Microsomal Drug Metabolizing Enzymes in Insects

Effect of geraniol against arecoline induced toxicity in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ) Bg9

In the present study geraniol at the final concentration of 10, 20, 30, and 40 µM was mixed in the diet along with 80 µM of arecoline and the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ) Bg⁹ were allowed to feed on it for 24 hrs. After the exposure of 24 hrs the larvae were subjected to ONPG, X-gal, trypan blue exclusion test, oxidative stress markers and apoptotic and comet assays. The exposure of larvae to geraniol showed a dose dependent decrease in the activity of β-galactosidase, tissue damage and oxidative stress markers. A dose dependent decrease in apoptosis and DNA damage was also observed. Molecular docking studies also support the protective role of geraniol against the arecoline induced toxicity. The results suggest that geraniol is potent in reducing the toxicity induced by arecoline in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ) Bg⁹.

Resistance irrelevant CYP417A2v2 was found degrading insecticide in Laodelphax striatellus

Cytochrome P450 monooxygenases (CYPs) usually overexpressed in resistant strain were found involved in oxidative detoxification of insecticides. In this study, an investigation was conducted to confirm if resistance irrelevant CYPs which were not overexpressed in resistant strain before, were capable of degrading insecticides. Three resistance irrelevant CYPs viz. CYP417A2v2, CYP425A1v2, and CYP4DJ1 from CYP4 family of Laodelphax striatellus were randomly selected for experiments. CYP417A2v2 and CYP425A1v2 were found expressed successfully in Sf9 cell line while CYP4DJ1 was not expressed successfully and out of two expressed CYPs, only CYP417A2v2 showed its efficient catalytic activity. For catalytic activity, three traditional model probe substrates and five insecticides were assayed. For the probe substrates screened, p-nitroanisole and ethoxycoumarin were preferentially metabolized by CYP417A2v2 (specific activity 3.76 ± 1.22 and 1.63 ± 0.37 nmol min-1 mg protein-1, respectively) and they may be potential diagnostic probes for this enzyme. Among insecticides, only imidacloprid was efficiently degraded by CYP417A2v2. Incubation of imidacloprid with CYP417A2v2 of L. striatellus and subsequent HPLC, LC-MS, and MS/MS analysis revealed the formation of imidacloprid metabolites, that is, 4' or 5'hydroxy-imidacloprid by hydroxylation. This result implies the exemption of CYPs character that it is not always, all the CYPs degrading insecticides being selected and overexpressed in resistant strains and the degrading CYPs without mutations to upregulate could be candidates during insecticide resistance evolution. This characterization of individual insect CYPs in insecticide degradation can provide insight for better understand of insecticide resistance development.

Evaluation of the toxic potential of calcium carbide in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ)Bg(9)

In the present study the toxic potential of calcium carbide (CaC2) was studied on the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ)Bg(9). The third instar larvae were exposed to 2, 4, 8, 16 and 32×10(-3)g/ml of CaC2 in diet for 24h. The results reveal that the dose 2×10(-3)g/ml was not toxic but the remaining doses showed a dose dependent significant increase in the hsp70 expression, β-galactosidase activity, tissue damage, oxidative stress markers (lipid peroxidation and protein carbonyl content), glutathione-S-transferase activity, expression of Caspase 3 and 9, apoptotic index and DNA damage (midgut cells). A significant reduction as compared to control group in total protein, glutathione content and acetylcholinesterase activity was also observed. The Inductively Coupled Plasma Atomic Emission Spectroscopy analysis (ICPAES) reveals the presence of copper, iron, sodium, aluminium, manganese, calcium, nickel and mercury. The toxic effects of CaC2 in the present study may be attributed to the impurities present in it.

Identification of a functional homolog of the mammalian CYP3A4 in locusts

Insects have been proposed as a new tool in early drug development. It was recently demonstrated that locusts have an efflux transporter localized in the blood-brain barrier (BBB) that is functionally similar to the mammalian P-glycoprotein efflux transporter. Two insect BBB models have been put forward, an ex vivo model and an in vivo model. To use the in vivo model it is necessary to fully characterize the locust as an entire organism with regards to metabolic pathways and excretion rate. In the present study, we have characterized the locust metabolism of terfenadine, a compound that in humans is specific to the cytochrome P450 enzyme 3A4. Using high-resolution mass spectrometry coupled to ultra-high-performance liquid chromatography, we have detected metabolites identical to human metabolites of terfenadine. The formation of human metabolites in locusts was inhibited by ketoconazole, a mammalian CYP3A4 inhibitor, suggesting that the enzyme responsible for the human metabolite formation in locusts is functionally similar to human CYP3A4. Besides the human metabolites of terfenadine, additional metabolites were formed in locusts. These were tentatively identified as phosphate and glucose conjugates. In conclusion, not only may locusts be a model useful for determining BBB permeation, but possibly insects could be used in metabolism investigation. However, extensive characterization of the insect model is necessary to determine its applicability.

Toxic potential of synthesized graphene zinc oxide nanocomposite in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ)Bg9

In the present study the graphene zinc oxide nanocomposite (GZNC) was synthesized, characterized, and evaluated for its toxic potential on third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ)Bg⁹. The synthesized GZNC was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The GZNC in 0.1% dimethyl sulphoxide (DMSO) was sonicated for 10 minutes and the final concentrations 0.033, 0.099, 0.199, and 3.996 μg/μL of diet were established. The third instar larvae were allowed to feed on it separately for 24 and 48 hr. The hsp70 expression was measured by o-nitrophenyl-β-D-galactopyranoside assay, tissue damage was measured by trypan blue exclusion test, and β-galactosidase activity was monitored by in situ histochemical β-galactosidase staining. Oxidative stress was monitored by performing lipid peroxidation assay and total protein estimation. Ethidium bromide/acridine orange staining was performed on midgut cells for apoptotic index and the comet assay was performed for the DNA damage. The results of the present study showed that the exposure of 0.199 and 3.996 μg/μL of GZNC was toxic for both 24 hr and 48 hr of exposure. The doses of 0.033 μg/μL and 0.099 of GZNC showed no toxic effects on its exposure to the third instar larvae for 24 hr as well as 48 hr of duration.

Metabolism of uscharidin, a milkweed cardenolide, by tissue homogenates of monarch butterfly larvae,Danaus plexippus L

Midgut and fat body homogenates of monarch butterfly larvae,Danaus plexippus L. (Lepidoptera:Danaidae), were examined for microsomal monooxygenase activity usingp-chloro-N-methylanilineN-demethylation and for the ability to metabolize a milkweed (Asclepias spp.) cardenolide (C23 steroid glycoside), uscharidin. All homogenates tested had bothN-demethylation and uscharidin biotransformation activities. Both transformations required NADPH. The monooxygenase inhibitors sesamex, SKF525A, and carbon monoxide inhibitedN-demethylation but not uscharidin biotransformation. Subsequent subcellular fractionation revealed the uscharidin biotransformation occurs in the soluble fraction and not the microsomal fraction, whileN-demethylation occurs in the microsomal fraction and not the soluble fraction. The larval NADPH-dependent microsomal monooxygenase apparently is not involved in the metabolism of uscharidin.

Comparison of larval and adult P-450 activity levels for alkaloid metabolism in desertDrosophila

The cytochrome P-450 monooxygenase system has been implicated in plant utilization by at least three species ofDrosophila (D. nigrospiracula, D. mettleri, andD. mojavensis) that are endemic to the Sonoran Desert of the southwestern United States and northwestern Mexico. Basal and induced levels of total cytochrome P-450 were determined for third-instar and decapitated 2- to 5-day post eclosion adults of the three desert species. Total P-450 levels, both basal and induced for all species assayed, were significantly higher for adults than for larvae by up to 20-fold. On a per organism basis, the levels of in vitro metabolism of the cactus alkaloid, carnegine, and patterns of response to induction by cactus tissue for adult desertDrosophila approximated those of larvae. Induction by phenobarbital, however, resulted in levels of in vitro carnegine metabolism that were up to 5.6-fold higher in adults than in larvae.

Validation of 1-methyl-2-phenylindole method for estimating lipid peroxidation in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ)Bg (9.)

Background:

A method using 1-methyl-2-phenylindole was developed for the estimation of lipid peroxidation in third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ)Bg⁹. The method is specific for the estimation of malonaldehyde.

Materials and Methods:

The larvae were exposed to 0.0025, 0.025, 0.050, and 0.100 μl/ml of cyclophosphamide for 24 and 48 h. The homogenate was prepared of the larvae tissue explant and the absorbance was noted at 586 nm.

Results:

A significant dose-dependent increase in the mean absorbance values was observed for both 24 and 48 h of exposure as compared to the untreated group.

Conclusions:

On the basis of results obtained, it is suggested that the present method is more precise, accurate, and robust for the estimation of lipid peroxidation in the third instar larvae of transgenic D. melanogaster (hsp70-lacZ)Bg⁹.

Turning the 'mustard oil bomb' into a 'cyanide bomb': aromatic glucosinolate metabolism in a specialist insect herbivore

Plants have evolved a variety of mechanisms for dealing with insect herbivory among which chemical defense through secondary metabolites plays a prominent role. Physiological, behavioural and sensorical adaptations to these chemicals provide herbivores with selective advantages allowing them to diversify within the newly occupied ecological niche. In turn, this may influence the evolution of plant metabolism giving rise to e.g. new chemical defenses. The association of Pierid butterflies and plants of the Brassicales has been cited as an illustrative example of this adaptive process known as 'coevolutionary armsrace'. All plants of the Brassicales are defended by the glucosinolate-myrosinase system to which larvae of cabbage white butterflies and related species are biochemically adapted through a gut nitrile-specifier protein. Here, we provide evidence by metabolite profiling and enzyme assays that metabolism of benzylglucosinolate in Pieris rapae results in release of equimolar amounts of cyanide, a potent inhibitor of cellular respiration. We further demonstrate that P. rapae larvae develop on transgenic Arabidopsis plants with ectopic production of the cyanogenic glucoside dhurrin without ill effects. Metabolite analyses and fumigation experiments indicate that cyanide is detoxified by β-cyanoalanine synthase and rhodanese in the larvae. Based on these results as well as on the facts that benzylglucosinolate was one of the predominant glucosinolates in ancient Brassicales and that ancient Brassicales lack nitrilases involved in alternative pathways, we propose that the ability of Pierid species to safely handle cyanide contributed to the primary host shift from Fabales to Brassicales that occured about 75 million years ago and was followed by Pierid species diversification.

Effect of the steroid K-canrenoate on hsp70 expression and tissue damage in transgenic Drosophila melanogaster (hsp70-lacZ) Bg9

In the present study the effect of 0.1, 0.2, 0.4, 0.8, and 1.0 µL/mL of the steroid K-canrenoate was evaluated in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ) Bg(9) for 6, 24, and 48 hours of duration. The treatment of 0.1, 0.2, and 0.4 µL/mL of K-canrenoate did not induce the activity of hsp70 significantly compared to the control. The treatments of 0.8 and 1.0 µL/mL of K-canrenoate not only caused tissue damage but also induced a significant increase in the expression of hsp70 for the different durations of exposure. The results of the present study suggest that the K-canrenoate at 0.8 and 1.0 µL/mL is cytotoxic and caused tissue damage in the third instar larvae of transgenic D. melanogaster (hsp70-lacZ) Bg(9).

Herbivore-plant interactions: mixed-function oxidases and secondary plant substances

The mixed-function oxidases of a polyphagous insect larva (the southern armyworm, Spodoptera eridania) were found to be induced by a diversity of secondary plant substances. The induction proceeds rapidly and in response to a small quantity of secondary substance. Following induction, the larva is less susceptible to dietary poisoning. It is argued that mixed-function oxidases play a major role in protecting herbivores against chemical stress from secondary plant substances.

Insecticide resistance: challenge to pest management and basic research

The agricultural use of synthetic insecticides usually protects crops but imposes strong selection pressures that can result in the development of resistance. The most important resistance mechanisms are enhancement of the capacity to metabolically detoxify insecticides and alterations in target sites that prevent insecticides from binding to them. Insect control methods must incorporate strategies to minimize resistance development and preserve the utility of the insecticides. The most promising approach, integrated pest management, includes the use of chemical insecticides in combination with improved cultural and biologically based techniques.

Resistance monitoring in codling moth: a need for standardization

BACKGROUND

A wide range of methods has been used for monitoring resistance in Cydia pomonella L. However, the results from specific tests are not always concordant and they generate doubts over the extent to which the data represent the field situation. Therefore, the variability in the expression of resistance mechanisms and in resistance ratios between developmental stages and their reliability in a field situation were studied.

RESULTS

Neonate larvae showed the highest insecticide resistance, except to spinosad. Resistance ratios were from 2 to 9.5, depending on the insecticide and the strain tested, when compared with the susceptible strain. Spinosad exhibited the highest resistance ratio for diapausing larvae (4.4, 12.2 and 4.4 for Raz, Rdfb and RDelta strains, respectively). Enzymatic ratios were higher in adults than in neonates or diapausing larvae, and the highest enzymatic ratios were observed for mixed-function oxidases (7.3-36.7 for adults of resistant strains).

CONCLUSIONS

The sensitivity of different instars depends on the insecticides used, and there is a differential expression of resistance mechanisms between stages. For the population considered in this study, tests on neonates provided the most consistent results to the field situation, but adult response to chlorpyrifos-ethyl and azinphos-methyl in the laboratory was also consistent with the field results.

Aldrin epoxidation and dihydroisodrin hydroxylation as probes of in vivo and in vitro oxidative metabolic capability of some caterpillars

Comparative biochemical studies are productive means to study factors that limit both beneficial and harmful effects of chemicals. Reactions such as aldrin epoxidation and dihydroisodrin hydroxylation are valuable assays of oxidative metabolism in scientific studies of chemical biology in insects, subhuman primates and other living things. The tissue distribution of activity in caterpillars may have functional significance. Localization of relatively high concentrations of these cytochrome P450 monooxygenases in gut tissue of lepidoptera may represent an important means to minimize absorption of lipophilic foreign chemicals in food. Some polychlorocycloalkanes permit in vivo and in vitro studies owing to their stability, acceptable toxicity and relatively simple pattern of metabolism. In vivo studies to assess the significance of in vitro findings are feasible with substrates such as aldrin, dihydroisodrin (DHI) and oxidative methylenedioxyphenyl inhibitors such as piperonyl butoxide (PBO) or carbon monoxide. Biphasic dose-dependent decreased and increased DHI-OH formation resulted from PBO pretreatment by gut, fat body, head and Malpighian tubule homogenates of cutworms and gut and fat body (the only tissues tested) of cabbage looper Trichplusia ni (Hübner) and black cutworm Agrotis ipsilon (Hüfnagel). The biphasic in vivo responses of caterpillars to PBO are a reminder of the complexity of biochemical and physiological responses of organisms coexposed to chemicals that are classified, often glibly, as toxic substances and metabolic inhibitors and inducers. Knowledge of dose and time relationships demands very careful evaluation in living things in the environment.

In vivo metabolic fate of [14C]-acetamiprid in six biological compartments of the honeybee, Apis mellifera L

The in vivo metabolism of acetamiprid was studied in the honeybee, Apis mellifera L. The distribution of acetamiprid and its metabolites was monitored over a 72-h period in six biological compartments: head, thorax, abdomen, haemolymph, midgut and rectum. Honeybees were treated orally with 100 microg [14C]-acetamiprid kg(-1) bee, a dose which is about 1500 times lower than the median lethal dose. After 72 h, only 40% of the total radioactivity was eliminated, suggesting that acetamiprid and its metabolites tended to persist in the honeybee. Acetamiprid was rapidly distributed in all compartments and metabolized. Just after administration, radioactivity was mainly localized in the abdomen and subsequently in the rectum. After 72 h, the maximum amount of radioactivity (about 20% of the ingested dose) was detected again in the abdomen, whereas the lowest level of total radioactivity was detected in the haemolymph. Radioactivity in the head did not exceed 7.6% of total ingested radioactivity. More than 50% of acetamiprid was metabolised in less than 30 min, indicating a very short half-life for the compound. During the first hours, acetamiprid was mainly detected in nicotinic acetylcholine receptor-rich tissues: abdomen, thorax and head. Of the seven metabolites detected, the major ones were 6-choronicotinic acid and an unknown metabolite called U1, which was present mainly in the rectum, the thorax and the head. Our results indicate that the low toxicity of acetamiprid may reflect its rapid metabolism.

Toxicity of cypermethrin: hsp70 as a biomarker of response in transgenic Drosophila

Heat shock protein induction is often associated with a cellular response to a harmful environment or to adverse life conditions. The main aims of our study were (1) to evaluate the cytotoxic potential of cypermethrin; and (2) to investigate the suitability of stress-induced heat shock protein Hsp70 as a biomarker for environmental pollutants in transgenic Drosophila melanogaster (Hsp70-lacZ)Bg(9). Different concentrations of cypermethrin (0.002, 0.2, 0.5 and 50.0 p.p.m.) were mixed with food. Third instar larvae of transgenic Drosophila melanogaster were allowed to feed on these mixtures for different time intervals (2, 4, 6, 12, 24 and 48 h). Following feeding, hsp70 induction and tissue damage were evaluated. In the highest concentration treatment group (50 p.p.m.), 100% larval mortality was recorded after 12 h exposure. Hsp70 was found to be induced even at the lowest concentration (0.002 p.p.m.) of the insecticide, while tissue damage was observed in the larvae exposed for 48 h. While an insignificant decline in hsp70 expression was observed in the larvae exposed to cypermethrin at a dietary concentration of 0.002 p.p.m. after 48 h compared with those exposed for 24 h, in the next two higher concentrations of the toxicant, a similar but significant decline in hsp70 expression was evident in the exposed larvae after 48 h. The present study reveals the cytotoxic potential of cypermethrin and further proposes that hsp70 induction in transgenic Drosophila melanogaster could be used as a sensitive biomarker in risk assessment.

The inhibitory effect of the fungicides captan and captafol on eukaryotic topoisomerases in vitro and lack of recombinagenic activity in the wing spot test of Drosophila melanogaster

In studies on the mechanisms of mutagenic and carcinogenic action of captan and captafol-related chloroalkylthiocarboximide fungicides, two effects were tested: (i) the effect of both compounds on the activity of eukaryotic topoisomerases I and II in vitro, and (ii) their mutagenic and recombinagenic activity in the somatic mutation and recombination test (SMART) in wing cells of Drosophila melanogaster. Only captafol inhibited the activity of topoisomerase I (10-20% inhibition of activity in the range of 10-100microM). In contrast, both chemicals decreased the activity of topoisomerase II already at 1microM concentration (50 and 20% inhibition of activity by captafol and captan, respectively).Genotoxicity was tested in vivo by administrating both compounds by acute (3h) and chronic feeding (48h) of 3-day-old larvae. In acute feeding, captan and captafol demonstrated positive results only for small single and total spots in 10-100mM exposure concentration range. Both chemicals were inconclusive for large single spots, as well as for twin spots. In chronic treatment, captan showed positive results only for small single and total spots at 2.5 and 5mM concentrations. Captafol gave inconclusive results over all concentrations tested. The results of the acute treatment experiments which have been performed at very high doses (50% toxicity at higher doses) indicate very weak overall mutagenic activity of both test fungicides.

Effect of three chlorinated pesticides on hsromega stress gene in transgenic Drosophila melanogaster

Expression of hsromega stress gene in the third-instar larvae of 951-lacZ2 (hsromega-lacZ having-844pb sequence) and 498-lacZ1 (hrsomega-lacZ having -498bp sequence) strains of Drosophila melanogaster at LC(50) and lower dietary concentrations of hexachlorocyclohexane (HCH) pentachlorophenol (PCP), and endosulfan was examined in relation to larval mortality by beta galactosidase activity, vital dye staining, and salivary gland polytene chromosome puffing. Our results showed that both HCH and PCP at lower concentrations evoked strong hsromega stress gene expression in the larval tissues while endosulfan did not. On the other hand, puffing data revealed that endosulfan at lower doses, induced well-developed puff at the resident site (93D) of the hsromega gene but the transgenic sites (30B in 951-lacZ2 and 44B in 498-lacZ1 strain) did not show any well-developed puff. Regression in hsromega stress gene expression in 951-lacZ2 strain at LC(50) concentrations of HCH and PCP after 48 h was concurrent with extensive tissue damage as evident by trypan blue staining. Similarly, strong hsromega expression was accompanied by insignificant trypan blue staining in the larval tissues of this strain after shorter duration of exposure (2-12 h) to these toxicants. Although endosulfan under similar experimental condition did not induce hsromega, strong trypan blue staining indicated extensive tissue damage after 48 h of exposure. The present study suggests that all the three toxicants pose cytotoxic potential to Drosophila. While protective role of this stress gene was evident at the initial stages of exposure, extensive tissue damage in the later stages of intoxication accompanied by autorepression of hsromega led to larval mortality. The study further suggests that -844bp upstream sequence of the gene is adequate for hsromega inducibility against HCH and PCP but not for endosulfan for which responsive elements may be searched further upstream.

Atrazine induction of cytochrome P450 in Chironomus tentans larvae

Cytochrome P450-dependent aldrin epoxidation was characterized in third instar larvae of the aquatic midge, Chironomus tentans. Optimal in vitro assay conditions for the epoxidase were pH 7.6 and 31 degrees C. Activity was linear up to 40 min of incubation time and 0.5 mg microsomal protein per incubation. The activity was concentrated in the microsomal fraction of whole body homogenates and was NADPH-dependent. The effect of atrazine exposure on aldrin epoxidase was measured to determine if this herbicide induces cytochrome P450-dependent activity. Comparisons of control and atrazine-exposed midges indicated increased epoxidase activity as a result of atrazine exposure, and a 45 kDa protein of increased intensity was observed after SDS-PAGE of microsomal protein. The molecular weight of this protein was similar in size to cytochrome P450 enzymes reported for other insects. Heme staining of SDS-PAGE gels and immunochemical studies using a Drosophila melanogaster anti-P450 polyclonal antiserum, further support the cytochrome P450 nature of this inducible 45 kDa protein.

Insect P450 enzymes

The P450 enzymes (mixed function oxidases, cytochrome P450 monooxygenases), a diverse class of enzymes found in virtually all insect tissues, fulfill many important tasks, from the synthesis and degradation of ecdysteroids and juvenile hormones to the metabolism of foreign chemicals of natural or synthetic origin. This diversity in function is achieved by a diversity in structure, as insect genomes probably carry about 100 P450 genes, sometimes arranged in clusters, and each coding for a different P450 enzyme. Both microsomal and mitochondrial P450s are present in insects and are best studied by heterologous expression of their cDNA and reconstitution of purified enzymes. P450 genes are under complex regulation, with induction playing a central role in the adaptation to plant chemicals and regulatory mutations playing a central role in insecticide resistance. Polymorphisms in induction or constitutive expression allow insects to scan their P450 gene repertoire for the appropriate response to chemical insults, and these evolutionary pressures in turn maintain P450 diversity.

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.

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.

In vivo metabolism of aminopyrine by the larvae of the helminth Heligmosomoides polygyrus

The in vivo N-dealkylation of [13C-2]-labeled aminopyrine by the L1-L2 larvae of Heligmosomoides polygyrus was demonstrated by the use of a sensitive gas chromatography-mass spectrometry method. This is the first evidence for the possible existence of a cytochrome P-450-dependent activity in helminths.

Tissue distribution of microsomal cytochrome P-450 monooxygenases and their inducibility by phenobarbital in the insecticide resistant LPR strain of house fly, Musca domestica L

Cytochrome P-450s, cytochrome b5, P-450 reductase, and four P-450 monooxygenase activities were abundant in proximal intestine (PI), Malpighian tubules (MT), and fat bodies (FB) in the insecticide resistant (LPR) strain of house fly. Insecticide resistance in LPR flies was associated with increases in P-450 monooxygenase activities in nearly all of these tissues relative to an insecticide susceptible strain. Cytochrome P-450lpr was present in all tissues and at higher levels in LPR than in susceptible flies suggesting that the role of P-450lpr in monooxygenase-mediated insecticide resistance is not due to its higher levels in a single tissue. However, it appears that fat bodies may have a major role in resistance due to their relatively large size and the abundance of P-450lpr in this tissue. Total cytochrome P-450s, cytochrome P-450lpr and P-450 monooxygenase activities were generally refractory to phenobarbital (PB) induction in LPR. This contrasts with a previous report in susceptible house flies showing that P-450 monooxygenases were induced by PB. It appears that total P-450s, cytochrome P-450lpr and monooxygenase activities were expressed at constitutive high levels in several tissues of LPR house flies.

Use of differential DNA-repair host mediated assays to investigate the biotransformation of xenobiotics in Drosophila melanogaster. I. Genotoxic effects of nitrosamines

A rapid differential DNA-repair assay procedure was developed to investigate the biotransformation of xenobiotics in Drosophila melanogaster in vivo. Indicator of genotoxic activity was a pair of streptomycin-dependent Escherichia coli strains differing vastly in DNA repair capacity (uvr+/rec+ vs. uvrB/recA). Prior to the experiments with test compounds, mixtures of the two strains were injected into the abdomina of untreated animal hosts (male Berlin-K flies) and the time-dependent recovery kinetics determined. Subsequently, different aliphatic and aromatic nitrosamines were tested. Solutions of the compounds were injected simultaneously with the indicator cells. Three hours later, the flies were killed, homogenized and the induction of (repairable) DNA damage determined by comparison of the survival rates of the two strains in single animals. Eight carcinogenic compounds (nitrosodiethylamine, NDEA; nitrosodimethylamine, NDMA; nitrosodi-npropylamine, NDPA; nitrosodiethanolamine, NDELA; nitrosomethylaniline, NMA; 4-methyl-nitrosopiperidine, MNPIP; nitrosopyrrolidine, NPYR; nitrosomorpholine, NMOR) and one whose tumorigenic activities are still controversially discussed (nitrosodiphenylamine, NDPhA) induced dose-dependent differential killing effects in the present system. One agent which has not been found carcinogenic in rodents (2.6-dimethyl-nitrosopiperidiine. NDMPIP) gave negative results. The ranking order of genotoxic activities of the nitrosamines found in Drosophila in vivo is in good agreement with those of carcinogenic potencies established on the basis of experiments with rats. The most pronounced exceptions are the rather weak response towards NMA and the stronger DNA damaging activity of NMPIP compared to NDMA. Phenobarbital (5-ethyl-5-phenyl-2,4,6-trioxohepatahydropyramidine) (PB) feeding of the flies resulted in an increase of the DNA damaging potencies of all nitrosamines tested. Substantial enhancement of the induction of DNA damage was however, restricted to NDEA, NPYR and NMOR, whereas with nitrosodiphenylamine (NDPhA), NDELA and NDMA only a moderate (less than 25%) increase of differential killing effects was found. In the case of the two latter compounds, these results might be due to the fact that enzymes other than the MFO are involved in their activation. Attempts to localize the formation and/or distribution of metabolites in the bodies of fruitflies by separation of the tagmata of chemically treated animals and determination of genotoxic effects in the different segments indicate that the most pronounced effects occur in the abdomina whereas in heads and thoraxes comparatively lower activities are detectable.

Cytochrome P-450 monooxygenases in insects

1. The insect monooxygenase system is involved in the oxidative metabolism of both endogenous and exogenous substrates. 2. Monooxygenases appear to be important in insect growth and development, in adaptation to multiple food plants in phytophagous insects and in pesticide resistance. 3. The genetics, purification and properties of cytochrome P-450 isozymes of the house fly (Musca domestica) are discussed in relation to their role in resistance to xenobiotics. 4. Induction of insect cytochrome P-450 isozymes is discussed in comparison to induction in mammals and with reference to polyphagy.

Cytochrome P-450-dependent monooxygenase activity in the velvetbean caterpillar, Anticarsia gemmatalis Hübner

The activity of microsomal aldrin epoxidase was observed in various tissues of the last instar larva of the velvetbean caterpillar (Anticarsia gemmatalis Hübner) with the midgut being the most active. Optimum conditions for assaying the enzyme were established. Allelochemicals (monoterpenes, indoles and flavones) and host plants (cowpeas, cotton, peanuts, soybeans and hairy indigo) were found to induce the monooxygenase. Piperonyl butoxide decreased and increased the toxicity of methomyl and methyl parathion, respectively, suggesting that the monooxygenase system of velvetbean caterpillars plays a major role in the detoxification of xenobiotics.

Induction of cytochrome P-450 mediated mono-oxygenase reactions and conjugation activities in freshwater crayfish (Astacus astacus)

The inducibility of the freshwater crayfish (Astacus astacus) biotransformation enzymes with model inducers (Aroclor 1254, beta-Naphthoflavone, Phenobarbital) were investigated three days after intra cephalothoracic injection in the fasting crayfish at 5 degrees C. Of the monooxygenase activities, 7-ethoxycoumar in O-deethylase increased in the hepatopancreas significantly (p less than 0.05) after beta-naphthoflavone administration. Benzo(a)pyrene hydroxylase did not change. Aroclor 1254 and phenobarbital injection elevated hepatopancreatic glutathione S-transferase activity (p less than 0.05).

A rapid method for preparing insect microsomes

The 1000 g supernatant of a tissue homogenate is layered on top of a small (less than 5 ml) sucrose density gradient and centrifuged for 20 min at very high centrifugal forces in a vertical rotor. Microsomes can be recovered rapidly in suspended form from the middle of the gradient, well separated from mitochondria and soluble (cytosolic) components. Applications to cockroach midgut microsomes and mosquito abdominal tissue microsomes are described, and the method is compared to the classical differential centrifugation method. Cytochrome P-450 monooxygenase activities can be measured on microsomes prepared from midgut tissue of 2-3 Diploptera punctata using this method.

Species variations in the metabolism of liposoluble organochlorine compounds by hepatic microsomal monooxygenase: comparative kinetics in four vertebrate species

Liposoluble organochlorine compounds were used as substrates in a kinetic study of the hepatic microsomal monooxygenases of the male rat, feral pigeon (Columbia livia), frog (Rana pipiens) and rainbow trout (Salmo gairdnerii). Substrate concentrations were taken down to environmentally realistic levels. Lineweaver-Burk plots gave straight lines for both aldrin (HHDN) and the dieldrin analogue HCE. For both substrates activities followed the order rat greater than feral pigeon greater than trout over the entire concentration range. The metabolism of PCB isomers in microsomes from these uninduced animals was very slow. These results give evidence of cytochrome P-450 forms which can metabolize organochlorine compounds at low substrate concentrations.

Polysubstrate monooxygenases in Drosophila, mammals and man

There is overwhelming evidence that polysubstrate monooxygenases play a central role in the metabolism of endogenous compounds as well as in the biotransformation of xenobiotics. These enzyme systems are of great importance in such diverse fields as insecticide resistance, mutagenicity, carcinogenicity, drug metabolism, etc. The constitutive and, in particular, the induced forms represent various products from a multigene family. This has first been shown for the mouse, but evidence is accumulating that this is also true for other mammals and for man. Also in insects a similar picture is emerging. If the regulation of cytochrome P-450 induction resembles in any way the other methods by which prokaryotes and eukaryotes cope genetically with the many forms of environmental selective pressures, it is very likely that most organisms have the genetic capacity to produce not only hundreds but probably thousands of inducible forms of cytochrome P-450 (Nebert et al., 1981). Doubtless, many fields from pest control to cancer prevention to drug safety will profit from the elucidation of the genetic mechanisms involved.

In vitro studies on the metabolism of aflatoxin B1 and aldrin in testes of genetically different strains of Drosophila melanogaster

As Drosophila melanogaster occupies an important position within the test battery for mutagens and carcinogens, it is of interest to study the xenobiotics metabolism of this insect. Likewise, the genetic control of these important enzyme systems falls within this interest. Our attempt was to get new strains, which show changes in their xenobiotics metabolism. This was done by a mutagenization and selection procedure for the second chromosome. The 44 fertile homozygous inbred strains produced by this selection were first tested for DDT resistance. Some of them showed LT50 values which were remarkably higher than that of the original strain Berlin K. Aflatoxin B1 metabolism in two of the new strains (H349 and H362), Berlin K, and Hikone-R was compared, whilst aldrin epoxidase activity was compared in strains H349, H362, Berlin K, vestigial, and Karsnäs-R. The metabolism studies were carried out in vitro with testes tissue of the different strains. The metabolism in testes is of specific interest because this tissue is most often used in mutagenicity testing. In the AFB1 assays of the up to 12 observed metabolites three could be identified as AFB2a, AFM1, and AFR0. Hikone-R produced mostly AFR0 (3.43% of the initial AFB1 concentration) and small amounts of AFM1 (0.59% AF and AFB2a (0.36% AF). The strain Berlin K showed only a low production of AFB2a (0.48% AF), while the strain H349 formed AFR0 (6.02% AF) and AFM1 (0.75% AF). The AFM1 appeared in even higher amounts than with Hikone-R. On the other hand, H362 showed the lowest activity in AFB1 metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)