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PubMed1976

Some properties of a detergent-solubilized NADPH-cytochrome c(cytochrome P-450) reductase purified by biospecific affinity chromatography.

NADPH-cytochrome c (cytochrome P-450) reductase (EC 1.6.2.4) has been purified to homogeneity, as judged by sodium dodecyl sulfate disc gel electrophoresis, from detergent-solubilized rat and pig liver microsomes using an affinity chromatography procedure. Treatment of microsomes with a polyethoxynonylphenyl ether plus either cholate or deoxycholate and subsequent batch-wise DEAE-cellulose chromatography followed by biospecific affinity chromatography on Sepharose 4B-bound N6-(6-aminohexyl)-adenosine 2',5'-bisphosphate (2'5'-ADP-Sepharose 4B) result in a greater than 30% yield of purified reductase from microsomes. The enzyme contains 1 mol each of FAD and FMN and exhibits a molecular weight of 78,000 g mol-1 estimated by comparison with protein standards on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The turnover numbers calculated on the basis of flavin are 1360 min-1 and 1490 min-1 at 25 degrees for the pig and rat liver enzymes, respectively. Titration of these purified preparations aerobically with both NADPH and potassium ferricyanide demonstrated unequivocally that the air-stable, reduced form of NADPH-cytochrome c (P-450) reductase contains 2 electron equivalents, confirming recent results obtained by Masters et al. (Masters, B. S. S., Prough, R. A., and Kamin, H. (1975) Biochemistry 14, 607-613) for the proteolytically solubilized enzyme. In addition, these preparations are capable of reconstituting benzphetamine N-demethylation activity in the presence of partially purified cytochrome P-450 and dilauroylphosphatidylcholine, as measured by formaldehyde formation from benzphetamine.

Aerobiosis↗
PubMed1993

[Phosphorylation of cytochrome P-450SCC by protein kinase C. The protective effect of adrenodoxin and cytochrome b5].

Cytochrome P-450scc from bovine adrenal cortex mitochondria was shown to be selectively phosphorylated by protein kinase C. The amino acid residues most accessible to phosphorylation by protein kinase C are located in the N-terminal sequence of cytochrome P-450scc. Adrenodoxin and cytochrome b5 protect cytochrome P-450scc from phosphorylation, this effect being dependent on the protein concentration.

Adrenal Glands↗
PubMed2001

Construction of Salmonella typhimurium YG7108 strains, each coexpressing a form of human cytochrome P450 with NADPH-cytochrome P450 reductase.

A series of Salmonella typhimurium (S. typhimurium) YG7108 strains, each coexpressing a form of human cytochrome P450 (CYP) (CYP1A1, CYP1A2, CYP1B1, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, or CYP3A5) together with human NADPH-cytochrome P450 reductase (OR), was established. The parental S. typhimurium YG7108, derived from TA1535, lacks two O(6)-methylguanine-DNA methyltransferase genes, ada and ogt, and is highly sensitive to the mutagenicity of alkylating agents. The expression levels of CYP holo-protein in the genetically engineered S. typhimurium YG7108 cells, determined by carbon monoxide (CO) difference spectra, ranged from 62 nmol/L culture for CYP2C19 to 169 nmol/L culture for CYP3A4. The expression level of the OR varied, depending on the form of CYP coexpressed, and ranged from 214 to 1029 units/L culture. Each form of CYP expressed in the S. typhimurium YG7108 cells catalyzed the oxidation of a representative substrate at an efficient rate. The rates appeared comparable to the reported activities of CYP expressed in human liver microsomes or CYP in other heterologous systems, indicating that the OR was sufficiently expressed to support the catalytic activity of CYP. These S. typhimurium strains may be useful not only for predicting the metabolic activation of promutagens catalyzed by human CYP but also for identifying the CYP form involved.

Alkylating Agents↗
PubMed1995

Transient kinetics of intracomplex electron transfer in the human cytochrome b5 reductase-cytochrome b5 system: NAD+ modulates protein-protein binding and electron transfer.

Transient kinetics of reduction and interprotein electron transfer in the human cytochrome b5 reductase-cytochrome b5 (b5R-b5) system was studied by laser flash photolysis in the presence of 5-deazariboflavin and EDTA at pH 7.0. Flash-induced reduction of the FAD cofactor of b5R by deazariboflavin semiquinone (in the absence of b5) occurred in a rapid second-order reaction (k2 = 3.1 x 10(8) M-1 s-1) and resulted in a neutral (blue) FAD semiquinone. The heme of cytochrome b5 (in the absence of b5R) was also rapidly reduced in this system with k2 = 3.1 x 10(8) M-1 s-1. When the two proteins were mixed at low ionic strength, a strong complex was formed. Although the heme of complexed b5 could be directly reduced by deazariboflavin semiquinone, the second-order rate constant was nearly an order of magnitude smaller than that of free b5 (k2 = 3.4 x 10(7) M-1 s-1). In contrast, access to the FAD of b5R by the external reductant was decreased by considerably more than an order of magnitude (k2 < 1 x 10(7) M-1 s-1). When an excess of b5R was titrated with small increments of b5 and then subjected to laser flash photolysis in the presence of deazariboflavin/EDTA, interprotein electron transfer from the b5R FAD semiquinone to the heme of b5 could be observed. At low ionic strength (I = 16 mM), the reaction showed saturation behavior with respect to the b5 concentration, with a limiting first-order rate constant for interprotein electron transfer k1 = 375 s-1, and a dissociation constant for protein-protein transient complex formation of approximately 1 microM. The observed rate constants for interprotein electron transfer decreased 23-fold when the ionic strength was increased to 1 M, indicating a plus-minus electrostatic interaction between the two proteins. Saturation kinetics were also observed at I = 56, 96, and 120 mM, with limiting first-order rate constants of 195, 155, and 63 s-1, respectively. In the presence of NAD+, the transient protein-protein complex was stabilized by approximately a factor of two, and limiting first-order rate constants of 360 s-1 were obtained at both I = 56 mM and I = 96 mM and 235 s-1 at I = 120 mM. Thus, NAD+ appears to stabilize as well as to optimize the protein-protein complex with respect to electron transfer. Another effect of NAD+ is to appreciably slow autoxidation and disproportionation of the FAD semiquinone.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome Reductases↗
PubMed2001

An enzymatically active chimeric protein containing the hydrophilic form of NADPH-cytochrome P450 reductase fused to the membrane-binding domain of cytochrome b5.

The microsomal flavoprotein NADPH-cytochrome P450 reductase (CPR) contains an N-terminal hydrophobic membrane-binding domain required for reconstitution of hydroxylation activities with cytochrome P450s. In contrast, cytochrome b5 (b5) contains a C-terminal hydrophobic membrane-binding domain required for interaction with P450s. We have constructed, expressed and purified a chimeric flavoprotein (hdb5-CPR) where the C-terminal 45 amino acid residues of b5 have replaced the N-terminal 56 amino acid domain of CPR. This hybrid flavoprotein retains the catalytic properties of the native CPR and is able to reconstitute fatty acid and steroid hydroxylation activities with CYP4A1 and CYP17A. However hdb5-CPR is much less effective than CPR for reconstituting activity with CYP3A4. We conclude that differences on the surface of the P450s reflect unique and specific information essential for the recognition needed to establish reactions of intermolecular electron transfer from the flavoprotein CPR.

Amino Acid Sequence↗
PubMed1996

Impact of tamoxifen on peripubertal androgen imprinting of rat hepatic cytochrome P450 2C11, cytochrome P450 3A2, and steroid 5 alpha-reductase.

Expression of sex-dependent rat hepatic cytochromes P450 and steroid 5 alpha-reductase is regulated mainly by the sex-specific pattern of growth hormone (GH) secretion and is subject to androgen imprinting. Since tamoxifen suppresses GH pulse amplitude and nadir levels, we investigated the effect of tamoxifen on peripubertal testosterone imprinting of hepatic CYP2C11, CYP3A2, CYP2A1, and steroid 5 alpha-reductase. Prepubertal tamoxifen administration (5 mg once daily s.c. on days 28 and 29 of age) to non-ovariectomized female Sprague-Dawley rats did not affect hepatic microsomal CYP2C11-dependent testosterone 2 alpha-hydroxylase, CYP3A-mediated testosterone 6 beta-hydroxylase, CYP2A1-dependent testosterone 7 alpha-hydroxylase, or steroid 5 alpha-reductase activity in adult rats. Testosterone treatment (5 mumol/kg, s.c., once daily) of intact female rats during either puberty (days 35-49 of age) or adult life (days 69-77 of age) had no effect on these enzyme activities in adult (78-day-old) female rats, but the same treatment given during both of these periods induced the male-specific testosterone 2 alpha- and 6 beta-hydroxylase activities and suppressed the female-predominant testosterone 7 alpha-hydroxylase and steroid 5 alpha-reductase activities, indicating that peripubertal testosterone administration imprints the adult androgen responsiveness but not the basal levels of these enzyme activities in non-ovariectomized female rats. However, peripubertal androgen imprinting of the basal levels of testosterone 2 alpha-hydroxylase and steroid 5 alpha-reductase activities was observed in female rats administered tamoxifen prepubertally. Tamoxifen pretreatment also enhanced testosterone imprinting of the adult androgen responsiveness of testosterone 2 alpha- and 6 beta-hydroxylase and steroid 5 alpha-reductase activities. The enhanced testosterone hydroxylase activities were, however, not associated with an increase in microsomal NADPH-cytochrome P450 reductase activity, but were accompanied by elevated hepatic CYP2C11 and CYP3A2 protein levels. Overall, the present study indicates that prepubertal tamoxifen administration does not interfere with the normal sex differentiation of the gender-dependent hepatic cytochromes P450 and steroid 5 alpha-reductase, but this drug modulates peripubertal androgen imprinting of CYP2C11, CYP3A2, and steroid 5 alpha-reductase in adult female rats.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed1987

fbc operon, encoding the Rieske Fe-S protein cytochrome b, and cytochrome c1 apoproteins previously described from Rhodopseudomonas sphaeroides, is from Rhodopseudomonas capsulata.

Detailed comparison of the 'Rhodopseudomonas sphaeroides GA' strain used by Gabellini et al. (1985) with genuine R. sphaeroides and R. capsulata strains indicated that the previously reported fbc operon of R. sphaeroides (Gabellini and Sebald, 1986) encoding the structural genes for the Rieske Fe-S protein, cytochrome b and cytochrome c1 subunits of the ubiquinol:cytochrome c2 oxidoreductase, is not from R. sphaeroides, but is rather from a strain of R. capsulata. Consequently, the genuine bc1 genes from R. sphaeroides were cloned using corresponding R. capsulata genes as probes, and a partial nucleotide sequence for the Rieske Fe-S protein of R. sphaeroides was determined and compared with that of R. capsulata.

Amino Acid Sequence↗
PubMed2003

Effects of cytochrome b(5) on drug oxidation activities of human cytochrome P450 (CYP) 3As: similarity of CYP3A5 with CYP3A4 but not CYP3A7.

Effects of cytochrome b(5) (b(5)) on catalytic activities of human cytochrome P450 (CYP) 3A5, CYP3A4, and CYP3A7 coexpressed with human NADPH-cytochrome P450 reductase in Escherichia coli membranes were investigated using 14 substrates. The activities of CYP3A5 were enhanced by addition of b(5) in approximately one third of the substrates employed in this study. Such enhancement by b(5) was roughly similar to that of CYP3A4, while the activities of CYP3A7 were not enhanced by b(5) with any substrates employed. V(max) values for midazolam 1'-hydroxylation and amitriptyline N-demethylation by CYP3A5 were increased about twice by addition of b(5), which was also seen with CYP3A4, although the extent of the effects of b(5) on S(50) (K(m)) and Hill coefficient differed dependent on substrates used. In contrast, b(5) did not alter any of these kinetic parameters of CYP3A7. The effects of b(5) on kinetic parameters of CYP3A5 were similar to those of CYP3A4 but not CYP3A7. These results suggest that roles of b(5) in drug oxidation activities of CYP3A5 and CYP3A4 are different from those of CYP3A7.

Adjuvants, Anesthesia↗
PubMed1986

Complete analysis of the cytochrome components of beef heart mitochondria in terms of spectra and redox properties. The c1-cytochromes.

Using newer techniques for conducting and analyzing potentiometric titrations, we have studied the thermodynamic and spectral properties of cytochrome c1 in beef heart mitochondria. We find two species of cytochrome c1, both with n = 2 values for the number of electrons involved in their oxidation or reduction. One has an Em approximately 210 mV and a spectral peak near 555 nm and the other has an Em approximately 255 mV and a spectral peak nearer 553 nm. These Em values are pH-independent in the range of pH 6 to 8. The Em and n values of these two components are indistinguishable from those of two species of cytochrome aa3 (i.e. spectral feature of 605 nm).

Animals↗
PubMed1999

Q-Band resonance Raman investigation of turnip cytochrome f and Rhodobacter capsulatus cytochrome c1.

The results of a comprehensive Q-band resonance Raman investigation of cytochrome c1 and cytochrome f subunits of bc1 and b6f complexes are presented. Q-band excitation provides a particularly effective probe of the local heme environments of these species. The effects of protein conformation (particularly axial ligation) on heme structure and function were further investigated by comparison of spectra obtained from native subunits to those of a site directed c1 mutant (M183L) and various pH-dependent species of horse heart cytochrome c. In general, all species examined displayed variability in their axial amino acid ligation that suggests a good deal of flexibility in their hemepocket conformations. Surprisingly, the large scale protein rearrangements that accompany axial ligand replacement have little or no effect on macrocycle geometry in these species. This indicates the identity and/or conformation of the peptide linkage between the two cysteines that are covalently linked to the heme periphery may determine heme geometry.

Brassica↗
PubMed1993

Structurally engineered cytochromes with unusual ligand-binding properties: expression of Saccharomyces cerevisiae Met-80-->Ala iso-1-cytochrome c.

A strategy has been developed to express and purify a recombinant, nonfunctional axial-ligand mutant of iso-1-cytochrome c (Met-80-->Ala) in Saccharomyces cerevisiae in quantities necessary for extensive biophysical characterization. It involves coexpressing in the same plasmid (YEp213) the nonfunctional gene with a functional gene copy for complementation in a selective medium. The functional gene encodes a product with an engineered metal-chelating dihistidine site (His-39 and Leu-58-->His) that enables efficient separation of the two isoforms by immobilized metal-affinity chromatography. The purified Met-80-->Ala protein possesses a binding site for dioxygen and other exogenous ligands. Absorption spectra of several derivatives of this mutant show striking similarities to those of corresponding derivatives of horseradish peroxidase, myoglobin, and cytochrome P450. The use of a dual-gene vector for cytochrome c expression together with metal-affinity separation opens the way for the engineering of variants with dramatically altered structural and catalytic properties.

Alanine↗
PubMed1989

A bacterial c-type cytochrome can be translocated to the periplasm as an apo form; the biosynthesis of cytochrome cd1 (nitrite reductase) from Paracoccus denitrificans.

An apo form of cytochrome cd1 (nitrite reductase) of Paracoccus denitrificans has been detected immunologically in the periplasm of a mutant that lacks all c-type cytochromes. A method for the preparation of apo-nitrite reductase (lacking both c- and d-type haem) from the holoenzyme of wild-type cells has been developed. The apoprotein synthesized by the mutant is indistinguishable from the chemically prepared apoprotein in respect of: (i) subunit molecular weight; (ii) formation of a homodimer; (iii) properties on anion exchange chromatography. The holoenzyme has similar properties in respect of (i) and (ii) but behaves differently during anion exchange. A suggested mode of assembly of cytochrome cd1 is translocation into the periplasm of a precursor polypeptide, maturation by a signal peptidase to give an apoprotein identical to that prepared chemically from the holoenzyme, followed by insertion of c-type and d-type haem in an as yet unknown order.

Apoproteins↗
PubMed2005

Role of cytochrome B5 in modulating peroxide-supported cyp3a4 activity: evidence for a conformational transition and cytochrome P450 heterogeneity.

The role of cytochrome b(5) (b(5)) in the alpha-naphthoflavone (alpha-NF)-mediated inhibition of H(2)O(2)-supported 7-benzyloxyquinoline (7-BQ) debenzylation by heterologously expressed and purified cytochrome P450 3A4 (CYP3A4) was studied. Although alpha-NF showed negligible effect in an NADPH-dependent reconstituted system, inhibition of 7-BQ oxidation was observed in the H(2)O(2) system. Analysis of the effect of various constituents of a standard reconstituted system on H(2)O(2)-supported activity showed that b(5) alone resulted in a 2.5-fold increase in the k(cat) value and reversed the inhibitory effect of alpha-NF. In addition, titration with b(5) suggested that only 65% of the CYP3A4 participated in the interaction with b(5), consistent with cytochrome P450 (P450) heterogeneity. Study of the influence of b(5) on the kinetics of H(2)O(2)-dependent destruction of the P450 heme moiety suggested two distinct conformers of CYP3A4 with different sensitivity to heme loss. In the absence of b(5), 66% of the wild-type enzyme was bleached in the fast phase, whereas the addition of b(5) decreased the fraction of the fast phase to 16%. Finally, to locate amino acid residues that might influence b(5) action, several active site mutants were tested. Substitution of Ser-119, Ile-301, Ala-305, Ile-369, or Ala-370 with the larger Phe or Trp decreased or even abolished the activation by b(5). Ser-119 is in the B'-C loop, a predicted b(5)-P450 interaction site, and Ile-301 and Ala-305 are closest to the heme. In conclusion, the interaction of b(5) with P450 apparently leads to a conformational transition, which results in redistribution of the CYP3A4 pool.

Allosteric Regulation↗
PubMed2001

Kinetics of testosterone 6beta-hydroxylation in the reconstituted system with similar ratios of purified CYP3A4, NADPH-cytochrome p450 oxidoreductase and cytochrome B5 to human liver microsomes.

Kinetics of testosterone 6beta-hydroxylation were determined using a reconstituted system that consisted of CYP3A4, cytochrome b5 and NADPH-cytochrome P450 oxidoreductase (OR) with similar ratios as those seen in human liver microsomes and compared with those determined using human liver microsomes. Two reconstituted systems were constructed in accordance with two human liver microsomal samples that showed extremely high and low ratios of OR/CYP3A4. The Km values of testosterone 6beta-hydroxylation obtained from the reconstituted systems with high and low OR/CYP3A4 ratios were 29.3 and 35.2 microM, respectively, which were similar to that of the corresponding human liver microsomal samples (23.2 and 40.0 microM, respectively). However, Vmax values obtained from the reconstituted systems (3.7 and 0.8 pmol/min/pmol CYP3A4) were much lower than those from the human liver microsomes (44.2 and 31.1 pmol/min/pmol CYP3A4). The results suggest that the interaction between substrate and CYP3A4 in the reconstituted systems appear to be similar to human liver microsomes but that the velocity of the substrate metabolism in the reconstituted systems is different from that in human liver microsomes. In conclusion, our reconstituted systems could be used for the determination of affinity but not for the determination of the maximum velocity of substrate metabolism. Further studies on the protein-protein interactions between CYP3A4, OR, cytochrome b5 and/or a specific lipid environment are required to establish a reconstituted system showing similar kinetic properties to those of human liver microsomes.

Cytochrome P-450 CYP3A↗
PubMed1981

On the evolutionary relationship of the 4-alpha-helical heme proteins. The comparison of cytochrome b562 and cytochrome c'.

The atomic models of the cytochrome b562 and cytochrome c' monomers have been compared. When the respective heme groups are superimposed, the four alpha-helices of each nearly coincide. Four aromatic side chains, including the heme ligands, and a methionine occur in spatially equivalent positions in contact with the heme groups. This structural evidence suggests that the two cytochrome families may have diverged from a common molecular ancestor.

Biological Evolution↗
PubMed1984

Kinetic isotope effects on cytochrome P-450-catalyzed oxidation reactions. Evidence for the irreversible formation of an activated oxygen intermediate of cytochrome P-448.

Substitution of deuterium for hydrogen on the alpha-carbon of 7-ethoxycoumarin results in an intrinsic isotope effect of approximately 14 during the cytochrome P-448-catalyzed O-deethylation of this substrate (G. T. Miwa, J. S. Walsh, and A. Y. H. Lu (1984) J. Biol. Chem. 259, 3000-3004). This dramatic decrease in the C-H bond cleavage rate does not, however, alter the rate of 7-ethoxycoumarin disappearance or the rates of NADPH and oxygen consumption indicating that the catalytic turnover of this enzyme is unaffected. Moreover, hydrogen peroxide formation and the concentrations of an oxycytochrome P-448 complex (lambda max = 440 nm) are also unchanged demonstrating that the steady state concentrations of various oxy intermediates of cytochrome P-448 are also unchanged. An inescapable conclusion from these data is that an irreversible step exists between the formation of these intermediates and the oxidation of the substrate. These data are in agreement with the view that an irreversible cleavage of the dioxygen bond precedes substrate oxidation. Moreover, the oxidatively competent form of the cytochrome must then be committed to substrate oxidation. The latter conclusion is substantiated from high performance liquid chromatography studies which demonstrate the formation of a second metabolite, 6-hydroxy-7-ethoxycoumarin, from the deuterated substrate arising from the metabolic switching away from the O-ethyl group to the aromatic ring of this substrate.

Animals↗
PubMed2003

Cytochrome c oxidase of mammals contains a testes-specific isoform of subunit VIb--the counterpart to testes-specific cytochrome c?

Sperm motility is highly dependent on aerobic energy metabolism, of which the apparent rate-limiting step of the mitochondrial respiratory chain is catalyzed by cytochrome c oxidase (COX). COX is the only electron transport chain complex to display isoforms, consistent with its suggested rate-limiting role. Isoforms were previously described for four of the 13 subunits. We now report the discovery that COX subunit VIb displays a testes-specific isoform in human, bull, rat, and mouse (COX VIb-2). Analysis of a variety of rat and mouse tissues, including ovaries, demonstrates exclusive expression of VIb-2 in testes, whereas VIb-1 transcripts are absent in rodent testes, even at early developmental stages. In contrast, both isoforms are transcribed in human testes. In situ hybridizations with human, rat, and mouse testes sections reveal VIb-2 transcripts in all testicular cell types. Within the seminiferous tubules, VIb-1 shows stronger signals in the periphery than in the lumen. Previously, cytochrome c was the only component of the mitochondrial respiratory chain known to express a testes-specific isoform in mammals. COX subunit VIb connects the two COX monomers into the physiological dimeric form, and is the only COX subunit that, like cytochrome c, is solely located in the inter-membrane space. Significant differences between the isoform sequences, in particular changes in charged amino acids, suggest interactions with cytochrome c and sperm-specific energy requirements.

Animals↗
PubMed1993

Identification and characterization of the ctaC (coxB) gene as part of an operon encoding subunits I, II, and III of the cytochrome c oxidase (cytochrome aa3) in the cyanobacterium Synechocystis PCC 6803.

The gene (coxII = coxB = ctaC) encoding subunit II of Synechocystis PCC 6803 cytochrome c oxidase has been isolated by screening a genomic DNA library in pUC18 with a 17-bp oligonucleotide probe (probe C) derived from coxI of Paracoccus denitrificans after Southern blots with a 19-kb oligonucleotide (probe A) derived from coxII of P. denitrificans had given equivocal results. A 2.2 kb PstI-KpnI restriction fragment was subcloned into pUC 18 and the resulting plasmid pDAUV26, which contained the probe C-binding site near the downstream end was found also to contain the whole coxII gene upstream of this site. The novel plasmid pDAUV 26 was used to transform competent E. coli cells, propagated therein, and the sequence determined. The 2.2 kb insert contained the entire coding region for the coxII gene together with a GAG start codon, a TAA stop codon, and a putative Shine-Dalgarno sequence. The deduced COII polypeptide is composed of 319 aa (calculated molecular mass of 32,800) plus a N-terminal leader sequence of 20 aa. The hydropathy plot suggests two lipophilic transmembrane domains near the N-terminus connected with an extremely hydrophilic aa stretch on the cytosolic side, while an unusually long (> 50 aa) aa stretch on the periplasmic (= intrathylakoidal) side leads to a typical cyanobacterial threonine in place of the first conserved glutamate of the cytochrome c-binding region in all other COII proteins. Together with a considerably shortened and interrupted aromatic aa stretch in this region, these differences are discussed in terms of the peculiar affinity of cyanobacterial cytochrome oxidases for acidic c-type cytochromes. Other invariant features such as the strictly conserved CuA-binding aa, however, are found in correct positions.

Amino Acid Sequence↗