A potent superoxide dismutase mimic: manganese beta-octabromo-meso-tetrakis-(N-methylpyridinium-4-yl) porphyrin

The ortho effect makes manganese(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin a powerful and potentially useful superoxide dismutase mimic

The ortho, meta, and para isomers of manganese(III) 5,10,15, 20-tetrakis(N-methylpyridyl)porphyrin, MnTM-2-PyP5+, MnTM-3-PyP5+, and MnTM-4-PyP5+, respectively, were analyzed in terms of their superoxide dismutase (SOD) activity in vitro and in vivo. The impact of their interaction with DNA and RNA on the SOD activity in vivo and in vitro has also been analyzed. Differences in their behavior are due to the combined steric and electrostatic factors. In vitro catalytic activities are closely related to their redox potentials. The half-wave potentials (E1/2) are +0.220 mV, +0.052 mV, and +0.060 V versus normal hydrogen electrode, whereas the rates of dismutation (kcat) are 6.0 x 10(7), 4.1 x 10(6), and 3.8 x 10(6) M-1 s-1 for the ortho, meta, and para isomers, respectively. However, the in vitro activity is not a sufficient predictor of in vivo efficacy. The ortho and meta isomers, although of significantly different in vitro SOD activities, have fairly close in vivo SOD efficacy due to their similarly weak interactions with DNA. In contrast, due to a higher degree of interaction with DNA, the para isomer inhibited growth of SOD-deficient Escherichia coli.

Molecular actions of a Mn(III)Porphyrin superoxide dismutase mimetic and peroxynitrite scavenger: reaction with nitric oxide and direct inhibition of NO synthase and soluble guanylyl cyclase

Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin (MnTMPyP), described as a superoxide dismutase mimetic and peroxynitrite scavenger, has been used previously to investigate the cytotoxic potential of superoxide and peroxynitrite in several pathological models. Here we report on the interference of MnTMPyP with NO/cGMP signaling using cultured endothelial cells as well as purified soluble guanylyl cyclase (sGC) either activated by the NO donor 2,2-diethyl-1-nitroso-oxyhydrazine sodium salt (DEA/NO) or reconstituted with nitric oxide synthase (NOS). MnTMPyP inhibited endothelial cGMP accumulation induced by A23187 (0.3 microM) with an IC50 of 75.0 +/- 10.4 microM but had no significant effect on the potency of the Ca2+ ionophore. Purified NOS was inhibited by MnTMPyP (IC50 = 5.5 +/- 0.8 microM) because of an interference of the Mn-porphyrin with the reductase domain of the enzyme. The most pronounced actions of MnTMPyP were direct inhibition of sGC and scavenging of NO. Purified sGC stimulated with either Ca2+/calmodulin-activated NOS (in the presence of GSH) or DEA/NO (in the absence of GSH) was inhibited with IC50 values of 0.8 +/- 0.09 microM and 0.6 +/- 0.2 microM, respectively. In the presence of GSH, MnTMPyP was reduced to the Mn(II) complex, resulting in efficient scavenging of NO under these conditions. Our data demonstrate that MnTMPyP (i) interferes with the reductase domain of NOS, (ii) scavenges NO in the presence of GSH, and (iii) is a potent direct inhibitor of sGC. These results cast doubt on the usefulness of MnTMPyP and related Mn-porphyrin complexes as probes to study the involvement of peroxynitrite/superoxide in biological systems.

The dark side of dioxygen biochemistry

The cellular biochemistry of dioxygen is Janus-faced. The good side includes numerous enzyme-catalyzed reactions of dioxygen that occur in respiration and normal metabolism, while the dark side encompasses deleterious reactions of species derived from dioxygen that lead to damage of cellular components. These reactive oxygen species have historically been perceived almost exclusively as agents of the dark side, but it has recently become clear that they play beneficial roles as well.