CYP12A1, a mitochondrial cytochrome P450 from the house fly

Eukaryotic P450 proteins are membrane proteins found predominantly in the endoplasmic reticulum. In vertebrates, several biosynthetic P450s are found in mitochondria as well. We cloned three putative insect mitochondrial P450s from larval house fly cDNA. These P450s are members of a new P450 family, CYP12. The CYP12 proteins are most closely related to the mammalian mitochondrial P450 of the CYP11, CYP24, and CYP27 families. The most abundant cDNA, CYP12A1, was expressed in Escherichia coli and purified. NADPH-dependent reduction of CYP12A1 was rapid and efficient with the bovine mitochondrial proteins adrenodoxin reductase and adrenodoxin as electron transfer partners. In contrast, house fly microsomal NADPH cytochrome P450 reductase reduced CYP12A1 only poorly. In a reconstituted system with the bovine mitochondrial electron donors, CYP12A1 metabolized a variety of insecticides and other xenobiotics, but did not metabolize ecdysteroids, juvenoids, or fatty acids. Subcellular localization of CYP12A1 by immunogold histochemistry established the mitochondrial nature of this protein. CYP12A1 mRNA levels are constitutively higher in an insecticide-resistant strain than in a susceptible strain, and this trait maps to chromosome II in the house fly, where the constitutive overexpression of the pesticide-metabolizing microsomal CYP6A1 also maps. Multiple mitochondrial P450s have evolved in insects and may play a role in the metabolism of xenobiotics in addition to their possibly ancestral functions in steroidogenesis.

Systematic reverse genetics of transfer-DNA-tagged lines of Arabidopsis. Isolation of mutations in the cytochrome p450 gene superfamily

We have developed an efficient reverse-genetics protocol that uses expedient pooling and hybridization strategies to identify individual transfer-DNA insertion lines from a collection of 6000 independently transformed lines in as few as 36 polymerase chain reactions. We have used this protocol to systematically isolate Arabidopsis lines containing insertional mutations in individual cytochrome P450 genes. In higher plants P450 genes encode enzymes that perform an exceptionally wide range of functions, including the biosynthesis of primary metabolites necessary for normal growth and development, the biosynthesis of secondary products, and the catabolism of xenobiotics. Despite their importance, progress in assigning enzymatic function to individual P450 gene products has been slow. Here we report the isolation of the first 12 such lines, including one (CYP83B1-1) that displays a runt phenotype (small plants with hooked leaves), and three insertions in abundantly expressed genes. The DNAs used in this study are publicly available and can be used to systematically isolate mutants in Arabidopsis.

A factor causing stable stimulation of juvenile hormone synthesis by Diploptera punctata corpora allata in vitro

Co-incubation of corpora allata (CA) from the cockroach, Diploptera punctata, with ovaries, fat body or muscle but not brain or testis, leads to a substantial increase in juvenile hormone synthesis. Incubation of the glands in medium pre-conditioned with ovaries also stimulates JH synthesis. The ovary was used as a convenient source of stimulatory factor for a detailed analysis of its physiological effects on the CA. The increase in JH synthesis is stable, maintained over 24h after exposure to the stimulatory factor. Stimulation is dose-dependent, and the corpora allata show an exquisite relationship between sensitivity to this factor and developmental stage. Day 0 and day 1 glands, as well as glands from post-vitellogenic females, are sensitive to stimulation, whereas glands from vitellogenic females are not sensitive. Corpora allata attached to the brain do not respond to the stimulatory factor, and denervation in vivo leads to an increase in JH synthesis by the glands and a loss in sensitivity to the factor. These data suggest that glands from pre- and post-vitellogenic females are inhibited by their nervous connection to the brain. In contrast, glands from vitellogenic females are normally responding to the endogenous stimulatory factor and are thus no longer stimulated in vitro. Co-incubation of CA with allatostatin and conditioned medium still leads to a stimulation of JH synthesis, suggesting that the restraining effect of the nervous connections to the brain is not caused by allatostatin. The CA cell number increases between emergence and day 2, then remains stable until after oviposition. The stimulatory factor accelerates the increase in cell number in young adult females. The results are interpreted as providing evidence for a constitutive change in CA activity caused by a humoral factor produced by various tissues including the ovary, and modulated by nervous connections to the brain.

Cytochrome P450 monooxygenases and insecticide resistance in insects

Cytochrome P450 monooxygenases are involved in many cases of resistance of insects to insecticides. Resistance has long been associated with an increase in monooxygenase activities and with an increase in cytochrome P450 content. However, this increase does not always account for all of the resistance. In Drosophila melanogaster, we have shown that the overproduction of cytochrome P450 can be lost by the fly without a corresponding complete loss of resistance. These results prompted the sequencing of a cytochrome P450 candidate for resistance in resistant and susceptible flies. Several mutations leading to amino-acid substitutions have been detected in the P450 gene CYP6A2 of a resistant strain. The location of these mutations in a model of the 3D structure of the CYP6A2 protein suggested that some of them may be important for enzyme activity of this molecule. This has been verified by heterologous expression of wild-type and mutated cDNA in Escherichia coli. When other resistance mechanisms are considered, relatively few genetic mutations are involved in insecticide resistance, and this has led to an optimistic view of the management of resistance. Our observations compel us to survey in more detail the genetic diversity of cytochrome P450 genes and alleles involved in resistance.

A cytochrome P450 terpenoid hydroxylase linked to the suppression of insect juvenile hormone synthesis

A cDNA encoding a cytochrome P450 enzyme was isolated from a cDNA library of the corpora allata (CA) from reproductively active Diploptera punctata cockroaches. This P450 from the endocrine glands that produce the insect juvenile hormone (JH) is most closely related to P450 proteins of family 4 and was named CYP4C7. The CYP4C7 gene is expressed selectively in the CA; its message could not be detected in the fat body, corpora cardiaca, or brain, but trace levels of expression were found in the midgut and caeca. The levels of CYP4C7 mRNA in the CA, measured by ribonuclease protection assays, were linked to the activity cycle of the glands. In adult females, CYP4C7 expression increased immediately after the peak of JH synthesis, reaching a maximum on day 7, just before oviposition. mRNA levels then declined after oviposition and during pregnancy. The CYP4C7 protein was produced in Escherichia coli as a C-terminal His-tagged recombinant protein. In a reconstituted system with insect NADPH cytochrome P450 reductase, cytochrome b5, and NADPH, the purified CYP4C7 metabolized (2E,6E)-farnesol to a more polar product that was identified by GC-MS and by NMR as (10E)-12-hydroxyfarnesol. CYP4C7 converted JH III to 12-trans-hydroxy JH III and metabolized other JH-like sesquiterpenoids as well. This omega-hydroxylation of sesquiterpenoids appears to be a metabolic pathway in the corpora allata that may play a role in the suppression of JH biosynthesis at the end of the gonotrophic cycle.

Isolation, heterologous expression and functional characterization of a novel cytochrome P450 3A enzyme from a canine liver cDNA library

A cDNA encoding a new member of the cytochrome P450 3A subfamily, P450 3A26, has been isolated from phenobarbital-induced canine liver. The sequence encodes a protein of 503 amino acids with 33 nucleotide differences conferring 22 amino acid substitutions when compared with the previously identified canine CYP3A12 enzyme. Nine of the amino acid differences are within the substrate recognition sites (SRSs) identified for P450 family 2, with five residue substitutions clustered within SRS-6. To facilitate heterologous expression in Escherichia coli, the N-terminus of 3A26 was modified. The expressed protein comigrated with a 3A-immunoreactive protein in dog liver microsomes with a slightly greater electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than 3A12, which suggests that 3A26 corresponds to a previously noted but never characterized 3A enzyme in dogs. Functional characterization of 3A26 was undertaken with use of progesterone, testosterone and androstenedione as substrates. Assays of expressed 3A26 and 3A12 demonstrated that 3A26 displays low steroid hydroxylase activity. Identification of an additional canine 3A enzyme should increase our understanding of xenobiotic metabolism in this important animal model. These findings also suggest that 3A26 and 3A12 may be an interesting model system for the investigation of structure-function relationships involved in steroid metabolism catalyzed by members of the cytochrome P450 3A subfamily.

The Drosophila cytochrome P450 gene Cyp6a2: structure, localization, heterologous expression, and induction by phenobarbital

The cytochrome P450 gene Cyp6a2 from Drosophila melanogaster is located on the right arm of chromosome 2 at position 43A1-2 and comprises two exons separated by a 69-bp intron. Phenobarbital treatment of flies leads to a rapid increase in the level of CYP6A2 mRNA and to an increased production of the CYP6A2 protein. DNA from the Cyp6a2 promoter region was functional when linked to a luciferase reporter gene and transfected into D. melanogaster Schneider cells. Moreover, a dose-dependent induction of luciferase activity by phenobarbital indicated that elements necessary for phenobarbital induction are located within 428 bp of the translation start site. Heterologous expression of the CYP6A2 protein in lepidopteran cells infected with a Cyp6a2-recombinant baculovirus was observed by Western blotting of cell lysates and by spectral characterization of the reduced-CO complex of the P450. The CYP6A2 protein produced in this system metabolized aldrin and heptachlor to their epoxides and metabolized the insecticide diazinon by desulfuration to diazoxon and by oxidative ester cleavage to 2-isopropyl-4-methyl-6-hydroxypyrimidine. Metabolism in lysates of cells infected with recombinant baculovirus was greatly enhanced by the addition of purified housefly NADPH cytochrome P450 reductase and cytochrome b5. These results show that CYP6A2 catalyzes the metabolism of organophosphorus insecticides and they implicate Cyp6a2 overexpression in metabolic resistance. The Cyp6a2 gene appears to be a suitable model for a genetic analysis of the phenobarbital induction process.

A valine421 to methionine mutation in IS6 of the hscp voltage-gated sodium channel associated with pyrethroid resistance in Heliothis virescens F

Multiple mutations in a locus encoding a voltage-gated sodium channel have been predicted for pyrethroid resistance in insects. Previously we reported a mutation associated with pyrethroid resistance, Leu1029 to His, in domain II transmembrane segment S6 (IIS6) of the Heliothis virescens F. sodium channel (para homologue) hscp locus. Sequence analysis of additional resistance haplotypes 5' to this mutation in the hscp locus has uncovered a G to A transition leading to a Val to Met mutation at amino acid position 421 in IS6 (V421M, numbering from Drosophila para). The V421M mutation is found only in a unique resistant haplotype, but not in two susceptible and a distinct resistant haplotype carrying the L1029H mutation. Implications of this finding in the evolution and mechanisms of pyrethroid resistance are discussed.

Allatostatin-like-immunoreactive neurons of the tobacco hornworm, Manduca sexta, and isolation and identification of a new neuropeptide related to cockroach allatostatins

The YXFGLamide C-terminus serves to define most members of a family of structurally related neuropeptides, the YXFGLamides. These peptides have been identified from the nervous system of various insects and include the allatostatins of cockroaches and crickets, the schistostatins of locusts, and the callatostatins of blowflies. The YXFGLamides have been shown to have various functions, including inhibition of juvenile hormone biosynthesis in cockroaches and crickets and inhibition of contraction of certain insect visceral muscles. We wanted to know if these peptides occur in Manduca sexta and what functions they might have. A new peptide, AKSYNFGLamide, was isolated and identified from M. sexta and has been named "lepidostatin-1"; this is the first YXFGLamide to be found in a lepidopteran, and there are indications that additional YXFGLamides occur in M. sexta. An antiserum to cockroach allatostatins (YXFGLamides) was shown to recognize lepidostatin-1 of M. sexta and was used to map YXFGLamide-immunoreactive neurons in larvae. Because immunoreactive interneurons were found to form an extensive neuropil, YXFGLamides probably function as neuromodulators in M. sexta. Neuroendocrine cells in the brain, abdominal ganglia, and their respective neurohemal organs were YXFGLamide immunoreactive and appear to release YXFGLamides as neurohormones. Immunoreactivity to YXFGLamides and M. sexta diuretic hormone were found to be colocalized and appear to be coreleased in these neuroendocrine cells, indicating that YXFGLamides may be involved in regulation of fluid transport. Innervation of the corpora allata by YXFGLamide-immunoreactive processes was very sparse, suggesting that this innervation does not play an important role in allatostasis. Many thoracic motor neurons were YXFGLamide immunoreactive, suggesting that YXFGLamides may have a myomodulatory or myotrophic function in larvae. However, this immunoreactivity disappeared early in metamorphosis and did not reappear in the adult. The YXFGLamide-immunoreactive neurons in the terminal abdominal ganglion were found to innervate the hindgut, indicating that YXFGLamides may be involved in the control of the rate of myogenic contractions of the larval hindgut.

Functional interactions in cytochrome P450BM3: flavin semiquinone intermediates, role of NADP(H), and mechanism of electron transfer by the flavoprotein domain

Cytochrome P450BM3 is a self-sufficient soluble fatty acid hydroxylase from Bacillus megaterium utilizing tightly bound FAD and FMN cofactors to transfer reducing equivalents from NADPH to the heme active site. Active-inactive transitions of cytochrome P450BM3 were exploited to identify catalytic intermediates of the enzyme. Shortly upon reduction by NADPH, a two-electron reduced active P450BM3 is formed with two flavin semiquinones, anionic and neutral, present simultaneously. P450BM3 inactivated by NADPH has a three-electron reduced flavoprotein domain. NADPH is unable to reduce P450BM3 rapidly unless the flavoprotein domain is fully oxidized. During steady-state hydroxylation of a poor substrate, tetradecanol, the flavoprotein reduction state does not exceed two, with two flavin semiquinones, anionic and neutral, present. Absorbance and EPR spectroscopic characterization of both anionic and neutral flavin semiquinone is presented. NADPH and NADH were compared as electron donors for P450BM3-catalyzed fatty acid hydroxylation and cytochrome c and heme iron reduction. The Km for NADH of 3-5 mM is about 3000 times higher than the Km of 1-1.5 microM for NADPH. Although NADH can support cytochrome c reduction and fatty acid hydroxylation with the rates as high as 22 and 13 s-1, respectively, these turnover numbers are only about 20% of those observed with NADPH. The results suggest that nucleotide binding plays an important role in catalysis by controlling electron-transfer properties of the flavin cofactors. In W574G and G570D mutant P450BM3 enzymes that are deficient in FMN, NADP+ binding stabilizes fully reduced FAD. P450BM3 catalyzes single-turnover and steady-state laurate hydroxylation with near stoichiometric product formation at NADPH concentrations below that of the enzyme. A mechanism of electron transfer by the flavoprotein domain of P450BM3 is proposed with the reduction state of the flavoprotein domain cycling in a 0-2-1-0 sequence. We also propose that an interaction of bound NADP+ with anionic FAD semiquinone is essential for splitting a pair of electrons that are then transferred in two one-electron transfer steps to the heme catalytic site.

Structure-activity studies reveal two allatostatin receptor types in corpora allata of Diploptera punctata

Synthetic variants of the octadecapeptide amide ASB2 (AYSYVSEYKRLPVYNFGL-NH(2)), a cockroach allatostatin, were assayed in vitro on corpora allata (CA) from 2-day-old (vitellogenic) and 10-day-old (post-vitellogenic) female Diploptera punctata. The analogs [(17)psi(18),CH(2)-S]ASB2, [D-Trp(17)]ASB2 and [Ile(18)]ASB2 inhibited juvenile hormone (JH) synthesis with simple dose-response curves on sensitive CA from 10-day-old females. These analogs were fully effective but less potent than ASB2. When tested on CA from 2-day-old mated females, which are only partially (65-70%) sensitive to ASB2, the three analogs gave biphasic dose-response curves and elicited a maximal effect only at higher concentrations. The dose-response curve for ASB2 on CA from 2-day-old females had a Hill plot slope of only 0.78+/-0.03. These findings suggested that the observed CA sensitivity to ASB2 may be the result of two partial responses having an IC(50) of approximately 0.35 and 3nM respectively. One partial response, or receptor type, appeared more sensitive than the other to adverse modification of the "message" segment of the peptide. The activity of shorter allatostatins was also studied, indicating that pentapeptides of the YXFGL-amide structure are fully effective, albeit at low potency, as inhibitors of JH biosynthesis.

Substrate specificity for the epoxidation of terpenoids and active site topology of house fly cytochrome P450 6A1

Heterologous expression in Escherichia coli, purification, and reconstitution of house fly P450 6A1 and NADPH-cytochrome P450 reductase were used to study the metabolism of terpenoids. In addition to the epoxidation of cyclodiene insecticides demonstrated previously [Andersen et al. (1994) Biochemistry 33, 2171-2177], this cytochrome P450 was shown to epoxidize a variety of terpenoids such as farnesyl, geranyl, and neryl methyl esters, juvenile hormones I and III, and farnesal but not farnesol or farnesoic acid. P450 6A1 reconstituted with NADPH-cytochrome P450 reductase and phosphatidylcholine did not metabolize alpha-pinene, limonene, of the insect growth regulators hydroprene and methoprene. The four geometric isomers of methyl farnesoate were metabolized predominantly to the 10,11-epoxides, but also the 6,7-epoxides and to the diepoxides. The 10,11-epoxide of methyl (2E,6E)-farnesoate was produced in a 3:1 ratio of the (10S) and (10R) enantiomers. Monoepoxides of methyl farnesoate were metabolized efficiently to the diepoxides. Methyl farnesoate epoxidation was strongly inhibited by a bulky substituted imidazole. The active site topology of P450 6A1 was studied by the reaction of the enzyme with phenyldiazene to form a phenyl-iron complex. Ferricyanide-induced in situ migration of the phenyl group showed formation of the N-phenylprotopor-phyrinporphyrin IX adducts in a 17:25:33:24 ratio of the NB:NA:NC:ND isomers. These experiments suggest that metabolism of xenobiotics by this P450, constitutively overexpressed in insecticide-resistant strains of the house fly, is not severely limited by stereochemically constrained access to the active site.

Identification of three allatostatins and their cDNA from the mosquito Aedes aegypti

Three allatostatins have been isolated from the mosquito Aedes aegypti. These peptides have the following structures: Ser-Pro-Lys-Tyr-Asn-Phc-Gly-Leu-amide, Leu-Pro-His-Tyr-Asn-Phe-Gly-Leu-amide, and Arg-Val-Tyr-Asp-Phe-Gly-Leu-amide. A cDNA encoding these peptides was isolated from an abdominal ganglia cDNA library and sequenced. It was found to encode two additional allatostatins: Ala-Ser-Ala-Tyr-Arg-Tyr-His-Phe-Gly-Leu-amide and Leu-Pro-Asn-Arg-Tyr-Asn-Phe-Gly-Leu-amide. Northern analysis of whole mosquito mRNA revealed a single prepro-allatostatin message of around 3,000 bases. Identification of a partial prepro-allatostatin cDNA from a midgut cDNA library shows that the same gene is also expressed in the mosquito midgut.

Functional interactions in cytochrome P450BM3. Fatty acid substrate binding alters electron-transfer properties of the flavoprotein domain

P450BM3 is a bacterial fusion protein between a cytochrome P450 fatty acid hydroxylase (CYP102) and an FAD- and FMN-containing flavoprotein homologous to NADPH: cytochrome P450 reductase. It has been shown that incubation of P450BM3 with NADPH in the absence of a fatty acid substrate results in inhibition of hydroxylase activity [Narhi, L. O., & Fulco, A. J. (1986) J. Biol. Chem. 261, 7160-7169]. We show that laurate-dependent oxidation of NADPH and oxygen consumption are also inhibited under those conditions. The inhibited enzyme is unable to transfer electrons to the heme iron, but reduces artificial electron acceptors such as cytochrome c, 2,6-dichlorophenolindophenol, or ferricyanide. Incubation with these acceptors rapidly restores hydroxylase activity of P450BM3. The active enzyme is able to catalyze the reduction of cytochrome c and hydroxylation of laurate simultaneously. Cytochrome c has no effect on the K(m) and Vmax of laurate hydroxylation. Laurate and other substrates stimulate cytochrome c reduction by 50-70%. Carbon monoxide inhibits hydroxylase activity, but stimulates cytochrome c reduction 3-4 fold and has no effect on the K(m) for cytochrome c. This stimulation requires binding of a substrate at the heme catalytic site. Laurate binding induces conformational changes in the flavoprotein domain as shown by a 2-fold increase of the flavin fluorescence. Inactivation of P450BM3 by NADPH abolishes the stimulation of cytochrome c reduction by laurate and CO. Complete inhibition of hydroxylase activity correlates with complete lack of stimulation of cytochrome c reduction. The results suggest that a specific conformation of the two domains is maintained in the active P450BM3, ensuring high hydroxylase activity. Cytochrome c reductase and hydroxylase activities of P450BM3 involve different sites of interaction with the flavoprotein domain, different catalytic intermediates, and different rate-limiting steps.

Molecular cloning, overexpression in Escherichia coli, structural and functional characterization of house fly cytochrome b5

A microsomal cytochrome b5 cDNA from the house fly, Musca domestica, was cloned and sequenced. The deduced amino acid sequence of the full-length house fly cytochrome b5 (134 residues) is 48% identical to that of rat microsomal cytochrome b5. The house fly cytochrome b5 protein was overexpressed in Escherichia coli, purified, and characterized. Absorption and EPR spectroscopy reveal properties very similar to cytochromes b5 from vertebrates. NMR spectra indicate that the orientation of the heme in the protein relative to its alpha,gamma meso axis is about 1:1. A redox potential of -26 mV versus standard hydrogen electrode was measured by cyclic voltammetry on a modified gold electrode in the presence of hexamminechromium(III) chloride. The cytochrome b5 is reduced by house fly cytochrome P450 reductase in a reconstituted system at a high rate (5.5 s-1), and it stimulates heptachlor epoxidation when reconstituted with house fly cytochrome P450 reductase, cytochrome P450 6A1, phospholipid, and detergent. Cytochrome b5 decreases the apparent Km for P450 reductase and increases the Vmax for heptachlor epoxidation at constant cytochrome P450 6A1 concentrations. The results indicate that cytochrome b5 stimulates a step following the first electron transfer during cytochrome P450 6A1 turnover.

Target of cockroach allatostatin in the pathway of juvenile hormone biosynthesis

Allatostatins, peptides with an YXFGL-amide C-terminus, cause a rapid and reversible inhibition of the sesquiterpenoid juvenile hormone III biosynthesis by cockroach corpora allata in vitro. Inhibition is relieved by farnesol and by mevalonate, and neither HMG-CoA reductase nor HMG-CoA synthesis (HMG-CoA synthase and acetoacetyl-CoA lyase) are inhibited by allatostatin. The neuropeptide is more effective as an inhibitor of JH III synthesis in glucose or amino acid-driven glands than in acetate-driven glands. Incorporation of label from acetate into JH III is increased whereas incorporation of label from glucose is decreased by allatostatin. Citrate is a very effective precursor for HMG-CoA synthesis in homogenates of corpora allata. We propose that inhibition of JH III biosynthesis by the tridecapeptide APSGAQRLYGFGL-amide occurs at the first committed step(s) of JH III synthesis, i.e. the transfer of 2C units from mitochondria to the cytoplasm by the tricarboxylate carrier and/or the ATP-citrate lyase.

Molecular biology and evolution of resistance of toxicants

To the prevailing biochemical/physiological classification of mechanisms of organismal resistance to toxicants, an additional molecular dimension is proposed. Predictions are developed regarding the relative prevalence of different classes of mutations and are found to compare favorably with reports from the literature. In particular, point mutations in target loci were the dominant form of resistance for both lab and field selection. Amplifications of target loci were less common than structural mutations, and more common for lab-selected than for field-selected strains. Amplification was the most common mechanism of up-regulation of metabolizing enzymes. In comparison, only one mutation involving cis-regulation and several involving trans-acting regulation were found. Mutations involving gene disruption and down-regulation were uncommon, but were found in appropriate cases, i.e., when toxicants stimulated rather than inhibited target function and when metabolizing enzymes converted toxicants into more toxic metabolites. Additional phenomena of likely but uncertain importance are genetic "succession," recombinational limitation, and negative cross-resistance. More work on these phenomena and on quantification of fitness costs of resistance is recommended.

Cytochrome P450 gene clusters in Drosophila melanogaster

Twelve cytochrome P450 cDNA fragments were cloned from Drosophila melanogaster by reverse transcriptase/PCR (RT/PCR) using degenerate oligonucleotide primers. The corresponding genes belong to several subfamilies of the CYP4 and CYP9 P450 families. Only two of these genes, Cyp4dl and Cyp4d2, have previously been described. In situ hybridization of each of the cDNA fragments showed two clusters of genes; one near the tip of the X chromosome and the other on the left arm of chromosome 2. Interestingly the latter cluster comprises widely divergent genes belonging both to the CYP9 and CYP4 families and also to the CYP6 family (Cyp6a2). Putative allelic variants of several of the genes were found in different insecticide-resistant and -susceptible strains (Hikone R, Haag 79 and Oregon R). The identification of these genes and alleles will allow us to clarify the involvement of P450s in xenobiotic metabolism and will facilitate a genetic analysis of P450 functions in insects.

P450 superfamily: update on new sequences, gene mapping, accession numbers and nomenclature

We provide here a list of 481 P450 genes and 22 pseudogenes, plus all accession numbers that have been reported as of October 18, 1995. These genes have been described in 85 eukaryote (including vertebrates, invertebrates, fungi, and plants) and 20 prokaryote species. Of 74 gene families so far described, 14 families exist in all mammals examined to date. These 14 families comprise 26 mammalian subfamilies, of which 20 and 15 have been mapped in the human genome and the mouse genome, respectively. Each subfamily usually represents a cluster of tightly linked genes widely scattered throughout the genome, but there are exceptions. Interestingly, the CYP51 family has been found in mammals, filamentous fungi and yeast, and plants-attesting to the fact that this P450 gene family is very ancient. One functional CYP51 gene and two processed pseudogenes, which are the first examples of intronless pseudogenes within the P450 superfamily, have been mapped to three different human chromosomes. This revision supersedes the four previous updates in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene, we recommend that the italicized root symbol "CYP' for human ("Cyp' for mouse and Drosophila), representing "cytochrome P450', be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen is no longer recommended in mouse gene nomenclature. "P' ("ps' in mouse and Drosophila) after the gene number denotes a pseudogene; "X' after the gene number means its use has been discontinued. If a gene is the sole member of a family, the subfamily letter and gene number would be helpful but need not be included. The human nomenclature system should be used for all species other than mouse and Drosophila. The cDNAs, mRNAs and enzymes in all species (including mouse) should include all capital letters, and without italics or hyphens. This nomenclature system is similar to that proposed in our previous updates.

Molecular biology of insecticide resistance

The widespread use of insecticides has amounted to a large scale 'experiment' in natural selection of insects by chemicals of toxicological importance to humans. Specific examples in which the molecular basis of insecticide resistance has been studied in detail are presented here. The biochemical/physiological mechanisms of resistance can be categorized as target site insensitivity, increased metabolic detoxification and sequestration or lowered availability of the toxicant. These are achieved at the molecular level by: point mutations in the ion channel portion of a GABA receptor subunit (cyclodiene insecticides); point mutations in the vicinity of the acetylcholinesterase (AChE) active site (organophosphorus and carbamate insecticide resistance); amplification of esterase genes (organophosphorus and carbamate insecticides); mutations linked genetically to a sodium channel gene (DDT and pyrethroid insecticides); and yet uncharacterized mutations leading to the up-regulation of detoxification enzymes, such as cytochrome P450 and glutathione S-transferases (many classes of insecticides). In several cases, the selection of a precisely homologous mutation has been observed in different insect species.

Expression of cytochrome P450 genes of the CYP4 family in midgut and fat body of the tobacco hornworm, Manduca sexta

Two conserved regions in the alignment of cytochrome P450 family 4 (CYP4) proteins served as guide to the synthesis of degenerate oligonucleotide primers. The primers were used in PCR from a midgut cDNA library and RT-PCR from fat body mRNA, both from last instar larvae of the tobacco hornworm, Manduca sexta. The PCR products of 443-449 bp were cloned and sequenced. Nine P450 clones representing four new genes were obtained from the midgut. Fifteen P450 clones representing three new genes were obtained from the fat body. Two genes were expressed in both tissues. A number of putative allelic variants were also observed for three of the P450 genes. The resulting sequences of 130-132 amino acids were aligned to generate a parsimony analysis of CYP4 P450 proteins. Two new subfamilies of CYP4 were designated from M. sexta by these procedures, CYP4L and CYP4M. The sequence of a full-length cDNA clone for CYP4M2 (41.2% identity to CYP4C1) confirmed that the PCR products obtained by this method were P450s belonging to the CYP4 family. The developmental expression of the CYP4 genes appeared to be coordinately regulated in both fat body and midgut. In the fat body, CYP4 mRNA levels declined after the first day of the final larval instar, peaked during the wandering stage, and fell again until the prepupal molt. Midgut CYP4 mRNA levels were higher during the active feeding, midwandering, prepupal, and pupal stages. Addition of 2-tridecanone or 2-undecanone to the diet induced several P450s in the midgut and in the fat body. Phenobarbital induced CYP4M1 in the fat body and dietary clofibrate induced the mRNA levels of CYP4M1 and CYP4M3 in the midgut. The results indicate that at least four CYP4 genes are expressed in single tissues of a Lepidopteran insect. Several of these P450 may be involved in tissue responses to xenobiotics.

Photoaffinity labeling of methyl farnesoate epoxidase in cockroach corpora allata

The last enzyme in the biosynthetic pathway to juvenile hormone III in the corpora allata of hemimetabolous insects is methyl farnesoate epoxidase, a cytochrome P450 monooxygenase. Assays with intact glands incubated in vitro and with gland homogenates have identified a series of 1,5-disubstituted imidazoles as potent inhibitors of the enzyme. We have designed, synthesized and tested two imidazoles, diazirine-Ice T and benzophenone-Ice T, in which a radiolabeled and photoactivatable diazirine or benzophenone group was introduced to label the hydrophobic substrate binding site of the enzyme. Our results show that these bifunctional compounds inhibit JH III synthesis by intact glands as well as methyl farnesoate epoxidation by gland homogenates. Moreover both compounds selectively label a protein of ca. 55 kDa in corpora allata of the cockroach, Diploptera punctata. These photoaffinity labels, which use an imidazole to coordinate to the heme iron and a photoreactive group to modify the hydrophobic substrate binding pocket, are specific and effective probes for the molecular analysis of methyl farnesoate epoxidase.