Structural modifications of human beta 2 microglobulin treated with oxygen-derived radicals

Treatment of human beta 2 microglobulin (beta 2m) with defined oxygen-derived species generated by treatment with gamma-radiation was studied. As assessed by SDS/PAGE, the hydroxyl radicals (.OH) caused the disappearance of the protein band at 12 kDa that represents beta 2m, and cross-linked the protein into protein bands stable to both SDS and reducing conditions. However, when .OH was generated under oxygen in equimolar combination with the superoxide anion radical (O2.-), the high-molecular-mass protein products were less represented, and fragmented derivatives were not obviously detectable. Exposure to .OH alone, or to .OH + O2.- in the presence of O2, induced the formation of beta 2m protein derivatives with a more acidic net electrical charge than the parent molecule. In contrast, O2.- alone had virtually no effect on molecular mass or pI. Changes in u.v. fluorescence during .OH attack indicated changes in conformation, as confirmed by c.d. spectrometry. A high concentration of radicals caused the disappearance of the beta-pleated sheet structure and the formation of a random coil structure. Loss of tryptophan and significant production of dityrosine (2,2'-biphenol type) were noted, exhibiting a clear dose-dependence with .OH alone or with .OH + O2.-. The combination of .OH + O2.- induced a pattern of changes similar to that with .OH alone, but more extensive for c.d. and tryptophan oxidation (2 Trp/beta 2m molecule), and more limited for dityrosine formation. Lower levels of these oxidative agents caused the reproducible formation of species at 18 and 25 kDa which were recognized by antibodies against native beta 2m. These findings provide a model for the protein pattern observed in beta 2m amyloidosis described in the literature.

Transient kinetics of the one-electron transfer reaction between reduced flavocytochrome b2 and oxidized cytochrome c. Evidence for the existence of a protein complex in the reaction

The one-electron transfer reaction from reduced flavocytochrome b2 (fully reduced by three electron equivalents) to ferricytochrome c, both purified from the yeast Hansenula anomala, has been studied using stopped-flow spectrophotometry in the course of a single turnover, for reactants initially mixed in a heme molar ratio equal to one. The cytochrome c reduction proceeded to completion through an apparently first-order process. Depending on the experimental conditions (concentrations and or ionic strength), the reduction is of second-order or first-order character. To interpret these kinetic results computer simulation studies have been performed based on a kinetic scheme involving, besides the formation of a complex before the electron transfer step, intramolecular electron transfer steps within flavocytochrome b2 to maintain the concentration of the specific electron donor center, the reduced cytochrome b2. As far as the cytochrome c reduction rate constant, ka, and its variations were concerned the simulated data showed that this complicated scheme could approximate a mechanism which is by far the simplest, involving only the two former steps. Such a scheme accounts firstly for the hyperbolic dependence of the rate of reduction of cytochrome c, ka, upon reductant concentrations which had provided clear evidence for the kinetic existence of a complex in the reaction pathway. At 5 degrees C the rate constant for the electron transfer is 380 s-1 with an activation energy of 13.8kJ mol-1 (3.3 kcal mol-1). Secondly it predicts the observed variations of ka with ionic strength and provides estimates of the rate constants of the binding step.

Flavocytochrome b2-cytochrome c interactions: the electron transfer reaction revisited

This review is concerned with the kinetics and mechanism of electron transfer processes which occur intermolecularly between reduced flavocytochrome b2 and cytochrome c molecules within an encounter complex. Analyses are given of previous reports which aimed at describing the formation of stable complexes obtained at low ionic strength in solution and in the crystalline state with a binding stoichiometry of 1 to 1 heme ratio. Relevant data allow to define the respective role of flavin and heme b2 in the electron transfer towards cytochrome c and give a description of the recognition areas on the two redox partners. The paper also refers to a recent computer model of their postulated interactions as based on the three-dimensional structure of the Saccharomyces cerevisiae single molecules. Special emphasis is given to rapid kinetic investigations of the electron transfer reaction between Hansenula anomala flavocytochrome b2 and cytochrome c studied as a function of concentration, ionic strength and temperature. Data showed that reaction rates were modulated by ionic strength, reaching a saturation behaviour at low ionic strength. In the present paper the temperature effects on Kd and kET have been re-examined. Thermodynamic analysis of the dissociation constant points out the importance of hydrophobic interactions in the complex formation. Analysis of the variations of rate constants in terms of semiclassical theory of electron-transfer reaction yields values of 1.12 eV for the reorganization energy and 0.05 cm-1 for the electronic coupling factor. Interpretation of the electronic coupling in terms of through-bond and/or through-space pathways takes into account the hypothetical model proposed for the binary complex. The functional implications of this model in the electron transfer reaction are discussed. Finally the existence of a conformational equilibrium between the initial binding complex and the complex from which electron transfer occurs is considered.

Reduction of ferric haemoproteins by tetrahydropterins: a kinetic study

We previously showed that one-electron transfer from tetrahydropterins to iron porphyrins is a very general reaction, with formation of an intermediate cation radical similar to the one detected in NO synthase. As a model reaction, the rates of reduction of eight haemoproteins by diMePH4 (6,7-dimethyltetrahydropterin) have been studied and correlated with their one-electron reduction potentials, E(m) (Fe(III)/Fe(II)). On the basis of kinetic data analyses, a bimolecular collisional mechanism is proposed for the electron transfer from diMePH4 to ferrihaemoproteins. Haemoproteins with reduction potentials below -160 mV were shown not to be reduced by diMePH4 to the corresponding ferrohaemoproteins. For haemoproteins with reduction potentials more positive than -160 mV, such as chloroperoxidase, cytochrome b5, methaemoglobin and cytochrome c, there was a good correlation between the second-order reduction rate constant and the redox potential, E(m) (Fe(III)/Fe(II)): [formula: see text]. The rate of reduction of cytochrome c by BH4 [(6R)-5,6,7,8-tetrahydrobiopterin] was determined to be similar to that of the reduction of cytochrome c by diMePH4. These results confirm the role of tetrahydropterins as one-electron donors to Fe(III) porphyrins.

Molecular characteristics of cytochrome b558 isolated from human granulocytes, monocytes and HL60 and U937 cells differentiated into monocyte/macrophages

Enriched cytochrome b558 preparations were obtained from human mature monocytes (MN) and retinoic acid plus interferon gamma induced human myeloid leukemia cell lines HL-60 and U937, using an adaptation of the procedure described by A.W. Segal (Nature (1987) 326, 88-91) for purification of cytochrome b558 from human polymorphonuclear leukocytes (PMN). Spectral characteristics of cytochrome b558 were determined and found to be independent of cell type and specific heme b content of the preparation. To increase the sensitivity of the spectral assay, analysis in the gamma band were used and delta epsilon 427-413 was determined to be equal to 158 mM-1 cm-1. An alpha beta type heterodimeric cytochrome b558 was found for PMN and MN by the concordant elution of heme b spectral activity from heparin agarose and the detection of two polypeptide chains by SDS-PAGE. The expression of the lighter polypeptide alpha chain in the various human monocyte-like cell lines was assessed and its identity, as a component of cytochrome b, was confirmed by immunodetection using a rabbit polyclonal antibody reacting with the alpha subunit of PMN cytochrome b558. Immunoblotting studies detected the alpha subunit in monocyto-macrophagic differentiated HL-60 and U 937 cells and mature MN at 22 kDa, but not in uninduced cells which did not express the respiratory burst. Whatever the specific content or the cell origin of the cytochrome b558-enriched preparations, the heme b binding site was shown to be associated with the alpha subunit defined by a constant molecular mass of 22 kDa, as evidenced by the finding of a constant ratio between the silver stained band intensity and the corresponding heme b amount. The heavy polypeptide beta chain from MN cytochrome b was found to have a significantly higher molecular weight than the beta subunit from PMN at 94 +/- 5 kDa instead of 90 +/- 4 kDa. In contrast, in cytochrome b preparations from induced monocyto-macrophagic cells, isolated with a low heme specific content, the variability in the detection of the staining intensity of the beta band either in SDS-PAGE or immunodetection reactivities precludes accurate definition of its molecular mass and estimation of the stoichiometry between the alpha and beta subunits in the differentiated cells. However, wheat-germ agglutinin binding studies indicated the presence of N-glycosylated protein in the range of 85-110 kDa.

The reduction of flavocytochrome b2 by carboxylate radicals. A pulse radiolysis study

The reaction of flavocytochrome b2 with carboxylate radicals has been studied by the pulse-radiolysis technique. The absorbance difference spectra observed 2, 10 and 90 mus after the pulse were very similar to the (reduced-minus-oxidized) difference spectrum of the cytochrome b2 core, a deflavoderivative prepared from flavocytochrome b2. The heme b2 group was reduced in a bimolecular reaction with a rate constant of (2.1 +/- 0.2) X 10(8) M-1 X s-1. Simulation data allowed us to assign this reduction mainly to a direct reaction of CO-2 on heme b2 without flavin involvement. However, this heme b2 reduction was accelerated in a flavocytochrome b2 sample of low molar activity. This observation seemed to reflect modifications of the heme b2 environment, in particular a closer contact with the solvent. Moreover, in such samples the flavin became reducible to the semiquinone state as deduced from absorbance difference spectra in the 440-490 nm region. The agreement between experimental and computed time-courses allowed to estimate the reaction rate of bound flavin to be equal to 2 X 10(8) M-1 X s-1 in this studied sample. Thus the reactivity of bound heme b2 and flavin within flavocytochrome b2 with CO-2 seems to be sensitive to the physical alterations of the polypeptide chain.

Hydroxylation reaction catalyzed by the Burkholderia cepacia AC1100 bacterial strain. Involvement of the chlorophenol-4-monooxygenase

The Burkholderia cepacia AC1100 strain, known to degrade the herbicide, 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T), is able to metabolize 4-hydroxyarylaldehyde, not only into the corresponding acid, but also into a new hydroquinone, 2,5-dihydroxyarylaldehyde. When incubated with resting AC1100 cells or cell-free extracts, syringaldehyde and 3,5-dimethoxy-4-hydroxybenzaldehyde were converted into such metabolites, identified by comparison of their mass and 1H-NMR spectra with those of authentic chemically synthesized samples. With 5-bromovanillin, only one metabolite was formed, the structure of which was identified as 2, 5-dihydroxy-4-methoxy-6-bromobenzaldehyde through 1H-NMR two-dimensional NOESY experiments. All these products result formally from a para hydroxylation of the phenol followed by the cis migration of the aldehyde. This reaction is the only one to be associated with the 2,4,5-T degradation pathway, as the acid formation was retained when the AC1100 strain had lost its degradation ability. Through competitive experiments with halophenols and methimazole, an alternative substrate of flavin monooxygenase, the chlorophenol-4-monooxygenase was recognized to be the enzyme involved in the hydroxylation of 4-hydroxyarylaldehyde. The purified enzyme, previously reported to catalyze the para hydroxylation or dehalogenating hydroxylation of chlorophenols, also promotes this hydroxylation reaction in the presence of NADH and FAD. The kcat value determined for the best substrate, syringaldehyde, 0. 08 s-1, was about 20% of that obtained for 2,6-dichlorophenol hydroxylation (0.38 s-1).

Interactions substrat-flavine-hemoproteine et stabilite thermique du cytochrome b(2) (L-lactate deshydrogenase de la levure)

We studied the thermal inactivation of yeast cytochrome b(2), the native flavohaemoprotein (FHP(n)) and its derivatives, the haemoprotein (HP) obtained by dissociation of FMN from FHP(n) and the reconstituted flavohaemoprotein (FHP(r)) (haemoprotein + FMN). The initial velocity of inactivation is faster in the presence of FMN. The substrate (L-lactate) or the competitive inhibitor (oxolate) protects only the flavohaemoproteins (FHP(n) and FHP(r)) from thermal inactivation by decreasing this velocity. In each case, the "protection constant", K(p), is determined.Several interpretations can be proposed to explain the lack of protection of the haemoprotein by its substrate: 1) the structure of the substrate-binding site depends on the presence of the FMN; 2) the substrate binding site exists, but the affinity is too small to be detected; 3) a hypothetical scheme is proposed, suggesting that the protein can exist under different thermally sensitive forms following whether FMN or substrate is present.