Immunohistochemical studies on electron transport proteins associated with cythochromes P-450 in steroidogenic tissues II. Microsomal NADPH-cytochrome c reductase in the rat adrenal

NADPH-cytochrome c reductase (NADPH : ferricytochrome oxido-reductase, EC 1.6.2.4), the flavoprotein which mediates the NADPH-dependent reduction of cytochromes P-450 in adrenocortical microsomes, has been localized immunohistochemically at the light microscopic level in rat adrenal glands. Localization was achieved through the use of sheep antiserum procued against purified, trypsin-solubilized rat hepatic microsomal NADPH-cytochrome c reductase in both an unlabeled antibody peroxidase-antiperoxidase techniques and an indirect fluorecent antibody method. The sheep antibody to rat hepatic microsomal NADPH-cytochrome c reductase concomitantly inhibited the NADPH-cytochrome c reductase and progesterone 21-hydroxylase activities catalyzed by isolated rat adrenal microsomes. When sections of rat adrenal glands were exposed to the reductase antiserum in both immunohistochemical procedures, positive staining for NADPH-cytochrome c reductase was observed in parenchymal cells of the three cortical zones but not in medullary chromaffin cells. The intensity of staining, however, was found to differ among the three cortical zones, with the most intense staining being found in the zona fasciculata and the least in the zona glomerulosa. The intensity of staining was also found differ among cells within the zona fasciculata. These immunohistochemical observations demonstrate that microsomal NADPH-cytochrome c reductase is not distributed uniformly throughout the rat adrenal cortex.     

Structure and function of NADPH-cytochrome P450 reductase and nitric oxide synthase reductase domain

NADPH-cytochrome P450 reductase (CPR) and the nitric oxide synthase (NOS) reductase domains are members of the FAD-FMN family of proteins. The FAD accepts two reducing equivalents from NADPH (dehydrogenase flavin) and FMN acts as a one-electron carrier (flavodoxin-type flavin) for the transfer from NADPH to the heme protein, in which the FMNH*/FMNH2 couple donates electrons to cytochrome P450 at constant oxidation-reduction potential. Although the interflavin electron transfer between FAD and FMN is not strictly regulated in CPR, electron transfer is activated in neuronal NOS reductase domain upon binding calmodulin (CaM), in which the CaM-bound activated form can function by a similar mechanism to that of CPR. The oxygenated form and spin state of substrate-bound cytochrome P450 in perfused rat liver are also discussed in terms of stepwise one-electron transfer from CPR. This review provides a historical perspective of the microsomal mixed-function oxidases including CPR and P450. In addition, a new model for the redox-linked conformational changes during the catalytic cycle for both CPR and NOS reductase domain is also discussed.     

The obligatory requirement of cytochrome b5 in the p-nitroanisole O-demethylation reaction catalyzed by cytochrome P-450 with a high affinity for cytochrome b5

The role of cytochrome $\text{b}$₅ in the $\text{p}$-nitroanisole O-demethylation was studied with a reconstituted system containing a unique cytochrome P-450, isolated from rabbit liver microsomes as a species with a high affinity for cytochrome $\text{b}$₅. The maximal activity was obtained in the complete system consisting of cytochrome P-450, NADPH-cytochrome P-450 reductase, NADH-cytochrome $\text{b}$₅ reductase, and Triton X-100 in addition to cytochrome $\text{b}$₅. The omission of cytochrome $\text{b}$₅ from the complete system entirely abolished the activity. These results clearly show that cytochrome $\text{b}$₅ is obligatory in the reconstitute p-nitroanisole O-demethylation system, and this cytochrome P-450 probably interacts with cytochrome $\text{b}$₅ in such a way that the second electron is transferred from cytochrome $\text{b}$₅ and thus exhibits the demethylase activity.     

Interaction between NADPH-cytochrome P-450 reductase and cytochrome P-450 in the membrane of phosphatidylcholine vesicles.

Cytochrome P-450 and NADPH-cytochrome P-450 REDUctase, both purified from liver microsomes of phenobarbital-pretreated rabbits, have been incorporated into the membrane of phosphoaditylcholine vesicles by the cholate dialysis method. The reduction of cytochrome P-450 by NADPH in this system is biphasic, consisting of two first-order reactions. The rate constant of the fast phase, in which 80--90% of the total cytochrome is reduced, increases as the molar ratio of the reductase to the cytochrome is increased at a fixed ratio of the cytochrome to phosphatidylcholine, suggesting that the rate-limiting step of the fast phase is the interaction between the reductase and the cytochrome. The rate constant of the fast phase also increases when the amount of phosphatidylcholine, relative to those of the two proteins, is decreased. This latter observation suggests that the interaction between the two proteins is effected by their random collision caused by their lateral mobilities on the plane of the membrane of phosphatidylcholine vesicles. The rate constant of the slow phase as well as the fraction of cytochrome P-450 reducible in the slow phase, on the other hand, remains essentially constant even upon alteration in the ratio of the reductase to the cytochrome or in that of the two proteins to phosphatidylcholine. No satisfactory explanation is as yet available for the cause of the slow-phase reduction of cytochrome P-450. The overall activity of benzphetamine N-demethylation catalyzed by the reconstituted vesicles responds to changes in the composition of the sysTEM IN A SIMILAR WAY TO THE FAST-PHASE REDUCTION OF CYTOCHROME P-450, though the latter is not the rate-limiting step of the overall reaction.     

Interactions of mammalian cytochrome P450, NADPH-cytochrome P450 reductase, and cytochrome b(5) enzymes

An immobilized system was developed to detect interactions of human cytochromes P450 (P450) with the accessory proteins NADPH-P450 reductase and cytochrome b(5) (b(5)) using an enzyme-linked affinity approach. Purified enzymes were first bound to wells of a polystyrene plate, and biotinylated partner enzymes were added and bound. A streptavidin-peroxidase complex was added, and protein-protein binding was monitored by measuring peroxidase activity of the bound biotinylated proteins. In a model study, we examined protein-protein interactions of Pseudomonas putida putidaredoxin (Pdx) and putidaredoxin reductase (PdR). A linear relationship (r(2)=0.96) was observed for binding of PdR-biotin to immobilized Pdx compared with binding of Pdx-biotin to immobilized PdR (the estimated K(d) value for the Pdx.PdR complex was 0.054muM). Human P450 2A6 interacted strongly with NADPH-P450 reductase; the K(d) values (with the reductase) ranged between 0.005 and 0.1muM for P450s 2C19, 2D6, and 3A4. Relatively weak interaction was found between holo-b(5) or apo-b(5) (devoid of heme) with NADPH-P450 reductase. Among the rat, rabbit, and human P450 1A2 enzymes, the rat enzyme showed the tightest interaction with b(5), although no increases in 7-ethoxyresorufin O-deethylation activities were observed with any of the P450 1A2 enzymes. Human P450s 2A6, 2D6, 2E1, and 3A4 interacted well with b(5), with P450 3A4 yielding the lowest K(d) values followed by P450s 2A6 and 2D6. No appreciable increases in interaction between human P450s with b(5) or NADPH-P450 reductase were observed when typical substrates for the P450s were included. We also found that NADPH-P450 reductase did not cause changes in the P450.substrate K(d) values estimated from substrate-induced UV-visible spectral changes with rabbit P450 1A2 or human P450 2A6, 2D6, or 3A4. Collectively, the results show direct and tight interactions between P450 enzymes and the accessory proteins NADPH-P450 reductase and b(5), with different affinities, and that ligand binding to mammalian P450s did not lead to increased interaction between P450s and the reductase.     

Fructose utilization and altered cytochrome P-450 in cultured hepatocytes from adult rats

Cultured adult rat hepatocytes incubated in media containing fructose exhibit increased levels of cytochrome P-450, relative to cells incubated with equimolar glucose, and the effect of fructose is proportional to its concentration between 2 and 10 mM. For investigating the mechanism of the effect of fructose on cytochrome P-450 in cultured cells, [U-¹⁴C]fructose or [U-¹⁴C]-glucose were added to the incubation medium, and their uptake and utilization were compared. While the uptake kinetics of the two hexoses were similar, the rate of phosphorylation of fructose was more than 10-fold that of glucose. Similarly, the appearance of fructose carbon in metabolic pools, as well as its conversion to CO₂ and cellular glycerolipid, was increased. The latter finding suggested that fructose might alter cytochrome P-450 by stimulating glycerolipid synthesis, since the stability of the cytochrome is lipid-dependent. However, the changes in glycerolipid formation failed to parallel changes in the level of cytochrome P-450 in fructose-treated cells. Moreover, the relative distribution of ¹⁴C into specific lipids was similar for both hexoses, suggesting that an increased carbon flux in cells incubated with fructose did not directly impose a qualitative change in cellular lipid synthesis. We conclude that the fructose-mediated alteration of cytochrome P-450 in cultured rat hepatocytes reflects a process other than increased incorporation of fructose carbon into metabolic pools.     

Purification and characterization of NADPH-cytochrome P-450 reductase from rat epidermis

NADPH-cytochrome P-450 oxidoreductase (P-450 red) transfers reducing equivalents from NADPH to cytochrome P-450 (P-450) in the monooxygenase system. Detergent solubilized proteins from the membrane fraction of neonatal rat epidermis were purified by 2′,5′-ADP-agarose affinity column chromatography. The purified protein showed an apparent homogeneity on sodium dodecylsulfate-polyacrylamide gel electrophoresis and molecular weight was estimated to be 78 kDa. NADPH-cytochrome c reductase activity increased by 95-fold in the purified enzyme. Epidermal P-450 red in vitro reconstituted benzo(a)pyrene hydroxylase activity in a dose dependent manner with P-450 purified from either rat liver or epidermis. Western blot analysis demonstrated that epidermal P-450 red immunologically cross reacts to liver P-450 red. Immunohistochemical staining showed that the enzyme was predominantly localized in the epidermis. The intensity of immunohistochemical staining of rat skin sections and tissue distribution did not change in the skin treated with β-naphtoflavone, which results in a substantial increase in P-450 1A1 activity. Quantitative assessment of P-450 red in treated and untreated epidermis also showed no change. These findings indicate that constitutive P-450 red, fully capable of supporting P-450, exists in rat epidermis, and can function in metabolism of endogenous and exogenous compounds.     

微粒体细胞色素P-450、NADPH-细胞色素P-450还原酶的纯化与重组活性

经苯巴比妥钠诱导的雄性大白鼠的肝微粒体纯化的细胞色素P-450同功酶组份,经SDS-PAGE鉴定呈电泳纯,分子量为55kD。部分纯化的NADPH-细胞色素P-450还原酶,含72和77kD两个蛋白质组分。上述细胞色素P-450和NADPH-细胞色素P-450还原酶与卵磷脂制备的脂质体重组后的活性试验表明,对艾氏剂有环氧化作用,对环已烷有羟化作用,对溴氰菊酯的羟化作用微弱。当重组系统中缺少细胞色素P-450组份时,对环已烷不再起作用。同时还研究了纯化的细胞色素P-450的光谱特性。     

Multiplicity of cytochrome P-450 in microsomal membranes from the housefly, Musca domestica

The heterogeneity of cytochrome P-450 in abdominal microsomes from the CSMA, SBO, Fc, Rutgers and Baygon strains of the housefly was examined by three different methods. Examination of ‘apparent absolute absorption spectra’ indicated at least two types of cytochrome in all strains, one with an absorption maximum at about 394 nm, being present in greater quantity in the insecticide-resistant strains, while the other, with an absorption maximum at about 412 nm, predominates in the insecticide-susceptible strains.Controlled tryptic digestion of microsomes followed by spectral examination at various time intervals indicated a heterogeneous population of cytochromes P-450 in CSMA, Fc and Rutgers strains.Subfractionation of microsomes from houseflies of the CSMA and Fc strains by a two-step discontinuous sucrose gradient centrifugation method provided evidence for cytochromes P-450 of different spectral characteristics. The concentration of cytochrome P-450, as well as its spectral characteristics varied between fractions and strains.     

Effect of selenolipoic acid on peroxynitrite-dependent inactivation of NADPH-cytochrome P450 reductase

Seleno-organic compounds are known as efficient “scavengers” of peroxynitrite (PN). Here we studied the protective effect of selenolipoic acid (SeLA), the seleno-containing analogue of lipoic acid, on peroxynitrite-dependent inactivation of NADPH-cytochrome P450 reductase. 3-Morpholinosydnonimine hydrochloride (SIN-1) was used as a source of peroxynitrite. The reductase was irreversibly inactivated by PN generated from SIN-1. The inactivation occurred with the rate constant of about 3 × 10⁴M^-1s^-1. The presence of SeLA at low concentration (0.5 μM) led to synergistic increase of the reductase inactivation by PN. Our results suggest the formation of a reactive derivative of SeLA in the reaction of SeLA with PN, probably selenolseleninate, that mediates the aggravation of reductase inactivation. In the presence of SeLA, the inactivation was reversible under the action of thiols, allowing us to conclude that the observed action of SeLA may be considered as protective.     

NADPH:cytochrome P-450(c) reductase: Biochemical characterization in rat brain and cultured neurons and evolution of activity during development

NADPH:cytochrome P-450 (c) reductase is a microsomal enzyme which is involved in the cytochrome P-450-dependent biotransformation of many exogenous agents as well as of some endogenous molecules. Using cytochromec as a substrate, the kinetic parameters of this enzyme were determined in brain microsomes. The comparison of the NADPH:cytochrome P-450 reductase's V_max values and cytochrome P-450 contents in both fractions, suggests a role of cerebral NADPH:cytochrome P-450 reductase in cytochrome P-450 independent pathways. This is also supported by the different developmental pattern of brain enzyme as compared to the liver enzyme, and by the presence of a relatively high NADPH:cytochrome P-450 reductase activity in immature rat brain and neuronal cultures, while cytochrome P-450 was hardly detectable in these preparations. The enzyme activity was not induced by a phenobarbital chronic treatment neither in the adult brain nor in cultured neurons, suggesting a different regulation of the brain enzyme expression.     

Alcohol-mediated effects on the level of cytochrome P-450 and heme biosynthesis in cultured rat hepatocytes

Interaction of alcohol and drugs in the liver appears to involve common microsomal oxidative enzymes which utilize cytochrome P-450. Since alcohol augments the toxicity of a variety of drugs, the regulation of the P-450 hemoprotein, a primary component in hepatic drug metabolizing systems, may play a vital role in this phenomenon. We utilize an adult rat liver culture system as a model to explore the action of levels of alcohol below that which is necessary to produce intoxication in humans. The addition of 16 mM ethanol (70 mg/dl) to these hepatocytes results in a 49.5% decrease in cytochrome P-450 activity after 24 h, and a 3-fold increase in the activity of δ-aminolevulinate synthase, the rate-limiting enzyme in hepatic heme biosynthesis. Furthermore, ethanol treatment also causes a transient decrease in the level of intracellular heme. However, the diminished level of total heme does not appear to act as a repressor for δ-aminolevulinate synthase, since it occurs after the initial stimulation of the enzyme by ethanol.     

Purification, cDNA cloning and functional expression of NADPH-cytochrome P450 reductase from Centaurium erythraea cell cultures

Solubilised NADPH-cytochrome P450 reductase (CPR) was purified from the microsomal fraction of centaury ( Centaurium erythraea ) cell cultures by Q-anion exchange chromatography and affinity chromatography on adenosine 2',5'-diphosphate agarose. SDS-PAGE demonstrated the presence of three CPR isoforms with molecular masses of 77, 79 and 81 kDa. The 79- and 81-kDa isoforms were identified as glycoproteins when blotted following SDS-PAGE and subjected to a sugar detection procedure. A homology-based approach led to the isolation of a CPR cDNA encoding the 77-kDa isoform. The enzyme was a class I CPR, possessing a short N-terminus upstream of the membrane anchor. The amino acid sequence contained a putative N -glycosylation site, indicating that the two major isoforms of 77 and 79 kDa are related through attachment of an oligosaccharide chain. This glycosylation process was also found upon heterologous expression in yeast. When co-expressed in yeast together with centaury coniferyl alcohol 5-hydroxylase, CPR efficiently supported the activity of the P450 enzyme. The genome of C. erythraea was found to contain a second CPR gene. RT-PCR experiments using gene-specific primers revealed differential regulation of the two CPR genes. While CPR 2 mRNA was strongly induced by the addition of methyl jasmonate to the cell cultures, the CPR 1 expression level did not change after this elicitation.     

云芝NADPH-细胞色素P450还原酶在大肠杆菌中表达及酶学特性分析

云芝Trametes versicolor具有很强的环境有机污染物降解能力,其烟酰胺腺嘌呤二核苷酸-细胞色素P450还原酶(NADPH-cytochrome P450 reductase,CPR)为细胞色素P450酶(Cytochrome P450s,CYPs)提供电子,参与有机污染物的降解过程。序列分析显示,云芝基因组拥有1个潜在CPR序列和多个潜在CYP序列。为深入研究云芝CPR参与细胞降解有机污染物的分子机制,实验进行了云芝CPR在大肠杆菌中异源表达和酶学特性分析。结果表明,经IPTG诱导后,去除预测的N端膜锚定区域(氨基酸残基1–24)的CPR蛋白(CPRΔ24)可在重组菌中实现可溶性表达,且表达蛋白的分子量与理论值(78 kDa)一致。镍离子亲和层析和分子筛层析纯化后测得其比活性为5.82 U/mg。酶学性质分析显示,重组CPRΔ24的最适温度和pH分别为35℃和8.0,并对一些金属离子及有机溶剂具有不同程度的耐受性。酶在35℃、pH 8.0反应条件下对NADPH的动力学参数K_m和k_(cat)分别为19.7μmol/L、3.31/s;对底物细胞色素c的动力学参数K_m和k_(cat)分别为25.9μmol/L、10.2/s。以上研究为探究云芝CPR在环境有机污染物降解途径中的功能机制奠定基础。     

Phosphorylation of rabbit liver cytochrome P-450 LM2 and its effect on monooxygenase activity

The phosphorylation of rabbit liver microsomal cytochrome P-450 LM2 by catalytic subunit of cyclic AMP-dependent protein kinase (W. Pyerin et al. (1983) Carcinogenesis 4, 573) has now been studied in detail with purified soluble form of cytochrome P-450 as well as with the purified protein incorporated into model membranes. The apparent Km values for P-450 of the phosphorylation reaction in all experimental systems were in a range of 2-8 microM, while the Vmax values were dependent on the state of P-450. Upon phosphorylation, the reconstituted enzyme activities with benzphetamine (N-demethylation) and 7-ethoxycoumarin (O-deethylation) as substrates were reduced to 30-40% of control.     

Engineering of proteolytically stable NADPH-cytochrome P450 reductase

NADPH-cytochrome P450 reductase (CPR) is a membrane-bound flavoprotein that interacts with the membrane via its N-terminal hydrophobic sequence (residues 1-56). CPR is the main electron transfer component of hydroxylation reactions catalyzed by microsomal cytochrome P450s. The membrane-bound hydrophobic domain of NADPH-cytochrome P450 reductase is easily removed during limited proteolysis and is the subject of spontaneous digestion of membrane-binding fragment at the site Lys56-Ile57 by intracellular trypsin-like proteases that makes the flavoprotein very unstable during purification or expression in E. coli. The removal of the N-terminal hydrophobic sequence of NADPH-cytochrome P450 reductase results in loss of the ability of the flavoprotein to interact and transfer electrons to cytochrome P450. In the present work, by replacement of the lysine residue (Lys56) with Gln using site directed mutagenesis, we prepared the full-length flavoprotein mutant Lys56Gln stable to spontaneous proteolysis but possessing spectral and catalytic properties of the wild type flavoprotein. Limited proteolysis with trypsin and protease from Staphylococcus aureus of highly purified and membrane-bound Lys56Gln mutant of the flavoprotein as well as wild type NADPH-cytochrome P450 reductase allowed localization of some amino acids of the linker fragment of NADPH-cytochrome P450 reductase relative to the membrane. During prolong incubation or with increased trypsin ratio, the mutant form showed an alternative limited proteolysis pattern, indicating the partial accessibility of another site. Nevertheless, the membrane-bound mutant form is stable to trypsinolysis. Truncated forms of the flavoprotein (residues 46-676 of the mutant or 57-676 of wild type NADPH-cytochrome P450 reductase) are unable to transfer electrons to cytochrome P450c17 or P4503A4, confirming the importance of the N-terminal sequence for catalysis. Based on the results obtained in the present work, we suggest a scheme of structural topology of the N-terminal hydrophobic sequence of NADPH-cytochrome P450 reductase in the membrane.     

Amino acid sequence of COOH-terminal 20K Da fragment from pig liver microsomal NADPH-cytochrome P-450 reductase

We have determined the complete amino acid sequence of a 20K Da COOH-terminal fragment of porcine NADPH-cytochrome P-450 reductase. The 20K Da fragment is probably produced by a proteolytic cleavage of the intact protein in porcine liver microsomes, and since the cleavage does not affect enzymatic activity, the fragment has been studied as a distinct domain. The sequence comprises 175 amino acids including three cysteine residues, one of which has been previously identified as protected by NADPH from S-carboxymethylation. The NADPH-protected cysteine lies in a stretch of 12 residues with partial homology to glutathione reductase, and is adjacent to a hydrophobic region containing a glycine-rich stretch homologous to other FAD-containing proteins. The predicted secondary structure over this entire region is beta-sheet/beta-turn/beta-sheet/alpha-helix/beta-sheet/beta-turn/alpha-h elix corresponding to hydrophobic residues 21-28/glycine-rich residues 29-33/residues 34-38/residues 39-54/residues 56-61/NADPH-protected cysteine residues 62-78/residues 71-82. It is possible that the 20K Da domain provided a significant portion of the sequence responsible for binding FAD and NADPH in the intact enzyme. This data provides a basis for further active site studies.     

Inhibition of the O2−-generating NADPH oxidase of guinea-pig polymorphonuclear leukocytes by rabbit antibody to homologous liver NADPH-cytochrome c (P-450) reductase

Rabbit antibody highly specific for guinea-pig liver NADPH-cytochrome c (P-450) reductase was found to inhibit dose-dependently the O₂^?-generating activity of the membrane fraction isolated from phorbol-myristate acetate-stimulated, homologous polymorphonuclear leukocytes. In addition, the antibody also could inhibit the NADPH-cytochrome c (Nitroblue tetrazolium) reductase from the membrane fractions and phagosomes of leukocytes by polyacrylamide gel electrophoresis or gel filtration on a Sephacryl S-300 column in the presence of 0.2% Triton X-100. These results demonstrate that the NADPH-cytochrome c reductase in the membrane fractions of leukocytes is antigenically cross-reactive with homologous liver NADPH-cytochrome c reductase, and also suggest that the enzyme of leukocytes participates in the respiratory burst.     

Role of cytochrome b5 in the cytochrome P-450-mediated C21-steroid 17,20-lyase reaction

The cytochrome P-450 (P-450_sccII) and its reductase, NADPH-cytochrome reductase [EC 1.6.2.4], associated with conversion of progesterone to 4-androstene-3,17-dione, were extensively purified from pig testis microsomes. Higher lyase activity (turnover number of 15 mol of the product formed/min/mol of P-450) could be restored by mixing the P-450_sccII, its reductase, pig liver cytochrome b₅ and cytochrome b₅-reductase [EC 1.6.2.2], and phospholipid in the presence of NADPH, NADH, and O₂. Omission of either cytochrome b₅ or NADH resulted in a significant loss of the lyase activity indicating actual participation of cytochrome b₅ in this P-450-mediated steroidogenic system in the testis.