Nitric oxide transfer from the NO-donor S-nitroso- N-acetylpenicillamine to monomers and dimers of water-soluble iron-porphyrins

Abstract

The nitrosylation of two water-soluble iron-porphyrins, the anionic Fe(III)-meso-tetrakis( p-sulfonatophenyl)porphyrin (FeTPPS 4) and the cationic Fe(III)-meso-tetrakis(4- N-methylpyridiniumyl)porphyrin (FeTMPyP), by the nitric oxide donor S-nitroso- N-acetylpenicillamine (SNAP) was studied using optical absorption spectroscopy. The influence of ionic and non-ionic micelles on rates of nitric oxide transfer was investigated. Initially, the effect of the micelles on the pH-dependent equilibrium between monomeric and μ-oxo-dimeric species of the iron-porphyrins was examined. It is not affected in micelle–porphyrin systems with electric charges identical in sign. The non-ionic micelles of polidocanol induce a small negative pK shift. In contrast, the dimerization equilibrium of porphyrins in oppositely charged micellar phases is displaced to lower pH by ∼2 units. Nitric oxide binding to monomers and μ-oxo-dimers was examined at pH 5.0 and 8.0, respectively. Contrary to nitrosylation by NO gas, SNAP induces reductive nitrosylation. There was no observed NO-Fe(III)porphyrin intermediate. Nitrosylation rates were obtained and compared in aqueous buffer and in micellar systems. Monomers nitrosylate much faster than μ-oxo-dimers. Oppositely charged micelles prevent nitrosylation of the iron-porphyrins or considerably enhance nitrosylation times. Nitrosylation rates are comparable to transnitrosylation rates between several S-nitrosothiols and thiol-containing proteins, suggesting biological relevance for the process.