The different biological behavior of cationic Fe and Mn pyridylporphyrins in Escherichia coil 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, E(1/)2 for (MP)-P-III/(MP)-P-II redox couple (M = Fe, Mn, P = porphyrin), k(cat) for the catalysis of O-2(center dot-) stability toward 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(111) porphyrins orders of magnitude more acidic than the analogous Mn(BI) porphyrins, as revealed by the pK(at) of axially coordinated waters. The 5 log units difference in the acidity between the Mn and Fe sites in polphyrin translates into the predominance of tetracationic (OH)(H2O)FeP complexes relative to pentacationic (H2O)(2)MnP species at pH This is additionally evidenced in large differences in the E-1/2 values of M-III/(MP)-P-II redox couples. 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 Of dismutation (log kw) between Fe and Mn porphyrins is modest, 2.5-5fold, due to predominantly outer-sphere, with partial inner-sphere character of two reaction steps. However, the rate constant for the innersphere H2O2 -based potphyrin oxidative degradation is 18 fold larger for (OH)(H2O)FeP than for (H20)2MnP. The in vivo consequences of the differences between the Fe and Mn polphyrins were best demonstrated in SOD-deficient E. coil growth. On the basis of fairly similar log k(cat)(O-2(center dot-)) values, a very similar effect on the growth of SOD-deficient E. coil was anticipated by both metalloporphyrins. Yet, while (H2O)(2)MnTE-2-PyP5+ was fully efficacious at >= 20 mu M, the Fe analogue (OH)(H2O)FeTE-2-PyP4+ supported SOD-deficient E. coil growth at as much as 200-fold lower doses in the range of 0.1-1 mu M. Moreover the pattern of SOD-deficient E. coil growth was different with Mn and Fe polphyrins. Such results suggested a different mode of action of these metallopotphyrins. Further exploration demonstrated that (1) 0.1 mu M (OH)(H2O)FeTE-2-Pyr(4+) provided similar growth stimulation as the 0.1 mu M Fe salt, while the 20 mu M Mn salt provides no protection to E. coli; and (2) 1 pM Fe porphyrin is fully degraded by 12 h in E. coil cytosol and growth medium, while Mn potphyrin is not. Stimulation of the aerobic growth of SOD deficient E colt by the Fe porphyrin is therefore due to iron acquisition. Our data suggest that in vivo, redox-driven degradation of Fe potphyrins resulting in Fe release plays a major role in their biological action. Possibly, iron reconstitutes enzymes bearing [4Fe-4S] dusters as active sites. Under the same experimental conditions, (OH)(H20)FePs do not cause mouse arterial hypotension, whereas (1420)2MnPs do, which greatly limits the application of Mn porphyrins in vivo.