Protonation of two adjacent tyrosine residues influences the reduction of cytochrome c by diphenylacetaldehyde: a possible mechanism to select the reducer agent of heme iron

UV-light effects on cytochrome c modulated by the aggregation state of phenothiazines

The present study shows the factors that modulate the photodamage promoted by phenothiazines. Cytochrome c was irradiated with UV light for 120 min, over a pH range from 4.0 to 8.0, in the absence and in the presence of different concentrations of thioridazine (TR) and fluphenazine (FP). In the absence of phenothiazines, the maximal rate of a Soret band blue shift (nm/min) from 409 to 406 nm was obtained at pH 4.0 (0.028 nm/min). The presence of phenothiazines at the concentration range 10-25 µmol/L amplified and accelerated a cytochrome c blue shift (409 to 405 nm, at a rate = 0.041 nm/min). Above 25 µmol/L, crescent concentrations of phenothiazines contributed to cytochrome c protection with (maximal at 2500 µmol/L). Scanning electronic microscopy revealed the formation of nanostructures. The pH also influenced the effect of low phenothiazine concentrations on cytochrome c. Thus, the predominance of phenothiazine-promoted cytochrome c damage or protection depends on a balance of the following factors: the yield of photo-generated drug cation radicals, which is favored by acidic pH; the stability of the cation radicals, which is favored by the drug aggregation; and the cytochrome c structure, modulated by the pH.

Ferricytochrome (c) directly oxidizes aminoacetone to methylglyoxal, a catabolite accumulated in carbonyl stress

Age-related diseases are associated with increased production of reactive oxygen and carbonyl species such as methylglyoxal. Aminoacetone, a putative threonine catabolite, is reportedly known to undergo metal-catalyzed oxidation to methylglyoxal, NH4(+) ion, and H2O2 coupled with (i) permeabilization of rat liver mitochondria, and (ii) apoptosis of insulin-producing cells. Oxidation of aminoacetone to methylglyoxal is now shown to be accelerated by ferricytochrome c, a reaction initiated by one-electron reduction of ferricytochrome c by aminoacetone without amino acid modifications. The participation of O2(•-) and HO (•) radical intermediates is demonstrated by the inhibitory effect of added superoxide dismutase and Electron Paramagnetic Resonance spin-trapping experiments with 5,5'-dimethyl-1-pyrroline-N-oxide. We hypothesize that two consecutive one-electron transfers from aminoacetone (E0 values = -0.51 and -1.0 V) to ferricytochrome c (E0 = 0.26 V) may lead to aminoacetone enoyl radical and, subsequently, imine aminoacetone, whose hydrolysis yields methylglyoxal and NH4(+) ion. In the presence of oxygen, aminoacetone enoyl and O2(•-) radicals propagate aminoacetone oxidation to methylglyoxal and H2O2. These data endorse the hypothesis that aminoacetone, putatively accumulated in diabetes, may directly reduce ferricyt c yielding methylglyoxal and free radicals, thereby triggering redox imbalance and adverse mitochondrial responses.

Topography of tyrosine residues and their involvement in peroxidation of polyunsaturated cardiolipin in cytochrome c/cardiolipin peroxidase complexes

Formation of cytochrome c (cyt c)/cardiolipin (CL) peroxidase complex selective toward peroxidation of polyunsaturated CLs is a pre-requisite for mitochondrial membrane permeabilization. Tyrosine residues - via the generation of tyrosyl radicals (Tyr) - are likely reactive intermediates of the peroxidase cycle leading to CL peroxidation. We used mutants of horse heart cyt c in which each of the four Tyr residues was substituted for Phe and assessed their contribution to the peroxidase catalysis. Tyr67Phe mutation was associated with a partial loss of the oxygenase function of the cyt c/CL complex and the lowest concentration of H(2)O(2)-induced Tyr radicals in electron paramagnetic resonance (EPR) spectra. Our MS experiments directly demonstrated decreased production of CL-hydroperoxides (CL-OOH) by Tyr67Phe mutant. Similarly, oxidation of a phenolic substrate, Amplex Red, was affected to a greater extent in Tyr67Phe than in three other mutants. Tyr67Phe mutant exerted high resistance to H(2)O(2)-induced oligomerization. Measurements of Tyr fluorescence, hetero-nuclear magnetic resonance (NMR) and computer simulations position Tyr67 in close proximity to the porphyrin ring heme iron and one of the two axial heme-iron ligand residues, Met80. Thus, the highly conserved Tyr67 is a likely electron-donor (radical acceptor) in the oxygenase half-reaction of the cyt c/CL peroxidase complex.

Spectroscopic, structural, and functional characterization of the alternative low-spin state of horse heart cytochrome C

The alternative low-spin states of Fe(3+) and Fe(2+) cytochrome c induced by SDS or AOT/hexane reverse micelles exhibited the heme group in a less rhombic symmetry and were characterized by electron paramagnetic resonance, UV-visible, CD, magnetic CD, fluorescence, and Raman resonance. Consistent with the replacement of Met(80) by another strong field ligand at the sixth heme iron coordination position, Fe(3+) ALSScytc exhibited 1-nm Soret band blue shift and epsilon enhancement accompanied by disappearance of the 695-nm charge transfer band. The Raman resonance, CD, and magnetic CD spectra of Fe(3+) and Fe(2+) ALSScytc exhibited significant changes suggestive of alterations in the heme iron microenvironment and conformation and should not be assigned to unfold because the Trp(59) fluorescence remained quenched by the neighboring heme group. ALSScytc was obtained with His(33) and His(26) carboxyethoxylated horse cytochrome c and with tuna cytochrome c (His(33) replaced by Asn) pointing out Lys(79) as the probable heme iron ligand. Fe(3+) ALSScytc retained the capacity to cleave tert-butylhydroperoxide and to be reduced by dithiothreitol and diphenylacetaldehyde but not by ascorbate. Compatible with a more open heme crevice, ALSScytc exhibited a redox potential approximately 200 mV lower than the wild-type protein (+220 mV) and was more susceptible to the attack of free radicals.

Divalent metal requirements for catalysis and stability of the RNA triphosphatase from Trypanosoma cruzi

RNA triphosphatases act in the first step of the mRNA capping process, removing the gamma-phosphoryl group from the 5' end of nascent RNA. A metal-dependent catalysis is found in the enzymes from trypanosomes and several other lower eukaryotes. This contrasts with the cysteine-dependent activity of the corresponding enzymes of mammals, a difference that points to these enzymes as potential targets for drug design. This work describes the identification, expression, purification, enzyme kinetics, and the role of divalent metal in the ATPase activity of the RNA triphosphatase from Trypanosoma cruzi, the agent of Chagas' disease, and compares it with the previously characterized enzyme from Trypanosoma brucei. Sequence similarity of the T. cruzi enzyme with the RNA triphosphatase of Saccharomyces cerevisiae indicates that a tunnel domain containing the divalent metal forms its active site. Based on enzyme kinetics, circular dichroism, and intrinsic fluorescence analysis, a kinetic mechanism for the ATPase activity of the T. cruzi tunnel triphosphatase is proposed. A single metal is sufficient to interact with the enzyme through the formation of a productive MnATP-enzyme complex, while free ATP inhibits activity. Manganese is also required for the tunnel stability of the T. cruzi enzyme, while the T. brucei homologue remains stable in the absence of metal, as shown for other triphosphatases. These findings may be useful to devise specific triphosphatase inhibitors to the T. cruzi enzyme.

Reaction route control by microperoxidase-9/CTAB micelle ratios

Microperoxidases (MP) as water-soluble models attract interest to studying the reaction mechanism of peroxidases because these heme peptides are able to form the same enzyme intermediates during the reaction with peroxides. In this work we have demonstrated that the association of Fe(III)MP-9 and Fe(III)MP-11 with CTAB micelles (MP-9/CTAB and MP11/CTAB) provides a microenvironment with an alkaline interface and a hydrophobic core that exhibits peroxidase behavior. This microenvironment shifts positively the redox potential of microperoxidases by approximately 100 mV. tert-Butylhydroperoxide (t-BuOOH) when added to the medium, converted Fe(III)MP-9/CTAB to MP-9/CTAB Compound II, a high valence oxidized intermediate of the heme peptide. Subsequent addition of diphenylacetaldehyde (DPAA) to MP-9/CTAB Compound II regenerated the native form of the enzyme, Fe(III)MP-9/CTAB, what characterizes the occurrence of a peroxidase cycle. Fe(III)MP-9/CTAB regenerated during the peroxidase cycle reacted with residual DPAA in the medium to form Fe(II)MP-9/CTAB, which indicates that both Fe(III)MP-9/CTAB and its oxyferryl form can use aldehydes as reducing agents. According to the determined reduction potential, Fe(III)MP-9 and Fe(III)MP-9/CTAB should be able to oxidize DPAA (reduction potential -630 mV). The reaction of MP-9/CTAB with DPAA produced benzophenone as final product, detected by infrared spectroscopy and mass spectrometry. Interestingly, a significant difference was observed in the benzophenone yield according to the micelle/MP-9 molar ratio.

Changes in the spin state and reactivity of cytochrome C induced by photochemically generated singlet oxygen and free radicals

This work compares the effect of photogenerated singlet oxygen (O(2)((1)Delta(g))) (type II mechanism) and free radicals (type I mechanism) on cytochrome c structure and reactivity. Both reactive species were obtained by photoexcitation of methylene blue (MB(+)) in the monomer and dimer forms, respectively. The monomer form is predominant at low dye concentrations (up to 8 microm) or in the presence of an excess of SDS micelles, while dimers are predominant at 0.7 mm SDS. Over a pH range in which cytochrome c is in the native form, O(2) ((1)Delta(g)) and free radicals induced a Soret band blue shift (from 409 to 405 nm), predominantly. EPR measurements revealed that the blue shift of the Soret band was compatible with conversion of the heme iron from its native low spin state to a high spin state with axial symmetry (g approximately 6.0). Soret band bleaching, due to direct attack on the heme group, was only detected under conditions that favored free radical production (MB(+) dimer in SDS micelles) or in the presence of a less structured form of the protein (above pH 9.3). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of the heme group and the polypeptide chain of cytochrome c with Soret band at 405 nm (cytc405) revealed no alterations in the mass of the cytc405 heme group but oxidative modifications on methionine (Met(65) and Met(80)) and tyrosine (Tyr(74)) residues. Damage of cytc405 tyrosine residue impaired its reduction by diphenylacetaldehyde, but not by beta-mercaptoethanol, which was able to reduce cytc405, generating cytochrome c Fe(II) in the high spin state (spin 2).