Fluorescence quenching by metal centered porphyrins and poryphyrin enzymes.

Fluorescence spectroscopy and microscopy have been used extensively to monitor biomolecules, especially reactive oxygen species (ROS) and, more recently, reactive sulfide (RSS) species. Nearly all fluorophores are either excited by or emit light between 450 and 550 nm, which is similar to the absorbance of heme proteins and metal-centered porphyrins. Here we examined the effects of catalase (Cat), reduced and oxidized hemoglobin (Hb and metHb), albumin (alb), manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP), iron protoporphyrin IX (hemin), and copper protoporphyrin IX (CuPPIX) on the fluorescence properties of fluorescein. We also examined the effects of catalase and MnTBAP on fluorophores for ROS (dichlorofluorescein, DCF), polysulfides (3',6'-di(O-thiosalicyl)fluorescein, SSP4), and H2S (7-azido-4-methylcoumarin, AzMC) previously activated by H2O2, a mixed polysulfide (H2Sn, n = 1-7) and H2S, respectively. All except albumin concentration dependently inhibited fluorophore fluorescence and absorbed light between 450 and 550 nm, suggesting that the inhibitory effect was physical not catalytic. Catalase inhibition of fluorescein fluorescence was unaffected by sodium azide, dithiothreitol, diamide, tris(2-carboxyethyl)phosphine (TCEP), or iodoacetate, supporting a physical inhibitory mechanism. Catalase and TBAP augmented, then inhibited DCF fluorescence, but only inhibited SSP4 and AzMC fluorescence indicative of a substrate-specific catalytic oxidation of DCF and nonspecific fluorescence inhibition of all three fluorophores. These results suggest caution must be exercised when using any fluorescent tracers in the vicinity of metal-centered porphyrins.

Manganese Porphyrin-Based SOD Mimetics Produce Polysulfides from Hydrogen Sulfide.

Manganese-centered porphyrins (MnPs), MnTE-2-PyP5+ (MnTE), MnTnHex-2-PyP5+ (MnTnHex), and MnTnBuOE-2-PyP5+ (MnTnBuOE) have received considerable attention because of their ability to serve as superoxide dismutase (SOD) mimetics thereby producing hydrogen peroxide (H2O2), and oxidants of ascorbate and simple aminothiols or protein thiols. MnTE-2-PyP5+ and MnTnBuOE-2-PyP5+ are now in five Phase II clinical trials warranting further exploration of their rich redox-based biology. Previously, we reported that SOD is also a sulfide oxidase catalyzing the oxidation of hydrogen sulfide (H2S) to hydrogen persulfide (H2S2) and longer-chain polysulfides (H2Sn, n = 3-7). We hypothesized that MnPs may have similar actions on sulfide metabolism. H2S and polysulfides were monitored in fluorimetric assays with 7-azido-4-methylcoumarin (AzMC) and 3',6'-di(O-thiosalicyl)fluorescein (SSP4), respectively, and specific polysulfides were further identified by mass spectrometry. MnPs concentration-dependently consumed H2S and produced H2S2 and subsequently longer-chain polysulfides. This reaction appeared to be O2-dependent. MnP absorbance spectra exhibited wavelength shifts in the Soret and Q bands characteristic of sulfide-mediated reduction of Mn. Taken together, our results suggest that MnPs can become efficacious activators of a variety of cytoprotective processes by acting as sulfide oxidation catalysts generating per/polysulfides.

Metabolism of hydrogen sulfide (H2S) and Production of Reactive Sulfur Species (RSS) by superoxide dismutase.

Reactive sulfur species (RSS) such as H2S, HS•, H2Sn, (n = 2-7) and HS2•- are chemically similar to H2O and the reactive oxygen species (ROS) HO•, H2O2, O2•- and act on common biological effectors. RSS were present in evolution long before ROS, and because both are metabolized by catalase it has been suggested that "antioxidant" enzymes originally evolved to regulate RSS and may continue to do so today. Here we examined RSS metabolism by Cu/Zn superoxide dismutase (SOD) using amperometric electrodes for dissolved H2S, a polysulfide-specific fluorescent probe (SSP4), and mass spectrometry to identify specific polysulfides (H2S2-H2S5). H2S was concentration- and oxygen-dependently oxidized by 1µM SOD to polysulfides (mainly H2S2, and to a lesser extent H2S3 and H2S5) with an EC50 of approximately 380µM H2S. H2S concentrations > 750µM inhibited SOD oxidation (IC50 = 1.25mM) with complete inhibition when H2S > 1.75mM. Polysulfides were not metabolized by SOD. SOD oxidation preferred dissolved H2S over hydrosulfide anion (HS-), whereas HS- inhibited polysulfide production. In hypoxia, other possible electron donors such as nitrate, nitrite, sulfite, sulfate, thiosulfate and metabisulfite were ineffective. Manganese SOD also catalyzed H2S oxidation to form polysulfides, but did not metabolize polysulfides indicating common attributes of these SODs. These experiments suggest that, unlike the well-known SOD-mediated dismutation of two O2•- to form H2O2 and O2, SOD catalyzes a reaction using H2S and O2 to form persulfide. These can then combine in various ways to form polysulfides and sulfur oxides. It is also possible that H2S (or polysulfides) interact/react with SOD cysteines to affect catalytic activity or to directly contribute to sulfide metabolism. Our studies suggest that H2S metabolism by SOD may have been an ancient mechanism to detoxify sulfide or to regulate RSS and along with catalase may continue to do so in contemporary organisms.

Catalase as a sulfide-sulfur oxido-reductase: An ancient (and modern?) regulator of reactive sulfur species (RSS).

Catalase is well-known as an antioxidant dismutating H2O2 to O2 and H2O. However, catalases evolved when metabolism was largely sulfur-based, long before O2 and reactive oxygen species (ROS) became abundant, suggesting catalase metabolizes reactive sulfide species (RSS). Here we examine catalase metabolism of H2Sn, the sulfur analog of H2O2, hydrogen sulfide (H2S) and other sulfur-bearing molecules using H2S-specific amperometric electrodes and fluorophores to measure polysulfides (H2Sn; SSP4) and ROS (dichlorofluorescein, DCF). Catalase eliminated H2Sn, but did not anaerobically generate H2S, the expected product of dismutation. Instead, catalase concentration- and oxygen-dependently metabolized H2S and in so doing acted as a sulfide oxidase with a P50 of 20mmHg. H2O2 had little effect on catalase-mediated H2S metabolism but in the presence of the catalase inhibitor, sodium azide (Az), H2O2 rapidly and efficiently expedited H2S metabolism in both normoxia and hypoxia suggesting H2O2 is an effective electron acceptor in this reaction. Unexpectedly, catalase concentration-dependently generated H2S from dithiothreitol (DTT) in both normoxia and hypoxia, concomitantly oxidizing H2S in the presence of O2. H2S production from DTT was inhibited by carbon monoxide and augmented by NADPH suggesting that catalase heme-iron is the catalytic site and that NADPH provides reducing equivalents. Catalase also generated H2S from garlic oil, diallyltrisulfide, thioredoxin and sulfur dioxide, but not from sulfite, metabisulfite, carbonyl sulfide, cysteine, cystine, glutathione or oxidized glutathione. Oxidase activity was also present in catalase from Aspergillus niger. These results show that catalase can act as either a sulfide oxidase or sulfur reductase and they suggest that these activities likely played a prominent role in sulfur metabolism during evolution and may continue do so in modern cells as well. This also appears to be the first observation of catalase reductase activity independent of peroxide dismutation.