Cytochrome b5 potentiation of cytochrome P-450 catalytic activity demonstrated by a vaccinia virus-mediated in situ reconstitution system

Modulation of CYP2C9 activity and hydrogen peroxide production by cytochrome b5

Cytochromes P450 (CYP) play a major role in drug detoxification, and cytochrome b₅ (cyt b5) stimulates the catalytic cycle of mono-oxygenation and detoxification reactions. Collateral reactions of this catalytic cycle can lead to a significant production of toxic reactive oxygen species (ROS). One of the most abundant CYP isoforms in the human liver is CYP2C9, which catalyzes the metabolic degradation of several drugs including nonsteroidal anti-inflammatory drugs. We studied modulation by microsomal membrane-bound and soluble cyt b5 of the hydroxylation of salicylic acid to gentisic acid and ROS release by CYP2C9 activity in human liver microsomes (HLMs) and by CYP2C9 baculosomes. CYP2C9 accounts for nearly 75% of salicylic acid hydroxylation in HLMs at concentrations reached after usual aspirin doses. The anti-cyt b5 antibody SC9513 largely inhibits the rate of salicylic acid hydroxylation by CYP2C9 in HLMs and CYP2C9 baculosomes, increasing the K_M approximately threefold. Besides, soluble human recombinant cyt b5 stimulates the Vmax nearly twofold while it decreases nearly threefold the Km value in CYP2C9 baculosomes. Regarding NADPH-dependent ROS production, soluble recombinant cyt b5 is a potent inhibitor both in HLMs and in CYP2C9 baculosomes, with inhibition constants of 1.04 ± 0.25 and 0.53 ± 0.06 µM cyt b5, respectively. This study indicates that variability in cyt b5 might be a major factor underlying interindividual variability in the metabolism of CYP2C9 substrates.

Soybean Cytochrome b5 Is a Restriction Factor for Soybean Mosaic Virus

Soybean mosaic virus (SMV) is one of the most destructive viral diseases in soybeans (Glycine max). In this study, an interaction between the SMV P3 protein and cytochrome b5 was detected by yeast two-hybrid assay, and bimolecular fluorescence complementation assay showed that the interaction took place at the cell periphery. Further, the interaction was confirmed by co-immunoprecipitation analysis. Quantitative real-time polymerase chain reaction analysis revealed that GmCYB5 gene was differentially expressed in resistant and susceptible soybean plants after inoculation with SMV-SC15 strain. To test the involvement of this gene in SMV resistance, the GmCYB5 was silenced using a bean pod mottle virus (BPMV)-based vector construct. Results showed that GmCYB5-1 was 83% and 99% downregulated in susceptible (NN1138-2) and resistant (RN-9) cultivars, respectively, compared to the empty vector-treated plants. Silencing of GmCYB5 gene promotes SMV replication in soybean plants. Our results suggest that during SMV infection, the host CYB5 protein targets P3 protein to inhibit its proliferation. Taken together, these results suggest that CYB5 is an important factor in SMV infection and replication in soybeans, which could help soybean breeders develop SMV resistant soybean cultivars.

Spectroscopy of NOX Protein Family Members

All members of the NOX protein family contain a unique b-type cytochrome that mediates the electron transport that characterizes the activity of the multicomponent oxidase complexes. Referred to as cytochrome b558, because of its signature spectral absorbance at 558 nm in reduced-minus-oxidized difference spectroscopy, or cytochrome b(-245), because of its very low midpoint potential of -245 mV at pH 7.0, the protein possesses two stacked inequivalent hemes ligated by pairs of histidine residues in membrane helices h3 and h5. In a flavin-dependent fashion, cytochrome b558 shuttles electrons from cytoplasmic NADPH across membranes to molecular oxygen and thereby generates superoxide anion. By performing reduced-minus-oxidized difference spectroscopy and using the millimolar extinction coefficient, E _559-540 nm = 21.6 cm^-1 mM^-1, one can calculate the amount of cytochrome b558 in intact cells or partially purified membrane preparations. Measurements in samples where cytochrome b558 is relatively high and the presence of unrelated heme-containing proteins low, as in neutrophils, are straightforward. However, low levels of cytochrome b558 expression combined with an abundance of mitochondria and other sources of heme proteins make spectral detection of cytochrome b558 in non-phagocytic cells extremely challenging.

Quinone and nitrofurantoin redox cycling by recombinant cytochrome b5 reductase

NADH cytochrome b₅ reductase mediates electron transfer from NADH to cytochrome b₅ utilizing flavin adenine dinucleotide as a redox cofactor. Reduced cytochrome b₅ is an important cofactor in many metabolic reactions including cytochrome P450-mediated xenobiotic metabolism, steroid biosynthesis and fatty acid metabolism, hemoglobin reduction, and methionine and plasmalogen synthesis. Using recombinant human enzyme, we discovered that cytochrome b5 reductase mediates redox cycling of a variety of quinones generating superoxide anion, hydrogen peroxide, and, in the presence of transition metals, hydroxyl radicals. Redox cycling activity was oxygen-dependent and preferentially utilized NADH as a co-substrate; NADH was 5-10 times more active than NADPH in supporting redox cycling. Redox cycling activity was greatest for 9,10-phenanthrenequinone and 1,2-naphthoquinone, followed by 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone (menadione), nitrofurantoin and 2-hydroxyestradiol. Using menadione as the substrate, quinone redox cycling was found to inhibit reduction of cytochrome b₅ by cytochrome b₅ reductase, as measured by heme spectral changes in cytochrome b₅. Under anaerobic conditions where redox cycling is inhibited, menadione had no effect on the reduction of cytochrome b₅. Chemical redox cycling by cytochrome b₅ reductase may be important in generating cytotoxic reactive oxygen species in target tissues. This activity, together with the inhibition of cytochrome b₅ reduction by redox-active chemicals and consequent deficiencies in available cellular cytochrome b₅, are likely to contribute to tissue injury following exposure to quinones and related redox active chemicals.

Disruption of biomolecule function by nanoparticles: how do gold nanoparticles affect Phase I biotransformation of persistent organic pollutants?

The potential influence of nanoparticles on cytochrome P-450 (CYP) isozyme mediated Phase I biotransformation of persistent organic pollutants (POPs) in vitro was investigated using citrate-capped gold nanoparticles (AuNPs) and 2,2',3,5',6-pentachlorobiphenyl (PCB 95) as the probe nanoparticle and compound, respectively. AuNPs affected the biotransformation activity of rat CYP2B1 and changed the atropisomeric composition of PCB 95, depending on the incubation time and the AuNP concentration. Electrostatic repulsion between citrate-coated AuNPs and rat CYP2B1 may influence the active conformation of the isozyme and consequently affect its activity and stereoselectivity. In addition, the effects of AuNPs on rat CYP2B1 activity also appeared to be through interference with the CYP catalytic cycle's electron transfer chain. Incubations with AuNPs had a decline in buffer conductance and an absorbance band red shift of AuNPs, from electrostatic interactions of K(+) with negatively-charged AuNP aggregates. These ionic strength changes affected the formation rate of nicotinamide adenine dinucleotide phosphate, which provides electrons for the oxidative reaction cycle, and the biotransformation activity and stereoselectivity of CYP. This study suggests that charged nanoparticles may be able to alter the functions of biomolecules directly, by electrostatic interaction, or indirectly, by changes to the surrounding ionic strength. These factors should be taken into account for further understanding and prediction of the environmental behavior and fate of POPs and nanoparticles.

The interaction of microsomal cytochrome P450 2B4 with its redox partners, cytochrome P450 reductase and cytochrome b(5)

Cytochrome P450 2B4 is a microsomal protein with a multi-step reaction cycle similar to that observed in the majority of other cytochromes P450. The cytochrome P450 2B4-substrate complex is reduced from the ferric to the ferrous form by cytochrome P450 reductase. After binding oxygen, the oxyferrous protein accepts a second electron which is provided by either cytochrome P450 reductase or cytochrome b(5). In both instances, product formation occurs. When the second electron is donated by cytochrome b(5), catalysis (product formation) is ∼10- to 100-fold faster than in the presence of cytochrome P450 reductase. This allows less time for side product formation (hydrogen peroxide and superoxide) and improves by ∼15% the coupling of NADPH consumption to product formation. Cytochrome b(5) has also been shown to compete with cytochrome P450 reductase for a binding site on the proximal surface of cytochrome P450 2B4. These two different effects of cytochrome b(5) on cytochrome P450 2B4 reactivity can explain how cytochrome b(5) is able to stimulate, inhibit, or have no effect on cytochrome P450 2B4 activity. At low molar ratios (<1) of cytochrome b(5) to cytochrome P450 reductase, the more rapid catalysis results in enhanced substrate metabolism. In contrast, at high molar ratios (>1) of cytochrome b(5) to cytochrome P450 reductase, cytochrome b(5) inhibits activity by binding to the proximal surface of cytochrome P450 and preventing the reductase from reducing ferric cytochrome P450 to the ferrous protein, thereby aborting the catalytic reaction cycle. When the stimulatory and inhibitory effects of cytochrome b(5) are equal, it will appear to have no effect on the enzymatic activity. It is hypothesized that cytochrome b(5) stimulates catalysis by causing a conformational change in the active site, which allows the active oxidizing oxyferryl species of cytochrome P450 to be formed more rapidly than in the presence of reductase.

Defining the in Vivo Role for cytochrome b5 in cytochrome P450 function through the conditional hepatic deletion of microsomal cytochrome b5

In vitro, cytochrome b₅ modulates the rate of cytochrome P450-dependent mono-oxygenation reactions. However, the role of this enzyme in determining drug pharmacokinetics in vivo and the consequential effects on drug absorption distribution, metabolism, excretion, and toxicity are unclear. In order to resolve this issue, we have carried out the conditional deletion of microsomal cytochrome b₅ in the liver to create the hepatic microsomal cytochrome b₅ null mouse. These mice develop and breed normally and have no overt phenotype. In vitro studies using a range of substrates for different P450 enzymes showed that in hepatic microsomal cytochrome b₅ null NADH-mediated metabolism was essentially abolished for most substrates, and the NADPH-dependent metabolism of many substrates was reduced by 50–90%. This reduction in metabolism was also reflected in the in vivo elimination profiles of several drugs, including midazolam, metoprolol, and tolbutamide. In the case of chlorzoxazone, elimination was essentially unchanged. For some drugs, the pharmacokinetics were also markedly altered; for example, when administered orally, the maximum plasma concentration for midazolam was increased by 2.5-fold, and the clearance decreased by 3.6-fold in hepatic microsomal cytochrome b₅ null mice. These data indicate that microsomal cytochrome b₅ can play a major role in the in vivo metabolism of certain drugs and chemicals but in a P450- and substrate-dependent manner.

Critical roles for p22phox in the structural maturation and subcellular targeting of Nox3

Otoconia are small biominerals in the inner ear that are indispensable for the normal perception of gravity and motion. Normal otoconia biogenesis requires Nox3, a Nox (NADPH oxidase) highly expressed in the vestibular system. In HEK-293 cells (human embryonic kidney cells) transfected with the Nox regulatory subunits NoxO1 (Nox organizer 1) and NoxA1 (Nox activator 1), functional murine Nox3 was expressed in the plasma membrane and exhibited a haem spectrum identical with that of Nox2, the electron transferase of the phagocyte Nox. In vitro Nox3 cDNA expressed an approximately 50 kDa primary translation product that underwent N-linked glycosylation in the presence of canine microsomes. RNAi (RNA interference)-mediated reduction of endogenous p22phox, a subunit essential for stabilization of Nox2 in phagocytes, decreased Nox3 activity in reconstituted HEK-293 cells. p22phox co-precipitated not only with Nox3 and NoxO1 from transfectants expressing all three proteins, but also with NoxO1 in the absence of Nox3, indicating that p22phox physically associated with both Nox3 and with NoxO1. The plasma membrane localization of Nox3 but not of NoxO1 required p22phox. Moreover, the glycosylation and maturation of Nox3 required p22phox expression, suggesting that p22phox was required for the proper biosynthesis and function of Nox3. Taken together, these studies demonstrate critical roles for p22phox at several distinct points in the maturation and assembly of a functionally competent Nox3 in the plasma membrane.

Genetically engineered bacterial cells co-expressing human cytochrome P450 with NADPH-cytochrome P450 reductase: prediction of metabolism and toxicity of drugs in humans

Genetically engineered bacterial cells expressing human cytochrome P450 (CYP) have been developed as new tools to predict the metabolism and toxicity of drugs in humans. There are various host cells for the heterologous expression of a form of CYP. Among them, bacterial cells such as Escherichia coli (E. coli) have advantages with regard to ease of use and high yield of protein. CYP protein could be first expressed by the modification of the N-terminal amino acid sequence in E. coli cells in 1991. Since then, many forms of human CYP have been successfully expressed in E. coli cells. Since the E. coli cells do not possess endogeneous electron transport systems to support the full catalytic activity of CYP, E. coli strains co-expressing both human CYP and NADPH-cytochrome P450 reductase (OR) have been established. Each form of CYP expressed in the E. coli cells efficiently catalyzed the oxidation of a representative substrate at an efficient rate, indicating that the OR was sufficiently expressed to support the catalytic activity of CYP. According to the studies performed so far, the modification of the N-terminal amino acid sequence of CYP did not seem to affect the catalytic properties of CYP. The human CYP expressed in the E. coli cells were applicable for studies to determine a metabolic pathway(s) of drugs and to estimate kinetic parameters of drug metabolism by human CYP. Drug-drug interactions caused by inhibition of the metabolism of drugs by human CYP could also be examined by in vitro inhibition studies with CYP expressed in the E. coli cells. Recently, human CYP was co-expressed with the OR in Salmonella typhimurium (S. typhimurium) cells used for mutation assay (Ames test) by applying the technology for the expression of human CYP and the OR in E. coli cells, to evaluate whether chemicals including drugs are metabolically activated by human CYP and show mutagenicity. These strains of bacteria are considered as useful tools to study the metabolism and the toxicity of drugs in humans.

The cationic charges on Arg347, Arg358 and Arg449 of human cytochrome P450c17 (CYP17) are essential for the enzyme's cytochrome b5-dependent acyl-carbon cleavage activities

CYP17 (17alpha-hydroxylase-17,20-lyase; also P450c17 or P450(17alpha)) catalyses the17alpha-hydroxylation of progestogens and the subsequent acyl-carbon cleavage of the 17alpha-hydroxylated products (lyase activity) in the biosynthesis of androgens. The enzyme also catalyses another type of acyl-carbon cleavage (direct cleavage activity) in which the 17alpha-hydroxylation reaction is by-passed. Human CYP17 is heavily dependent on the presence of the membrane form of cytochrome b(5) for both its lyase and direct cleavage activities. In the present study it was found that substitution of human CYP17 amino acids, Arg(347), Arg(358) and Arg(449), with non-cationic residues, yielded variants that were impaired in the two acyl-carbon bond cleavage activities, quantitatively to the same extent and these were reduced to between 3 and 4% of the wild-type protein. When the arginines were replaced by lysines, the sensitivity to cytochrome b(5) was restored and the acyl-carbon cleavage activities were recovered. All of the human mutant CYP17 proteins displayed wild-type hydroxylase activity, in the absence of cytochrome b(5). The results suggest that the bifurcated cationic charges at Arg(347), Arg(358) and Arg(449) make important contributions to the formation of catalytically competent CYP17.cytochrome b(5) complex. The results support our original proposal that the main role of cytochrome b(5) is to promote protein conformational changes which allow the iron-peroxo anion to form a tetrahedral adduct that fragments to produce the acyl-carbon cleavage products.

The roles of cytochrome b5 in cytochrome P450 reactions

Cytochrome b(5), a 17-kDa hemeprotein associated primarily with the endoplasmic reticulum of eukaryotic cells, has long been known to augment some cytochrome P450 monooxygenase reactions, but the mechanism of stimulation has remained controversial. Studies in recent years have clarified this issue by delineating three pathways by which cytochrome b(5) augments P450 reactions: direct electron transfer of both required electrons from NADH-cytochrome b(5) reductase to P450, in a pathway separate and independent of NADPH-cytochrome P450 reductase; transfer of the second electron to oxyferrous P450 from either cytochrome b(5) reductase or cytochrome P450 reductase; and allosteric stimulation of P450 without electron transfer. Evidence now indicates that each of these pathways is likely to operate in vivo.

Assessment of porcine and human 16-ene-synthase, a third activity of P450c17, in the formation of an androstenol precursor. Role of recombinant cytochrome b5 and P450 reductase

Recently, we have shown that the biosynthesis of androstenol, a potential endogenous ligand for the orphan receptors constitutive androstane receptor and pregnane-X-receptor, requires the presence of enzymes of the steroidogenic pathway, such as 3 beta-hydroxysteroid dehydrogenase, 5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase. In this report, we examine at the molecular level whether the enzyme 17 alpha-hydroxylase/17,20-lyase (P450c17), which possesses dual 17 alpha-hydroxylase and 17,20-lyase activities and catalyzes the production of precursors for glucocorticoids and sex steroids, is also able to catalyze the formation of a third class of active steroids, 16-ene steroids (including androstenol). The role of components of the P450 complex is also assessed. We transfected human embryonic kidney (HEK-293) cells with various amounts of vectors expressing P450c17, NADPH-cytochrome P450 reductase, and cytochrome b5. Our results showed that P450c17 possesses a 16-ene-synthase activity able to transform pregnenolone into 5,16-androstadien-3 beta-ol, without the formation of the precursor 17-hydroxypregnenolone. Cytochrome b5 has a much stronger effect on the 16-ene-synthase activity than on the 17 alpha-hydroxylase/17,20-lyase activities. On the other hand, P450reductase has a drastic effect on the latter, but a negligible one on 5,16-androstadien-3 beta-ol synthesis. Our results therefore demonstrate that human P450c17, as other enzymes of the classical steroidogenic pathway, is involved in the biosynthetic pathway leading to the formation of androstenol.

Cytochrome b(5) coexpression increases the CYP2E1-dependent mutagenicity of dialkylnitrosamines in methyltransferase-deficient strains of Salmonella typhimurium

Addition of cytochrome b(5) to recombinant cytochrome P450 2E1 systems has been shown to enhance the metabolism of dialkylnitrosamines in vitro. To determine if this effect could be observed with recombinant expression systems in vivo, we have constructed mutagenicity tester strains that coexpress full-length human cytochrome P450 2E1 (CYP2E1), rat cytochrome P450 reductase, and human cytochrome b(5) in Salmonella typhimurium lacking ogt and ada methyltransferases (YG7104, ogt(-); and YG7108, ogt(-), ada(-)). These new recombinant strains exhibit a four- to five-fold greater mutagenic response to dimethylnitrosamine, diethylnitrosamine, and dipropylnitrosamine than strains that contain only CYP2E1 and reductase, and are over 100-fold more sensitive to nitrosamines than the parental strains in the presence of an exogenous activating system (S9 fraction). The four-fold increase in mutagenicity in the presence of cytochrome b(5) was consistent with increasing alkyl chain length up to dibutylnitrosamine, which was poorly activated by CYP2E1. The greatest enhancement was obtained with a tricistronic construct in which the b(5) cDNA preceded the P450 and reductase cDNAs; placing the b(5) cDNA after the reductase cDNA was substantially less effective. These new, highly sensitive strains may prove useful in the detection of nitrosamine contamination of food and environmental samples.

Enantioselectivity of bunitrolol 4-hydroxylation is reversed by the change of an amino acid residue from valine to methionine at position 374 of cytochrome P450-2D6

The enantioselectivity of 4-hydroxylation of bunitrolol (BTL), a beta-adrenoceptor blocking drug, was studied in microsomes from human liver, human hepatoma (Hep G2) cells expressing CYP2D6, and lymphoblastoid cells expressing CYP2D6. Kinetics in human liver microsomes showed that the Vmax value for (+)-BTL was 2.1-fold that of (-)-BTL, and that the Km value for (+)-BTL was lower than that for the (-)-antipode, resulting in the intrinsic clearance (Vmax/Km) of (+)-BTL being 2.1-fold over its (-)-antipode. CYP2D6 (CYP2D6-met) expressed in Hep G2 cells had a methionine residue at position 373 of the amino acid sequence and a rat-type N-terminal peptide (MELLNGTGLWSM) instead of the human-type (MGLEALVPLAVIV), and showed enantioselectivity of [(+)-BTL < (-)-BTL] for the rate of BTL 4-hydroxylation. In contrast, enantioselectivity [(+)-BTL > (-)-BTL] for Hep G2-CYP2D6 (CYP2D6-val) with a human-type N-terminal peptide that had a valine residue at 374, which corresponds to the methionine of the CYP2D6-met variant, was the same as that for human liver microsomes. We further confirmed that CYP2D6-met and CYP2D6-val expressed in human lymphoblastoid cells, both of which have methionine and valine, respectively, at position 374 and a human-type N-terminal peptide, exhibited the same enantioselectivities as those obtained from CYP2D6-met and CYP2D6-val expressed in the Hep G2 cell system. These results indicate that the amino acid at 374 of CYP2D6 is one of the key factors influencing the enantioselectivity of BTL 4-hydroxylation.

The molecular basis of premature adrenarche: an hypothesis

Adrenarche is characterized by a prepubertal rise in adrenal secretion of dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS) that is independent of the gonads or gonadotropins. Adrenopause is the corresponding diminution in DHEA and DHEAS concentrations in later life. The mechanisms by which adrenarche and adrenopause are induced and regulated are unknown. Early work focused on identifying hypothetical adrenal androgen regulatory hormones that would induce DHEA in much the same way that adrenocorticotropin induces cortisol, but no such factors have been found. Current studies of adrenarche focus on intra-adrenal events, particularly those concerning 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and 17alpha-hydroxylase/17,20-lyase (P450c17). Molecular data implicate a decrease in 3beta-HSD specifically in the adrenal zona reticularis. However, a decrease in 3beta-HSD is insufficient to explain why the reticularis catalyzes 17,20-lyase activity and hence makes DHEA, rather than catalyzing only 17alpha-hydroxylase activity, as does the zona fasciculata. P450c17 appears to catalyze 17,20-lyase activity only if P450c17 has undergone serine phosphorylation and has access to cytochrome b5 as an allosteric cofactor. Although these two factors have not yet been investigated in adrenarche, it appears that both a zone-specific diminution in 3beta-HSD and a zone-specific induction of 17,20-lyase activity are required to account for the physiological data. Exaggerated premature adrenarche appears to be an early sign of polycystic ovary syndrome (PCOS). Mechanistic considerations of PCOS suggest a key role for serine phosphorylation of P450c17 in both adrenarche and some forms of heritable PCOS.

Effects of human cytochrome b5 on CYP3A4 activity and stability in vivo

Cytochrome P450s (P450) form a superfamily of membrane-bound proteins that play a key role in the primary metabolism of both xenobiotics and endogenous compounds such as drugs and hormones, respectively. To be enzymically active, they require the presence of a second membrane-bound protein, NADPH P450 reductase, which transfers electrons from NADPH to the P450. Because of the diversity of P450 enzymes, much of the work on individual forms has been carried out on purified proteins, in vitro, which requires the use of complex reconstitution mixtures to allow the P450 to associate correctly with the NADPH P450 reductase. There is strong evidence from such reconstitution experiments that, when cytochrome b5 is included, the turnover of some substrates with certain P450s is increased. Here we demonstrate that allowing human P450 reductase, CYP3A4, and cytochrome b5 to associate in an in vivo-like system, by coexpressing all three proteins together in Escherichia coli for the first time, the turnover of both nifedipine and testosterone by CYP3A4 is increased in the presence of cytochrome b5. The turnover of testosterone was increased by 166% in whole cells and by 167% in preparations of bacterial membranes. The coexpression of cytochrome b5 also resulted in the stabilization of the P450 during substrate turnover in whole E. coli, with 109% of spectrally active CYP3A4 remaining in cells after 30 min in the presence of cytochrome b5 compared with 43% of the original P450 remaining in cells in the absence of cytochrome b5.

A cytochrome b5 is required for full activity of flavonoid 3', 5'-hydroxylase, a cytochrome P450 involved in the formation of blue flower colors

The substitution pattern of anthocyanin pigments is a main determinant of flower color. Flavonoid 3',5'-hydroxylase (F3'5'H) is a cytochrome P450 enzyme (Cyt P450) that catalyzes the 3', 5'-hydroxylation of dihydroflavonols, the precursors of purple anthocyanins. Species such as rose and carnation lack F3'5'H activity and are, therefore, unable to generate purple or blue flowers. Petunia, on the other hand, contains two loci, termed hf1 and hf2, that encode a Cyt P450 with F3'5'H activity. Here we report the identification of an additional petunia gene that is required for 3',5' substitution of anthocyanins and purple flower colors. It encodes a cytochrome b5 and is expressed exclusively in the flower. Inactivation of the gene by targeted transposon mutagenesis reduced F3'5'H enzyme activity and the accumulation of 5'-substituted anthocyanins, resulting in an altered flower color. However, no phenotypic effect on the activity of other Cyt P450s, involved in the synthesis of hormones or general phenylpropanoids, was observed. These data provide in vivo evidence for the regulation of the activity of specific Cyt P450s by a cytochrome b5.

Suppression of CYP1A1 transcription by H2O2 is mediated by xenobiotic-response element

We have previously demonstrated that H2O2 downregulates CYP1A1 and CYP1A2 transcription in isolated rat hepatocytes (C. W. Barker, et al., 1994, J. Biol. Chem. 269, 3985-3990). In the present study, induction of chloramphenicol acetyltransferase (CAT) expression driven by 3.1 kb of rat CYP1A1 upstream regulatory sequences was suppressed by 56% in Hepa-1 cells treated with H2O2. Similarly, H2O2 inhibited CAT expression from vectors containing two copies of either xenobiotic-response element (XRE) 1 or XRE2. H2O2 did not inhibit basal CAT expression in cells that were not treated with the inducer beta-napthoflavone. Electrophoretic mobility shift assays demonstrated that the suppression of XRE-dependent transcription by H2O2 was not due to changes in nuclear aryl hydrocarbon (Ah) receptor DNA binding activity. Several types of experiments indicated that modulation of XRE enhancer strength by various means could modify H2O2-dependent suppression of CAT expression. Conditions that increased the transactivation potential of the Ah receptor (increase in XRE copy number or shortening of the distance between XREs and the minimal CYP1A1 promoter) attenuated the action of H2O2, while conditions that reduced XRE-mediated transactivation potential (decrease in XRE copy number, increase of the distance between the XRE and the promoter, or reduction of the number of bound Ah receptors by lowering the concentration of inducer) potentiated the inhibitory action of H2O2.

The effect of acute stress and opioid antagonist on the activity of NADPH-P450 reductase in rat Leydig cells

Previous studies indicate that acute immobilization stress (IMO; 2 h) impaired testicular steroidogenesis primarily at the testicular level decreasing the activity of certain steroidogenic enzymes. In the present study unstressed rats as well as IMO rats (2 h) were treated by intratesticular injection of naltrexone methobromide (NMB; peripheral opioid receptor antagonist; 36 microg/testis) or vehicle at the beginning of and at 1 h of the IMO period. In IMO rats the activity of P450c17 was significantly reduced as well as the activity of NADPH-P450 reductase (which catalyzes the transfer of electrons from NADPH to cytochrome P450), while the activity of NADH-b5 reductase was not affected. Present data confirmed previous results that acute IMO reduced testicular P450c17 activity and implicate that decreased activity of NADPH-P450 reductase could be responsible for the inhibition of P450c17 under IMO conditions, while NADH-b5 reductase is probably not involved. NMB treatment antagonized the inhibitory effect of IMO on P450c17 and NADPH-P450 reductase activities. Such results put forward the implication that endogenous opioid peptides are involved in mediating the inhibitory effect of IMO on testicular steroidogenesis, and allow the speculation that NADPH-P450 reductase could be a possible site of such an inhibition.

High catalytic activity of human cytochrome P450 co-expressed with human NADPH-cytochrome P450 reductase in Escherichia coli

Forms of human cytochrome P450 (P450 or CYP), such as CYP1A1, CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, were expressed or co-expressed together with human NADPH-P450 reductase in Escherichia coli. When P450 was expressed alone in E. coli, the expression level of holo-P450 ranged from 310 to 1620 nmol/L of culture. The expression level of holo-P450 decreased by co-expression with the reductase, and the level ranged from 66 to 381 nmol/L of culture. The expression level of the reductase varied depending on the forms of P450 co-expressed, and ranged from 204 to 937 U/L of culture. We assayed the catalytic activity of P450 using E. coli cells disrupted by freeze-thaw. When co-expressed with the reductase, human P450 catalyzed the oxidation of representative substrates at efficient rates. The rates appeared comparable to the reported activities of P450 in a reconstituted system containing purified preparations of P450 and the reductase.

Cytochrome b5 augments the 17,20-lyase activity of human P450c17 without direct electron transfer

In the biosynthesis of steroid hormones, P450c17 is the single enzyme that catalyzes both the 17alpha-hydroxylation of 21-carbon steroids and the 17,20-lyase activity that cleaves the C17-C20 bond to produce C19 sex steroids. Cytochrome b5 augments the 17,20-lyase activity of cytochrome P450c17 in vitro, but this has not been demonstrated in membranes, and the mechanism of this action is unknown. We expressed human P450c17, human P450-oxidoreductase (OR), and/or human cytochrome b5 in Saccharomyces cerevisiae and analyzed the 17alpha-hydroxylase and 17,20-lyase activities of the resulting yeast microsomes. Yeast expressing only P450c17 have 17alpha-hydroxylase and trace 17,20-lyase activities toward both Delta4 and Delta5 steroids. Coexpression of human OR with P450c17 increases the Vmax of both the 17alpha-hydroxylase and 17,20-lyase reactions 5-fold; coexpression of human b5 with P450c17 also increases the Vmax of the 17,20-lyase reactions but not of the 17alpha-hydroxylase reactions. Simultaneous expression of human b5 with P450c17 and OR, or addition of purified human b5 to microsomes from yeast coexpressing human P450c17 and OR, further increases the Vmax of the 17,20-lyase reaction without altering 17alpha-hydroxylase activity. Genetically engineered yeast and mixing experiments demonstrate that OR is both necessary and sufficient for microsomal 17,20-lyase activity. Addition of purified human holo-b5, apo-b5, or cytochrome c to microsomes containing both human P450c17 and OR demonstrate that the stimulatory action of b5 does not require electron transfer from b5 to P450c17. These data suggest that human b5 acts principally as an allosteric effector that interacts primarily with the P450c17.OR complex to stimulate 17, 20-lyase activity.

High levels of recombinant CYP3A4 expression in Chinese hamster ovary cells are modulated by coexpressed human P450 reductase and hemin supplementation

Expression of recombinant cytochrome P450s (P450s) in mammalian cells has been used as a powerful tool to study these enzymes. However, the activity of CYP3A4 expressed in several stable mammalian cell lines was much lower than native enzyme in human liver. The low level of recombinant CYP3A4 may have been due to the low copy number of the cDNA. In addition, the low activity is caused by the low level of P450 reductase in these cells. To achieve high levels of CYP3A4 expression, we employed gene amplification of the CYP3A4 cDNA in Chinese hamster ovary (CHO) cells followed by transfection of the P450 reductase cDNA. Using this strategy, we have obtained a cell line, designated D3A4, with high levels of recombinant CYP3A4. The content of spectrally active P450 was 14 pmol/mg total cellular protein. Hemin treatment increased the P450 content 2-fold. Upon coexpression of P450 reductase in DHR/3A4 cells, enzyme activity of CYP3A4 was stimulated 15-fold, despite a 40% decrease in spectrally active P450. Interestingly, the latter effect was not due to a decrease in CYP3A4 mRNA. Treatment of these cells with hemin, however, counteracted the P450 reductase-mediated decrease of spectrally active P450. These data demonstrate that P450 reductase has a strong influence on the levels of recombinant P450 holoenzyme, possibly by modulating the level of heme in CHO cells. Concomitantly our results show that the gene amplification strategy provides a powerful approach to obtain a high level of functional recombinant P450.

Molecular characterization of mitochondrial trifunctional protein deficiency: formation of the enzyme complex is important for stabilization of both alpha- and beta-subunits

Mitochondrial trifunctional protein (TP) is an enzyme complex with three activities: enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase. Studies on defects in this enzyme in patients with TP deficiency suggest that there are two types of defect. Patients in group 1 have normal amount of cross-reacting material by immunoblot and lack only long-chain 3-hydroxyacyl-CoA dehydrogenase activity. Patients in group 2 have a trace amount of cross-reacting material, with all three activities being low. We identified three patients in group 2, and analysis was made at the cDNA level. In patient 2, there was a heterozygous 71-bp deletion at position 110-180 in the alpha-subunit. In patients 1 and 3, there was an abnormal beta-subunit; patient 1 had an A-788-to-G substitution, and patient 3 had G-182-to-A and G-740-to-A substitutions in each of separate alleles. This is the first demonstration of disease-causing mutations in the beta-subunit. cDNA-expression experiments in patients' fibroblasts, using a vaccinia virus system, and gel filtration analysis, using patients' fibroblasts, revealed that the existence of both normal alpha- and beta-subunits, and possibly their association, are important for stabilizing TP and that A-788-to-G substitution on the beta-subunit in patient 1 seems to interfere with the association, the result being a rapid decomposition of TP.

Serine phosphorylation of human P450c17 increases 17,20-lyase activity: implications for adrenarche and the polycystic ovary syndrome

Microsomal cytochrome P450c17 catalyzes both steroid 17 alpha-hydroxylase activity and scission of the C17-C20 steroid bond (17,20-lyase) on the same active site. Adrenal 17 alpha-hydroxylase activity is needed to produce cortisol throughout life, but 17,20-lyase activity appears to be controlled independently in a complex, age-dependent pattern. We show that human P450c17 is phosphorylated on serine and threonine residues by a cAMP-dependent protein kinase. Phosphorylation of P450c17 increases 17,20-lyase activity, while dephosphorylation virtually eliminates this activity. Hormonally regulated serine phosphorylation of human P450c17 suggests a possible mechanism for human adrenarche and may be a unifying etiologic link between the hyperandrogenism and insulin resistance that characterize the polycystic ovary syndrome.

The stoichiometry of the cytochrome P-450-catalyzed metabolism of methoxyflurane and benzphetamine in the presence and absence of cytochrome b5

The complete stoichiometry of the metabolism of the cytochrome b5 (cyt b5)-requiring substrate, methoxyflurane, by purified cytochrome P-450 2B4 was compared to that of another substrate, benzphetamine, which does not require cyt b5 for its metabolism. Cyt b5 invariably improved the efficiency of product formation. That is, in the presence of cyt b5 a greater percentage of the reducing equivalents from NADPH were utilized to generate substrate metabolites, primarily at the expense of the side product, superoxide. With methoxyflurane, cyt b5 addition always resulted in an increased rate of product formation, while with benzphetamine the rate of product formation remained unchanged, increased or decreased. The apparently contradictory observations of increased reaction efficiency but decrease in total product formation for benzphetamine can be explained by a second effect of cyt b5. Under some experimental conditions cyt b5 inhibits total NADPH consumption. Whether stimulation, inhibition, or no change in product formation is observed in the presence of cyt b5 depends on the net effect of the stimulatory and inhibitory effects of cyt b5. When total NADPH consumption is inhibited by cyt b5, the rapidly metabolized, highly coupled (approximately equal to 50%) substrate, benzphetamine, undergoes a net decrease in metabolism not counterbalanced by the increase in the efficiency (2-20%) of the reaction. In contrast, in the presence of the slowly metabolized, poorly coupled (approximately equal to 0.5-3%) substrate, methoxyflurane, inhibition of total NADPH consumption by cyt b5 was never sufficient to overcome the stimulation of product formation due to an increase in efficiency of the reaction.

Cloning of human very-long-chain acyl-coenzyme A dehydrogenase and molecular characterization of its deficiency in two patients

Two overlapping cDNA clones (1,991 bp and 736 bp, respectively) encoding the precursor of human mitochondrial very-long-chain acyl-coenzyme A dehydrogenase (VLCAD) were cloned and sequenced. The cDNA inserts of these clones together encompass a region of 2,177 bases, encoding the entire protein of 655 amino acids, including a 40-amino acid leader peptide and a 615-amino acid mature polypeptide. PCR-amplified VLCAD cDNAs were sequenced in cultured fibroblasts from two VLCAD-deficient patients. In both patients, a 105-bp deletion encompassing bases 1078-1182 in VLCAD cDNA was identified. The deletion seems to occur due to exon skipping during processing of VLCAD pre-mRNA. This is the first demonstration of a mutation causing VLCAD deficiency. Quantitative cDNA expression of normal human VLCAD was performed in the patients' fibroblasts, using vaccinia viral system, which demonstrated that the deficiency of the normal VLCAD protein causes impaired long-chain fatty acid beta-oxidation activity in the patients' fibroblasts. In patient fibroblasts, raising VLCAD activity to approximately 20% of normal control fibroblast activity raised palmitic acid beta-oxidation flux to the level found in control fibroblasts, which may offer important information for the rational design of future somatic gene therapy for VLCAD deficiency.

Specificity of the cytochrome P-450 interaction with cytochrome b5

The specificity of the interaction of cytochrome b5 with different forms of cytochrome P-450 was examined. Immunopurification of cytochromes P-450 1A1, 2B1 and 2E1 from rat liver microsomes resulted in co-purification of cytochrome b5 with cytochrome P-450 forms 2B1 and 2E1 but not 1A1. This specificity was evaluated in conjunction with multiple sequence alignment of the three cytochrome P-450s and a molecular model of the cytochrome P-450-cytochrome b5 complex [(1989) Biochemistry 28, 8201-8205]. These analyses suggest two basic residues in the arginine cluster region of P-450, which are present in P-450s 2B1 and 2E1 but are absent in P-450 1A1, as potential binding sites for cytochrome b5.

Isolation and complete sequence of CBR, a gene encoding a putative cytochrome b reductase in Saccharomyces cerevisiae

We have isolated and characterised a novel yeast gene, CBR (cytochrome b reductase), encoding a 35-kDa yeast novobiocin-binding protein. The predicted protein sequence of CBR displays considerable similarity to both plant nitrate reductases and mammalian cytochrome b5 reductases indicating that it is a putative member of the flavoprotein pyridine-nucleotide-cytochrome-reductase family. Disruption of CBR is not lethal under various growth conditions, suggesting the presence of some functional overlap with other reductases, possibly with the cytochrome P-450 reductase.

The human cytochrome b5 gene and two of its pseudogenes are located on chromosomes 18q23, 14q31-32.1 and 20p11.2, respectively

Using very high stringency hybridization conditions for the Southern blot hybridization analysis of hamster-human cell hybrid DNA, we were able to map the human cytochrome b5 gene and two of its pseudogenes (psgb(5)1 and psgb(5)2) unambiguously to chromosomes 18, 14, and 20. These localizations were confirmed and extended to 18q23, 14q31-32.1, and 20p11.2 by using a combination of nonisotopic in situ hybridization of chromosomal spreads and the polymerase chain reaction analysis of DNA samples isolated from somatic cell hybrids retaining deletions or translocations of chromosome 18.

A model system for studying membrane biogenesis. Overexpression of cytochrome b5 in yeast results in marked proliferation of the intracellular membrane

Cytochrome b5 is an amphipathic microsomal protein that is anchored to the endoplasmic reticulum by a single hydrophobic transmembrane alpha-helix located near the carboxyl terminus of the protein. In yeast, cytochrome b5 provides electrons for fatty acid desaturation and ergosterol biosynthesis. High level expression of cytochrome b5 in Saccharomyces cerevisiae was achieved using the yeast metallothionein promoter and a synthetic cytochrome b5 gene. In order to accommodate the markedly increased amount of the membrane-bound cytochrome b5, the yeast cell proliferated its nuclear membrane. As many as 20 pairs of stacked membranes could be observed to partially encircle the nucleus. This morphological arrangement of membrane around the nucleus is known as a karmella. In an effort to understand which part of the cytochrome b5 molecule, i.e. the membrane anchor or the soluble heme domain, which is competent in electron transfer, provided the signal for the de novo membrane biogenesis, a series of studies, including site-directed mutagenesis, was undertaken. The results of these experiments demonstrated that the inactive hemedeficient apo form of the membrane-bound protein stimulates membrane proliferation to the same extent as the holo wild-type protein, whereas cytosolic forms of cytochrome b5 did not induce membrane synthesis. These data demonstrate that membrane proliferation is a consequence of the cell's ability to monitor the level of membrane proteins and to compensate for alterations in these levels rather than the result of the ability of the extra cytochrome b5 to catalyze synthesis of extra lipid that had to be accommodated in new membrane. Site-directed mutagenesis studies of the membrane binding domain of cytochrome b5 provided additional clues about the nature of the signal for membrane proliferation. Replacement of the membrane anchor by a non-physiological nonsense sequence of 22 leucines gave rise to a mutant protein that triggered membrane biosynthesis. The conclusion from these experiments is clear; the signal for membrane proliferation does not reside in some specific amino acid sequence but instead in the hydrophobic properties of the proliferant. Interestingly, these membranes are somewhat diminished in quantity and have a slightly altered morphology compared to those induced by the wild-type protein. It was also observed that disruption of the putative alpha helix of the membrane anchor by an Ala116Pro mutation, which gives rise to two sequential prolines at positions 115 and 116 results in a protein with diminished capacity to induce membrane formation.(ABSTRACT TRUNCATED AT 400 WORDS)

NADH-cytochrome b5 reductase and cytochrome b5 isoforms as models for the study of post-translational targeting to the endoplasmic reticulum

Cytochrome b5 and NADH-cytochrome b5 reductase are integral membrane proteins with cytosolic active domains and short membrane anchors, which are inserted post-translationally into their target membranes. Both are produced as different isoforms, with different localizations, in mammalian cells. In the rat, the reductase gene generates two transcripts by an alternative promoter mechanism: a ubiquitous mRNA coding for the myristylated membrane-bound form, and an erythroid mRNA which generates both the soluble form and a nonmyristylated membrane-binding form. The available evidence indicates that the ubiquitous myristylated form binds to the cytosolic face of both outer mitochondrial membranes and ER. In contrast, two genes code for two homologous forms of cytochrome b5, one of which is found on outer mitochondrial membranes, the other on the ER. The gene specifying the ER form probably also generates an erythroid-specific mRNA by alternative splicing, which codes for soluble cytochrome b5. Possible molecular mechanisms responsible for the observed localizations of these different enzyme isoforms are discussed.

Polymorphism in stereoselective hydroxylations of mephenytoin and hexobarbital by Japanese liver samples in relation to cytochrome P-450 human-2 (IIC9)

1. Stereoselective 4'-hydroxylations of R-(--)-mephenytoin and S-(+)-mephenytoin were determined in liver microsomes of 19 Japanese subjects. 2. The content of P-450 human-2 assessed by Western-blots correlated with microsomal S-(+)-mephenytoin 4'-hydroxylation. Antibody raised against P-450 human-2 effectively inhibited microsomal S-(+)-mephenytoin 4'-hydroxylation, but was less efficient for inhibition of R-(--)-mephenytoin 4'-hydroxylation in extensive metabolizers, and 4'-hydroxylation of both mephenytoin enantiomers in poor metabolizers. 3. Similar results were observed on the stereoselective hydroxylations of R-(--)- and S-(+)-hexobarbital. Clear correlations were observed for the content of P-450 human-2 and microsomal R-(--)-hexobarbital 3'alpha-hydroxylation and S-(+)-hexobarbital 3'beta-hydroxylation. 4. Moreover, yeast microsomes expressing P-450 human-2 cDNA showed high stereoselectivities for hydroxylations of mephenytoin and hexobarbital similar to those observed in human liver. 5. Two other cytochromes P-450(IIC 9/10) expressed in yeast, whose cDNA were synthesized by site-directed mutagenesis from human-2 cDNA, showed no stereoselectivity for the hydroxylations of mephenytoin and hexobarbital, in spite of the modification of only two amino acid substitutions or deletions in the whole sequence. 6. Only a cytochrome derived from P-450 human cDNA corresponding to P-450 human-2 was expressed in human livers, the two cytochromes of the three related IIC9/10 forms were not expressed. 7. These findings indicate that P-450 human-2 is the major cytochrome P-450 responsible for the polymorphisms in stereoselective hydroxylations of mephenytoin and hexobarbital.