Structural and functional analysis of NADPH-cytochrome P-450 reductase from human liver: complete sequence of human enzyme and NADPH-binding sites

Enzymatic activity on valsartan of 38 CYP2C9 variants from the Chinese population

BACKGROUND AND OBJECTIVE

Valsartan is widely used for the treatment of moderate hypertension. However, previous studies have found that efficacy of the valsartan depends on the dose and intake. Cytochrome P450 (CYP) 2C9 metabolizes ∼15% of the clinical drugs. Genetic polymorphisms of CYP2C9 markedly affect the safety and effectiveness of many drugs, which might lead to adverse reactions and therapeutic failure. Twenty-four novel CYP2C9 variants (*36-*60) had been previously discovered via gene sequencing in the Han population. Our study aims to evaluate the impact of 38 CYP2C9 variants from the Chinese population on valsartan metabolism compared with CYP2C9*1 in vitro.

METHODS

Wild-type CYP2C9*1 and other CYP2C9 variants were expressed in Spodoptera frugiperda 21 insect cells. Incubations were performed at 37 °C with 20-2000 μM substrate for 30 min. The metabolite 4-OH valsartan was determined via UPLC-MS/MS.

RESULTS

Among the 38 CYP2C9 variants, the enzymatic activities of most variants were significantly altered compared with the wild-type. Three variants (CYP2C9*27, *40 and *49) exhibited increased intrinsic clearance values (134-153% relative clearance). However, 12 variants (CYP *8, *13, *16, *19, *33, *36, *42, *43, *45, *52, *54, *58) caused >90% decreases in the relative clearance of valsartan compared to CYP2C9*1.

CONCLUSIONS

Our research provides systematic data for evaluating the effects of CYP2C9 variants on valsartan metabolism in the Chinese population. These results will expand our understanding of the impact of CYP2C9 genetic polymorphisms on valsartan metabolism and will contribute to precision medicine.

Response of phase I and II detoxification enzymes, glutathione, metallothionein and acetylcholine esterase to mercury and dimethoate in signal crayfish (Pacifastacus leniusculus)

Metals and pesticides are common pollutants and the modulation of biomarkers can indicate sub-lethal influences on the physiology of organisms inhabiting impacted aquatic systems. We examined the effects of mercury and the organophosphate pesticide dimethoate on EROD, MROD, glutathione S-transferase (GST), acetylcholine esterase (AChE), metallothionein (MT) and glutathione (GSH) in the signal crayfish (Pacifastacus leniusculus). Crayfish were injected with mercury chloride or dimethoate (0.3, 0.6, 0.9 μg kg-1) and dissected after 72 h. EROD activity in the hepatopancreas did not change in response to mercury chloride treatment but exhibited a dose dependent decrease at all concentrations of dimethoate tested. MROD (hepatopancreas) exhibited a significant decrease at the 0.9 μg kg-1 treatment for both chemicals. GST (hepatopancreas) demonstrated a significant dose dependent decrease at all concentrations of both mercury chloride and dimethoate. AChE (tail muscle) decreased at the 0.6 and 0.9 μg kg-1 concentrations of dimethoate and 0.9 μg kg-1 mercury chloride. In gill tissue, MT increased in response to 0.3 and 0.6 μg kg-1 of mercury chloride but no effect was observed at the 0.9 μg kg-1 concentration of mercury chloride or any concentrations of dimethoate tested. MT did not change in response to mercury or dimethoate in tail tissue. Furthermore, neither chemical modulated GSH concentrations. Our results indicate that, apart from GSH, these markers are sensitive to the pollutants tested and that animals exposed in the wild are potentially compromised in their ability to detoxify environmental contaminants and carry out normal cellular processes.

Study on the in vitro effects of the mixtures of polycyclic aromatic hydrocarbons (PAHs) and heavy metals on ethoxyresorufin-O-deethylase (EROD) activity in Mossambica tilapia liver

This paper reports in vitro effects of individual heavy metals (Cd(2+), Cu(2+) and Hg(2+)), and PAHs, including benzo[a]pyrene(BaP), indeno[1,2,3-cd]pyrene (IP) and fluoranthene (FL), and their mixtures on ethoxyresorufin-O-deethylase (EROD) activities using a plate-reader method. The results showed that all three metals inhibited EROD activity, while BaP/IP significantly induced the enzyme. However, FL alone decreased EROD activity. Moreover, co-treatment with BaP/IP and heavy metals inhibited PAH-induced EROD activities, while combined exposure to FL and heavy metals induced FL-inhibited EROD activities.

NADPH P450 oxidoreductase: structure, function, and pathology of diseases

Cytochrome P450 oxidoreductase (POR) is an enzyme that is essential for multiple metabolic processes, chiefly among them are reactions catalyzed by cytochrome P450 proteins for metabolism of steroid hormones, drugs and xenobiotics. Mutations in POR cause a complex set of disorders that often resemble defects in steroid metabolizing enzymes 17α-hydroxylase, 21-hydroxylase and aromatase. Since our initial reports of POR mutations in 2004, more than 200 different mutations and polymorphisms in POR gene have been identified. Several missense variations in POR have been tested for their effect on activities of multiple steroid and drug metabolizing P450 proteins. Mutations in POR may have variable effects on different P450 partner proteins depending on the location of the mutation. The POR mutations that disrupt the binding of co-factors have negative impact on all partner proteins, while mutations causing subtle structural changes may lead to altered interaction with specific partner proteins and the overall effect may be different for each partner. This review summarizes the recent discoveries related to mutations and polymorphisms in POR and discusses these mutations in the context of historical developments in the discovery and characterization of POR as an electron transfer protein. The review is focused on the structural, enzymatic and clinical implications of the mutations linked to newly identified disorders in humans, now categorized as POR deficiency.

Cytochrome P450s in the synthesis of cholesterol and bile acids--from mouse models to human diseases

The present review describes the transgenic mouse models that have been designed to evaluate the functions of the cytochrome P450s involved in cholesterol and bile acid synthesis, as well as their link with disease. The knockout of cholesterogenic Cyp51 is embrionally lethal, with symptoms of Antley-Bixler syndrome occurring in mice, whereas the evidence for this association is conflicting in humans. Disruption of Cyp7a1 from classic bile acid synthesis in mice leads to either increased postnatal death or a milder phenotype with elevated serum cholesterol. The latter is similar to the case in humans, where CYP7A1 mutations associate with high plasma low-density lipoprotein and hepatic cholesterol content, as well as deficient bile acid excretion. Disruption of Cyp8b1 from an alternative bile acid pathway results in the absence of cholic acid and a reduced absorption of dietary lipids; however, the human CYP8B1 polymorphism fails to explain differences in bile acid composition. Unexpectedly, apparently normal Cyp27a1(-/-) mice still synthesize bile acids that originate from the compensatory pathway. In humans, CYP27A1 mutations cause cerebrotendinous xanthomatosis, suggesting that only mice can compensate for the loss of alternative bile acid synthesis. In line with this, Cyp7b1 knockouts are also apparently normal, whereas human CYP7B1 mutations lead to a congenital bile acid synthesis defect in children or spastic paraplegia in adults. Mouse knockouts of the brain-specific Cyp46a1 have reduced brain cholesterol excretion, whereas, in humans, CYP46A1 polymorphisms associate with cognitive impairment. At present, cytochrome P450 family 39 is poorly characterized. Despite important physiological differences between humans and mice, mouse models prove to be an invaluable tool for understanding the multifactorial facets of cholesterol and bile acid-related disorders.

Network analysis of hepatic genes responded to high-fat diet in C57BL/6J mice: nutrigenomics data mining from recent research findings

Obesity and its associated complications, including diabetes, dyslipidemia, atherosclerosis, and some cancers, have been a global health problem with a rapid increase of the obese population. In this study, we selected 31 obesity candidate genes in the liver of high-fat-induced obese C57BL/6J mice through investigation of literature search and analyzed functional protein-protein interaction of the genes using the STRING database. Most of the obesity candidate genes were closely connected through lipid metabolism, and in particular acyl-coenzyme A oxidase 1 appeared to be a core obesity gene. Overall, genes involved in fatty acid beta-oxidation, fatty acid synthesis, and gluconeogenesis were up-regulated, and genes involved in sterol biosynthesis, insulin signaling, and oxidative stress defense system were down-regulated with a high-fat diet. Future identification of core obesity genes and their functional targets is expected to provide a new way to prevent obesity by phytochemicals or functional foods on the basis of food and nutritional genomics.

Regulation of P450 oxidoreductase by gonadotropins in rat ovary and its effect on estrogen production

BACKGROUND

P450 oxidoreductase (POR) catalyzes electron transfer to microsomal P450 enzymes. Its deficiency causes Antley-Bixler syndrome (ABS), and about half the patients with ABS have ambiguous genitalia and/or impaired steroidogenesis. POR mRNA expression is up-regulated when mesenchymal stem cells (MSCs) differentiate into steroidogenic cells, suggesting that the regulation of POR gene expression is important for steroidogenesis. In this context we examined the regulation of POR expression in ovarian granulosa cells by gonadotropins, and its possible role in steroidogenesis.

METHODS

Changes in gene expression in MSCs during differentiation into steroidogenic cells were examined by DNA microarray analysis. Changes in mRNA and protein expression of POR in the rat ovary or in granulosa cells induced by gonadotropin treatment were examined by reverse transcription-polymerase chain reaction and western blotting. Effects of transient expression of wild-type or mutant (R457H or V492E) POR proteins on the production of estrone in COS-7 cells were examined in vitro. Effects of POR knockdown were also examined in estrogen producing cell-line, KGN cells.

RESULTS

POR mRNA was induced in MSCs following transduction with the SF-1 retrovirus, and was further increased by cAMP treatment. Expression of POR mRNA, as well as Cyp19 mRNA, in the rat ovary were induced by equine chorionic gonadotropin and human chorionic gonadotropin. POR mRNA and protein were also induced by follicle stimulating hormone in primary cultured rat granulosa cells, and the induction pattern was similar to that for aromatase. Transient expression of POR in COS-7 cells, which expressed a constant amount of aromatase protein, greatly increased the rate of conversion of androstenedione to estrone, in a dose-dependent manner. The expression of mutant POR proteins (R457H or V492E), such as those found in ABS patients, had much less effect on aromatase activity than expression of wild-type POR proteins. Knockdown of endogenous POR protein in KGN human granulosa cells led to reduced estrone production, indicating that endogenous POR affected aromatase activity.

CONCLUSION

We demonstrated that the expression of POR, together with that of aromatase, was regulated by gonadotropins, and that its induction could up-regulate aromatase activity in the ovary, resulting in a coordinated increase in estrogen production.

Clinical, structural and functional implications of mutations and polymorphisms in human NADPH P450 oxidoreductase

Cytochrome P450 proteins are involved in metabolism of drugs and xenobiotics. In the endoplasmic reticulum a single nicotinamide adenine dinucleotide phosphate (NADPH) P450 oxidoreductase (POR) supplies electrons to all microsomal P450s for catalytic activity. POR is a flavoprotein that contains both flavin mononucleotide and flavin adenine dinucleotide as cofactors and uses NADPH as the source of electrons. We have recently reported a number of POR mutations in the patients with disordered steroidogenesis. In the first report we had described missense mutations (A287P, R457H, V492E, C569Y, and V608F) identified in four patients with defects in steroid production. Each POR variant was produced as recombinant N-27 form of the enzyme in bacteria and as full-length form in yeast. Membranes from bacteria or yeast expressing normal or variant POR were purified and their activities were characterized in cytochrome c and CYP17A1 assays. Later we have published a larger study that described a whole range of POR mutations and characterized the mutants/polymorphisms A115V, T142A, M263V, Y459H, A503V, G539R, L565P, R616X, V631I, and F646del from the sequencing of patient DNA. We also studied POR variants Y181D, P228L, R316W, G413S, and G504R that were available in public databases or published literature. Three-dimensional structure of rat POR is known and we have used this structure to deduce the structure-function correlation of POR mutations in human. The missense mutations found in patients with disordered steroidogenesis are generally in the co-factor binding and functionally important domains of POR and the apparent polymorphisms are found in regions with lesser structural importance. A variation in POR can alter the activity of all microsomal P450s, and therefore, can affect the metabolism of drugs and xenobiotics even when the P450s involved are otherwise normal. It is important to study the genetic and biochemical basis of POR variants in human population to gain information about possible differences in P450 mediated reactions among the individuals carrying a variant or polymorphic form of POR that could impact their metabolism.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Biochemical analysis of mutations in P450 oxidoreductase

All microsomal P450s require POR (cytochrome P450 reductase) for catalytic activity. Most of the clinically used drugs are metabolized by a small number of P450s and polymorphisms in the cytochrome P450s are known to cause changes in drug metabolism. We have recently found a number of POR missense mutations in the patients with disordered steroidogenesis. Our initial report described five missense mutations (A284P, R454H, V489E, C566Y and V605F) identified in four patients. We built bacterial expression vectors for each POR variant, purified the membranes expressing normal or variant POR and characterized their activities with cytochrome c and P450c17 assays. We have recently completed an extensive study of the range of POR mutations and characterized the mutants/polymorphisms A112V, T139A, M260V, Y456H, A500V, G536R, L562P, R613X, V628I and F643del from sequencing of patient DNA. We also studied POR variants Y179D, P225L, R313W, G410S and G501R that were available in databases or the published literature. We analysed the mutations with a three-dimensional model of human POR that was based on an essentially similar rat POR with known crystal structure. The missense mutations found in patients with disordered steroidogenesis mapped to functionally important domains of POR and the apparent polymorphisms mapped to less crucial regions. Since a variation in POR can alter the activity of all microsomal P450s, it can also affect the drug metabolism even with a normal P450. Understanding the genetic and biochemical basis of POR-mediated drug metabolism will provide valuable information about possible differences in P450-mediated reactions among the individuals carrying a variant or polymorphic form of POR.

Alpha-benzene hexachloride exerts hormesis in preneoplastic lesion formation of rat hepatocarcinogenesis with the possible role for hepatic detoxifying enzymes

Recently there has been a shift in the prevailing paradigm regarding the dose dependence of carcinogen action with increasing acceptance of hormesis phenomenon, although underlying mechanisms remain to be established. To ascertain whether alpha-benzene hexachloride (alpha-BHC) might act by hormesis, rats were initiated with diethylnitrosamine and then alpha-BHC ranging from 0.01 to 500 ppm was administered in the diet for 10 weeks. The highest concentration of alpha-BHC significantly increased the number and area of glutathione S-transferase placental form (GST-P) positive foci, preneoplastic lesions in the liver, but its low dose, 0.05 ppm, caused significant reduction, showing a J-shape dose-response curve. The proliferating cell nuclear antigen positive index for GST-P positive foci in the low dose-treated group was significantly reduced. The dose response curves of CYP450 content, NADPH-P450 reductase activity and 8-hydroxydeoxyguanosine formation revealed the same pattern as GST-P positive foci data. The response curves of CYP2B1 and 3A2 in their activities, protein and mRNA expression showed a threshold but CYP2C11 activity exhibited an inverted J-shape. These results might suggest the possibility of hormesis of alpha-BHC at early stages of rat hepatocarcinogenesis. The possible mechanism involves induction of detoxifying enzymes at low dose, influencing free radical production and oxidative stress, and consequently pathological change in the liver.

Diversity and function of mutations in p450 oxidoreductase in patients with Antley-Bixler syndrome and disordered steroidogenesis

P450 oxidoreductase (POR) is the obligatory flavoprotein intermediate that transfers electrons from reduced nicotinamide adenine dinucleotide phosphate (NADPH) to all microsomal cytochrome P450 enzymes. Although mouse Por gene ablation causes embryonic lethality, POR missense mutations cause disordered steroidogenesis, ambiguous genitalia, and Antley-Bixler syndrome (ABS), which has also been attributed to fibroblast growth factor receptor 2 (FGFR2) mutations. We sequenced the POR gene and FGFR2 exons 8 and 10 in 32 individuals with ABS and/or hormonal findings that suggested POR deficiency. POR and FGFR2 mutations segregated completely. Fifteen patients carried POR mutations on both alleles, 4 carried mutations on only one allele, 10 carried FGFR2 or FGFR3 mutations, and 3 patients carried no mutations. The 34 affected POR alleles included 10 with A287P (all from whites) and 7 with R457H (four Japanese, one African, two whites); 17 of the 34 alleles carried 16 "private" mutations, including 9 missense and 7 frameshift mutations. These 11 missense mutations, plus 10 others found in databases or reported elsewhere, were recreated by site-directed mutagenesis and were assessed by four assays: reduction of cytochrome c, oxidation of NADPH, support of 17alpha-hydroxylase activity, and support of 17,20 lyase using human P450c17. Assays that were based on cytochrome c, which is not a physiologic substrate for POR, correlated poorly with clinical phenotype, but assays that were based on POR's support of catalysis by P450c17--the enzyme most closely associated with the hormonal phenotype--provided an excellent genotype/phenotype correlation. Our large survey of patients with ABS shows that individuals with an ABS-like phenotype and normal steroidogenesis have FGFR mutations, whereas those with ambiguous genitalia and disordered steroidogenesis should be recognized as having a distinct new disease: POR deficiency.

Inhibitory effects of divalent metal ions on liver microsomal 7-ethoxyresorufin O-deethylase (EROD) activity of leaping mullet

The purpose of the present study was to elucidate in vitro effects of Hg(2+), Zn(2+), Ni(2+) and Cd(2+) on cytochrome P4501A1 (CYP1A1) dependent EROD activities in leaping mullet liver microsomes. Fish captured from the most polluted part of Izmir Bay, had highly elevated EROD activities, and induced CYP1A1 protein levels as determined by Western blotting. Although all of the metal ions caused inhibition of the initial velocity of the reaction, Hg(2+) and Cd(2+) exhibited much higher inhibitory effect at lower concentrations and they were evidently more potent inhibitors than others. The inhibitor concentration giving 50% inhibition (IC(50) values) of Zn(2+), Ni(2+), Cd(2+) and Hg(2+) of initial EROD activity were 107, 16, 1.3 and 0.15 micromolar, respectively. Glutathione (GSH) at 0.5 mM final concentration, completely reversed Ni(2+) and Cd(2+) inhibition of EROD activity indicating the protective action of GSH.

The function of the small insertion in the hinge subdomain in the control of constitutive mammalian nitric-oxide synthases

Control of nitric oxide (NO) synthesis in the constitutive nitric-oxide synthases (NOS) by calcium/calmodulin is exerted through the regulation of electron transfer from NADPH through the reductase domains. This process has been shown previously to involve the calmodulin binding site, the autoinhibitory insertion in the FMN binding domain, and the C-terminal tail. Smaller sequence elements also appear to correlate with control. Although some of these elements appear well positioned to function in control, they are poorly conserved; their role in control is neither well established nor defined by available information. In this study mutations have been induced in the small insertion of the hinge subdomain, which has been shown recently to form a beta hairpin in structural studies of the neuronal NOS reductase domains adjacent to the calmodulin site and the autoinhibitory element. Modification of the small insertion in neuronal NOS tends to increase cytochrome c reduction but not NO synthetic activity; some modifications or deletions in the corresponding region in endothelial NOS modestly increase activity under some conditions. Unexpectedly, some minor changes in the sequence introduce a loss in the content of heme relative to flavin cofactors. Taken together, these results suggest that the small insertion protects the calmodulin binding site and that it may be a modulator of NOS activity.

Changes in apoptosis-related pathways in acute myelocytic leukemia

Expression analysis of apoptotic genes was performed for 15 patients with acute myelocytic leukemia (AML) at the time of diagnosis to identify genes and signaling pathways involved in the regulation of cell survival and apoptosis during leukemogenesis. cDNA array analysis revealed 34 genes whose expression was significantly different compared to others. Tumor suppressor genes TP53 and CDKN2A were downregulated and protooncogenes JUN and GRB10 were upregulated. Furthermore, several cellular signaling pathways acting either in cell cycle regulation or in apoptosis were altered. Deregulation was found in pathways that contribute to genomic stability (by downregulation of either TP53 or CSE1L and by upregulation of GADD45A) and regulate cell cycle progression (by downregulation of CDKN2A and upregulation of RBBP4, CDC37, and NEDD5). Alterations at the transcriptional level were identified, namely, upregulation of JUN and E2F5. Abnormalities were observed in the regulation of the caspases through upregulation of CASP8 and by altered expression of BCL2-related pathway. Extrinsic apoptotic signals mediated by IGFs were deregulated and the glutathione detoxification pathway was downregulated. These findings provide insight into the regulation of balance between apoptosis and cell proliferation signals, and suggest that these genes and pathways may have an important role in the pathogenesis of AML.

Metabolism of linear and angular furanocoumarins by Papilio polyxenes CYP6B1 co-expressed with NADPH cytochrome P450 reductase

One challenge in the heterologous expression of microsomal cytochrome P450 monooxygenases (P450s) is fulfilling their obligatory requirement for electrons transferred from NADPH P450 reductase. We have established co-expression parameters for Papilio polyxenes CYP6B1 and house fly P450 reductase in baculovirus-infected Sf9 cells that allow for efficient expression of both components and significantly enhance metabolic turnover of this insect P450's substrates. These expression conditions have allowed us to reexamine the turnover capacities of CYP6B1 toward linear and angular furanocoumarins present in the host plants for the specialist caterpillar P. polyxenes. Coexpression of CYP6B1 and P450 reductase at equivalent viral concentrations [MOI (multiplicity of infection) ratio of 1] results in turnover rates for the linear furanocoumarins xanthotoxin and psoralen, which are increased 32-33 fold over the turnover rates obtained with CYP6B1 expressed alone. The turnover rate for the angular furanocoumarin angelicin is also significantly increased to 4.76 nmol/min/nmol P450 compared to its barely detectable level obtained with CYP6B1 expressed alone. Substrate binding analyses indicate that all three of these compounds elicit typical type I binding spectra but with varying magnitudes and affinities that are indicative of each substrate's effectiveness at coordinating with the heme iron. The relative proportions of high spin state generated with these substrates are consistent with CYP6B1 metabolizing these furanocoumarins in the rank order xanthotoxin>psoralen>angelicin. These differential activities for CYP6B1 suggest that it may have been an ancient participant in the coevolutionary arms race between papilionid butterflies and their apiaceous host plants. Due to its ability to handle a range of furanocoumarin structures, CYP6B1 may have contributed to P. polyxenes' early colonization of linear furanocoumarin-containing plants and to its subsequent colonization of angular furanocoumarin-containing plants.

Induction of NADPH cytochrome P450 reductase by the Alzheimer beta-protein. Amyloid as a "foreign body"

A large body of data suggests that the Alzheimer's amyloid peptide (Abeta) causes degeneration and death of neurons by mechanisms that involve reactive oxygen species. The pathways involved in Abeta-mediated oxidative injury are only partially understood. We theorized that abnormal microaggregates and/or pathological conformations of Abeta peptides may behave as xenobiotics and trigger the induction of NADPH cytochrome P450 reductase (CP450r), an enzyme which, if induced by non-physiological substrates (such as xenobiotics like drugs or other 'foreign molecules'), is known to cause oxidative stress. In order to test this hypothesis, i.e. that Abeta can increase the expression of CP450r, SK-N-SH human neuroblastoma cells were exposed to Abeta25-35 and Abeta1-42 and then examined for induction of this enzyme in immunoblots, using specific antibodies. Following exposure to Abeta peptides, neuroblastoma cells showed a clear-cut induction of CP450r. To determine whether this mechanism is operational in vivo, we investigated the expression of CP450r in a transgenic mouse model of Alzheimer's disease (AD) and in brains from patients afflicted with AD, using an immunocytochemical approach. Tissue sections from brains of transgenic mice exhibited strong immunoreactivity for CP450r, surrounding amyloid deposits. The pattern of expression of CP450r was similar to that exhibited by neuritic and oxidative stress markers. Sections from non-transgenic mice showed no detectable immunoreactivity. Immunostaining of sections from four brains with neuropathologically confirmed AD showed a pattern of abnormality different from transgenic mice that was characterized by abnormal immunoreactivity for CP450r within the cytoplasm of cortical neurons. No labeling was seen in sections from aged-matched control brains. The data showed that CP450r is induced by Alzheimer amyloid peptide and that such a response must be considered as one possible mechanism whereby Abeta causes oxidative stress.

Competition between C-terminal tyrosine and nicotinamide modulates pyridine nucleotide affinity and specificity in plant ferredoxin-NADP(+) reductase

Chloroplast ferredoxin-NADP(+) reductase has a 32,000-fold preference for NADPH over NADH, consistent with its main physiological role of NADP(+) photoreduction for de novo carbohydrate biosynthesis. Although it is distant from the 2'-phosphoryl group of NADP(+), replacement of the C-terminal tyrosine (Tyr(308) in the pea enzyme) by Trp, Phe, Gly, and Ser produced enzyme forms in which the preference for NADPH over NADH was decreased about 2-, 10-, 300-, and 400-fold, respectively. Remarkably, in the case of the Y308S mutant, the k(cat) value for the NADH-dependent activity approached that of the NADPH-dependent activity of the wild-type enzyme. Furthermore, difference spectra of the NAD(+) complexes revealed that the nicotinamide ring of NAD(+) binds at nearly full occupancy in the active site of both the Y308G and Y308S mutants. These results correlate well with the k(cat) values obtained with these mutants in the NADH-ferricyanide reaction. The data presented support the hypothesis that specific recognition of the 2'-phosphate group of NADP(H) is required but not sufficient to ensure a high degree of discrimination against NAD(H) in ferredoxin-NADP(+) reductase. Thus, the C-terminal tyrosine enhances the specificity of the reductase for NADP(H) by destabilizing the interaction of a moiety common to both coenzymes, i.e. the nicotinamide.

Cloning and characterization of the cytochrome P450 oxidoreductase gene from the zygomycete fungus Cunninghamella

The filamentous fungus Cunninghamella utilizes cytochrome P450 system(s) in the metabolism of a broad range of polyaromatic and aliphatic pollutants and a variety of drugs, but prior attempts at isolation of P450 system components of this fungus have been generally unsuccessful. We report upon the cytochrome P450 oxidoreductase (CPR) gene from two widely studied species, C. elegans and C. echinulata. The C. elegans CPR gene was obtained by screening a genomic library using as probe a PCR amplicon obtained with degenerate primers based on known CPRs. The 2420 bp coding region contained two apparent introns (149 bp and 138 bp). Northern blot analysis showed that the CPR gene is transcriptionally expressed in C. elegans and appears to be inducible by an alkane substrate, n-tetradecane. Phylogenetic comparison of the deduced C. elegans CPR (710 aa) suggested that it is more closely related to animal CPRs (41-42%) than to yeast (38-41%) and plant (35-36%) forms. A 2074 bp sequence containing most of the CPR gene homolog from C. echinulata was also isolated.

Autoinhibition of endothelial nitric-oxide synthase. Identification of an electron transfer control element

The primary sequences of the three mammalian nitric- oxide synthase (NOS) isoforms differ by the insertion of a 52-55-amino acid loop into the reductase domains of the endothelial (eNOS) and neuronal (nNOS), but not inducible (iNOS). On the basis of studies of peptide derivatives as inhibitors of.NO formation and calmodulin (CaM) binding (Salerno, J. C., Harris, D. E., Irizarry, K., Patel, B., Morales, A. J., Smith, S. M., Martasek, P., Roman, L. J., Masters, B. S., Jones, C. L., Weissman, B. A., Lane, P., Liu, Q., and Gross, S. S. (1997) J. Biol. Chem. 272, 29769-29777), the insert has been proposed to be an autoinhibitory element. We have examined the role of the insert in its native protein context by deleting the insert from both wild-type eNOS and from chimeras obtained by swapping the reductase domains of the three NOS isoforms. The Ca2+ concentrations required to activate the enzymes decrease significantly when the insert is deleted, consistent with suppression of autoinhibition. Furthermore, removal of the insert greatly enhances the maximal activity of wild-type eNOS, the least active of the three isoforms. Despite the correlation between reductase and overall enzymatic activity for the wild-type and chimeric NOS proteins, the loop-free eNOS still requires CaM to synthesize.NO. However, the reductive activity of the CaM-free, loop-deleted eNOS is enhanced significantly over that of CaM-free wild-type eNOS and approaches the same level as that of CaM-bound wild-type eNOS. Thus, the inhibitory effect of the loop on both the eNOS reductase and. NO-synthesizing activities may have an origin distinct from the loop's inhibitory effects on the binding of CaM and the concomitant activation of the reductase and.NO-synthesizing activities. The eNOS insert not only inhibits activation of the enzyme by CaM but also contributes to the relatively low overall activity of this NOS isoform.

Cytochromes P450 in crustacea

Since the last review of this topic, further insight has been gained into the presence and functions of cytochrome P450 proteins in the hepatopancreas and other organs of aquatic crustacean species, although progress has been slow relative to the advances in other species. Recent studies with several lobster, shrimp, crab and crayfish species suggest that cytochromes P450 in the 2 and 3 families are the most abundant forms in hepatopancreas microsomes. Substrates normally metabolized by CYP2 and CYP3 family members are monooxygenated more rapidly by crustacea than substrates normally metabolized by CYP1 family enzymes, e.g. erythromycin, testosterone and aminopyrine are much more rapidly monooxygenated than ethoxyresorufin. Some progress has been made in cloning and sequencing crustacean P450 forms. CYP2L1 and CYP2L2 cDNA sequences have been cloned from spiny lobster hepatopancreas libraries, and there was evidence for at least two more cytochromes P450 in spiny lobster hepatopancreas. An area of continued interest, but of no consensus or general findings, relates to the presence and inducibility of CYP1 family members in crustacea. Some studies indicate weak induction of total cytochrome P450 and increased turnover of substrates normally associated with CYP1, while others show no effect of the classic inducers that act at the Ah receptor in vertebrates. A few studies of the roles of cytochromes P450 in the biosynthesis and degradation of steroids, including ecdysteroids, have been published. Further studies are needed to understand the regulation and normal function of the crustacean cytochromes P450.

Membrane topology of NADPH-cytochrome P450 reductase on the endoplasmic reticulum

Topology of the membrane-anchoring segment of mouse NADPH-cytochrome P450 reductase in the endoplasmic reticulum membrane was elucidated. An N-glycosylation site was generated in the amino-terminal hydrophilic sequence of the reductase, and the mutated protein was expressed in a cell-free system in the presence of microsomal vesicles. The in vitro synthesized reductase protein was integrated into the microsomal membrane and N-glycosylated depending on the presence of signal recognition particles. We conclude that the amino-terminal membrane-anchoring segment of the reductase is a type I signal-anchor sequence which shows amino-terminus-lumen and carboxy-terminus-cytoplasm topology.

Reductase gene sequences and protein structures: p-cymene methyl hydroxylase

Oxygenases are critical to cycling carbon in the biosphere and dependent on reductase action, principally from flavoprotein enzymes. Oxygenase diversity among organisms and strains carries a common theme of protein sequence and folding. p-Cymene (para-isopropyl toluene) was chosen as a point of convergence in terpene-aromatic mineralization to characterize a methyl hydroxylase electron transport system with the aerobe Pseudomonas aureofaciens. The cymA hydroxylase reductase gene was isolated and sequenced and the protein primary structure deduced. Optimized amino acid sequence alignments of flavoprotein reductases revealed major similarities over protein length, in the binding domains for NAD(P)H, and the flavine centers of pro- and eukaryote systems.

Role of Ser457 of NADPH-cytochrome P450 oxidoreductase in catalysis and control of FAD oxidation-reduction potential

Site-directed mutagenesis of Ser457 of NADPH-cytochrome P450 oxidoreductase demonstrates that this residue plays a major role in both hydride transfer from NADPH to FAD and modulation of FAD redox potential. Substitution of Ser457 with alanine or cysteine decreases the rates of reduction of the substrates cytochrome c and potassium ferricyanide approximately 100-fold, while substitution with threonine produces a 20-fold decrease in activity. No changes are observed in k(m)NADPH, KiNADP+, or flavin content, indicating that these substitutions have no effect on cofactor binding but affect catalysis only. k(m)cyt c values are decreased in parallel with the observed decreases in the rates of the reductive half-reaction. Stopped-flow studies with the S457A mutant show a 100-fold decrease in the rate of flavin reduction. The primary deuterium isotope effect on Kcat for cytochrome c reduction increases from 2.7 for the wild-type enzyme to 9.0 for the S457A mutant, consistent with a change in the rate-determining step from NADP+ release in the wild-type enzyme to hydride transfer in the S457A mutant. The primary deuterium isotope effect on K1 for flavin reduction at high ionic strength (I = 535 mM) increases from 12.2 for the wild-type enzyme to > 20 for the S457A mutant, consistent again with an increase in the relative rate limitation of hydride transfer. Furthermore, anaerobic titration of S457A indicates that the redox potential of the FAD semiquinone has been decreased. Data presented in this study support the hypothesis that Ser457 is involved in hydrogen bonding interactions which stabilize both the transition state for hydride transfer and the reduced FAD.

Is the NAD(P)H:flavin oxidoreductase from Escherichia coli a member of the ferredoxin-NADP+ reductase family? Evidence for the catalytic role of serine 49 residue

The NAD(P)H:flavin oxidoreductase from Escherichia coli, Fre, is a monomer of 26.1 kDa which catalyzes the reduction of free flavins by NADPH or NADH. The flavin reductase Fre is the prototype of a new class of flavin reductases able to transfer electrons with no prosthetic group. It has been suggested that the flavin reductase could belong to the ferredoxin-NADP+ reductase (FNR) family, on the basis of limited sequence homologies. A sequence, conserved within the ferredoxin-NADP+ reductase family and present in the flavin reductase, is important for recognition of the isoalloxazine ring. Within this sequence, we have mutated serine 49 of the flavin reductase into alanine or threonine. kcat value of the S49A mutant was 35-fold lower than kcat of the wild-type enzyme. Determination of real Kd values for NADPH and lumichrome, a flavin analog, showed that recognition of the flavin is strongly affected by the S49A mutation, whereas affinity for the nicotinamide cofactor is only weakly modified. This suggests that serine 49 is involved in the binding of the isoalloxazine ring. Moreover, the Kd value for 5-deazariboflavin, in which the N-5 position of the isoalloxazine ring has been changed to a carbon atom, is not affected by the serine 49 to alanine mutation. This is consistent with the concept that the N-5 position is the main site for serine 49-flavin interaction. In the ferredoxin-NADP+ reductase family, the equivalent serine residue, which has been shown to be essential for activity, is hydrogen-bonded to the N-5 of the FAD cofactor. Taken together, these data provide the first experimental support to the hypothesis that the flavin reductase Fre may belong to the ferredoxin-NADP+ reductase family.

Cloning and characterization of the NADPH cytochrome P450 oxidoreductase gene from the filamentous fungus Aspergillus niger

In this paper, we describe the cloning and molecular characterization of the Aspergillus niger cytochrome P450 reductase (CPR) gene, cprA. Attempts to clone the cprA gene by heterologous hybridization techniques were unsuccessful. Using the polymerase chain reaction (PCR) with degenerate primers based on conserved regions found in cpr genes from other organisms, we were able to isolate a fragment that contained part of the gene. With the aid of this fragment, a genomic fragment containing the entire coding region and 5' and 3' untranslated ends of the cprA gene was isolated and sequenced. The cprA gene was introduced in multiple copies in A. niger strain N402 using the amdS transformation system. One of the resulting transformants, AB2-2, showed a 14-fold increase in CPR activity, indicating that the cloned cprA gene is functional. We analyzed the induction of cprA gene expression by several generally used cytochrome P450 inducers but did not find any induction of cprA gene expression. However, A. niger cprA gene expression could be induced by benzoic acid, which is the substrate of the highly inducible A. niger cytochrome P450 gene, bphA (cyp53). On the basis of a comparison of the deduced protein sequence of the A. niger cprA gene with CPR proteins isolated from other organisms, the structure-function relationship of some conserved regions is discussed.

Crystallization and preliminary x-ray studies of NADPH-cytochrome P450 reductase

NADPH-cytochrome P450 reductase (CPR; NADPH:ferrihemoprotein reductase, EC 1.6.2.4) catalyzes the transfer of electrons to all known microsomal cytochromes P450. CPR is unique in that it is one of only two mammalian enzymes known to contain both flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), the other being the various isoforms of nitric oxide synthase. Similarities in amino acid sequence and in functional domain arrangement with other key flavoproteins, including nitric oxide synthase, make CPR an excellent prototype for studies of interactions between two flavin cofactors. We have obtained diffraction-quality crystals of rat liver CPR, expressed in Escherichia coli and solubilized by limited proteolysis with trypsin. The crystals were grown in Hepes buffer (pH 7.0), containing polyethylene glycol 4500 and NaCl. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit cell dimensions a = 103.3 A, b = 116.1 A, and c = 120.4 A. If we assume that there are two molecules of the 72-kDa CPR polypeptide per asymmetric unit, the calculated value of Vm is 2.54 A3/Da.

NADPH-P-450 reductase: structural and functional comparisons of the eukaryotic and prokaryotic isoforms

The comparison of the properties of microsomal NADPH-P-450 reductase and the flavoprotein domain of P-450BM-3 (BMR) has revealed a significant difference in the mechanism of reduction of the hemoprotein P-450 by these flavoproteins. Microsomal NADPH-P-450 reductase transfers electrons to the hemoprotein by shuttling between hydroquinone and semiquinone forms of the FMN delivering one electron per cycle. Since the microsomal NADPH-P450 reductase has evolved as a component of multi-enzyme system, this type of mechanism may permit regulation of the steps of the P-450 reaction via variation in the affinity of the reductase for different P-450s, interaction with cytochrome b5, etc. In contrast, in the soluble, bacterial flavocytochrome P-450BM-3, the reductase domain has evolved together with a single unique heme domain. This enzyme was found to utilize the fastest and simplest way to reduce the heme iron, with the FMN moiety of BMR shuttling between the semiquinone and oxidized states. This mechanism of reduction provides the highest turnover number of any P-450 and tight coupling of the monooxygenation reaction. While there are clear differences in the intermediates involved in the reduction of P-450s by these two enzymes, the domain structure and presumably the mode of interaction between the reductase and P-450s has been maintained over evolutionary time.

Structural prototypes for an extended family of flavoprotein reductases: comparison of phthalate dioxygenase reductase with ferredoxin reductase and ferredoxin

The structure of phthalate dioxygenase reductase (PDR), a monomeric iron-sulfur flavoprotein that delivers electrons from NADH to phthalate dioxygenase, is compared to ferredoxin-NADP+ reductase (FNR) and ferredoxin, the proteins that reduce NADP+ in the final reaction of photosystem I. The folding patterns of the domains that bind flavin, NAD(P), and [2Fe-2S] are very similar in the two systems. Alignment of the X-ray structures of PDR and FNR substantiates the assignment of features that characterize a family of flavoprotein reductases whose members include cytochrome P-450 reductase, sulfite and nitrate reductases, and nitric oxide synthase. Hallmarks of this subfamily of flavoproteins, here termed the FNR family, are an antiparallel beta-barrel that binds the flavin prosthetic group, and a characteristic variant of the classic pyridine nucleotide-binding fold. Despite the similarities between FNR and PDR, attempts to model the structure of a dissociable FNR:ferredoxin complex by analogy with PDR reveal features that are at odds with chemical crosslinking studies (Zanetti, G., Morelli, D., Ronchi, S., Negri, A., Aliverti, A., & Curti, B., 1988, Biochemistry 27, 3753-3759). Differences in the binding sites for flavin and pyridine nucleotides determine the nucleotide specificities of FNR and PDR. The specificity of FNR for NADP+ arises primarily from substitutions in FNR that favor interactions with the 2' phosphate of NADP+. Variations in the conformation and sequences of the loop adjoining the flavin phosphate affect the selectivity for FAD versus FMN. The midpoint potentials for reduction of the flavin and [2Fe-2S] groups in PDR are higher than their counterparts in FNR and spinach ferredoxin, by about 120 mV and 260 mV, respectively. Comparisons of the structure of PDR with spinach FNR and with ferredoxin from Anabaena 7120, along with calculations of electrostatic potentials, suggest that local interactions, including hydrogen bonds, are the dominant contributors to these differences in potential.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.

Immunochemical specificity of placental NADPH cytochrome c (P-450) reductase in neoplastic and non-neoplastic human tissue

NADPH cytochrome c (P-450) reductase was purified from human placental microsomes using a combination of affinity and gel filtration chromatography. Affinity chromatography using agarose-hexane-adenosine 2'5 diphosphate resulted in two protein bands being detected by SDS-PAGE of approximate MwS 68 and 75 kDa. Fractions containing the two proteins were pooled, and then resolved using Sephacryl S-200. Both of the purified proteins displayed enzyme activity, measured by their ability to reduce cytochrome c. The 75 kDa protein obtained was used to immunize three female New Zealand white rabbits. The IgG fraction was partly purified from rabbit sera which suppressed placental microsomal NADPH cytochrome c reductase activity by > 80% using 33% ammonium sulphate. The procured antibody suppressed androstenedione aromatase activity in microsomal preparations of human placental and breast adipose tissue, and NADPH cytochrome c reductase activity in prostate (benign and malignant), MDA-MB-231 breast cancer cells, breast adipose, Hep G2 hepatoma cells and placental microsomal preparations. The extent of NADPH cytochrome c reductase inhibition varied in the order of malignant prostate < benign prostate < MDA < breast adipose < Hep G2 < placenta. The results suggest that human placental NADPH cytochrome c (P-450) reductase shares common antigenic epitopes pertinent to its capability of reducing cytochrome c in all of the above-mentioned tissues. In attempting to associate possible changes in NADPH cytochrome c reductase activity imposed by neoplasia to the obtained immunochemical cross reactivity and enzyme activity results, it was noted that microsomes obtained from MDA cells exhibited enzyme activity significantly less than that of breast adipose microsomes (1.6 and 8.1 nmol/min/mg protein, respectively) and by comparison showed 6% less homology towards the placental antibody. The results obtained for benign and malignant prostate showed no significant difference between the neoplastic states as adjudged by enzyme activity and immunochemical assays.

Cytochrome b558, a component of the phagocyte NADPH oxidase, is a flavoprotein

Cytochrome b558 is the only membrane component of the phagocyte O2(-)-producing NADPH oxidase. The O2- production by the oxidase reconstituted in vitro with the crude membrane fraction is enhanced several-fold by addition of FAD, whereas that with the partially purified cytochrome is completely dependent on exogenous FAD, suggesting that FAD acts through the membrane component, cytochrome b558. The alignments of the amino acid sequence of the large subunit of the cytochrome (gp91-phox) with those of previously characterized flavoproteins reveal that the middle and C-terminal portions of gp91-phox are likely to be FAD- and NADPH-binding domains, respectively. Cytochrome b558, thus, appears to be a flavoprotein with an NADPH-binding site, of the NADPH oxidase.

Cytochrome b558: the flavin-binding component of the phagocyte NADPH oxidase

The phagocyte respiratory burst oxidase is a flavin-adenine dinucleotide (FAD)-dependent dehydrogenase and an electron transferase that reduces molecular oxygen to superoxide anion, a precursor of microbicidal oxidants. Several proteins required for assembly of the oxidase have been characterized, but the identity of its flavin-binding component has been unclear. Oxidase activity was reconstituted in vitro with only the purified oxidase proteins p47phox, p67phox, Rac-related guanine nucleotide (GTP)-binding proteins, and membrane-bound cytochrome b558. The reconstituted oxidase required added FAD, and FAD binding was localized to cytochrome b558. Alignment of the amino acid sequence of the beta subunit of cytochrome b558 (gp91phox) with other flavoproteins revealed similarities to the nicotinamide adenine dinucleotide phosphate (reduced) (NADPH)-binding domains. Thus flavocytochrome b558 is the only obligate electron transporting component of the NADPH oxidase.

The haemoglobin-like protein (HMP) of Escherichia coli has ferrisiderophore reductase activity and its C-terminal domain shares homology with ferredoxin NADP+ reductases

Three soluble ferrisiderophore reductases (FsrA, FsrB and FsrC) were detected in Escherichia coli. FsrB was purified and identified as the haemoglobin-like protein (HMP) by size and N-terminal sequence analyses. HMP was previously isolated as a dihydropteridine reductase and is now shown to have ferrisiderophore reductase activity. Database searches revealed that the C-terminal region of HMP (FsrB) is homologous to members of a family of flavoprotein oxidoreductases which includes ferredoxin NADP+ reductase (FNR). The combination of FNR-like and haemoglobin-like regions in HMP (FsrB) represents a novel pairing of functionally and structurally distinct domains. Structure-function properties of other FNR-like proteins, including LuxG and VanB, are also discussed.

Quantification of cytochrome P450 reductase gene expression in human tissues

We have isolated and sequenced cDNA clones that code for a variant of human cytochrome P450 reductase. An RNase protection assay was used to quantify the corresponding mRNA in adult and fetal tissues. The results demonstrate that, in the samples analyzed, the cytochrome P450 reductase gene displays very little inter-individual variation in its expression in adult liver and is subject to little developmental or tissue-specific regulation.

Isolation and sequencing of a cDNA for an unusual hemoglobin from the parasitic nematode Pseudoterranova decipiens

A cDNA clone encoding a 333-amino acid hemoglobin was isolated from the nematode Pseudoterranova decipiens. The protein contains an 18-amino acid hydrophobic signal sequence and has a calculated mass of 37.6 kDa in the mature form. The predicted protein reveals an internal duplication of a 154-amino acid domain (51% identity). Both domains have significant sequence homology to other primitive hemoglobins, in agreement with a duplication event. Hydrophobicity plots reveal identical strongly hydrophobic regions in each domain, which are potential heme binding sites. This confirms previous suggestions that nematode hemoglobins can have two heme groups per molecule. In addition, each domain contains several conserved histidine motifs that may serve as potential copper binding sites. This result provides further evidence that hemoglobins may have evolved from a primitive cytochrome-like molecule.

Molecular cloning and sequence analysis of cDNA encoding delta 4-3-ketosteroid 5 beta-reductase of rat liver

A cDNA clone encoding delta 4-3-ketosteroid 5 beta-reductase was isolated from rat liver cDNA libraries using antibodies specific for the enzyme and oligonucleotides as probes. The cDNA contained 981-base pair open reading frame encoding 327 amino acid residues (Mr 37,376) and an unusually long 3'-untranslated region rich in AT sequence in the total length of 3189 base pairs. The predicted amino acid sequence contains the sequences similar to the putative NADPH- and steroid-binding regions.

An unusual yet strongly conserved flavoprotein reductase in bacteria and mammals

The recent determination of the amino acid sequences of the Bacillus megaterium cytochrome P-450 and the flavoprotein component of Salmonella typhimurium NADPH-sulfite reductase revealed that these enzymes contain a flavoprotein moiety remarkably similar to mammalian NADPH-cytochrome P-450 reductase. The presence of this oxidoreductase in these very different enzymes suggests that this flavoprotein arose early in evolution and was utilized as an enzymological building block. The multi-domain structure of the reductase further suggests that it arose through a fusion of genes encoding simple flavin electron-transport proteins.