Hiromasa Imaishi

Molecular characterization and chromosomal localization of cytochrome P450 genes involved in the biosynthesis of cyclic hydroxamic acids in hexaploid wheat.

Abstract. The cyclic hydroxamic acids, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), are defensive secondary metabolites found in gramineous plants including wheat, maize and rye. cDNAs for five cytochromes P450 (P450s) involved in DIBOA biosynthesis (CYP71C6, CYP71C7v2, CYP71C8v2, CYP71C9v1 and CYP71C9v2) were isolated from seedlings of hexaploid wheat [(Triticum aestivum L. cv. Chinese Spring (2n=6x=42, genomes AABBDD)] by RT-PCR and screening of a cDNA library. CYP71C9v1 and CYP71C9v2 are 97% identical to each other in amino acid and nucleotide sequences. The cloned P450 species showed 76–79% identity at the amino acid level to the corresponding maize P450 species CYP71C1–C4, which are also required for DIBOA biosynthesis. The wheat P450 cDNAs were heterologously expressed in the yeast (Saccharomyces cerevisiae) strain AH22. Microsome fractions from yeast cells expressing these P450 species catalyzed the same reactions as their maize orthologs. The chromosomes carrying the cyp71C6–C9v1 orthologs were identified by Southern hybridization using aneuploid lines of Chinese Spring wheat. The cyp71C9v1 orthologs were located on the chromosomes of wheat homoeologous group-4. The orthologs of the other P450 genes, cyp71C7v2, cyp71C6 and cyp71C8v2, were located on group-5 chromosomes. The same P450 genes were also present in the three ancestral diploid species of hexaploid wheat, T. monococcum (AA), Aegilops speltoides [BB (≈SS)] and Ae. squarrosa (DD).

Rearrangement of the genes for the biosynthesis of benzoxazinones in the evolution of Triticeae species

Gramineous plants, including the major agricultural crops wheat (Triticum aestivum L.), rye (Secale cereale L.) and maize (Zea mays L.), accumulate benzoxazinones (Bxs) as defensive compounds. Previously, we isolated cDNAs of the Bx biosynthetic genes in wheat, TaBx2–TaBx5, that encode the enzymes catalyzing the sequential hydroxylation of indole to Bxs. In this study we isolated a cDNA of TaBx1, which encodes the first enzyme of the Bx pathway of wheat. The level of identity (80%) in deduced amino-acid sequence between TaBx1 and the corresponding maize gene Bx1 was as high as those shown between TaBx2–TaBx5 and the corresponding maize genes Bx2–Bx5, respectively. Southern blot analysis using the TaBx1–TaBx5 cDNAs as probes was conducted with aneuploid lines of hexaploid wheat in order to determine sub-chromosomal locations of the five Bx biosynthetic genes in Triticeae species. In wheat, TaBx1 and TaBx2 co-existed in specific regions of chromosomes 4A, 4B and 4D, and TaBx3–TaBx5 were localized together in the distal regions of the short arms of chromosomes 5A, 5B and 5D. TaBx3 and TaBx5 were found to have duplicated loci in the long arm and the short arm of chromosome 5B, respectively. In rye, homoeoloci of TaBx1 and TaBx2 were located on chromosome 7R and those for TaBx3–TaBx5 were located on chromosome 5R. In barley, no Southern blot band was detected with any of the probes under the highly stringent hybridization conditions, suggesting that the non-Bx phenotype of barley is attributable to the loss of Bx biosynthetic genes.

Glucose concentration determination based on silica sol-gel encapsulated glucose oxidase optical biosensor arrays.

Optical biosensor arrays for rapidly determining the glucose concentrations in a large number of beverage and blood samples were developed by immobilizing glucose oxidase (GOD) on oxygen sensor layer. Glucose oxidase was first encapsulated in silica based gels through sol-gel approach and then immobilized on 96-well microarrays integrated with oxygen sensing film at the bottom. The oxygen sensing film was made of an organically modified silica film (ORMOSIL) doped with tris(4,7-diphenyl-1,10-phenanthroline) ruthenium dichloride (Ru(dpp)(3)Cl(2)). The oxidation reaction of glucose by glucose oxidase could be monitored through fluorescence intensity enhancement due to the oxygen consumption in the reaction. The luminescence changing rate evaluated by the dynamic transient method (DTM) was correlated with the glucose concentration with the wide linear range from 0.1 to 5.0mM (Y=13.28X-0.128, R=0.9968) and low detection limit (0.06 mM). The effects of pH and coexisting ions were systemically studied. The results showed that the optical biosensor arrays worked under a wide range of pH value, and normal interfering species such as Na(+), K(+), Cl(-), PO(4)(3-), and ascorbic acid did not cause apparent interference on the measurement. The activity of glucose oxidase was mostly retained even after 2-month storage, indicating their long-term stability.

5-epi-Aristolochene 3-hydroxylase from green pepper

Abstract 5-epi-Aristolochene 3-hydroxylase, one of the key enzymes of the biosynthetic pathway leading to capsidiol, a major phytoalexin of green pepper (Capsicum annuum), is localized in the microsomes of elicitor treated green pepper fruit. The enzyme was found to be a P-450-dependent enzyme with an estimated molecular mass of 59 kDa. The total microsomal P-450 content increased and the λmax of the CO-binding spectrum changed in response to elicitation. Immunoblot analysis of fruit microsomal proteins was performed with anti-tulip constitutive P-450 antibody during the elicitation period. A 48 kDa protein strongly immunoreacted with the P-450 antibody, and a decrease of this P-450-like 48 kDa protein was observed with time. These results suggest that elicitation induces the production of P-450 isoforms.

CYP92B1, A Cytochrome P450, Expressed in Petunia Flower Buds, That Catalyzes Monooxidation of Long-Chain Fatty Acids

In higher plants, long-chain fatty acid hydroperoxides are intermediates in the synthesis of a diverse group of bioactive compounds. We used the reverse trascriptase-polymerase chain reaction to isolate a gene responsible for the oxidization of fatty acids from Petunia hybrida. A P450 cDNA not isolated earlier, CYP92B1, contained an open reading frame predicted to encode a polypeptide consisting of 510 amino acid residues. The transcript of the cyp92B1 gene was expressed at a high level in the early stage of flower development. CYP92B1 cDNA was expressed in a yeast, Saccharomyces cerevisiae, and recombinant yeast microsomes containing CYP92B1, a hemoprotein, metabolized lauric acid, linoleic acid, and linolenic acid.

Vertically integrated human P450 and oxygen sensing film for the assays of P450 metabolic activities.

An assaying method of cytochrome P450 (P450 or CYP) monooxygenase activities for toxicological evaluation of drugs and environmental pollutants was developed by immobilizing P450 on an oxygen sensoring layer. Membrane fractions from E. coli expressing human P450 were entrapped in agarose or silica-based gels and immobilized on 96-well microarrays having an oxygen sensing film at the bottom. The oxygen sensing film was made of an organically modified silica film (ORMOSIL) doped with Tris(4,7-diphenyl-1,10-phenanthroline) ruthenium dichloride (Ru(dpp)(3)Cl(2)). P450 activity toward the substrates was monitored through the fluorescence intensity enhancement due to the oxygen consumption by the metabolic reactions. For the metabolism of chlortoluron, a selective herbicide used to control grass weeds, CYP1A1 immobilized in agarose gel showed a higher activity and stability compared with those in silica gels and free suspensions. The luminescence changing rate evaluated by the dynamic transient method (DTM) could be correlated with the substrate concentration. We also compared the metabolic responses of human P450s (CYP1A1,CYP2C8, CYP2E1, CYP3A4) toward various substances. The use of immobilized P450 on an oxygen sensing layer provides a versatile assaying platform owing to the following features. First, the oxygen sensor can detect metabolic reactions of any P450 species, in contrast with assays using fluorogenic substrates. Second, vertical integration of the oxygen sensor and immobilized P450 enhanced the sensitivity because of the effective depletion of oxygen in the vicinity of the oxygen sensing layer. Third, immobilization enables repeated use of P450 by replacing the substrate solutions using a flow cell. Furthermore, the activity of immobilized P450 was retained at least for 3 weeks at 4 °C, suggesting its long-term stability, which is an additional attractive feature.

Surface Functionalization of a Polymeric Lipid Bilayer for Coupling a Model Biological Membrane with Molecules, Cells, and Microstructures

We describe a stable and functional model biological membrane based on a polymerized lipid bilayer with a chemically modified surface. A polymerized lipid bilayer was formed from a mixture of two diacetylene-containing phospholipids, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DiynePC) and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine (DiynePE). DiynePC formed a stable bilayer structure, whereas the ethanolamine headgroup of DiynePE enabled functional molecules to be grafted onto the membrane surface. Copolymerization of DiynePC and DiynePE resulted in a robust bilayer. Functionalization of the polymeric bilayer provided a route to a robust and biomimetic surface that can be linked with biomolecules, cells, and three-dimensional (3D) microstructures. Biotin and peptides were grafted onto the polymeric bilayer for attaching streptavidin and cultured mammalian cells by molecular recognition, respectively. Nonspecific adsorption of proteins and cells on polymeric bilayers was minimum. DiynePE was also used to attach a microstructure made of an elastomer (polydimethylsiloxan: PDMS) onto the membrane, forming a confined aqueous solution between the two surfaces. The microcompartment enabled us to assay the activity of a membrane-bound enzyme (cyochrome P450). Natural (fluid) lipid bilayers were incorporated together with membrane-bound proteins by lithographically polymerizing DiynePC/DiynePE bilayers. The hybrid membrane of functionalized polymeric bilayers and fluid bilayers offers a novel platform for a wide range of biomedical applications including biosensor, bioassay, cell culture, and cell-based assay.

Bioconversion of small molecules by cytochrome P450 species expressed in Escherichia coli

P450 (cytochrome P450) enzymes catalyse the mono‐oxygenation of a wide range of compounds such as steroids, fatty acids, vitamins and drugs. In the present paper we demonstrate a system for bioconverting diverse compounds [flavanone, DHEA (dehydroepiandrosterone) and 7‐ethoxycoumarin] using P450 species expressed in Escherichia coli. First, we expressed four P450 species: rabbit CYP2B (P450 family 2, subfamily B), fruitfly (Drosophila) CYP317A, rat CYP3A23 and mouse CYP2J5. Next, we added substrates directly to the incubation medium. The resulting metabolites were extracted and analysed by HPLC and spectrofluorimetry. The first substrate, 7‐ethoxycoumarin, was de‐ethylated by CYP2B; CYP2J5 and CYP3A23 showed weak activity, and CYP317A had no activity for 7‐ethoxycoumarin. We next used flavanone, a flavonoid, as a substrate for these four P450 species and other P450 species expressed previously. As a result, CYP2B, CYP2C43 and CYP2C29 catalysed flavanone 2‐hydroxylation. CYP2A5 catalysed 2‐ and 4‐hydroxylations. Finally, to produce diverse modified compounds, variants of CYP2A5 with point mutations were incubated with a steroid (DHEA) and an antioxidant (flavanone) in vivo. HPLC analysis indicated that two P450 species produced a 7‐β‐hydroxy‐DHEA and two P450 species produced a 2‐α‐hydroxy‐DHEA. Four P450 species catalysed flavanone 2‐ and 4‐hydroxylations. These results indicate that bioconversion by P450 is a useful technique to modify small molecules (steroids, coumarin and flavanone) and produce new, diverse hydroxylated compounds, which could be used for high‐throughput screening for drug discovery.

The effects of single nucleotide polymorphisms in CYP2A13 on metabolism of 5-methoxypsoralen

A number of studies have demonstrated that cytochrome P450 (P450) converts furanocoumarin derivatives into reactive molecules, which form covalent bonds to biomolecules. 5-Methoxypsoralen (5-MOP) is a natural furanocoumarin from apiaceous plants. In this study, we examined the effect on 5-MOP metabolism of single nucleotide polymorphisms (SNPs) in CYP2A13. We used Escherichia coli-generated recombinant enzymes of wild-type CYP2A13*1 and five variants, CYP2A13*4 (R101Q), CYP2A13*5 (F453Y), CYP2A13*6 (R494C), CYP2A13*8 (D158E), and CYP2A13*9 (V323L). In high-performance liquid chromatography analyses of 5-MOP metabolic products, CYP2A13*1 converted 5-MOP into 5-MOP dihydrodiol; Km and Vmax values of the reaction were 1.44 ± 0.17 μM and 4.23 ± 0.36 nmol/(min · nmol P450), respectively. The generation of a dihydrodiol from 5-MOP implies that conversion by CYP2A13 causes toxicity due to the formation of covalent bonds with DNA or proteins. Most of the CYP2A13 variants could metabolize 5-MOP; Km values for CYP2A13*5, *6, *8, and *9 were 1.63 ± 0.12, 1.36 ± 0.10, 0.85 ± 0.09, and 0.58 ± 0.06 μM, respectively, and Vmax values were 3.20 ± 0.13, 4.69 ± 0.13, 2.34 ± 0.07, and 1.84 ± 0.09 nmol/(min · nmol P450), respectively. However, the processing of 5-MOP by CYP2A13*4 was not detectable. Based on this data, we hypothesize that SNPs within the CYP2A13 gene affect metabolism of 5-MOP in humans.

Metabolism of 7‐ethoxycoumarin, safrole, flavanone and hydroxyflavanone by cytochrome P450 2A6 variants

CYP 2A6 is a human enzyme that metabolizes many xenobiotics including coumarin, indole, nicotine and carcinogenic nitrosamines. The gene for CYP2A6 is polymorphic. There are few data available to clarify the relationship between P450 genetic variants and the metabolism of materials in food. The CYP 2A6 wild‐type protein and 13 mutants (CYP2A6.1, CYP2A6.2, CYP2A6.5, CYP2A6.6, CYP2A6.7, CYP2A6.8, CYP2A6.11, CYP2A6.15, CYP2A6.16, CYP2A6.17, CYP2A6.18, CYP2A6.21, CYP2A6.23 and CYP2A6.25) were co‐expressed with NADPH‐cytochrome P450 reductase in E. coli. The hydroxylase activities toward 7‐ethoxycoumarin, coumarin, safrole, flavanone and hydroxyflavanone were examined. Ten types of CYP2A6 variants except for CYP2A6.2, CYP2A6.5 and CYP2A6.6 showed Soret peaks (450 nm) typical of P450 in the reduced CO‐difference spectra and had 7‐ethoxycoumarin O‐deethylase activities. CYP2A6.15 and CYP2A6.18 showed higher activities for safrole 1′‐hydroxylation than CYP2A6.1. CYP2A6.25 and CYP2A6.7 had lower safrole 1′‐hydroxylase activities. CYP2A6.7 had lower flavanone 6‐ and 2′‐hydroxylase activities, whereas CYP2A6.25 had higher 6‐hydroxylase activity and lower 2′‐hydroxylase activity. Hydroxyflavanone was metabolized by CYP2A6.25, but was not metabolized by wild‐type CYP2A6.1. These results indicate that CYP2A6.25 possessed new substrate specificity toward flavonoids. Copyright