Response of hepatic microsomal mixed-function oxidases to various types of insecticide chemical synergists administered to mice

Pharmacokinetic analysis of acute and dietary exposure to piperonyl butoxide in the mouse

Piperonyl butoxide (PBO) is a popular insecticide synergist present in thousands of commercial, agricultural, and household products. PBO inhibits cytochrome P450 activity, impairing the ability of insects to detoxify insecticides. PBO was recently discovered to also inhibit Sonic hedgehog signaling, a pathway required for embryonic development, and rodent studies have demonstrated the potential for in utero PBO exposure to cause structural malformations of the brain, face, and limbs, or more subtle neurodevelopmental abnormalities. The current understanding of the pharmacokinetics of PBO in mice is limited, particularly with respect to dosing paradigms associated with developmental toxicity. To establish a pharmacokinetic (PK) model for oral exposure, PBO was administered to female C57BL/6J mice acutely by oral gavage (22-1800 mg/kg) or via diet (0.09 % PBO in chow). Serum and adipose samples were collected, and PBO concentrations were determined by HPLC-MS/MS. The serum concentrations of PBO were best fit by a linear one-compartment model. PBO concentrations in visceral adipose tissue greatly exceeded those in serum. PBO concentrations in both serum and adipose tissue decreased quickly after cessation of dietary exposure. The elimination half-life of PBO in the mouse after gavage dosing was 6.5 h (90 % CI 4.7-9.5 h), and systemic oral clearance was 83.3 ± 20.5 mL/h. The bioavailability of PBO in chow was 41 % that of PBO delivered in olive oil by gavage. Establishment of this PK model provides a foundation for relating PBO concentrations that cause developmental toxicity in the rodent models to Sonic hedgehog signaling pathway inhibition.

Neonicotinoids and Other Insect Nicotinic Receptor Competitive Modulators: Progress and Prospects

Neonicotinoids (neonics) are remarkably effective as plant systemics to control sucking insects and for flea control on dogs and cats. The nitroimines imidacloprid, clothianidin, thiamethoxam, and dinotefuran are the leaders among the seven commercial neonics that also include the nitromethylene nitenpyram, the nitromethylene-derived cycloxaprid, and the cyanoimines acetamiprid and thiacloprid. Honey bees are highly sensitive to the nitroimines and nitromethylenes, but the cyanoimines are less toxic. All neonics are nicotinic acetylcholine receptor (nAChR) agonists with a common mode of action, target-site cross-resistance, and much higher potency on insect than mammalian nAChRs at defined binding sites. The structurally related sulfoximine sulfoxaflor and butenolide flupyradifurone are also nAChR agonists, and the mesoionic triflumezopyrim is a nAChR competitive modulator with little or no target-site cross-resistance. Some neonics induce stress tolerance in plants via salicylate-associated systems. The neonics in general are readily metabolized and, except for pollinators, have favorable toxicological profiles.

The effect of insecticide synergist treatment on genome-wide gene expression in a polyphagous pest

Synergists can counteract metabolic insecticide resistance by inhibiting detoxification enzymes or transporters. They are used in commercial formulations of insecticides, but are also frequently used in the elucidation of resistance mechanisms. However, the effect of synergists on genome-wide transcription in arthropods is poorly understood. In this study we used Illumina RNA-sequencing to investigate genome-wide transcriptional responses in an acaricide resistant strain of the spider mite Tetranychus urticae upon exposure to synergists such as S,S,S-tributyl phosphorotrithioate (DEF), diethyl maleate (DEM), piperonyl butoxide (PBO) and cyclosporin A (CsA). Exposure to PBO and DEF resulted in a broad transcriptional response and about one third of the differentially expressed genes (DEGs), including cytochrome P450 monooxygenases and UDP-glycosyltransferases, was shared between both treatments, suggesting common transcriptional regulation. Moreover, both DEF and PBO induced genes that are strongly implicated in acaricide resistance in the respective strain. In contrast, CsA treatment mainly resulted in downregulation of Major Facilitator Superfamily (MFS) genes, while DEGs of the DEM treatment were not significantly enriched for any GO-terms.

Pesticide Interactions: Mechanisms, Benefits, and Risks

Interactions between pesticides at common molecular targets and detoxification systems often determine their effectiveness and safety. Compounds with the same mode of action or target are candidates for cross resistance and restrictions in their recommended uses. Discovery research is therefore focused on new mechanisms and modes of action. Interactions in detoxification systems also provide cross resistance and synergist and safener mechanisms illustrated with serine hydrolases and inhibitors, cytochrome P450 and insecticide synergists, and glutathione S-transferases and herbicide safeners. Secondary targets are also considered for inhibitors of serine hydrolases, aldehyde dehydrogenases, and transporters. Emphasis is given to the mechanistic aspects of interactions, not the incidence, which depends on potency, exposure, ratios, and timing. The benefits of pesticide interactions are the additional levels of chemical control to achieve desired organismal effects. The risks are the unpredictable interactions of complex interconnected biological systems. However, with care, two can be better than one.

Unexpected Metabolic Reactions and Secondary Targets of Pesticide Action

Pesticides provide a fascinating combination of substituents not present in other environmental chemicals, leading to unexpected metabolites and toxicological effects in pests, mammals, and other organisms. The parent compound and/or metabolites of some pesticides have multiple targets, requiring identification of the causal agents and their modes of action. This review considers a few of the author's observations in the past six decades, some solved and others still puzzling. It illustrates that a new substituent combination not only confers specific chemical and physical properties to a class of compounds but often yields metabolites with a surprising variety of biological activities. Examples considered include proinsecticides, procyclic phosphates, CYP inhibitors as synergists, thiocarbamate sulfoxides, promutagens, carcinogens, and hepatotoxins, and stress tolerance inducers in plants. Although the discoveries considered are based on pesticide toxicology, they are broadly applicable to environmental toxicology and xenobiotics in animals, plants, and microorganisms.

Physiological and biochemical effects of botanical extract from Piper nigrum Linn (Piperaceae) against the dengue vector Aedes aegypti Liston (Diptera: Culicidae)

The leaves of Piper nigrum L. (Piperaceae) were evaluated for chemical constituents and mosquito larvicidal activity against the larvae of Aedes aegypti. GC and GC-MS analyses revealed that the crude extracts contain 16 compounds. Thymol (20.77%) and ç-elemene (10.42%) were identified as the major constituents followed by cyclohexene, 4-ethenyl-4-methyl-3-(1-methylethenyl)-1-(1 methylethyl)-, (3R-trans) (7.58%), 4,6-octadienoic acid, 2-acetyl-2-methyl-, ethyl ester (6.98), 2(3H)-furanone, 3,4-bis(1,3-benzodioxol-5-ylmethyl) dihydro-, (3R-trans) (6.95%), 1-naphthalenol, 1,2,3,4,4a,7,8,8a-octahydro-1,6-dimethyl-4-(1-methylethyl)-, [1R-(1à,4á,4aá,8aá)]-(Cedreanol) (5.30%), trans-2-undecen-1-ol (4.48%), phytol (4.22%), 1,6-cyclodecadiene, 1-methyl-5-methylene-8-(1-methylethyl)-,[s-(E,E)] (3.78%) and 2,6-dimethyl-3,5,7-octatriene-2-ol, Z,Z (2.39%). Larval mortality was observed after 3 h of exposure period. The crude extract showed remarkable larvicidal activity against Ae. aegypti (LC50 = 34.97). The larvae of Ae. aegypti exposed to the P. nigrum, significantly reduced the activities of α- and β-carboxylesterases and superdioxide. Further, P. nigrum extract was severely affecting the mosquito gut cellular organelles. Based on the results, the chemical constituents of crude extracts of P. nigrum can be considered as a new source of larvicide for the control of Ae. aegypti.

Analysis of putative inhibitors of anthelmintic resistance mechanisms in cattle gastrointestinal nematodes

Effects of the cytochrome P450 inhibitor piperonyl butoxide and the P-glycoprotein inhibitor verapamil on the efficacy of ivermectin and thiabendazole were studied in vitro in susceptible and resistant isolates of the cattle parasitic nematodes Cooperia oncophora and Ostertagia ostertagi. The effects of combined use of drug and piperonyl butoxide/verapamil, respectively, were investigated in the Egg Hatch Assay, the Larval Development Assay and the Larval Migration Inhibition Assay. The effects of piperonyl butoxide and verapamil as inhibitors of thiabendazole and ivermectin responses were particularly marked for larval development, where both inhibitors were able to completely eliminate all differences between susceptible and resistant isolates. Even the lowest concentrations of anthelmintics used in combination with inhibitors caused complete inhibition of development. Differences and/or similarities among responses in different isolates were only obtained in the two other assays: in the Egg Hatch Assay piperonyl butoxide caused a shift in concentration-response curves obtained with thiabendazole to the left for all isolates tested, changing relative differences between isolates. In contrast, an effect of verapamil in the Egg Hatch Assay was only apparent for benzimidazole-resistant isolates. In the Larval Migration Inhibition Assay only ivermectin was tested and piperonyl butoxide shifted the concentration-response curves for all isolates to the left, again eliminating differences in EC50 values between susceptible and resistant isolates. This was not the case using verapamil as an inhibitor, where curves for both susceptible and benzimidazole-resistant isolates shifted to the left in Ostertagia isolates. In Cooperia the picture was more complex with ivermectin-resistant isolates showing a larger shift than the susceptible isolate. Single nucleotide polymorphisms in the β-tubulin isotype 1 gene were investigated. Significantly increased frequencies of resistance-associated alleles were observed for the codons 167 and 200 in one benzimidazole-resistant isolate but not in an isolate selected for benzimidazole resistance at an early stage of selection.

Insect nicotinic receptor interactions in vivo with neonicotinoid, organophosphorus, and methylcarbamate insecticides and a synergist

The nicotinic acetylcholine (ACh) receptor (nAChR) is the principal insecticide target. Nearly half of the insecticides by number and world market value are neonicotinoids acting as nAChR agonists or organophosphorus (OP) and methylcarbamate (MC) acetylcholinesterase (AChE) inhibitors. There was no previous evidence for in vivo interactions of the nAChR agonists and AChE inhibitors. The nitromethyleneimidazole (NMI) analog of imidacloprid, a highly potent neonicotinoid, was used here as a radioligand, uniquely allowing for direct measurements of house fly (Musca domestica) head nAChR in vivo interactions with various nicotinic agents. Nine neonicotinoids inhibited house fly brain nAChR [(3)H]NMI binding in vivo, corresponding to their in vitro potency and the poisoning signs or toxicity they produced in intrathoracically treated house flies. Interestingly, nine topically applied OP or MC insecticides or analogs also gave similar results relative to in vivo nAChR binding inhibition and toxicity, but now also correlating with in vivo brain AChE inhibition, indicating that ACh is the ultimate OP- or MC-induced nAChR active agent. These findings on [(3)H]NMI binding in house fly brain membranes validate the nAChR in vivo target for the neonicotinoids, OPs and MCs. As an exception, the remarkably potent OP neonicotinoid synergist, O-propyl O-(2-propynyl) phenylphosphonate, inhibited nAChR in vivo without the corresponding AChE inhibition, possibly via a reactive ketene metabolite reacting with a critical nucleophile in the cytochrome P450 active site and the nAChR NMI binding site.

The synergistic effects of insecticidal essential oils and piperonyl butoxide on biotransformational enzyme activities in Aedes aegypti (Diptera: Culicidae)

The biochemical mechanisms underlying the increased toxicity of several plant essential oils (thymol, eugenol, pulegone, terpineol, and citronellal) against fourth instar of Aedes aegypti L. when exposed simultaneously with piperonyl butoxide (PBO) were examined. Whole body biotransformational enzyme activities including cytochrome P450-mediated oxidation (ethoxyresorufin O-dethylase [EROD]), glutathione S-transferase (GST), and beta-esterase activity were measured in control, essential oil-exposed only (single chemical), and essential oil + PBO (10 mg/liter) exposed larvae. At high concentrations, thymol, eugenol, pulegone, and citronellal alone reduced EROD activity by 5-25% 16 h postexposure. Terpineol at 10 mg/liter increased EROD activity by 5 +/- 1.8% over controls. The essential oils alone reduced GST activity by 3-20% but PBO exposure alone did not significantly affect the activity of any of the measured enzymes. All essential oils in combination with PBO reduced EROD activity by 58-76% and reduced GST activity by 3-85% at 16 h postexposure. This study indicates a synergistic interaction between essential oils and PBO in inhibiting the cytochrome P450 and GST detoxification enzymes in Ae. aegypti.

Enzymes and inhibitors in neonicotinoid insecticide metabolism

Neonicotinoid insecticide metabolism involves considerable substrate specificity and regioselectivity of the relevant CYP450, aldehyde oxidase, and phase II enzymes. Human CYP450 recombinant enzymes carry out the following conversions: CYP3A4, 2C19, and 2B6 for thiamethoxam (TMX) to clothianidin (CLO); 3A4, 2C19, and 2A6 for CLO to desmethyl-CLO; 2C19 for TMX to desmethyl-TMX. Human liver aldehyde oxidase reduces the nitro substituent of CLO to nitroso much more rapidly than it does that of TMX. Imidacloprid (IMI), CLO, and several of their metabolites do not give detectable N-glucuronides but 5-hydroxy-IMI, 4,5-diol-IMI, and 4-hydroxythiacloprid are converted to O-glucuronides in vitro with mouse liver microsomes and UDP-glucuronic acid or in vivo in mice. Mouse liver cytosol with S-adenosylmethionine converts desmethyl-CLO to CLO but not desmethyl-TMX to TMX. Two organophosphorus CYP450 inhibitors partially block IMI, thiacloprid, and CLO metabolism in vivo in mice, elevating brain and liver levels of the parent compounds while reducing amounts of the hydroxylated metabolites.

Piperonyl butoxide induces the expression of cytochrome P450 and glutathione S-transferase genes in Drosophila melanogaster

Piperonyl butoxide (PBO) is an insecticide synergist known to inhibit the activity of cytochrome P450 enzymes. PBO is currently used in some insecticide formulations, and has also been suggested as a pretreatment for some pesticide applications. Little is known about how insects respond to PBO exposure at the gene transcription level. The authors have characterised the transcriptional response of the Drosophila melanogaster genome after PBO treatment, using both a custom-designed 'detox' microarray, containing cytochrome P450 (P450), glutathione S-transferase (GST) and esterase genes, and a full genome microarray. A subset of P450 and GST genes is identified, along with additional metabolic genes, that are induced by PBO. The gene set is an extremely similar gene set to that induced by phenobarbital, a compound for which pretreatment is known to confer tolerance to a range of insecticide compounds. The implications of the induction of gene families known to metabolise insecticides and the use of PBO in pest management programs are discussed.

Induction and inhibition of pesticide-metabolizing enzymes: roles in synergism of pesticides and pesticide action

Synergism of the toxic action of pesticides is discussed with particular emphasis on the methylenedioxyphenyl (benzodioxole) (MDP) synergists and on how studies of these compounds may relate to the general significance of pesticide synergism. MDP compounds function as both inhibitors and inducers of cytochrome P450 isoforms, the two processes proceeding, in vivo, at different rates. Inhibition is mediated through the formation of metabolite inhibitory complexes, and induction involves both aryl hydrocarbon (Ah)-dependent and Ah-independent mechanisms. The significance of this biphasic effect on multienzyme xenobiotic metabolizing systems is illustrated by considerations of the insecticide phorate.

Chronic toxicity studies of piperonyl butoxide in CD-1 mice: induction of hepatocellular carcinoma

Male and female CD-1 mice (51-104 mice/group) were administered piperonyl butoxide (alpha-[2-(2-butoxyethoxy)ethoxy-4,5-methylenedioxy-2-propyltol uene) in the diet at levels of 0 (control), 0.6 and 1.2% for 52 weeks (1 year). Hepatocellular carcinomas were induced in treated groups in a dose-dependent manner. The incidences of hepatocellular carcinoma were 11.3 and 52.0% in male mice given 0.6 and 1.2% piperonyl butoxide, and 41.2% in female mice given 1.2%. Piperonyl butoxide is thus a hepatocarcinogen to mice as it is known to be to rats.

Interaction between fenbendazole and piperonyl butoxide: pharmacokinetic and pharmacodynamic implications

The effect of the cytochrome P450 inhibitor, piperonyl butoxide on the pharmacokinetics and anthelmintic efficacy of the benzimidazole compound fenbendazole was studied in sheep and goats. Pretreatment of goats with the inhibitor caused a greater than three-fold increase in the relative bioavailability of fenbendazole and fenbendazole sulphoxide. A pharmacokinetic dose titration study was carried out in sheep with fenbendazole (5 mg kg-1) and piperonyl butoxide administered orally at 0, 15, 31, 63, 125 and 250 mg kg-1. The AUC of fenbendazole and the sulphoxide were significantly increased when fenbendazole was co-administered with piperonyl butoxide at dose rates equal to or higher than 31 mg kg-1. Peak plasma concentrations (Cmax) and mean residence time (MRT) were also significantly increased. The efficacy of the combination was assessed in sheep against two species of benzimidazole-resistant abomasal nematodes; Ostertagia circumcincta and Haemonchus contortus. The percentage reduction in the total number of O. circumcincta worms was 7.9% (fenbendazole) and 97.8% (fenbendazole-piperonyl butoxide). For H. contortus, the percentage reduction was 84.8% (fenbendazole) and 99.0% (fenbendazole-piperonyl butoxide). The in-vitro S-oxidation of fenbendazole and fenbendazole sulphoxide was studied using microsomal preparations from rat liver. Piperonyl butoxide inhibited significantly the sulphoxidation and sulphonation of fenbendazole. It was concluded that piperonyl butoxide inhibited the oxidative conversion of fenbendazole into inactive metabolites and this resulted in a potentiated anthelmintic action.

Biphasic responses in synergistic interactions

Synergistic reactions, as exemplified by the methylenedioxyphenyl (benzodioxole) insecticide synergists, are one of the more studied interactions between toxicants. This group of chemicals includes, in addition to synergists such as piperonyl butoxide, carcinogens such as safrole and isosafrole and many compounds occurring naturally in foods, such as myristicin and piperine. These compounds may function as cytochrome P450 substrates, inhibitors and/or inducers. The biphasic curve in cytochrome P450 activities following a single dose is the result of an initial inhibitory phase followed by a phase of induction, with an eventual return to baseline levels. Both inhibition and induction may be isozyme-specific and different isozymes may be involved in the two activities of the same chemical. Details of these activities are presented and their significance is discussed.

Piperonyl butoxide as a tool in aquatic toxicological research with organophosphate insecticides

Experiments were conducted to determine the effects of piperonyl butoxide, a synthetic methylenedioxyphenyl inhibitor of cytochrome(s) P450, on the toxicity of organophosphate insecticides to three cladoceran test species: Ceriodaphnia dubia. Daphnia magna, and Daphnia pulex. Coadministration of piperonyl butoxide effectively reduced the acute toxicity of four metabolically activated organophosphates (parathion, methyl parathion, diazinon, and malathion) and did not affect the toxicity of three organophosphates not requiring metabolic activation (dichlorvos, chlorfenvinphos, and mevinphos). These results indicate that piperonyl butoxide may be an effective tool in toxicological research focused upon identifying specific compounds responsible for toxicity in complex aqueous mixtures.

Drug metabolism components in liver microsomes from Hypostomus punctatus, a Brazilian benthic fish (Cascudo)

1. The major components of hepatic drug biotransformation system were identified in a Brazilian freshwater benthic fish. 2. Cytochrome P-450 difference spectra were obtained adding 0.02 mM phenazine ethosulphate and 2 mM ascorbate to microsomal suspensions. Basal levels of P-450 were high (0.9 nmol/mg of microsomal protein) and were not induced by 3-MC. 3. Microsomal NADPH-cytochrome C reductase activity was determined in presence of 1.3 x 10(-4) M NADPH, 3.3 x 10(-5) M cytochrome C, 1.0 x 10(-4) M EDTA, 66 micrograms of microsomal protein per ml in a 0.3 M Tris-HCl buffer, pH 8.6. Basal levels of NADPH-cytochrome C were 152.7 nmoles/min/mg of microsomal protein.

The effect of piperonyl butoxide on hepatic cytochrome P-450-dependent monooxygenase activities in rainbow trout (Salmo gairdneri)

Although piperonyl butoxide (PBO) is commonly used both in vivo and in vitro as an inhibitor of cytochrome P-450-dependent monooxygenase (MO) activity in a wide variety of species, the effect of PBO on the hepatic MO of fishes has never been characterized. The MO activity in hepatic microsomes from rainbow trout exposed to either 1 or 2 ppm PBO for 24 hr in a static system was induced to a similar level in both treatment groups. Conversely, when PBO was administered in a flow-through system to trout, the hepatic microsomes of treated animals contained MO activities that were induced in a dose-dependent manner. Furthermore, total cytochrome P-450 was significantly increased in the livers of trout treated in a flow-through system with 1 ppm or more of PBO. The in vitro inhibition kinetics of PBO toward the 7-ethoxycoumarin-O-deethylase (ECOD) and 7-ethoxyresorufin-O-deethylase (EROD) activities of hepatic microsomes from trout treated with beta-naphthoflavone (BNF) (100 mg/kg, ip) or PBO (4 ppm by flow-through) and untreated trout were compared with Dixon plots. With respect to ECOD activity, the slopes of Dixon plots from control, BNF- and PBO-treated animals were similar. However, the slopes of Dixon plots of EROD inhibition by PBO in microsomes from BNF- and PBO-treated trout were significantly different from each other. Treatment of trout with PBO in a flow-through system resulted in an increase in ECOD and EROD activity in hepatic microsomes while simultaneously decreasing their activity toward [14C]rotenone oxidation. These data suggest that the cytochrome P-450 isozyme composition in hepatic microsomes from PBO-treated rainbow trout may be qualitatively different from that of BNF-treated trout. Also, the activity of hepatic microsomes from PBO-treated trout toward a specific substrate may be either inhibited or induced.

Uroporphyrin accumulation produced by halogenated biphenyls in chick-embryo hepatocytes. Reversal of the accumulation by piperonyl butoxide

Cultures of chick-embryo hepatocytes were used to study the mechanism by which 3,4,3',4'-tetrachlorobiphenyl and 2,4,5,3',4'-pentabromobiphenyl cause accumulation of uroporphyrin. In a previous paper, an isoenzyme of cytochrome P-450 induced by 3-methylcholanthrene had been implicated in this process [Sinclair, Bement, Bonkovsky & Sinclair (1984) Biochem. J. 222, 737-748]. Cells treated with 3,4,3',4'-tetrachlorobiphenyl and 5-aminolaevulinate accumulated uroporphyrin and heptacarboxyporphyrin, whereas similarly treated cells accumulated protoporphyrin immediately after piperonyl butoxide was added. Piperonyl butoxide also restored haem synthesis as detected by incorporation of radioactive 5-aminolaevulinate into haem, and decrease in drug-induced 5-aminolaevulinate synthase activity. The restoration of synthesis of protoporphyrin and haem by piperonyl butoxide was not affected by addition of cycloheximide, indicating recovery was probably not due to protein synthesis de novo. Piperonyl butoxide also reversed uroporphyrin accumulation caused by 3,4,5,3',4',5'-hexachlorobiphenyl, mixtures of other halogenated biphenyls, lindane, parathion, nifedipine and verapamil. The effect of piperonyl butoxide was probably not due to inhibition of metabolism of these compounds, since the hexachlorobiphenyl was scarcely metabolized. Other methylenedioxyphenyl compounds, as well as ellipticine and acetylaminofluorene, also reversed the uroporphyrin accumulation caused by 3,4,3',4'-tetrachlorobiphenyl. SKF-525A (2-dimethylaminoethyl-2,2-diphenyl valerate) did not reverse the uroporphyrin accumulation caused by the halogenated biphenyls, but did reverse that caused by phenobarbital and propylisopropylacetamide. We conclude that the mechanism of the uroporphyrin accumulation cannot be due to covalent binding of activated metabolites of halogenated compounds to uroporphyrinogen decarboxylase.

Exposure to toxic agents: the heme biosynthetic pathway and hemoproteins as indicator

The heme biosynthetic pathway is closely controlled by levels of the end product of the pathway, namely, heme, and porphyrins are normally formed in only trace amounts. When control mechanisms are disturbed by xenobiotics, porphyrins accumulate and serve as a signal of the interaction between a xenobiotic and the heme biosynthetic pathway. For example, an increase in erythrocyte protoporphyrin is a useful measurement for early detection of exposure to lead and porphyrinuria was an early manifestation of a hexachlorobenzene-induced porphyria in Turkey. In recent years a variety of additional xenobiotics has been shown to interact with the heme biosynthetic pathway, namely, halogenated aromatic hydrocarbons, pesticides, sulfides, and a variety of metals. Moreover, different xenobiotics (e.g., dihydropyridines and compounds containing unsaturated carbon-carbon bonds) interact with the prosthetic heme of cytochrome P-450 forming novel N-alkylporphyrins.

Suicidal destruction of cytochrome p-450 by ethynyl substituted compounds

Compounds containing a terminal carbon-carbon triple bond, ranging in structure from the 17α-ethynyl substituted contraceptive steroids to acetylene gas, when administered to rats cause a selective and rapid time dependent loss of up to 50% of hepatic cytochrome P-450. Cytochrome b5 is not affected. Metabolic activation of the acetylenic substituent by the phenobarbital inducible, NADPH dependent, mixed function oxidases results in the formation of a reactive species which alkylates one of the tetrapyrrole nitrogen atoms of heme to form a 1 : 1 covalent adduct. Either cytochrome P-450 destruction or the formation of N-alkylated porphyrins (green pigments) have been used to assess factors affecting the extent of metabolic activation of the acetylenic group in rats, man, and other laboratory species. The chemical identity of a number of green pigments have been resolved. Those formed from 1-octyne consist of the protoporphyrin IX ring of heme substituted with the saturated 2-oxo-octyl group. In contrast, reactive metabolites of 1-octyne trapped with N-acetylcysteine contain the unsaturated 3-oxo-octenyl substituent. Two independent routes of activation of terminal acetylenes have been described to account for these results. Both pathways can lead to cytochrome P-450 loss but only one, probably involving an oxirene intermediate, leads to green pigment formation.

Effects of dihydrosafrole on cytochromes P-450 and drug oxidation in hepatic microsomes from control and induced rats

Changes in cytochromes P-450, aminopyrine N-demethylase (APDM), aromatic hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), and type III spectral complex formation were measured in hepatic microsomes of control, phenobarbital (PB)-, and beta-naphthoflavone (beta NF)-induced rats after a single dose of dihydrosafrole (4-n-propyl-1,2-methylenedioxybenzene, DHS). Time profiles of changes in these microsomal parameters were complex and showed that APDM activities and cytochrome P-450 levels decreased immediately after treatment and were associated with concurrent increases in the intensity of the type III methylenedioxyphenyl (MDP) metabolite/cytochrome P-450 spectral complex. In noninduced rats, both APDM activity and cytochrome P-450 levels returned to control levels between 12 and 24 hr after treatment with DHS and subsequently increased above control levels. In PB- and beta NF-induced animals, the inhibitory phases were more prolonged and activity never returned to levels higher than the corresponding controls. AHH activity was increased substantially (two- to three-fold) in all cases after DHS administration. Although displacement of the MDP metabolite/cytochrome P-450 complex with 2-methylbenzimidazole generally led to a marked restoration of cytochrome P-450 levels and partially reversed the inhibition of APDM, it had little or no effect on AHH activities.