| abstract |
To improve bioavailability of the catalytic metalloporphyrin-based SOD mimics Mn(III) 5,10,15,20-tetrakis[N-ethylpyridinium-2-yl]porphyrin (MnTE-2-PyP 5+ ) and Mn(III) 5,10,15,20-tetrakis[N,N′-diethylimidazolium-2-yl]porphyrin (MnTDE-2-ImP 5+ ), three new Mn(III) porphyrins, bearing oxygen atoms within side chains, were synthesized and characterized: Mn(III) 5,10,15,20-tetrakis[N-(2-methoxyethyl)pyridinium-2-yl]porphyrin (MnTMOE-2-PyP 5+ ), Mn(III) 5,10,15,20-tetrakis[N-methyl-N′-(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTM,MOE-2-ImP 5+ ) and Mn(III) 5,10,15,20-tetrakis[N,N′-di(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTDMOE-2-ImP 5+ ). The catalytic rate constants for O 2 ′ − dismutation (and the related metal-centered redox potentials vs NHE) for the new compounds are: log k cat =8.04 (E 1/2 =+251 mV) for MnTMOE-2-PyP 5+ , log k cat =7.98 (E 1/2 =+356 mV) for MnTM,MOE-2-ImP 5+ and log k cat =7.59 (E 1/2 =+365 mV) for MnTDMOE-2-ImP 5+ . At 30 μM levels none of the new compounds were toxic, and allowed SOD-deficient E. coli to grow nearly as well as wild type. At 3 μM levels, the MnTDMOE-2-ImP 5+ , bearing an oxygen atom within each of the eight side chains, was the most effective and offered much higher protection than MnTE-2-PyP 5+ ′ while MnTDE-2-ImP 5+ was inefficient. These new porphyrins were compared to Mn(III) N-alkylpyridylporphyrins. While longer-chain n-alkyl members of the series exerted toxicity at higher concentration levels, they were very effective at submicromolar levels. Thus, 0.3 μM Mn(III) tetrakis(N-n-hexyl-pyridinum-2-yl)porphyrin and its n-octyl analogue offered the same level of protection as did ≧10 μM methyl and ethyl porphyrins. The k cat of methyl and n-octyl porphyrins are identical, but n-octyl is ˜10-fold more lipophilic. Therefore, the 30-fold improvement in bioavailability appears to be due to the increase in lipophilicity. MnTDMOE-2-ImP 5+ and longer-chain Mn(III) N-alkylpyridylporphyrins may offer better treatment for oxidative stress injuries than the previously studied MnTE-2-PyP 5+ and MnTDE-2-ImP 5+ . |