Purification and characterization of the NADPH-cytochrome P-450 (cytochrome c) reductase from higher-plant microsomal fraction

Functional genomics of primary congenital glaucoma by pathway analysis and functional characterization of CYP1B1 mutations

CYP1B1 is the most common gene implicated in primary congenital glaucoma (PCG) - the most common form of childhood glaucoma. How CYP1B1 mutations cause PCG is not known. Understanding the mechanism of PCG caused by CYP1B1 mutations is crucial for disease management, therapeutics development, and potential prevention. We performed a comprehensive metabolome/reactome analysis of CYP1B1 to enlist CYP1B1-mediated processes in eye development. The identified metabolic events were classified into major pathways. Functional analysis of these metabolic pathways was performed after cloning the CYP1B1 wild-type gene and expressing the wild-type and selected novel mutants (previously reported by our group L24R, F190L, H279D, and G329D) in heterologous hosts. Stability and enzymatic functions were investigated. Structural modeling of the wild-type and the variants was also performed. Reactome analysis revealed a total of 166 metabolic processes which could be classified into four major pathways including estradiol metabolism, retinoic acid metabolism, arachidonic acid metabolism, and melatonin metabolism. Stability assay revealed rapid denaturing of mutant proteins compared to wild-type. Enzymatic assays showed functional deficit in mutant proteins in metabolizing estradiol, retinoids, arachidonate, and melatonin. Modeling revealed that the examined mutations induced structural changes likely causative in functional loss in CYB1B1 as observed in enzymatic assays. Hence, mutations in the CYP1B1 gene are associated with a functional deficit in critical pathways of eye development. These findings implicate the potential contributions of altered metabolic regulations of estradiol, retinoids, arachidonate and melatonin to the pathogenesis of PCG during the processes of the formation of ocular structures and function.

Functional and structural insight into the flexibility of cytochrome P450 reductases from Sorghum bicolor and its implications for lignin composition

Plant NADPH-dependent cytochrome P450 reductase (CPR) is a multidomain enzyme that donates electrons for hydroxylation reactions catalyzed by class II cytochrome P450 monooxygenases involved in the synthesis of many primary and secondary metabolites. These P450 enzymes include trans-cinnamate-4-hydroxylase, p-coumarate-3′-hydroxylase, and ferulate-5-hydroxylase involved in monolignol biosynthesis. Because of its role in monolignol biosynthesis, alterations in CPR activity could change the composition and overall output of lignin. Therefore, to understand the structure and function of three CPR subunits from sorghum, recombinant subunits SbCPR2a, SbCPR2b, and SbCPR2c were subjected to X-ray crystallography and kinetic assays. Steady-state kinetic analyses demonstrated that all three CPR subunits supported the oxidation reactions catalyzed by SbC4H1 (CYP73A33) and SbC3′H (CYP98A1). Furthermore, comparing the SbCPR2b structure with the well-investigated CPRs from mammals enabled us to identify critical residues of functional importance and suggested that the plant flavin mononucleotide–binding domain might be more flexible than mammalian homologs. In addition, the elucidated structure of SbCPR2b included the first observation of NADP⁺ in a native CPR. Overall, we conclude that the connecting domain of SbCPR2, especially its hinge region, could serve as a target to alter biomass composition in bioenergy and forage sorghums through protein engineering.

An inducible NADPH-cytochrome P450 reductase from Picrorhiza kurrooa - an imperative redox partner of cytochrome P450 enzymes

Picrorhiza kurrooa synthesizes a large array of pharmacologically important monoterpenoid iridoid glycosides called picrosides. Although chemical profile and pharmacological activities of P. kurrooa have been extensively studied, limited attempts have been made to decipher the biosynthetic route and to identify the key regulatory genes involved in picroside biosynthesis. In the present study, NADPH-cytochrome P450 reductase, a key enzyme involved in electron transfer to cytochrome P450s was identified from P. kurrooa. The full length cDNA (2679 bp) contained an open reading frame of 2133 bp, corresponding to 710 amino acids. PkCPR was heterologously expressed in Escherichia coli and the kinetic parameters of the recombinant enzyme were determined. Specific activity, V max and K m of PkCPR were found to be 5.8 ± 0.05 μmol min(-1) mg(-1), 8.1 ± 0.12 μmol min(-1) mg(-1) and 7.8 μM, respectively. PkCPR was found to be spatially regulated at transcript level, being maximally expressed in leaf tissues. Altitude was found to have a positive effect on the picroside concentration and the picroside content positively correlated with the PkCPR transcript levels in samples collected at varied altitudes. Further, transcript profiling under methyl jasmonate, salicylic acid, 2,4-dicholorophenoxy acetic acid and UV-B elicitations displayed differential transcriptional regulation of PkCPR that fully corroborated with the identified cis-elements within the PkCPR promoter. Expression of PkCPR was inducible by UV-B and phytohormone elicitation, indicating that the PkCPR is possibly related to defence reactions, including biosynthesis of secondary metabolites. Present study is so far the only report of identification and functional characterization of CPR ortholog from P. kurrooa.

A gain-of-function polymorphism controlling complex traits and fitness in nature

Identification of the causal genes that control complex trait variation remains challenging, limiting our appreciation of the evolutionary processes that influence polymorphisms in nature. We cloned a quantitative trait locus that controls plant defensive chemistry, damage by insect herbivores, survival, and reproduction in the natural environments where this polymorphism evolved. These ecological effects are driven by duplications in the BCMA (branched-chain methionine allocation) loci controlling this variation and by two selectively favored amino acid changes in the glucosinolate-biosynthetic cytochrome P450 proteins that they encode. These changes cause a gain of novel enzyme function, modulated by allelic differences in catalytic rate and gene copy number. Ecological interactions in diverse environments likely contribute to the widespread polymorphism of this biochemical function.

Characterization of two NADPH: cytochrome P450 reductases from cotton (Gossypium hirsutum)

Cytochrome P450 monooxygenases (P450s) are commonly involved in biosynthesis of endogenous compounds and catabolism of xenobiotics, and their activities rely on a partner enzyme, cytochrome P450 reductase (CPR, E.C.1.6.2.4). Two CPR cDNAs, GhCPR1 and GhCPR2, were isolated from cotton (Gossypium hirsutum). They are 71% identical to each other at the amino acid sequence level and belong to the Class I and II of dicotyledonous CPRs, respectively. The recombinant enzymes reduced cytochrome c, ferricyanide and dichlorophenolindophenol (DCPIP) in an NADPH-dependent manner, and supported the activity of CYP73A25, a cinnamate 4-hydroxylase of cotton. Both GhCPR genes were widely expressed in cotton tissues, with a reduced expression level of GhCPR2 in the glandless cotton cultivar. Expression of GhCPR2, but not GhCPR1, was inducible by mechanical wounding and elicitation, indicating that the GhCPR2 is more related to defense reactions, including biosynthesis of secondary metabolites.

Cloning and expression of koala (Phascolarctos cinereus) liver cytochrome P450 reductase

The cloning, expression and characterization of hepatic NADPH-cytochrome P450 reductase (CPR) from koala (Phascolarctos cinereus) is described. Two 2059 bp koala liver CPR cDNAs, designated CPR1 and CPR2, were cloned by reverse transcription-polymerase chain reaction and rapid amplification of cDNA ends. The koala CPR cDNAs encode proteins of 678 amino acids and share 85% amino acid sequence identity to human CPR. Transfection of the koala CPR cDNAs into Cos-7 cells resulted in the expression of proteins, which were recognized by a goat-antihuman CPR antibody. The koala CPR1 and 2 cDNA-expressed enzymes catalysed cytochrome c reductase at the rates of 4.9 +/- 0.5 and 2.6 +/- 0.4 nmol/min/mg protein (mean +/- SD, n = 3), respectively which were comparable to that of rat CPR cDNA-expressed enzyme. The apparent Km value for CPR activity in koala liver microsomes was 11.61 +/- 6.01 microM, which is consistent with that reported for rat CPR enzyme. Northern analysis detected a CPR mRNA band of approximately 2.6 kb. Southern analysis suggested a single PCR gene across species. The present study provides primary molecular data regarding koala CPR1 and CPR2 genes in this unique marsupial species.

Improved cloning and expression of cytochrome P450s and cytochrome P450 reductase in yeast

Combination of the pYeDP60 yeast expression system with a modified version of the improved uracil-excision (USER) cloning technique provides a new powerful tool for high-throughput expression of eukaryotic cytochrome P450s. The vector presented is designed to obtain an optimal 5' untranslated sequence region for yeast (Kozak consensus sequence), and has been tested to produce active P450s and NADPH-cytochrome P450 oxidoreductase (CPR) after 5' end silent codon optimization of the cDNA sequences. Expression of two plant cytochrome P450s, Sorghum bicolor CYP79A1 and CYP71E1, and S. bicolor CPR2 using the modified pYeDP60 vector in all three cases produced high amounts of active protein. High-throughput functional expression of cytochrome P450s have long been a troublesome task due to the workload involved in cloning of each individual P450 into a suitable expression vector. The redesigned yeast P450 expression vector (pYeDP60u) offers major improvements in cloning efficiency, speed, fidelity, and simplicity. The modified version of the USER cloning system provides great potential for further development of other yeast vectors, transforming these into powerful high-throughput expression vectors.

Cyanogenic glucosides: the biosynthetic pathway and the enzyme system involved

In vitro studies with a microsomal system obtained from etiolated sorghum seedlings have indicated a biosynthetic pathway for cyanogenic glucosides involving amino acids, N-hydroxyamino acids, aldoximes, nitriles and cyanohydrins. NADPH is an essential cofactor. Simultaneous measurements of tyrosine metabolism and oxygen consumption show that three molecules of oxygen are consumed for each molecule of p-hydroxymandelonitrile produced. This indicates the operation of three monooxygenases in the pathway and implies the involvement of one hitherto undetected intermediate in the pathway. The nature of this intermediate is unknown. Gel filtration and sucrose gradient centrifugation of the microsomal system resulted in a more than tenfold increase in specific activity. Attempts to further purify the system did not produce preparations of higher specific activity because of a simultaneous partial loss of essential components as demonstrated by reconstitution experiments.

Relevance of cytochrome P450s in plants: also one of Ron Estabrook's research interests

I worked with Dr. Ronald Estabrook for nearly 10 years at The University of Texas Southwestern Medical Center in Dallas, Texas. In Ron's lab, when I joined I was initially involved in the isolation, purification, and characterization of cytochrome P450s and NADPH-P450(c) reductase(s) from plants, which was his new exploratory project at the time. We developed methods for the isolation, solubilization, and purification of P450s and NADPH-P450(c) reductase from plant tissue microsomes. We carried out number of in vitro experiments to study the involvement P450s and NADPH-P450(c) reductase in the biosynthesis of number of phytoalexins. We successfully isolated, purified, and cloned NADPH-P450(c) reductase from etiolated mung bean (Vigna radiate) seedlings. In addition, a series of studies were undertaken to show that purified mung bean NADPH-P450(c) reductase was able to catalyze P450-supported reactions for mammalian and bacterial P450s. My stay in Ron's lab was very educational and productive. He provided the necessary support and led the way through the maze in different research projects in the lab, which allowed me to understand the roles of P450s in humans, animals, plants, and microorganisms. He liked to teach and discover new things everyday in the lab. He is a great scientist, as well as loving and caring mentor.

Coexpression in yeast of Taxus cytochrome P450 reductase with cytochrome P450 oxygenases involved in Taxol biosynthesis

To maximize redox coupling efficiency with recombinant cytochrome P450 hydroxylases from yew (Taxus) species installed in yeast for the production of the anticancer drug Taxol, a cDNA encoding NADPH:cytochrome P450 reductase from T. cuspidata was isolated. This single-copy gene (2,154 bp encoding a protein of 717 amino acids) resembles more closely other reductases from gymnosperms (approximately 90% similarity) than those from angiosperms (<80% similarity). The recombinant reductase was characterized and compared to other reductases by heterologous expression in insect cells and was shown to support reconstituted taxoid 10beta-hydroxylase activity with an efficiency comparable to that of other plant-derived reductases. Coexpression in yeast of the reductase along with T. cuspidata taxoid 10beta-hydroxylase, which catalyzes an early step of taxoid biosynthesis, demonstrated significant enhancement of hydroxylase activity compared to that supported by the endogenous yeast reductase alone. Functional transgenic coupling of the Taxus reductase with a homologous cytochrome P450 taxoid hydroxylase represents an important initial step in reconstructing Taxol biosynthesis in a microbial host.

Drosophila melanogaster CYP6A8, an insect P450 that catalyzes lauric acid (ω-1)-hydroxylation

Only a handful of P450 genes have been functionally characterized from the approximately 90 recently identified in the genome of Drosophila melanogaster. Cyp6a8 encodes a 506-amino acid protein with 53.6% amino acid identity with CYP6A2. CYP6A2 has been shown to catalyze the metabolism of several insecticides including aldrin and heptachlor. CYP6A8 is expressed at many developmental stages as well as in adult life. CYP6A8 was produced in Saccharomyces cerevisiae and enzymatically characterized after catalytic activity was reconstituted with D. melanogaster P450 reductase and NADPH. Although several saturated or non-saturated fatty acids were not metabolized by CYP6A8, lauric acid (C12:0), a short-chain unsaturated fatty acid, was oxidized by CYP6A8 to produce 11-hydroxylauric acid with an apparent V(max) of 25 nmol/min/nmol P450. This is the first report showing that a member of the CYP6 family catalyzes the hydroxylation of lauric acid. Our data open new prospects for the CYP6 P450 enzymes, which could be involved in important physiological functions through fatty acid metabolism.

Optimization of an in vivo plant P450 monooxygenase system in Saccharomyces cerevisiae

Cytochrome P450s are heme-thiolate oxygenases involved in a wide number of reactions such as epoxidation, hydroxylation, and demethylation. Heterologously expressed eukaryotic P450s are potentially useful biocatalysts for stereospecific oxygenation reactions under mild conditions. Numerous factors, such as intracellular pH, cytochrome P450, cytochrome P450 reductase, NADPH, and oxygen concentration all influence the in vivo activity. To systematically examine these factors, we selected ferulate 5-hydroxylase (F5H), a plant P450, with the Saccharomyces cerevisiae WAT11 strain as an expression host. Two media compositions and two cultivation procedures were investigated to optimize the in vivo activity of F5H. We modified a previously published selective growth medium (Pompon et al. [1996] Methods Enzymol 272:51-64) that increased the specific growth rate and cell yield of the host strain. A cultivation procedure with separate growth and induction stages that each contained selective media resulted in a 45% increase of whole cell F5H specific activity. In a medium designed for simultaneous growth and induction, we observed a 2.6-fold higher specific F5H activity, but substantially lower cell yield. Surprisingly, in this medium the higher specific F5H activity did not correlate with a higher P450 concentration. The effects of addition of the first committed heme precursor, delta-aminolevulinic acid, and Fe(III) at the beginning of induction period were also studied for our two-stage procedure. A small, but significant (P < 0.05) increase in whole cell F5H activity was observed following ALA addition.

An in silico assessment of gene function and organization of the phenylpropanoid pathway metabolic networks in Arabidopsis thaliana and limitations thereof

The Arabidopsis genome sequencing in 2000 gave to science the first blueprint of a vascular plant. Its successful completion also prompted the US National Science Foundation to launch the Arabidopsis 2010 initiative, the goal of which is to identify the function of each gene by 2010. In this study, an exhaustive analysis of The Institute for Genomic Research (TIGR) and The Arabidopsis Information Resource (TAIR) databases, together with all currently compiled EST sequence data, was carried out in order to determine to what extent the various metabolic networks from phenylalanine ammonia lyase (PAL) to the monolignols were organized and/or could be predicted. In these databases, there are some 65 genes which have been annotated as encoding putative enzymatic steps in monolignol biosynthesis, although many of them have only very low homology to monolignol pathway genes of known function in other plant systems. Our detailed analysis revealed that presently only 13 genes (two PALs, a cinnamate-4-hydroxylase, a p-coumarate-3-hydroxylase, a ferulate-5-hydroxylase, three 4-coumarate-CoA ligases, a cinnamic acid O-methyl transferase, two cinnamoyl-CoA reductases) and two cinnamyl alcohol dehydrogenases can be classified as having a bona fide (definitive) function; the remaining 52 genes currently have undetermined physiological roles. The EST database entries for this particular set of genes also provided little new insight into how the monolignol pathway was organized in the different tissues and organs, this being perhaps a consequence of both limitations in how tissue samples were collected and in the incomplete nature of the EST collections. This analysis thus underscores the fact that even with genomic sequencing, presumed to provide the entire suite of putative genes in the monolignol-forming pathway, a very large effort needs to be conducted to establish actual catalytic roles (including enzyme versatility), as well as the physiological function(s) for each member of the (multi)gene families present and the metabolic networks that are operative. Additionally, one key to identifying physiological functions for many of these (and other) unknown genes, and their corresponding metabolic networks, awaits the development of technologies to comprehensively study molecular processes at the single cell level in particular tissues and organs, in order to establish the actual metabolic context.

Engineering of a water-soluble plant cytochrome P450, CYP73A1, and NMR-based orientation of natural and alternate substrates in the active site

CYP73A1 catalyzes cinnamic acid hydroxylation, a reaction essential for the synthesis of lignin monomers and most phenolic compounds in higher plants. The native CYP73A1, initially isolated from Jerusalem artichoke (Helianthus tuberosus), was engineered to simplify purification from recombinant yeast and improve solublity and stability in the absence of detergent by replacing the hydrophobic N terminus with the peptitergent amphipathic sequence PD1. Optimized expression and purification procedures yielded 4 mg engineered CYP73A1 L(-1) yeast culture. This water-soluble enzyme was suitable for 1H-nuclear magnetic resonance (NMR) investigation of substrate positioning in the active site. The metabolism and interaction with the enzyme of cinnamate and four analogs were compared by UV-visible and 1H-NMR analysis. It was shown that trans-3-thienylacrylic acid, trans-2-thienylacrylic acid, and 4-vinylbenzoic acid are good ligands and substrates, whereas trans-4-fluorocinnamate is a competitive inhibitor. Paramagnetic relaxation effects of CYP73A1-Fe(III) on the 1H-NMR spectra of cinnamate and analogs indicate that their average initial orientation in the active site is parallel to the heme. Initial orientation and distances of ring protons to the iron do not explain the selective hydroxylation of cinnamate in the 4-position or the formation of single products from the thienyl compounds. Position adjustments are thus likely to occur during the later steps of the catalytic cycle.

Key substrate recognition residues in the active site of a plant cytochrome P450, CYP73A1. Homology guided site-directed mutagenesis

CYP73 enzymes are highly conserved cytochromes P450 in plant species that catalyse the regiospecific 4-hydroxylation of cinnamic acid to form precursors of lignin and many other phenolic compounds. A CYP73A1 homology model based on P450 experimentally solved structures was used to identify active site residues likely to govern substrate binding and regio-specific catalysis. The functional significance of these residues was assessed using site-directed mutagenesis. Active site modelling predicted that N302 and I371 form a hydrogen bond and hydrophobic contacts with the anionic site or aromatic ring of the substrate. Modification of these residues led to a drastic decrease in substrate binding and metabolism without major perturbation of protein structure. Changes to residue K484, which is located too far in the active site model to form a direct contact with cinnamic acid in the oxidized enzyme, did not influence initial substrate binding. However, the K484M substitution led to a 50% loss in catalytic activity. K484 may affect positioning of the substrate in the reduced enzyme during the catalytic cycle, or product release. Catalytic analysis of the mutants with structural analogues of cinnamic acid, in particular indole-2-carboxylic acid that can be hydroxylated with different regioselectivities, supports the involvement of N302, I371 and K484 in substrate docking and orientation.

Engineering herbicide metabolism in tobacco and Arabidopsis with CYP76B1, a cytochrome P450 enzyme from Jerusalem artichoke

The Jerusalem artichoke (Helianthus tuberosus) xenobiotic inducible cytochrome P450, CYP76B1, catalyzes rapid oxidative dealkylation of various phenylurea herbicides to yield nonphytotoxic metabolites. We have found that increased herbicide metabolism and tolerance can be achieved by ectopic constitutive expression of CYP76B1 in tobacco (Nicotiana tabacum) and Arabidopsis. Transformation with CYP76B1 conferred on tobacco and Arabidopsis a 20-fold increase in tolerance to linuron, a compound detoxified by a single dealkylation, and a 10-fold increase in tolerance to isoproturon or chlortoluron, which need successive catalytic steps for detoxification. Two constructs for expression of translational fusions of CYP76B1 with P450 reductase were prepared to test if they would yield even greater herbicide tolerance. Plants expressing these constructs had lower herbicide tolerance than CYP76B1 alone, which is apparently a consequence of reduced stability of the fusion proteins. In all cases, increased herbicide tolerance results from more extensive metabolism, as demonstrated with exogenously fed phenylurea. Beside increased herbicide tolerance, expression of CYP76B1 has no other visible phenotype in the transgenic plants. Our data indicate that CYP76B1 can function as a selectable marker for plant transformation, allowing efficient selection in vitro and in soil-grown plants. Plants expressing CYP76B1 may also be a potential tool for phytoremediation of contaminated sites.

Catalytic and immunochemical properties of NADPH-cytochrome P450 reductase from fungus Rhizopus nigricans

Flavoprotein NADPH-cytochrome P450 reductase (CPR, EC 1.6.2.4) from filamentous fungus Rhizopus nigricans is a membrane bound enzyme which is involved in the reduction of cytochrome P450 during the hydroxylation of progesterone at 11alpha position. After purification of the enzyme from induced mycelia three forms of fungal CPR were detected on SDS-PAGE: a predominant form with an apparent molecular mass of 78kDa and two truncated forms. N-terminal sequences of all three forms were determined as well as some internal sequences of 78kDa form. Dose-dependent immunoinhibition of NADPH-cytochrome c reductase and progesterone 11alpha-hydroxylase activities was observed with mouse anti-CPR antisera. No cross-reactions were obtained on Western blots between mouse anti-CPR antisera and protein preparations from noninduced mycelia and microsomal fraction from fungus Pleurotus osteatus, plant Ginkgo biloba or chicken liver. The kinetic mechanism of CPR was proposed on the basis of model reaction with cytochrome c(3+). Results obtained at high ionic strength suggest a nonclassical two-site ping pong mechanism and at low ionic strength a sequential mechanism of bisubstrate reaction.

Increasing expression of P450 and P450-reductase proteins from monocots in heterologous systems

Monocotyledonous crop plants are usually more resistant to herbicides than grass weeds and most dicots. Their resistance to herbicides is mediated in many cases by P450 oxygenases. Monocots thus constitute an appealing source of P450 enzymes for manipulating herbicide resistance and recombinant forms of the major xenobiotic metabolizing mooxygenases are potential tools for the optimization of new active molecules. We report here the isolation and functional characterization of the first P450 and P450 reductase coding sequences from wheat. The first attempts at expressing these cDNAs in yeast and tobacco led to levels of protein, which were extremely low, often not even detectable. The wheat P450 cDNAs were efficiently transcribed, but no protein or activity was found. Wheat coding sequences, like those of other monocots, are characterized by a high GC content and by a related strong bias of codon usage, different from that observed in yeast or dicots. Complete recoding of genes being costly, the reengineering their 5'-end using a single PCR megaprimer designed to comply with codon usage of the host was attempted. It was sufficient to relieve translation inhibition and to obtain good levels of protein expression. The same strategy also resulted in a dramatic increase in protein expression in tobacco. A basis for the success of such a partial recoding strategy, much easier and cheaper than complete recoding of the cDNA, is proposed.

Is Fusarium culmorum isotrichodermin-15-hydroxylase different from other fungal species?

Fusarium spp. are ubiquitous fungi infecting cereals and grains, and therefore constitute a major problem for agriculture. Their trichothecene metabolites, and in particular deoxynivalenol and its 3-acetylated derivative, are the mycotoxins involved. The major metabolite produced by Fusarium culmorum is 3-acetyldeoxynivalenol. Studies in vivo with Fusarium culmorum have established that its tricyclic intermediate, isotrichodermin, is a major biosynthetic precursor, which is hydroxylated at position 15 to give 15-deacetylcalonectrin, prior to being converted to the product. In a preliminary in vitro investigation of the cell-free system involved in this transformation, we suggested that cytochrome P450 enzymes are not involved. In this paper, the isotrichodermin-15-hydroxylase from the microsomal fraction of Fusarium culmorum was solubilized and partially purified (60 fold). Our studies with cofactors indicate that this enzyme is a flavoprotein, and the inducers tested highly indicate that indeed the hydroxylase is not attached to cytochrome P450. This is particularly interesting, since the only other enzyme catalyzing the same reaction isolated from Fusarium sporotrichiodes is attached to cytochrome P450.

Cytochromes P-450 from cassava (Manihot esculenta Crantz) catalyzing the first steps in the biosynthesis of the cyanogenic glucosides linamarin and lotaustralin. Cloning, functional expression in Pichia pastoris, and substrate specificity of the isolated recombinant enzymes

The first committed steps in the biosynthesis of the two cyanogenic glucosides linamarin and lotaustralin in cassava are the conversion of L-valine and L-isoleucine, respectively, to the corresponding oximes. Two full-length cDNA clones that encode cytochromes P-450 catalyzing these reactions have been isolated. The two cassava cytochromes P-450 are 85% identical, share 54% sequence identity to CYP79A1 from sorghum, and have been assigned CYP79D1 and CYP79D2. Functional expression has been achieved using the methylotrophic yeast, Pichia pastoris. The amount of CYP79D1 isolated from 1 liter of P. pastoris culture exceeds the amounts that putatively could be isolated from 22,000 grown-up cassava plants. Each cytochrome P-450 metabolizes L-valine as well as L-isoleucine consistent with the co-occurrence of linamarin and lotaustralin in cassava. CYP79D1 was isolated from P. pastoris. Reconstitution in lipid micelles showed that CYP79D1 has a higher k(c) value with L-valine as substrate than with L-isoleucine, which is consistent with linamarin being the major cyanogenic glucoside in cassava. Both CYP79D1 and CYP79D2 are present in the genome of cassava cultivar MCol22 in agreement with cassava being allotetraploid. CYP79D1 and CYP79D2 are actively transcribed, and production of acyanogenic cassava plants would therefore require down-regulation of both genes.

Cytochrome P450 monooxygenases of DIBOA biosynthesis: specificity and conservation among grasses

DIBOA and DIMBOA are secondary metabolites of grasses which function as natural pesticides. The four maize genes BX2 through BX5 encode cytochrome P450-dependent monooxygenases that catalyse four consecutive reactions in the biosynthesis of these secondary products. Although BX2-BX5 share significant sequence homology, the four enzymes have evolved into specific enzymes each catalysing predominantly only one reaction in the pathway. In addition to these natural reactions, BX3 hydroxylates 1,4-benzoxazin-3-one and BX2 shows pCMA demethylase activity. With respect to DIBOA biosynthesis, identical enzymatic reactions have been found in rye as compared to maize, indicating early evolution of the P450 enzymes in the grasses.

Substrate specificity of the cytochrome P450 enzymes CYP79A1 and CYP71E1 involved in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench

The two multifunctional cytochrome P450 enzymes, CYP79A1 and CYP71E1, involved in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench have been characterized with respect to substrate specificity and cofactor requirements using reconstituted, recombinant enzymes and sorghum microsomes. CYP79A1 has a very high substrate specificity, tyrosine being the only substrate found. CYP71E1 has less stringent substrate requirements and metabolizes aromatic oximes efficiently, whereas aliphatic oximes are slowly metabolized. Neither CYP79A1 nor CYP71E1 catalyze the metabolism of a range of different herbicides. The reported resistance of sorghum to bentazon is therefore not linked to the presence of CYP79A1 or CYP71E1. NADPH is a much better cofactor than NADH although NADH does support the entire catalytic cycle of both P450 enzymes. Km and Vmax values for NADPH when supporting CYP71E1 activity are 0.013 mM and 111 nmol/mg protein/s. For NADH, the corresponding values are 0. 3 mM and 42 nmol/mg protein/s. CYP79A1 is a fairly stable enzyme. In contrast, CYP71E1 is labile and prone to rapid denaturation at room temperature. CYP71E1 is isolated in the low spin form. CYP71E1 catalyzes an unusual dehydration reaction of an oxime to the corresponding nitrile which subsequently is C-hydroxylated. The oxime forms a peculiar reverse Type I spectrum, whereas the nitrile forms a Type I spectrum. Several compounds which do not serve as substrates formed Type I substrate binding spectra with the two P450 enzymes.

The chemically inducible plant cytochrome P450 CYP76B1 actively metabolizes phenylureas and other xenobiotics

Cytochrome P450s (P450s) constitute one of the major classes of enzymes that are responsible for detoxification of exogenous molecules both in animals and plants. On the basis of its inducibility by exogenous chemicals, we recently isolated a new plant P450, CYP76B1, from Jerusalem artichoke (Helianthus tuberosus) and showed that it was capable of dealkylating a model xenobiotic compound, 7-ethoxycoumarin. In the present paper we show that CYP76B1 is more strongly induced by foreign compounds than other P450s isolated from the same plant, and metabolizes with high efficiency a wide range of xenobiotics, including alkoxycoumarins, alkoxyresorufins, and several herbicides of the class of phenylureas. CYP76B1 catalyzes the double N-dealkylation of phenylureas with turnover rates comparable to those reported for physiological substrates and produces nonphytotoxic compounds. Potential uses for CYP76B1 thus include control of herbicide tolerance and selectivity, as well as soil and groundwater bioremediation.

Double triton X-114 phase partitioning for the purification of plant cytochromes P450 and removal of green pigments

A double Triton X-114 phase partitioning procedure that separates plant cytochromes P450 from green pigments and provides an extract highly enriched in total cytochromes P450 has been developed. Upon phase partitioning in Triton X-114, plant cytochromes P450 have previously been found to partition to the pigmented detergent rich phase. These partitionings were carried out using phosphate buffer. We found that the partitioning of the cytochromes P450 could be shifted to a pigment-free Triton X-114 poor phase by changing the buffer component to borate. The protein extract containing the cytochromes P450 but devoid of green pigment was subjected to a second phase partitioning step before which the buffer was changed from borate to phosphate. This second phase partitioning step produced a Triton X-114-rich phase highly enriched in cytochromes P450 proteins compared to the microsomal starting material as monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, cytochrome P450 reconstitution assays, and Western blotting. The yield of the double phase partitioning purification procedure is about 26% which is high compared to the yields obtained at similar stages of purification using column chromatography. The double phase partitioning procedure takes 3-4 h to complete. This is very fast compared to traditional purification schemes for cytochromes P450 which involve multiple of column chromatographic steps. Plant cytochromes P450 are labile, low abundant proteins that are difficult to isolate. The double Triton X-114 phase partitioning here reported thus constitutes a versatile, efficient purification procedure circumventing many of the problems previously encountered.

Differentially regulated NADPH:cytochrome P450 oxidoreductases in parsley

Two NADPH:cytochrome P450 oxidoreductases (CPRs) from parsley (Petroselinum crispum) were cloned, and the complete proteins were expressed and functionally identified in yeast. The two enzymes, designated CPR1 and CPR2, are 80% identical in amino acid sequence with one another and about 75% identical with CPRs from several other plant species. The mRNA accumulation patterns for CPR1 and CPR2 in fungal elicitor-treated or UV-irradiated cultured parsley cells and in developing or infected parsley plants were compared with those for cinnamate 4-hydroxylase (C4H), one of the most abundant CPR-dependent P450 enzymes in plants. All treatments strongly induced the mRNAs for C4H and CPR1 but not for CPR2, suggesting distinct metabolic roles of CPR1 and CPR2 and a functional relationship between CPR1 and C4H.

Cloning and heterologous expression of NADPH-cytochrome P450 reductases from the Papaveraceae

Cytochrome P450 reductase was purified to homogeneity from cell suspension cultures of the opium poppy Papaver somniferum, the enzyme was characterized (K(m) cytochrome c, 8.3 microM; K(m) NADPH, 4.2 microM; pH optimum, 8.0; M(r), 80 kDa), and the amino acid sequence of internal peptides was determined. Partial cDNA clones from P. somniferum and from Eschscholzia californica (California poppy) were then generated using the polymerase chain reaction and were used as hybridization probes to isolate full-length cDNAs. The Papaver and Eschscholzia cytochrome P450 reductases are 63% identical at the nucleotide level and 69% identical at the amino acid level. SDS-PAGE of the purified native P. somniferum enzyme as well as genomic DNA gel blot analysis indicate that two cytochrome P450 reductase isoforms are present in each species. This evidence is also supported by translation of nucleotide sequences obtained from the PCR-generated partial cDNAs and the full-length cDNAs isolated from lambda libraries. The Papaver and Eschscholzia cytochrome P450 reductases were functionally expressed in the yeast Saccharomyces cerevisiae and in the insect cell culture Spodoptera frugiperda Sf9. Coexpression of cytochrome P450 reductase with the C-O phenol coupling cytochrome P450 of bisbenzylisoquinoline alkaloid biosynthesis in Berberis stolonifera, berbamunine synthase (CYP80A1), in insect cell culture resulted in an alteration of the product profile as compared to that obtained by expression of berbamunine synthase in the absence of plant reductase.

Isolation and reconstitution of the heme-thiolate protein obtusifoliol 14alpha-demethylase from Sorghum bicolor (L.) Moench

The heme-thiolate (cytochrome P450) enzyme which catalyzes the 14alpha-demethylation of obtusifoliol has been isolated from microsomes prepared from etiolated seedlings of Sorghum bicolor (L.) Moench. The obtusifoliol 14alpha-demethylase is a key enzyme in plant sterol biosynthesis and a target for the design of phyla-specific sterol 14alpha-demethylase inhibitors. Microsomal cytochrome P450s were solubilized by using the detergents Renex 690 and reduced Triton X-100, and the obtusifoliol 14alpha-demethylase was isolated by DEAE ion exchange and dye affinity column chromatography. The isolated enzyme has an absorption spectrum characteristic for low spin cytochrome P450s and produces a Type I binding spectrum with obtusifoliol as substrate. Binding spectra were not obtained with lanosterol, campesterol, sitosterol, or stigmasterol. Obtusifoliol 14alpha-demethylase has an apparent molecular mass of 53 kDa and is estimated to constitute approximately 20% of the total cytochrome P450 content of the microsomal membranes and about 0.2% of the total microsomal protein. Gas chromatography-mass spectrometry analysis of reconstitution experiments with dilauroylphosphatidylcholine micelles containing isolated obtusifoliol 14alpha-demethylase and sorghum NADPHcytochrome P450 oxidoreductase demonstrated the conversion of obtusifoliol (4alpha,14alpha-dimethyl-5alpha-ergosta-8, 24(28)-dien-3beta-ol) to 4alpha-methyl-5alpha-ergosta-8,14, 24(28)-trien3beta-ol, the 14alpha-demethylated product of obtusifoliol with a double bond introduced at the Delta14 position. The N-terminal amino acid sequence of the protein is MDLADIPQ/KQQRLMAGXALVV. Five internal sequences were obtained after endoproteinase Lys-C and Glu-C digestion. The fragment AAGAFSYISFGGGRH aligns with the unique heme binding domain of mammalian and yeast sterol 14alpha-demethylases which belong to the CYP51 family. Therefore it is conceivable that the obtusifoliol 14alpha-demethylase from plants also belongs to the CYP51 family, the only P450 family so far known to be conserved across the phyla.

Cytochrome P-450TYR is a multifunctional heme-thiolate enzyme catalyzing the conversion of L-tyrosine to p-hydroxyphenylacetaldehyde oxime in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench

Cytochrome P-450TYR, which catalyzes the N-hydroxylation of L-tyrosine in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench has recently been isolated (Sibbesen, O., Koch, B., Halkier, B. A., and Møller, B. L. (1994) Proc. Natl. Acad. Sci. U.S.A. 92, 9740-9744). Reconstitution of the enzyme activity in lipid micelles containing cytochrome P-450TYR and NADPH-cytochrome P-450 oxidoreductase demonstrates that cytochrome P-450TYR catalyzes the conversion of L-tyrosine into p-hydroxyphenylacetaldehyde oxime. Earlier studies with microsomes have demonstrated that this conversion involves two N-hydroxylation reactions of which the first produces N-hydroxytyrosine. We propose that the product of the second N-hydroxylation reaction is N,N-dihydroxytyrosine. N,N-dihydroxytyrosine is dehydrated to 2-nitroso-3-(p-hydroxyphenyl) propionic acid which decarboxylates to p-hydroxyphenylacetaldehyde oxime. The dehydration and decarboxylation reactions may proceed non-enzymatically. The E/Z ratio of the p-hydroxyphenylacetaldehyde oxime produced by reconstituted cytochrome P-450TYR is 69:31. Lipid micelles made from L-alpha-dilauroyl phosphatidylcholine are more than twice as effective in reconstituting cytochrome P-450TYR activity as compared to other lipids. The Km and turnover number of the enzyme is 0.14 mM and 200 min-1, respectively, when assayed in the presence of 15 mM NaCl whereas the values are 0.21 mM and 230 min-1 when assayed in the absence of added salt. The multifunctional nature cytochrome P-450TYR is confirmed by demonstrating that binding of L-tyrosine or N-hydroxytyrosine mutually excludes binding of the other substrate. These results explain why the conversion of tyrosine to p-hydroxyphenylacetaldehyde oxime as earlier reported (Møller, B. L., and Conn, E. E. (1980) J. Biol. Chem. 255, 3049-3056) shows the phenomenon of catalytic facilitation ("channeling"). Cytochrome P-450TYR is the first isolated multifunctional heme-thiolate enzyme from plants. N-Hydroxylases of the cytochrome P-450 type with high substrate specificity have not previously been reported.

Purification and immunochemical characteristics of NADPH-cytochrome P-450 oxidoreductase from tobacco cultured cells

NADPH-cytochrome P-450 oxidoreductase (EC 1.6.2.4) was purified from the microsomal fraction of tobacco (Nicotiana tabacum) BY2 cells by chromatography on two anion-exchange columns and 2',5' ADP-Sepharose 4B column. The purified enzyme showed a single protein band with a molecular weight of 79 kDa on SDS-PAGE and exhibited a typical flavoprotein redox spectrum, indicating the presence of an equimolar quantity of FAD and FMN. This enzyme followed Michaelis-Menten Kinetics with Km values of 24 microM for NADPH and 16 microM for cytochrome c. An in vitro reconstituted system of the purified reductase with a partially purified tobacco cytochrome P-450 preparation showed the cinnamic acid 4-hydroxylase activity at the rate of 14 pmol min-1 nmol-1 P-450 protein and with a purified rabbit P-4502C14 catalyzed N-demethylation of aminopyrine at the rate of 6 pmol min-1 nmol-1 P-450 protein. Polyclonal antibodies raised against the purified reductase reacted with tobacco reductase but not with yeast reductase on Western blot analysis. Anti-yeast reductase antibodies did not react with the tobacco reductase. This result indicate that the tobacco reductase was immunochemically different from the yeast reductase. The anti-tobacco reductase antibodies totally inhibited the tobacco reductase activity, but not the yeast reductase. Also, Western blot analyses using the anti-tobacco reductase antibodies revealed that leaves, roots and shoots of Nicotiana tabacum plants contained an equal amount of the reductase protein. From these results, it was suggested that there are different antibody binding sites, which certainly participate in enzyme activity, between tobacco and yeast reductase.

Cytochrome P450 limonene hydroxylases of Mentha species

The oxygenation pattern of the monoterpenoids of mint (Mentha) species is determined by regiospecific cytochrome P450-catalyzed hydroxylation of the common olefinic precursor (-)-limonene. In peppermint, C3-allylic hydroxylation leads to (-)-trans-isopiperitenol that ultimately is converted to (-)-menthol, whereas in spearmint, C6-allylic hydroxylation leads to (-)-trans-carveol that is oxidized to (-)-carvone. The limonene-6-hydroxylase and the cytochrome P450 reductase were purified from the oil glands of spearmint, and the system was reconstituted. Amino acid sequences from the purified hydroxylase were utilized to design primers with which a large, non-degenerate PCR product was prepared. This probe was employed to screen a spearmint oil gland cDNA library from which the corresponding full-length cDNA was isolated. This clone provides the tool for isolating the homologous cDNA species from peppermint and related Mentha species.

Kinetic mechanism for the model reaction of NADPH-cytochrome P450 oxidoreductase with cytochrome c

The kinetic mechanism of NADPH-cytochrome P450 oxidoreductase (P450R) has been determined for the model reaction with cytochrome c3+. Although initial velocity studies show parallel patterns, consistent with a classical (one-site) ping-pong mechanism that precludes the formation of a ternary NADPH-P450R-cytochrome c3+ complex, product and dead-end inhibition results suggest a nonclassical (two-site) ping-pong mechanism [Northrop, D. B. (1969) J. Biol. Chem. 244, 5808-5819]. This mechanism is a hybrid of the random sequential (ternary complex) and ping-pong mechanisms, since ternary complexes can form as well as intermediate, modified forms of the enzyme that can be present in the absence of any bound substrate. The complete rate equation is derived for this mechanism, and values for Vmax, (V/K)NADPH, (V/K)cytc, and the corresponding Michaelis constants are presented in terms of microscopic rate constants along with the expected product inhibition patterns (Appendix). Inhibition by NADP+ is competitive versus NADPH and uncompetitive versus cytochrome c3+, while inhibition by cytochrome c2+ is competitive versus cytochrome c3+ and noncompetitive versus NADPH. These inhibition patterns are consistent with the proposed two-site mechanism. This mechanism would give the same initial velocity patterns as the classical one-site ping-pong mechanism, but it allows for the formation of a ternary complex, with NADPH and cytochrome c3+ reacting independently at two separate sites on P450R. The D(V/K)NADPH isotope effect is not affected by cytochrome c3+ concentration, consistent with our assumption (in deriving the rate equation) that binding at the two sites is independent. At the high ionic strength used in this study (850 mM), the mechanism is two-site ping-pong, with the electron acceptor site itself reacting with cytochrome c3+ in a tetra uni ping-pong manner.

Purification and partial characterization of NADPH-cytochrome c reductase from Petunia hybrida flowers

NADPH-cytochrome c reductase was solubilized from the microsomal fraction of Petunia hybrida flowers by 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate detergent and purified by adenosine 2',5'-bisphosphate-Sepharose chromatography, followed by high-performance anion-exchange chromatography. Two proteins with molecular sizes of 75 and 81 kD were detected in the purified preparation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Western blot analysis showed that both purified proteins cross-reacted with two different monoclonal antibodies raised against P. hybrida NADPH-cytochrome c reductase and rabbit anti-Jerusalem artichoke NADPH-cytochrome P450 reductase antibodies. Only one 84-kD protein was detected by western blot analysis of fresh microsomal extracts. Amino acid sequence analysis of tryptic peptides revealed significant similarity to the NADPH binding region of plant and animal NADPH-cytochrome P450 reductases and Bacillus megaterium cytochrome P450:NADPH-cytochrome P450 reductase. The pH optimum for reduction of ferricytochrome c was 7.4 and the Km values for the binding of NADPH and ferricytochrome c were 9.2 and 2.8 microM, respectively. We believe that the purified enzyme is a P. hybrida NADPH-cytochrome P450 reductase (EC 1.6.2.4).

Purification, characterization, and cDNA cloning of an NADPH-cytochrome P450 reductase from mung bean

We report here the isolation and deduced amino acid sequence of the flavoprotein, NADPH-cytochrome P450 (cytochrome c) reductase (EC 1.6.2.4), associated with the microsomal fraction of etiolated mung bean seedlings (Vigna radiata var. Berken). An 1150-fold purification of the plant reductase was achieved, and SDS/PAGE showed a predominant protein band with an apparent molecular mass of approximately 82 kDa. The purified plant NADPH-P450 reductase gave a positive reaction as a glycoprotein, exhibited a typical flavoprotein visible absorbance spectrum, and contained almost equimolar quantities of FAD and FMN per mole of enzyme. Specific antibodies revealed the presence of unique epitopes distinguishing the plant and mammalian flavoproteins as demonstrated by Western blot analyses and inhibition studies. Peptide fragments from the purified plant NADPH-P450 reductase were sequenced, and degenerate primers were used in PCR amplification reactions. Overlapping cDNA clones were sequenced, and the deduced amino acid sequence of the mung bean NADPH-P450 reductase was compared with equivalent enzymes from mammalian species. Although common flavin and NADPH-binding sites are recognizable, there is only approximately 38% amino acid sequence identity. Surprisingly, the purified mung bean NADPH-P450 reductase can substitute for purified rat NADPH-P450 reductase in the reconstitution of the mammalian P450-catalyzed 17 alpha-hydroxylation of pregnenolone or progesterone.

Production and Characterization of Monoclonal Antibodies against NADPH-Cytochrome P-450 Reductases from Helianthus tuberosus

Monoclonal antibodies (mAbs) against a plant NADPH-cytochrome P-450 (Cyt P-450) reductase from Jerusalem artichoke (Helianthus tuberosus) tuber were prepared. These antibodies were produced by hybridoma resulting from the fusion of spleen cells from a rat immunized with a purified preparation of the reductase and mouse myeloma cells. The mAbs thus obtained were screened for their interaction with the reductases, first in western dots and then in blots, and for their ability to inhibit the NADPH-cytochrome c (Cyt c) reductase activity from Jerusalem artichoke microsomes. Among the 11 clones giving a positive response on western blots, only 6 were also able to inhibit microsomal NADPH-Cyt c reductase activity, and the microsomal Cyt P-450 monooxygenase activities dependent upon electrons transferred by the reductase. Thus, two families of mAbs were characterized: a family of mAbs that interact with epitopes of the reductase implicated in the reduction of Cyt P-450 by NADPH (binding sites for NADPH, flavin mononucleotide, flavin adenine dinucleotide, and Cyt P-450), and a structural family, whose members recognize epitopes outside the active site of the reductases. These mAbs specifically recognize the reductase, and all of them interact with all of the isoforms, indicating that important primary or secondary structural analogies exist between the isoforms, not only at the active site, but also at the level of epitopes not directly associated with catalytic activity.

Cytochrome p-450-dependent hydroxylation of lauric Acid at the subterminal position and oxidation of unsaturated analogs in wheat microsomes

Microsomes from etiolated wheat (Triticum aestivum L. cv Etoile de Choisy) shoots catalyzed the reduced nicotinamide adenine dinucleotide phosphate-dependent hydroxylation of lauric acid predominantly at the subterminal or (omega-1) position (65%). Minor amounts of 10-hydroxy- (31%) and 9-hydroxylaurate (4%) were also formed. The reaction was catalyzed by cytochrome P-450, since enzyme activity was strongly inhibited by tetcyclacis, carbon monoxide, and antibodies against NADPH-cytochrome c (P-450)-reductase. The apparent K(m) for lauric acid was estimated to be 8.5 +/- 2.0 mum. Seed treatment with the safener naphthalic acid anhydride or treatment of seedlings with phenobarbital increased cytochrome P-450 content and lauric acid hydroxylase (LAH) activity of the microsomes. A combination of both treatments further stimulated LAH activity. A series of radiolabeled unsaturated lauric acid analogs (8-, 9-, 10-, and 11-dodecenoic acids) was used to explore the regioselectivity and catalytic capabilities of induced wheat microsomes. It has been found that wheat microsomes catalyzed the reduced nicotinamide adenine dinucleotide phosphate-dependent epoxidation of sp2 carbons concurrently with hydroxylation at saturated positions. The regioselectivity of oxidation of the unsaturated substrates and that of lauric acid were similar. Preincubation of wheat microsomes with reduced nicotinamide adenine dinucleotide phosphate and 11-dodecenoic acid resulted in a partial loss of LAH activity.

Regioselectivity of a plant lauric acid omega hydroxylase. Omega hydroxylation of cis and trans unsaturated lauric acid analogs and epoxygenation of the terminal olefin by plant cytochrome P-450

The study of the stereochemistry and regioselectivity of plant fatty acid hydroxylases is hampered by the difficulty to purify plant cytochrome P-450 enzymes. To provide an alternative, we have now defined an experimental plant system which expresses only one hydroxylase activity towards lauric acid: microsomes from clofibrate-induced Vicia sativa seedlings hydroxylate this fatty acid exclusively at the methyl terminus. To explore the catalytic capabilities of this laurate oxidase, a series of 1-14C-radiolabeled unsaturated lauric acid analogs (7-, 8-, 9-, 10- and 11-dodecenoic acids) were synthesized. Microsomes from clofibrate induced Vicia sativa seedling catalyzed the omega-oxidation of the lauric acid analogs in the presence of O2 and NADPH. The cis and trans forms of the four in-chain unsaturated analogs of lauric acid were 12-hydroxylated with similar efficiency. The terminal olefin was readily converted to the epoxide with only marginal autocatalytic inactivation of the enzyme. The formation of each metabolite was inhibited to the same extend when microsomes were incubated in presence of carbon monoxide or a suicide-substrate for omega LAH, suggesting that a single cytochrome P-450 isoenzyme from Vicia sativa microsomes is able to omega hydroxylate lauric acid and in-chain unsaturated analogs, and to epoxygenate 11-dodecenoic acid.

Further characterization of cytochrome P450 involved in phytoalexin synthesis in soybean: cytochrome P450 cinnamate 4-hydroxylase and 3,9-dihydroxypterocarpan 6a-hydroxylase

Two cytochrome P450 enzymes, cinnamate 4-hydroxylase (C4H) and 3,9-dihydroxypterocarpan 6a-hydroxylase (D6aH), were isolated from elicitor-challenged soybean (Glycine max) cell cultures (G. Kochs and H. Grisebach, 1989, Arch. Biochem. Biophys. 273, 543-553). An earlier purification protocol was improved by the use of new chromatographic media, leading to a higher yield of enzymatic activity. After separation of C4H from D6aH on hydroxyapatite, the C4H was identified using anti-C4H antibody from Jerusalem artichoke (Helianthus tuberosus) (B. Gabriac et al., 1991, Arch. Biochem. Biophys. 288, 302-309). The two proteins show molecular weights of about 58,000 for C4H and about 55,000 for D6aH on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both enzyme activities are dependent on NADPH:cytochrome P450 reductase and cross-react with their respective antibodies. Both cytochrome P450 subspecies show substrate binding and CO-difference spectra typical for cytochrome P450 and were found to be glycoproteins by their cross-reaction with biotinylated lectins in Western blot. The N-terminal sequence of C4H from soybean shows high similarity to the N-terminus of C4H from Jerusalem artichoke.

Purification and some properties of the squalene-tetrahymanol cyclase from Tetrahymena thermophila

The membrane-bound enzyme from Tetrahymena thermophila responsible for the conversion of squalene into the quasi-hopanoid tetrahymanol was purified 297-fold to near homogeneity. Purification involved solubilization by octylthioglucoside, chromatography on DEAE-trisacryl, hydroxyapatite and FPLC ion-exchange on Mono Q. The apparent KM was found to be 18 microM. 2,3-Iminosqualene and N,N-dimethyldodecylamine-N-oxide are effective inhibitors of the cyclase with I50 values of 50 and 30 nM, respectively. The cyclase has a molecular mass of 72 kDa as judged by electrophoresis in polyacrylamide gels under denaturating conditions. The optimal enzymatic activity was obtained at pH 7.0 and 30 degrees C. The solubilized enzyme needs the presence of detergent for maintaining activity. The influence of different detergents on cyclase activity was studied. Triton X-100 proved to be a strong inactivator of the enzyme. Solubilization of the cyclase in Tween 80 and digitonin inactivates the enzyme. However, its activity can be recovered by complementation of the assay buffer with octylthioglucoside above its critical micellar concentration. We suggest that this approach might be applicable to other membrane-bound proteins.

Purification and immunocharacterization of a plant cytochrome P450: the cinnamic acid 4-hydroxylase

Cinnamic acid 4-hydroxylase (CA4H) was purified from microsomes of manganese-induced Jerusalem artichoke (Helianthus tuberosus L.) tuber tissues. The three-step purification procedure involved solubilization and phase partitioning in Triton X-114, followed by chromatography on DEAE-Trisacryl and hydroxylapatite columns. Purification was monitored using carbon monoxide and type I substrate binding properties of the enzyme. The protein, purified to electrophoretic homogeneity, showed an Mr of about 57,000 on SDS-PAGE. Polyclonal antibodies raised against this protein selectively reacted with a 57-kDa polypeptide on Western blots of induced Jerusalem artichoke microsomes. The antibody selectively and strongly inhibited CA4H activity from several plant species.

Multiple forms of NADPH-cytochrome P450 reductase in higher plants

We report on the presence of multiple forms of NADPH-cyt P450 reductase in microsomes from higher plants. This contrasts with the animal cyt P450 monooxygenases, where the numerous cyt P450 isoforms are reduced by a single form of reductase. Three NADPH-cyt c reductases have been resolved from Jerusalem artichoke tuber microsomes by chromatography on Reactive Red Agarose and Concanavalin A-Sepharose. Their molecular weights, determined by sodium dodecylsulfate-gel electrophoresis, are 80,000, 82,000 and 84,000. The three proteins share common epitopes and are dependent upon FMN for catalytic activity. They are highly selective for NADPH as electron donor, and allowed effective reconstitution of trans-cinnamic acid and 3,9-dihydroxypterocarpan 6a-hydroxylase activities with purified cyt P450 fractions from Helianthus tuberosus and Glycine max, respectively. As such, they appear as true isoenzyme forms of NADPH-cyt P-450 reductase.

Involvement of Cytochrome P-450 in the Biosynthesis of Dhurrin in Sorghum bicolor (L.) Moench

The biosynthesis of the tyrosine-derived cyanogenic glucoside dhurrin involves N-hydroxytyrosine, (E)- and (Z)-p-hydroxyphenylacetaldehyde oxime, p-hydroxyphenylacetonitrile, and p-hydroxymandelonitrile as intermediates and has been studied in vitro using a microsomal enzyme system obtained from etiolated sorghum (Sorghum bicolor [L.] Moench) seedlings. The biosynthesis is inhibited by carbon monoxide and the inhibition is reversed by 450 nm light demonstrating the involvement of cytochrome P-450. The combined use of two differently prepared microsomal enzyme systems and of tyrosine, p-hydroxyphenylacetaldehyde oxime, and p-hydroxyphenylacetonitrile as substrates identify two cytochrome P-450-dependent monooxygenases: the N-hydroxylase which converts tyrosine into N-hydroxytyrosine and the C-hydroxylase converting p-hydroxyphenylacetonitrile into p-hydroxymandelonitrile. The inhibitory effect of a number of putative cytochrome P-450 inhibitors confirms the involvement of cytochrome P-450. Monospecific polyclonal antibodies raised toward NADPH-cytochrome P-450-reductase isolated from sorghum inhibits the same metabolic conversions as carbon monoxide. No cytochrome P-450-dependent monooxygenase catalyzing an N-hydroxylation reaction has previously been reported in plants. The metabolism of p-hydroxyphenylacetaldehyde oxime is completely dependent on the presence of NADPH and oxygen and results in the production of p-hydroxymandelonitrile with no accumulation of the intermediate p-hydroxyphenylacetonitrile in the reaction mixture. The apparent NADPH and oxygen requirements of the oxime-metabolizing enzyme are identical to those of the succeeding C-hydroxylase converting p-hydroxyphenylacetonitrile to p-hydroxymandelonitrile. Due to the complex kinetics of the microsomal enzyme system, these requirements may not appertain to the oxime-metabolizing enzyme, which may convert p-hydroxyphenylacetaldehyde oxime to p-hydroxyacetonitrile by a simple dehydration.

Two cytochrome P-450 isoforms catalysing O-de-ethylation of ethoxycoumarin and ethoxyresorufin in higher plants

The O-dealkylating activities of 7-ethoxycoumarin O-de-ethylase (ECOD) and 7-ethoxyresorufin O-de-ethylase (EROD) have been fluorimetrically detected in microsomes prepared from manganese-induced Jerusalem artichoke tubers. Cytochrome P-450 dependence of the reactions was demonstrated by light-reversed CO inhibition, NADPH-dependence, NADH-NADPH synergism and by use of specific inhibitors: antibodies to NADPH-cytochrome P-450 reductase, mechanism-based inactivators and tetcyclasis. Apparent Km values of 161 microM for 7-ethoxycoumarin and 0.4 microM for 7-ethoxyresorufin were determined. O-De-ethylase activity was also detected in microsomes prepared from several other plant species, including wheat, maize, tulip, avocado and Vicia. ECOD and EROD were low or undetectable in uninduced plant tissues, and both activities were stimulated by wounding or by chemical inducers. Two distinct cytochrome P-450 isoforms are involved in ECOD and EROD activities since (1) they showed different distributions among plant species; (2) they showed contrasting inhibition and induction patterns; and (3) ECOD but not EROD activity was supported by cumene hydroperoxide.

Phytoalexin synthesis in soybean: purification and reconstitution of cytochrome P450 3,9-dihydroxypterocarpan 6a-hydroxylase and separation from cytochrome P450 cinnamate 4-hydroxylase

Elicitor-challenged soybean (Glycine max) cell cultures were used for detergent solubilization and purification of cytochrome P450 3,9-dihydroxypterocarpan 6a-hydroxylase (D6aH). D6aH was purified to electrophoretic homogeneity from such cells by a five-step procedure. It could be separated from cytochrome P450 cinnamate 4-hydroxylase on hydroxyapatite. This is the first report on separation of two cytochrome P450 enzymes from a higher plant. On sodium dodecyl sulfate polyacrylamide gels D6aH migrated with a Mr about 55,000. For reconstitution experiments soybean NADPH:cytochrome P450 (cytochrome c) reductase was purified to homogeneity. Reconstitution of D6aH in the presence of NADPH was dependent on cytochrome P450 D6aH, the reductase, and lipid. Dilauroylphosphatidylcholine gave higher D6aH activity than soybean lipids (asolectin). The reconstituted D6aH system showed a much higher temperature stability than the microsomal system.

Biosynthesis of the Cyanogenic Glucoside Dhurrin in Seedlings of Sorghum bicolor (L.) Moench and Partial Purification of the Enzyme System Involved

The cyanogenic glucoside dhurrin is rapidly synthesized in etiolated seedlings of Sorghum bicolor (L.) Moench. The dhurrin content of the seedlings increases sigmoidally with the germination time. Shoots of 10 centimeters height contain 850 nanomoles of dhurrin per shoot corresponding to 6% of the dry weight. The biosynthetic activity sharply rises upon germination and reaches a maximum level of 10 nanomoles dhurrin/(hour x shoot) after 48 hours when the shoots are 3 centimeters high. This maximum level is followed by a sharp decline in activity when germination time exceeds 65 hours. Dhurrin and the dhurrin-synthesizing enzyme system are primarily located in the upper part of the etiolated shoot where both are evenly distributed between the coleoptile, the primary leaves and the upper 0.5 centimeter of the first internode including the shoot apex. Dhurrin constitutes 30% of the dry weight of the upper 1.2 centimeter of 10 centimeter high shoots. The seed and root contain neither dhurrin nor the dhurrin-synthesizing enzyme system. The codistribution of dhurrin and the enzyme system throughout the seedling indicates that production and storage sites are located within the same cell. Purification of the dhurrin-synthesizing enzyme by gel filtration or by sucrose gradient centrifugations results in a tenfold increase in specific activity. Further purification is accompained by a decline in specific activity due to loss of essential components as demonstrated by reconstitution experiments.

Immunochemical characterization of NADPH-cytochrome P-450 reductase from Jerusalem artichoke and other higher plants

Polyclonal antibodies were prepared against NADPH-cytochrome P-450 reductase purified from Jerusalem artichoke. These antibodies inhibited efficiently the NADPH-cytochrome c reductase activity of the purified enzyme, as well as of Jerusalem artichoke microsomes. Likewise, microsomal NADPH-dependent cytochrome P-450 mono-oxygenases (cinnamate and laurate hydroxylases) were efficiently inhibited. The antibodies were only slightly inhibitory toward microsomal NADH-cytochrome c reductase activity, but lowered NADH-dependent cytochrome P-450 mono-oxygenase activities. The Jerusalem artichoke NADPH-cytochrome P-450 reductase is characterized by its high Mr (82,000) as compared with the enzyme from animals (76,000-78,000). Western blot analysis revealed cross-reactivity of the Jerusalem artichoke reductase antibodies with microsomes from plants belonging to different families (monocotyledons and dicotyledons). All of the proteins recognized by the antibodies had an Mr of approx. 82,000. No cross-reaction was observed with microsomes from rat liver or Locusta migratoria midgut. The cross-reactivity generally paralleled well the inhibition of reductase activity: the enzyme from most higher plants tested was inhibited by the antibodies; whereas Gingko biloba, Euglena gracilis, yeast, rat liver and insect midgut activities were insensitive to the antibodies. These results point to structural differences, particularly at the active site, between the reductases from higher plants and the enzymes from phylogenetically distant plants and from animals.