent signaling events, affecting therefore the regulation of redox-dependent transcription factors and the suppression of secondary inflammatory- and oxidative stress-mediated immune responses. While the prevailing mechanistic view of the suppression of transcription factors activation is via anti-oxidative action (presumably in cytosol), the pro-oxidative action of MnPs in suppressing NF-􀀀B activation in nucleus has been proposed. The reducing nuclear environment assures that cysteine 62 residue of p50 NF-􀀀B subunit is in reduced form, which is critical for p50 DNA binding and NF-􀀀B activation. Based on previous work, we hypothesize that MnIIIP can oxidize thiol group of cysteine 62. We performed the pH dependent study of glutathione oxidation by MnP in aqueous solution. The pKa of glutathione is = 9.2. The reaction is slow at pH 7.8 where the protonated thiol is a reacting species. At higher pH, where there is a significant contribution of reactive thiolate, a reaction is much faster (Fig. 1A). Many proteins have domains that lower the average pKa of cysteine thiols significantly, so that under physiological conditions thiolate anion is a reactive species. Under such conditions (which would resemble the oxidation of GSH at higher pH) MnIIIP would readily oxidize cysteines in vivo. The electrostatic and thermodynamic effects of Mn porphyrins and alike compounds on the inhibition of p50 DNA binding were studied in a cell-free system (Fig. 1B). As the thiolate is presumably the reactive species of p50 cysteine, the positive charges on the porphyrins would favor the approach and thus the oxidation of cysteine, while the negatively charged compounds would be repelled from p50. The data clearly showed that compounds that are anionic or neutral (MnTBAP3-, MnhematoP-, Mn(salen)+ - EUK-8 and MnCl2) do not prevent NF-􀀀B p50 DNA binding, while all cationic porphyrins are suppressive (MnTM-3- PyP5+, MnTnHex-2-PyP5+, MnTE-2-PyP5+, MnTnBu-2-PyP5+, and MnTMOE-2-PyP5+).

Mn porphyrins; transcriptional activity; NF-kB

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