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Resonance Raman (RR) spectroscopy was used to investigate conformational characteristics of the hemes of several ferricytochromes of the cytochrome c3 family, electron transfer proteins isolated from the periplasm and membranes of sulfate-reducing bacteria. Our analysis concentrated on the low-frequency region of the RR spectra, a fingerprint region that includes vibrations for heme-protein C-S bonds [nu(C(a)S)]. It has been proposed that these bonds are directly involved in the electron transfer process. The three groups of tetraheme cytochrome c3 analyzed, namely Type I cytochrome c (3) (TpIc (3)s), Type II cytochrome c (3) (TpIIc (3)s) and Desulfomicrobium cytochromes c3, display different frequency separations for the two nu(C(a)S) lines that are similar among members of each group. These spectral differences correlate with differences in protein structure observed among the three groups of cytochromes c3. Two larger cytochromes of the cytochrome c3 family display RR spectral characteristics for the nu(C(a)S) lines that are closer to TpIIc3 than to TpIc3. Two other multiheme cytochromes from Desulfovibrio that do not belong to the cytochrome c3 family display nu(C(a)S) lines with reverse relative areas in comparison with the latter family. This RR study shows that the small differences in protein structure observed among these cytochrome c3 correlate to differences on the heme-protein bonds, which are likely to have an impact upon the protein function, making RR spectroscopy a sensitive and useful tool for characterizing these cytochromes.
Hybridomas were prepared from myeloma cells and spleen cells of BALB/c female mice immunized with hepatic cytochrome P-450E purified from the marine fish, Stenotomus chrysops (scup). Nine independent hybrid clones produced MAbs, either IgG1, IgG2b, or IgM, that bound to purified cytochrome P-450E in radioimmunoassay. Antibodies from one clone MAb (1-12-3), also strongly recognized rat cytochrome P-450MC-B (P-450BNF-B; P-450c). The nine antibodies inhibited reconstituted aryl hydrocarbon hydroxylase (AHH) and ethoxycoumarin O-deethylase of scup cytochrome P-450E to varying degrees, and inhibited AHH activity of beta-naphthoflavone-induced scup liver microsomes in a pattern similar to that in reconstitutions, indicating that cytochrome P-450E is identical to the AHH catalyst induced in this fish by beta-naphthoflavone. MAb 1-12-3 also inhibited the reconstituted AHH activity of the major BNF-induced rat isozyme. Conversely, MAb 1-7-1 to rat cytochrome P-450MC-B had little effect on AHH activity of scup cytochrome P-450E, and did not recognize cytochrome P-450E in radioimmunoassay nor in an immunoblot. Scup cytochrome P-450E and rat cytochrome P-450MC-B thus have at least one common epitope recognized by MAb 1-12-3, but the epitope recognized by Mab 1-7-1 is absent or recognized with low affinity in cytochrome P-450E. The various assays indicate that the nine MAbs against cytochrome P-450E are directed to different epitopes of the molecule. These MAbs should be useful in determining phylogenetic relationships of the BNF- or MC-inducible isozymes and their regulation by other environmental factors.
Photoautotrophically grown cyanobacterium Nostoc sp. strain Mac (PCC 8009) released up to about 10 nmol of a c-type cytochrome per ml packed cells after treatment with EDTA under conditions that left the plasma membrane absolutely intact as judged from the absence of cytosolic proteins in the supernatant. Spectra of the ascorbate reduced cytochrome revealed peaks at 553, 522 and 416 nm. The protein was purified to an A-553/A-275 ratio of 0.8. Midpoint potential (at pH 7), isoelectric point and apparent molecular weight of the cytochrome were +0.35 V, 8.6, and around 10,500, respectively. The cytochrome proved to be an excellent electron donor to the aa3-type cytochrome oxidase in both plasma and thylakoid membranes isolated and purified from Nostoc Mac. Chemoheterotrophic growth of the cells increased the level of periplasmic cytochrome c up to 10-fold and cytochrome oxidase activity of plasma membranes up to 90-fold. The periplasmic cytochrome also transferred electrons to photosystem I in illuminated thylakoid membranes. We conclude that cyanobacteria contain a periplasmic c-type cytochrome presumably identical to so-called cytochrome c6 or c-553 which has long been known as a photosynthetic (i.e. thylakoid-associated) redox protein in these organisms, and which is capable of donating electrons (from the periplasmic space) to the cytochrome oxidase in the plasma membrane and (from the thylakoid lumen) to both P700 and cytochrome oxidase in the thylakoid membrane.
The synthesis of 5,16-androstadien-3 beta-ol from pregnenolone occurs via a cytochrome P450-dependent reaction (andien-beta synthase) that is analogous to the C17-hydroxylase/lyase reaction. It is not known whether the andien-beta synthase activity in adult porcine testis involves cytochrome P450c17 or is unique to porcine testis. Andien-beta synthase activity in testis microsomes was inhibited by high pH and concentration of salt, while C17-hydroxylase/lyase activity was stimulated under these conditions. Cytochrome P450c17 purified from adult porcine testis and adrenal glands and bovine adrenal glands had only C17-hydroxylase/lyase activity in the absence of cytochrome b5. However, when cytochrome b5 isolated from porcine testis was added, andien-beta synthase activity was detected in all three preparations of cytochrome P450c17, with the highest activity found in the porcine preparations. The andien-beta synthase activity was further increased from 2.5 to 6 times when NADH cytochrome b5 reductase was added along with cytochrome b5. Levels of mRNA for cytochrome b5 relative to cytochrome P450c17 mRNA were five times higher in porcine testis than in porcine adrenal. It appears that the andien-beta synthase activity is catalysed by cytochrome P450c17, which is not unique to the porcine testis and is dependent upon adequate levels of cytochrome b5.
The reactions of bovine cytochrome c oxidase with horse cytochrome c derivatives labeled at specific lysine amino groups with (dicarboxybipyridine)bis(bipyridine)ruthenium (II) were studied by laser flash photolysis. All of the derivatives form complexes with cytochrome c oxidase at low ionic strength (5 mM sodium phosphate, pH 7). Excitation of Ru(II) to Ru(II*) with a short laser flash resulted in rapid electron transfer to the ferric heme group of cytochrome c, followed by electron transfer to cytochrome c oxidase. The photoreduced heme Fe(II) in the cytochrome c derivative modified at lysine 25 on the periphery of the heme crevice domain transferred an electron to CuA with a rate constant of 1.1 x 10(4) s-1. CuA then transferred an electron to cytochrome a with a rate constant of 2.3 x 10(4) s-1. The derivatives modified at lysines 7, 39, 55, and 60 remote from the heme crevice domain of cytochrome c have nearly the same kinetics. The rate constant for electron transfer from the cytochrome c heme to CuA is greater than 10(5) s-1, and the rate constant for electron transfer from CuA to cytochrome a is 2 x 10(4) s-1. The cytochrome c derivatives modified at lysines 13 and 27 in the heme crevice domain react much more slowly than the other derivatives, with intracomplex rate constants for oxidation of cytochrome c ranging from 1000 to 6000 s-1. The bulky ruthenium group at the heme crevice domain of these derivatives apparently alters the binding orientation, leading to smaller electron-transfer rates.(ABSTRACT TRUNCATED AT 250 WORDS)
Four mutations in the mitochondrial cytochrome b of Saccharomyces cerevisiae have been characterized with respect to catalytic properties, inhibitor resistance and subunit interaction. The respiratory-deficient mutant [G137E]cytochrome b and the pseudo-wild-type revertant [G137E, N256K]cytochrome b were described previously [di Rago, J.-P., Netter, P. & Slonimski, P. P. (1990) J. Biol. Chem. 265, 3332-3339; di Rago, J.-P., Netter, P. & Slonimski, P. P. (1990) J. Biol. Chem. 265, 15750-15757]. Two new mutants [N256K]cytochrome b and [N256I]cytochrome b were isolated by dissociation of the second-site suppressor from the original target mutation. The mutants [G137E]cytochrome b and [G137E, N256K]cytochrome b exhibited a high resistance against methoxyacrylate inhibitors, whereas the suppressors [N256K]cytochrome b and [N256I]cytochrome b showed only a slight resistance. Remarkably, all mutants exhibited stigmatellin cross-resistance. The electron-transfer activity from the substrate nonylubiquinol to cytochrome c of mitochondrial membranes was diminished in all mutants. The substitution G137-->E decreases Vmax/Km by one order of magnitude, indicating a reduced catalytic efficiency for ubiquinol. The amino acid exchange at position 256 to a positively charged lysine residue or to a hydrophobic isoleucine residue resulted mainly in a diminished specific activity. The iron-sulfur subunit and the 8.5-kDa subunit were detectable in all mutants at normal levels in immunoblots of membrane preparations, indicating proper assembly of the complex. However, after purification, the mutant bc1 complex lacked the iron-sulfur subunit and the 8.5-kDa subunit. In contrast, the iron-sulfur subunit can only be dissociated from the parental bc1 complex by harsh treatment. These data suggest that residues 137 and 256 in cytochrome b are crucial for cytochrome-b/iron-sulfur protein interaction.
Cytochrome p30, a novel hemoprotein isolated from rabbit peritoneal neutrophils [Escriou, V., Laporte, F., Garin, J., Brandolin, G. & Vignais, P. V. (1994) J. Biol. Chem. 269, 14007-14014] has been characterized by low-temperature (77 K) absorption and resonance Raman spectroscopies. The spectral data have been compared with those obtained with neutrophil cytochrome b-558. At room temperature, the absorption difference spectra (reduced minus oxidized) of cytochrome p30 and cytochrome b-558 could not been distinguished from each other. However, at 77 K, significant differences were observed. In particular, the alpha band of cytochrome p30 was split whereas that of cytochrome b-558 was symmetrical, but particularly broad. The resonance Raman spectra of cytochrome p30 provided evidence for the presence of two hemes both in the ferric and ferrous states. One of them was a six-coordinated low-spin heme either oxidized or reduced whereas the other one was a high-spin heme, five-coordinated in the reduced state and six-coordinated in the oxidized state. It is probable that two histidine residues constitute the axial ligands of the six-coordinated low-spin heme of cytochrome p30. The resonance Raman spectra of cytochrome b-558 allowed the detection of a six-coordinated low-spin heme, similar to that found in cytochrome p30. The component typical of the high-spin heme of cytochrome p30 was however absent in the spectra of oxidized and reduced cytochrome b-558.
Two abundant, low-redox-potential cytochromes c were purified from the facultative anaerobe Shewanella oneidensis strain MR1 grown anaerobically with fumarate. The small cytochrome was completely sequenced, and the genes coding for both proteins were cloned and sequenced. The small cytochrome c contains 91 residues and four heme binding sites. It is most similar to the cytochromes c from Shewanella frigidimarina (formerly Shewanella putrefaciens) NCIMB400 and the unclassified bacterial strain H1R (64 and 55% identity, respectively). The amount of the small tetraheme cytochrome is regulated by anaerobiosis, but not by fumarate. The larger of the two low-potential cytochromes contains tetraheme and flavin domains and is regulated by anaerobiosis and by fumarate and thus most nearly corresponds to the flavocytochrome c-fumarate reductase previously characterized from S. frigidimarina to which it is 59% identical. However, the genetic context of the cytochrome genes is not the same for the two Shewanella species, and they are not located in multicistronic operons. The small cytochrome c and the cytochrome domain of the flavocytochrome c are also homologous, showing 34% identity. Structural comparison shows that the Shewanella tetraheme cytochromes are not related to the Desulfovibrio cytochromes c(3) but define a new folding motif for small multiheme cytochromes c.
Cytochrome c oxidase activity and cytochromes b, (c+c1) and a(+a3) concentrations were determined in liver mitochondria from rats fed the following diets: controls (group 1) fed ad libitum, energy-restricted (group 2) and protein-deficient (group 3). The animals were fed for two time intervals, 3--5 and 7--9 weeks. At 3--5 weeks, the cytochrome oxidase specific activity (nmol cytochrome c oxidized/mg protein/min) and cytochrome concentrations (nmol/mg protein) were not different in groups 2 and 3 as compared to group 1. At 7--9 weeks, the cytochrome oxidase specific activity and concentrations of cytochromes b, (c+c1) and a(+a3) were significantly reduced in group 2 rats as compared to well-fed controls. The Michaelis-Menten constant, Km apparent for ferrocytochrome c, was significantly higher in group 2 as compared to group 1. In group 3 rats, cytochrome oxidase specific activity and cytochrome b, a(+a3) concentrations were not different from group 1 at 7-9 weeks. However, cytochrome (c+c1) concentration was higher in group 3, resulting in an elevated ratio of cytochrome (c+c1) to cytochrome a(+a3) as compared to groups 1 and 2.
To investigate the basis for the pattern of ovarian steroid production during the bovine estrous cycle, the tissue concentrations of major steroidogenic enzymes, 17 alpha-hydroxylase cytochrome P-450 and cholesterol side-chain cleavage cytochrome P-450 (cytochrome P-450scc), and their respective electron donors, NADPH-cytochrome P-450 reductase and adrenodoxin, were estimated and compared with those of the nonsteroidogenic enzymes, cytochrome c oxidase and F1-ATPase. The levels of these enzymes were estimated in medium sized (9-11 mm) and large (14-18 mm) follicles after removal of follicular fluid by centrifugation, and corpora lutea from the early, early-mid late-mid, and late stages of the luteal phase (n = 5 per group). The specific contents of all enzymes and electron donors were determined by immunoblot analysis, except for cytochrome c oxidase, which was quantified by determination of specific activity. The specific (per microgram of tissue homogenate protein) and total (per follicle or corpus luteum) tissue contents of 17 alpha-hydroxylase cytochrome P-450 increased 0.5- and 5-fold respectively from medium sized to large follicles, but then decreased to undetectable levels in corpora lutea in the early luteal phase, and remained undetectable throughout the luteal phase. In contrast, the specific content of NADPH-cytochrome P-450 reductase was similar between follicles and corpora lutea. The specific contents of cytochrome P-450scc, adrenodoxin and cytochrome c oxidase in follicles were similar to those of corpora lutea of the early luteal phase. However, by the early-mid luteal phase the specific contents of luteal cytochrome P-450scc (490 +/- 46 vs. 5709 +/- 982 cpm/micrograms protein) and adrenodoxin (44 +/- 15 vs. 705 +/- 229 cpm/micrograms protein) were increased, by 12- and 15-fold, respectively (P less than 0.05). In contrast, cytochrome c oxidase activity (29.1 +/- 10.1 vs. 108.6 +/- 20.7 nmol/mg tissue protein X min) and the specific content of F1-ATPase increased only 3- to 4-fold reflective of an increase in numbers of mitochondria. The levels of these enzymes remained elevated until the late luteal phase when they declined markedly. It is concluded that the induction of synthesis of P-450scc and adrenodoxin after ovulation is specific and does not merely reflect biogenesis of mitochondria during luteinization. Moreover the changes in the types of steroids produced by the ovarian compartments throughout the estrous cycle are a reflection of changes in the tissue content of steroidogenic enzymes.
1. The role of cytochrome P-450 in the one-electron reduction of mitomycin c was studied in rat hepatic microsomal systems and in reconstituted systems of purified cytochrome P-450. Formation of H2O2 from redox cycling of the reduced mitomycin c in the presence of O2 and the alkylation of p-nitrobenzylpyridine (NBP) in the absence of O2 were taken as parameters. 2. With liver microsomes from both 3-methylcholanthrene (MC)- and phenobarbital (PB)-pretreated rats, reverse type I difference spectra were observed, indicative of a weak interaction between mitomycin c and the substrate binding site of cytochrome P-450. Mitomycin c inhibited the oxidative dealkylation of aminopyrine and ethoxyresorufin in both microsomal systems. 3. Under aerobic conditions the H2O2 production in the microsomal systems was dependent on NADPH, O2 and mitomycin c, and was inhibited by the cytochrome P-450 inhibitors, metyrapone and SKF-525A. 4. Although purified NADPH-cytochrome P-450 reductase was also effective in reduction of mitomycin c and the concomitant reduction of O2, complete microsomal systems and fully reconstituted systems of cytochrome P-450b or P-450c and the reductase were much more efficient. 5. Under anaerobic conditions in the microsomal systems both reduction of mitomycin c (measured as the rate of substrate disappearance) and the reductive alkylation of NBP were dependent on cytochrome P-450. 6. The relative rate of reduction of mitomycin c by purified NADPH-cytochrome P-450 reductase was lower than that by a complete microsomal system containing both cytochrome P-450 and a similar amount of NADPH-cytochrome P-450 reductase. 7. It is concluded that although NADPH-cytochrome P-450 reductase is active in the one-electron reduction of mitomycin c, the actual metabolic locus for the reduction of this compound in liver microsomes under a relatively low O2 tension is more likely the haem site of cytochrome P-450.
Reduction of cytochrome b in isolated succinate-cytochrome c reductase is a triphasic reaction. Initially, there is a relatively rapid, partial reduction of the cytochrome b, the rate of which matches the rate of reduction of cytochrome c1. This is followed by partial or complete reoxidation of the b, which is then followed by slow rereduction. At very low concentrations of succinate, the initial partial reduction of b is followed by reoxidation, but the third (rereduction) phase is absent, owing to insufficient substrate to rereduce the cytochromes. If antimycin is added at various times during the triphasic reaction, it inhibits the reoxidation and also inhibits the rereduction phase. Antimycin does not inhibit the initial phase of b reduction and, if added before or during this phase, it causes reduction of b to proceed to completion as a monophasic reaction. Myxothiazol inhibits the first phase of b reduction and the subsequent reoxidation, but does not inhibit the third, slow phase of b reduction. The resulting monophasic reduction of b which is observed in the presence of myxothiazol is slower than that in the presence of antimycin. The combination of both inhibitors, whether added together or successively during the triphasic reaction, completely inhibits b reduction. The triphasic reduction of cytochrome b is consistent with electron transfer by a protonmotive Q cycle in which there are two pathways for cytochrome b reduction. One pathway allows the initial phase of cytochrome b reduction by a myxothiazol-sensitive reaction in which reduction of b by ubisemiquinone is linked to reduction of iron-sulfur protein and cytochrome c1 by ubiquinol. In the second phase of the triphasic reaction, the b cytochromes are reoxidized by ubiquinone or ubisemiquinone through an antimycin-sensitive reaction. If oxidation of ubiquinol by iron-sulfur protein is blocked, either by myxothiazol or by reduction of iron-sulfur protein and cytochrome c1, the b cytochromes can be reduced by reversal of the antimycin-sensitive pathway, thus accounting for the third phase of b reduction.
Complex formation between cytochrome c oxidase and cytochrome c perturbs the optical absorption spectrum of heme c and heme a in the region of the alpha-, beta, and gamma-bands. The perturbations have been used to titrate cytochrome c oxidase with cytochrome c. A stoichiometry of one molecule of cytochrome c bound per molecule of cytochrome c oxidase is obtained (1 heme c per heme aa3). In contrast, a stoichiometry of 2:1 was found earlier using a gel-filtration method (Rieder, R., and Bosshard, H.R. (1978) J. Biol. Chem. 253, 6045-6053). From the result of the spectrophotometric titration and from the wavelength position of the perturbation signals it is concluded that cytochrome c oxidase contains only a single binding site for cytochrome c which is close enough to heme a to function as an electron transfer site. The second site detected earlier by the gel-filtration method must be remote from this electron transfer site. Scatchard plots of the titration data are curvilinear, possibly indicating interactions between cytochrome c-binding sites on adjacent monomers of dimeric cytochrome c oxidase. The relationship between cytochrome c binding and the reaction of cytochrome c oxidase with ferrocytochrome c is discussed.
To identify the cytochrome b5 residues responsible for the electrostatic interaction with NADH-cytochrome b5 reductase (b5R), we prepared and characterized the cytochrome b5 mutants in which Glu41, Glu42, Glu63, Asp70, and Glu73 were replaced by Ala, utilizing site-directed mutagenesis and the expression system for cytochrome b5 in Escherichia coli. Apparent Km values of the wild type b5R for Glu42Ala cytochrome b5 and Asp70Ala cytochrome b5 were approximately three-fold and six-fold higher than that for the wild type cytochrome b5, respectively, while the kcat values for those mutants were not remarkably affected. In contrast, Glu41Ala, Glu63Ala, and Glu73Ala cytochrome b5 showed almost the same kinetic properties as the wild type cytochrome b5. Furthermore, kinetic studies on combinations of the cytochrome b5 and b5R mutants suggested the interaction between Glu42 and Asp70 of cytochrome b5 and Lys125 and Lys41 of b5R, respectively, in the reaction.
We have studied the role of NADPH cytochrome P-450 reductase in the metabolism of arachidonic acid and in two other monooxygenase systems: aryl hydrocarbon hydroxylase and 7-ethoxyresorufin-o-deethylase. Human liver NADPH cytochrome P-450 reductase was purified to homogeneity as evidenced by its migration as a single band on SDS gel electrophoresis, having a molecular weight of 71,000 Da. Rabbits were immunized with the purified enzyme and the resulting antibodies were used to evaluate the involvement of the reductase in cytochrome P-450-dependent arachidonic acid metabolism by bovine corneal epithelial and rabbit renal cortical microsomes. A highly sensitive immunoblotting method was used to identify the presence of NADPH cytochrome P-450 reductase in both tissues. We used these antibodies to demonstrate for the first time the presence of cytochrome c reductase in the cornea. Anti-NADPH cytochrome P-450 reductase IgG, but not anti-heme oxygenase IgG, inhibited the NADPH-dependent arachidonic acid metabolism in both renal and corneal microsomes. The inhibition was dependent on the ratio of IgG to microsomal protein where 50% inhibition of arachidonic acid conversion by cortical microsomes was achieved with a ratio of 1:1. A higher concentration of IgG was needed to achieve the same degree of inhibition in the corneal microsomes. The antibody also inhibited rabbit renal cortical 7-ethoxyresorufin-o-deethylase activity, a cytochrome P-450-dependent enzyme. However, the anti-NADPH cytochrome P-450 reductase IgG was much less effective in inhibiting rabbit cortical aryl hydrocarbon hydroxylase. Thus, the degree of inhibition of monooxygenases by anti-NADPH cytochrome P-450 reductase IgG is variable. However, with respect to arachidonic acid, NADPH cytochrome P-450 reductase appears to be an integral component for the electron transfer to cytochrome P-450 in the oxidation of arachidonic acid.