The role of substrate lipophilicity in determining type 1 microsomal P450 binding characteristics

Heterologous expression and characterization of plant Taxadiene-5α-Hydroxylase (CYP725A4) in Escherichia coli

Taxadiene-5α-Hydroxylase (CYP725A4) is a membrane-bound plant cytochrome P450 that catalyzes the oxidation of taxadiene to taxadiene-5α-ol. This oxidation is a key step in the production of the valuable cancer therapeutic and natural plant product, taxol. In this work, we report the bacterial expression and purification of six different constructs of CYP725A4. All six of these constructs are N-terminally modified and three of them are fused to cytochrome P450 reductase to form a chimera construct. The construct with the highest yield of CYP725A4 protein was then selected for substrate binding and kinetic analysis. Taxadiene binding followed type-1 substrate patterns with an observed K_D of 2.1 ± 0.4 μM. CYP725A4 was further incorporated into nanoscale lipid bilayers (nanodiscs) and taxadiene metabolism was measured. Taxadiene metabolism followed Michaelis-Menten kinetics with an observed V_max of 30 ± 8 pmol/min/nmol_CYP725A4 and a K_M of 123 ± 52 μM. Additionally, molecular operating environment (MOE) modeling was performed in order to gain insight into the interactions of taxadiene with CYP725A4 active site. Taken together, we demonstrate the successful expression and purification of the functional membrane-bound plant CYP, CYP725A4, in E. coli.

In Silico Prediction of Cytochrome P450-Drug Interaction: QSARs for CYP3A4 and CYP2C9

Cytochromes P450 (CYP) are the main actors in the oxidation of xenobiotics and play a crucial role in drug safety, persistence, bioactivation, and drug-drug/food-drug interaction. This work aims to develop Quantitative Structure-Activity Relationship (QSAR) models to predict the drug interaction with two of the most important CYP isoforms, namely 2C9 and 3A4. The presented models are calibrated on 9122 drug-like compounds, using three different modelling approaches and two types of molecular description (classical molecular descriptors and binary fingerprints). For each isoform, three classification models are presented, based on a different approach and with different advantages: (1) a very simple and interpretable classification tree; (2) a local (k-Nearest Neighbor) model based classical descriptors and; (3) a model based on a recently proposed local classifier (N-Nearest Neighbor) on binary fingerprints. The salient features of the work are (1) the thorough model validation and the applicability domain assessment; (2) the descriptor interpretation, which highlighted the crucial aspects of P450-drug interaction; and (3) the consensus aggregation of models, which largely increased the prediction accuracy.

Factors affecting phase I stereoselective biotransformation of chiral polychlorinated biphenyls by rat cytochrome P-450 2B1 isozyme

In vitro incubations of rat cytochrome P-450 (CYP) 2B1 isozyme with three chiral polychlorinated biphenyl (PCB) congeners (PCBs 45, 95, and 132) were performed to investigate factors affecting phase I stereoselective biotransformation. Rat CYP2B1 preferentially biotransformed the second-eluting atropisomers of PCBs 45 and 95 at low substrate concentration ranges (≤15 μM). Biotransformation competition by different congeners was also observed, with increasing competition at higher chlorination. Competition decreased the biotransformation rates of each congener stereoselectively, affecting atropisomeric composition. No atropisomeric enrichment was observed for PCB 132 upon incubation of the racemate. However, under the same conditions, significant differences in biotransformation kinetics were observed in individual atropisomer incubations, indicating that (+)-PCB 132 and (-)-PCB 132 were competitively biotransformed. Homology modeling and docking studies suggested that each atropisomer had different interactions with rat CYP2B1 and could dock with the isozyme at different locations. This is one possible explanation for stereoselective biotransformation and competition of chiral PCBs at the molecular level. Our results suggest that the lack of predictive capability for stereoselectivity of PCBs and other chiral pollutants in biota may be due to competitive and/or inhibitory activities of different substrates, including individual enantiomers of the same compound.

Structure-activity relationships for the inhibition of recombinant human cytochromes P450 by curcumin analogues

Inhibition of cytochrome P450 (CYP) is a major cause of drug-drug interactions. In this work, inhibitory potentials of 33 curcumin analogues, i.e. 2,6-dibenzylidenecyclohexanone (A series), 2,5-dibenzylidenecyclopentanone (B series) and 1,4-pentadiene-3-one (C series) substituted analogues of curcumin towards recombinant human CYP1A2, CYP3A4, CYP2B6, CYP2C9 and CYP2D6, all important for drug metabolism, were studied in vitro. Fluorescence plate reader and high performance liquid chromatography (HPLC) assays were used to evaluate CYP-inhibitory activities. MOE-based Quantitative structure-activity relationship (QSAR) analysis suggested that electrostatic and hydrophobic interactions and lipophilicity are important factors for CYP inhibition. Apart from insights in important molecular properties for CYP inhibition, the present results may also guide further design of curcumin analogues with less susceptibility to drug-drug interactions.

Quantitative structure-activity relationships (QSARs) within cytochromes P450 2B (CYP2B) subfamily enzymes: the importance of lipophilicity for binding and metabolism

The results of qualitative structure-activity relationship (QSAR) analysis are reported for several series of compounds which act as substrates for mammalian CYP2B subfamily enzymes, together with a homologous series of aliphatic primary amines which are known to inhibit CYP2B enzymes. It is found that the compound lipophilicity in the form of the log P value (where P is the octanol/water partition coefficient) is related, either linearly or quadratically, to equilibrium constants of inhibition (Ki), binding (Ks) or metabolism (Km) depending on the series of compounds in question. In some instances, the difference between frontier orbital energy levels (deltaE) also features in several of the log P expressions with biological activity. Also present in a small number of correlations are parameters which are likely to be related to logP: namely, Rm, which is the partitioning factor derived from thin layer chromatography (TLC) retention times, and also the compound molecular weight (Mr). All of these three parameters ((log P, Rm and Mr) are thought to be related to the compound's ability to desolvate the P450 active site when they bind to the enzyme. Although the linear relationships between lipophilicity and CYP2B-related activity point to a major role for desolvation of the enzyme binding site in the overall interaction, it is noted that there may be an optimal log P value displayed by preferred substrates as shown by parabolic relationships with this lipophilic parameter. In addition, there is a remarkable similarity in the coefficients for the log P term of any QSAR expression, which suggests that the hydrophobicity of CYP2B active sites may be broadly equivalent between the various mammalian species.

Compound lipophilicity for substrate binding to human P450s in drug metabolism

Compound lipophilicity is of key importance to P450 binding affinity and enzyme selectivity. Here, lipophilicity is discussed with reference to the human drug-metabolizing P450 enzymes of families CYP1, CYP2 and CYP3. From an extensive compilation of log P values for P450 substrates, and by analysis of relationships between partitioning energy and substrate-binding free energy, the relevance of lipophilicity and other factors pertaining to P450 binding affinity is explained, leading to the formulation of lipophilicity relationships within substrates of each human P450 enzyme involved in drug metabolism. Furthermore, log P values for P450 substrates appear to represent markers for enzyme selectivity. Together with the important roles of hydrogen bonding and pi-pi stacking interaction energies, the desolvation of the P450 active site makes a major contribution to the overall substrate-binding energy and, consequently, a good agreement with experimental information is reported based on this analysis.

Spectral interactions of tetrachlorobiphenyls with hepatic microsomal cytochrome p450 enzymes

This study investigated the spectral interactions of hepatic microsomal cytochrome p450 (CYP) enzymes with four symmetrical polychlorinated biphenyls (PCBs): 2,2',4,4'-tetrachlorobiphenyl (PCB 47); 2,2',5,5'-tetrachlorobiphenyl (PCB 52); 2,2',6,6'-tetrachlorobiphenyl (PCB 54); and 3,3',4,4'-tetrachlorobiphenyl (PCB 77). The PCBs were selected to explore structure-activity relationships and the effect of the chlorination pattern on PCB-CYP spectral interactions. To examine CYP enzyme specificity, difference spectra were measured with hepatic microsomes prepared from control, phenobarbital (PB)-, 3-methylcholanthrene (MC)-, and dexamethasone (DEX)-treated rats in the absence and presence of CYP-specific antibodies. The four PCB congeners elicited a type I spectral change with all hepatic microsomal preparations. The binding efficiency of the PCBs was highest with microsomes from PB-treated rats. The largest absorbance change and highest binding efficiency were observed with PCB 54, the most non-coplanar congener tested. Antibody inhibition and CYP immunoquantitation data showed that the PCBs bind to CYP1A, CYP2B, CYP2C and CYP3A enzymes to varying degrees. For example, PCB 47, 52, and 54 bind preferentially to CYP2B and to a lesser extent to CYP3A enzymes in microsomes from PB-treated male rats; PCB 52 binds primarily to CYP3A enzymes in microsomes from DEX-treated female rats; and PCB 54 binds to CYP3A and to CYP2C enzymes in microsomes from control male rats. The study demonstrated that the extent of PCB-CYP binding interaction was dependent on the chlorination pattern of the PCB and on the relative abundance of individual CYP enzymes in hepatic microsomes.

Substrate access channel topology in membrane-bound prostacyclin synthase

Results from our molecular-modelling and site-directed-mutagenesis studies of prostaglandin I(2) synthase (PGIS) have suggested that the large PGIS cytoplasmic domain is anchored to the endoplasmic reticulum (ER) membrane by the N-terminal segment in a way that orients the substrate access channel opening to face the membrane. To test this hypothesis we have explored the accessibility of the PGIS substrate channel opening to site-specific antibodies. The working three-dimensional PGIS model constructed by protein homology modelling was used to predict surface portions near the substrate access channel opening. Two peptides corresponding to the surface immediately near the opening [residues 66-75 (P66-75) and 95-116 (P95-116)], and two other peptides corresponding to the surface about 10-20 A (1 A=0.1 nm) away from the opening [residues 366-382 (P366-382) and 472-482 (P472-482)] were used to prepare site-specific antibodies. All four antipeptide antibodies specifically recognized the synthetic segments of human PGIS and recombinant PGIS, as shown by binding assays and Western-blot analysis. The site-specific antibodies were used to probe the accessibility of the substrate access channel opening in transiently transfected COS-1 cells expressing recombinant human PGIS, and in spontaneously transformed human endothelial cell line ECV cells expressing endogenous human PGIS. Immunofluorescence staining was performed for cells selectively permeabilized with streptolysin O and for cells whose membranes were permeabilized with detergent. Antibodies to peptides in the immediate vicinity of the substrate channel (P66-75 and P95-116) bound to their targets only after general permeabilization with Triton X-100. In contrast, the two antibodies to peptides further from the channel opening (P366-382 and P472-482) bound to their targets even in cells with intact ER membranes. These observations support our topology model in which the PGIS substrate access channel opening is positioned close to the ER membrane.

Hydroxylation of limonene enantiomers and analogs by recombinant (-)-limonene 3- and 6-hydroxylases from mint (Mentha) species: evidence for catalysis within sterically constrained active sites

Limonene enantiomers and substrate analogs, including specifically fluorinated derivatives, were utilized to probe active site interactions with recombinant (-)-(4S)-limonene-3-hydroxylase (CYP71D13) and (-)-(4S)-limonene-6-hydroxylase (CYP71D18) from mint (Mentha) species. (-)-(4S)-Limonene is hydroxylated by both enzymes at the designated C3- and C6-allylic positions, with strict regio- and stereospecificity and without detectable allylic rearrangement, to give the corresponding products (-)-trans-isopiperitenol and (-)-trans-carveol. CYP71D13-catalyzed hydroxylation of (+)-(4R)-limonene also yields the corresponding trans-3-hydroxylated product ((+)-transisopiperitenol); however, the C6-hydroxylase converts (+)-(4R)-limonene to a completely different product profile dominated by the enantiopure cis-6-hydroxylated product (+)-cis-carveol along with several minor products, including both enantiomers of the trans-6-hydroxylated product ((+/-)-trans-carveol), indicating allylic rearrangement during catalysis. These results demonstrate that the regiospecificity and facial stereochemistry of oxygen insertion is dictated by the absolute configuration of the substrate. Fluorinated limonene analogs are also tightly bound by both enzymes and hydroxylated at the topologically congruent positions in spite of the polarizing effect of the fluorine atom on substrate reactivity. This strict retention of oxygenation geometry suggests a rigid substrate orientation imposed by multiple hydrophobic active site contacts. Structurally simplified substrate analogs are hydroxylated at slower rates and with substantial loss of regiospecificity, consistent with a loss of active site complementarity. Evaluation of the product profiles generated allowed assessment of the role of hydrophobic contacts in orienting the substrate relative to the activated oxygen species.

The rate-determining step in P450 C21-catalyzing reactions in a membrane-reconstituted system

Adrenal cytochrome P450 C21 in a membrane-reconstituted system catalyzed 21-hydroxylation of 17alpha-hydroxyprogesterone at a rate higher than that for progesterone in the steady state at 37 degrees C. The rate of product formation in the steady state increased with the concentration of the complex between P450 C21 and the reductase in the membranes. The complex formation was independent of the volume of the reaction, showing that the effective concentrations of the membrane proteins should be defined with the volume of the lipid phase. The rates of conversion of progesterone and 17alpha-hydroxyprogesterone to the product in a single cycle of the P450 C21 reaction were measured with a reaction rapid quenching device. The first-order rate constant for the conversion of progesterone by P450 C21 was 4.3 +/- 0.7 s(-)1, and that for 17alpha-hydroxyprogesterone was 1.8 +/- 0.5 s(-)1 at 37 degrees C. It was found from the analysis of kinetic data that the rate-determining step in 21-hydroxylation of progesterone in the steady state was the dissociation of product from P450 C21, whereas the conversion to deoxycortisol was the rate-determining step in the reaction of 17alpha-hydroxyprogesterone. The difference in the rate-determining steps in the reactions for the two substrates was clearly demonstrated in the pre-steady-state kinetics.

Interaction of 7-n-alkoxycoumarins with cytochrome P-450(2) and their partitioning into liposomal membranes. Assessment of methods for determination of membrane partition coefficients

A study was made of the binding of 7-ethoxy-, 7-n-propoxy- and 7-n-pentoxy-coumarin to cytochrome P-450(2) reconstituted into large unilamellar liposomes composed of a mixture of egg L-alpha-phosphatidylcholine, egg phosphatidylethanolamine and dipalmitoyl phosphatidic acid (2:1:0.06, by weight). The apparent spectral dissociation constants Ksapp. increased linearly with increasing proteoliposomal concentration. When both cytochrome P-450(2) and NADPH:cytochrome P-450 reductase were reconstituted into liposomes, the apparent Michaelis constants Kmapp. for O-dealkylation of 7-methoxy-, 7-ethoxy- and 7-n-propoxy-coumarin showed a similar dependence on the proteoliposomal concentration. The results were in accordance with models for kinetic or equilibrium processes in biphasic systems containing membrane-bound catalytic or acceptor sites, in which a linear solute partition in the bilayer membrane is postulated. The methyl, ethyl and n-propyl ether were readily dealkylated. However, the O-dealkylation rate of 7-n-butoxycoumarin was low and became very small for longer alkyl ethers. Both the effective dissociation constants and effective Michaelis constants decreased with elongation of the alkyl side chain of the coumarins. From plots of the apparent dissociation constants and apparent Michaelis constants against the lipid volume fraction of the proteoliposomes, the membrane partition coefficients for several homologues were calculated. When protein-free liposomes were added to 7-n-alkoxycoumarin solutions, the fluorescence intensity of the coumarins decreased and eventually became negligible in the presence of an excess of liposomal material. On the assumption that the overall fluorescence can be ascribed exclusively to the fraction of 7-n-alkoxycoumarin molecules present in the aqueous phase, partition coefficients for liposomal accumulation of the test compounds could be determined directly. For several coumarin ethers, comparable values were derived for the membrane partition coefficients from binding, kinetic and fluorescence intensity measurements. The change in free energy per methylene group of the 7-n-alkoxycoumarins for partitioning between n-octanol and buffer was significantly different from the value for liposome partitioning.

In vitro covalent binding of new brain tracer, para-125I-amphetamine, to rat liver and lung microsomes

p-125I-amphetamine (I-Amp) is retained significantly in liver and lung during brain tomoscintigraphy. To attempt to explain this clinical observation, we have investigated the interaction of I-Amp with rat liver and lung microsomal proteins. Studies using spectral shift technique indicate that low concentration of I-Amp gives a type I complex and high concentration appears very stable type II complex with cytochrome P-450 Fe III. In the presence of NADPH, I-Amp gives rise to a 455 nm absorbing complex with similar properties to the Fe-RNO complexes. This complex formation was greatly enhanced with phenobarbital treated liver microsomes. The in vitro binding study shows that I-Amp and/or its metabolites was covalently bound to macromolecules in the presence of the molecular oxygen and NADPH-generating system. Incubation in the presence of glutathione, cystein and radical scavengers decreases binding. Mixed function oxydase (MFO) inhibitors diminish the amount of covalent binding and alter the extent of metabolite formation. The total covalent binding level increased with liver microsomes from PB pretreated rats as it was observed with the 455nm complex formation. The radioactivity distribution on microsomal proteins was examinated with SDS polyacrylamide gel electrophoresis and autoradiography. This experiment proves that the radiolabelled compounds are bound on the cytochrome P-450. The radioactivity bound increased when the PB induced rat liver microsomes were used. All these results indicate that I-Amp was activated by an oxydative process dependent on the MFO system which suggests a N-oxydation of I-Amp and the formation of reactive entities which covalently bind to proteins.

Interactions of the anti-tumor ametantrone and mitoxantrone with rat hepatic microsomes

The interaction of the anti-tumor anthraquinones, ametantrone and mitoxantrone, with rat hepatic microsomes has been studied with a fluorescence technique using 7,12-dimethylbenzanthracene as a new fluorescent probe. The two drugs were able to quench the intrinsic fluorescence of microsomal suspension. Mitoxantrone was able to displace dimethylbenzanthracene bound microsomes with a linear representation of one ligand-one acceptor model, whereas bimodal shape was found in the case of ametantrone. The mechanism of quenching and/or binding is discussed.

Comparative effects of antithrombitic and antimycotic N-substituted imidazoles on rat hepatic microsomal steroid and xenobiotic hydroxylases in vitro

N-Substituted imidazoles have been shown to be potent inhibitors of microsomal mixed-function oxidase activities in vitro and in vivo. In the present study the effects of two antithrombitic (dazmegrel and dazoxiben) and four antimycotic (ketoconazole, econazole, miconazole and clotrimazole) imidazoles on microsomal cytochrome P-450-mediated steroid and xenobiotic hydroxylases were studied in vitro. Despite the presence of the N-substituted imidazole moiety, the antithrombitic agents were essentially non-potent as inhibitors of all of the oxidase activities evaluated. In contrast, the antimycotic drugs were potent inhibitory compounds. Binding studies revealed that all six imidazoles elicited type II optical difference spectra and exhibited relatively high affinity for ferricytochrome P-450 in microsomal suspensions (Ks range 0.26-0.73 microM for the antimycotic agents and 6.5 microM and 21 microM for dazmegrel and dazoxiben, respectively). The structural feature that the antithrombitic compounds share is a carboxylate function so that, at physiological pH, less than 1% of the drug would be present in the unionised form. This functionality is absent from the structures of the antimycotic agents which possess much greater hydrophobic character. Even though the antithrombitic imidazoles elicit type II binding interactions of quite high affinity it would appear from this study that significant inhibition potency does not necessarily follow. The present findings also suggest that interesting differences exist between the active site binding regions in the cytochrome P-450 that catalyse thromboxane synthetase activity and those involved in microsomal drug oxidation. Inhibitor hydrophobicity is clearly an important factor in the inhibition of microsomal cytochromes P-450 whereas effective thromboxane synthetase inhibitors may be quite hydrophilic at physiological pH.

Identification of structural characteristics of some potential H2-receptor antagonists that determine the interaction with rat hepatic P-450

Several potential H2-receptor antagonists have been tested in vitro, using liver microsomal preparations from untreated rats, in order to study their interaction with P-450. The aim of this investigation was to establish structure-activity relationships for the P-450-inhibition developed by cimetidine and related drugs. Most of the compounds tested demonstrate an inhibitory activity and a binding ability to P-450, via type II (ligand type) binding. Our results strongly indicate that the cyano-guanidine moiety is an essential structural feature for both the inhibition of a ferrocytochrome P-450-metabolic intermediate complex formation occurring during the metabolism of tofenacine, and the binding of the compounds to the heme iron of P-450. The presence of an imidazole group is not necessary for these activities. Furthermore, it is pointed out that the lipophilic character of the cyano-guanidine side chain contributes to the interaction of the test compounds with P-450, since a trend for a parabolic relationship between lipophilicity and inhibitory activity or binding ability is observed. Finally, under the experimental conditions used, no increase of the inhibitory activity of cimetidine on the metabolism of tofenacine and 7-ethylresorufin is observed after preincubation of rat liver microsomes with cimetidine, confirming earlier results in similar studies.

In vitro effects of polyhalogenated hydrocarbons on liver mitochondria respiration and microsomal cytochrome P-450

The present study evidenced the critical levels of six major polyhalogenated hydrocarbons (PHH's), namely chloroform, carbon tetrachloride, 1,1,1-trichloroethane, 1,2-dibromoethane,perchloroethylene, hexachlorobutadiene, over which significant inhibitory effects of the mitochondrial respiratory chain take place in vitro. At these critical levels, even in PB-induced animals only a very little fraction of cytochrome P-450 is saturated by the compounds and therefore the microsomal metabolism plays no effective role either in decreasing the levels of the test chemicals under the threshold of clear direct adverse effects in mitochondria, nor to the formation of toxic metabolites. Our data show also that phenobarbital not only enhances both the direct and metabolism-mediated interaction of most tested PHH with microsomal cytochrome P-450, but also increases the affinity of hexachlorobutadiene, chloroform and carbon tetrachloride for the mitochondrial sites resulting in respiration inhibition.

New heterocyclic modifiers of oxidative drug metabolism--II. Steric factors in the interaction of isomeric 2-(naphthyl)methylbenzimidazoles with rat hepatic microsomal cytochrome P-450 and monooxygenase activities

The inhibitory potency of the two isomeric 2-(naphthyl)methylbenzimidazoles towards three monooxygenase activities (aminopyrine N-demethylase, 7-ethoxycoumarin O-deethylase and aniline p-hydroxylase) was assessed in hepatic microsomal fractions from untreated, phenobarbitone-induced and beta-naphthoflavone-induced rats. The isomers were essentially equipotent with each other as inhibitors of the phenobarbitone-induced monooxygenases (the ratio of the I50s of the isomers was about 1.0 in each case) but differences between the isomers were noted in the inhibition potencies against three monooxygenase activities from beta-naphthoflavone-induced liver. The isomer 2-(1'-naphthyl)methylbenzimidazole was approximately twice as potent as the 2'-naphthyl isomer against 7-ethoxyresorufin O-deethylase activity, whereas the opposite was observed with respect to 7-ethoxycoumarin O-deethylase inhibition; aniline p-hydroxylase was poorly inhibited by both isomers. The binding affinity and extent of binding, assessed from double-reciprocal plots of spectral binding studies, of the 1'-isomer was much greater than that of the 2'-isomer in beta-naphthoflavone-induced microsomes. Inhibition data in untreated hepatic microsomes were more complex and the finding of principal interest was that the 1'-isomer was poorly inhibitory towards aniline p-hydroxylase activity whereas the 2'-isomer enhanced this activity. These studies suggest that the steric conformations of the isomeric naphthylmethylbenzimidazoles at the cytochrome P-450 active centre determines the extent to which the inhibitors modulate a specific monooxygenase activity, and that multiple binding sites with the capacity to interact to different extents with benzimidazole derivatives are present in P-450 in beta-naphthoflavone-induced hepatic microsomes. The apparent importance of steric conformation as a determinant of inhibition and enhancement of aniline p-hydroxylase in untreated microsomal fractions may well reflect specific interactions with multiple binding sites.

Inhibition of hepatic microsomal monooxygenase activity by cinchocaine: mechanistic studies and effects of ionization

The quinoline-based local anaesthetic cinchocaine (dibucaine) was found to be a mixed-type inhibitor of microsomal aminopyrine N-demethylase and 7-ethoxycoumarin O-deethylase activities from control and phenobarbitone-induced rat liver in-vitro. Cinchocaine also elicited a characteristic type I optical difference spectrum in oxidized liver microsomes (Ks = 24 microM; delta Amax = 3.4 X 10(-3) absorbance units (nmol cytochrome P450)-1) but did not appear to bind to the reduced form of the cytochrome. Additional studies indicated that cinchocaine competitively inhibited the type I spectral binding of substrate (aminopyrine) to ferric cytochrome P450. Studies of monooxygenase inhibition by cinchocaine over a relatively narrow pH range (6.5-8.5) indicated that, as might be expected, the un-ionized form of the drug is associated with inhibitory potency superior to that of the ionized form. Thus 40% inhibition of aminopyrine N-demethylase activity was observed with 100 microM cinchocaine at pH 8.0 and 8.5 (24% and 50% un-ionized drug, respectively), whereas only 16% inhibition was observed at pH 6.5 (1% un-ionized drug). These findings suggest that the inhibitory action of cinchocaine is mediated exclusively via an interaction with ferric cytochrome P450 and that the extent of ionization is a determinant of mixed function oxidase inhibition.

Relation between the structure of benzphetamine analogues and their binding properties to cytochrome P-450 LM2

Twelve substrates of a homologous series of tertiary amines (type I substrates) have been reacted with cytochrome P-450 LM2 incorporated into unilamellar liposomes and in soluble form. The apparent spectral dissociation constants (Ks) of the substrate enzyme complexes and the induced high-spin shifts have been correlated with the electron density of distinct carbon atoms as monitored by 13C-NMR chemical shifts, the solubility of the amines and steric parameters of the substrate molecules. The results obtained led to the conclusion that two different intrinsic properties of the substrates can be discriminated in relation to the substrate-enzyme interaction. A diminished electron density at the nitrogen atom is accompanied by an increased binding affinity. The steric structure of the respective substrate determines its capability to shift the spin equilibrium to the high-spin state. Some characteristics of the active center of the enzyme are derived from the evidenced properties of the substrates.

In vivo and in vitro release of cyanide from neurotoxic aminonitriles

Cyanide release from neurotoxic aminonitriles was measured following in vitro incubation with both microsomes and liver slices. Investigation of cyanide released as urinary thiocyanate following ip aminonitrile administration to rats was also measured. The yield of cyanide in the in vivo study, as measured by the mole percent of administered dose, was greatest from dimethylaminonitrile (DMAA), followed by trimethylaminopropionitrile (TMAPN), dimethylaminopropionitrile (DMAPN), 3,3'-iminodipropionitrile (IDPN), dimethylaminobutyronitrile (DMABN), and monomethylaminopropionitrile (MMAPN). Urinary excretion of thiocyanate accounted for 48.9% of the administered DMAA, 11.6% of TMAPN, 8.0% of DMAPN, 6.8% of IDPN, 3.1% of DMABN, and 1.8% of MMAPN. Incubation of aminonitriles and related compounds with microsomes or liver slices from rats yielded measurable quantities of cyanide from all the compounds tested except for DMABN, TMABN, and succiononitrile. Quantitative evaluation of the yield of formaldehyde by demethylation following microsomal incubation was also determined. The signs of acute toxicity in rats after ip administration of KCN were similar only to those in rats administered DMAA.

Relationship between molecular structure and cytochrome P450-metabolic intermediate complex formation, studied with orphenadrine analogues

Complexation of ferrous cytochrome P450 by metabolic intermediates formed during NADPH-catalyzed metabolism of compounds structurally related to orphenadrine was studied. This so-called metabolic intermediate complexation was determined in rat liver microsomes, obtained from phenobarbital-pretreated rats, at 455 nm using 33 microM of the orphenadrine derivatives. Using secondary amine derivatives with various N-alkyl substituents, a parabolic relationship between the logarithm of percentage of cytochrome P450 complexation and hydrophobic fragmental constant was observed. The derivative with a bulky tertiary butyl group, however, was devoid of metabolic intermediate-complexing activity. This indicates that steric factors besides lipid solubility may govern the complexing activity; also substitution at the phenyl group affects metabolic intermediate complex formation.

Direct in vitro effects of bis(tri-n-butyltin)oxide on hepatic cytochrome P-450

Bis(tri-n-butyltin)oxide, an agriculturally important biocidal agent, when added in vitro to liver microsomes containing the phenobarbital-induced form of cytochrome P-450, produced a typical type I binding spectrum (an absorption maximum at 390 nm; an absorption minimum at 420 nm). Studies with microsomal preparations containing cytochrome P-448, induced by 3-methylcholanthrene or beta-naphthoflavone, revealed that this hemeprotein was more susceptible to direct degradation by bis(tri-n-butyltin)oxide than was the uninduced or phenobarbital-induced forms of cytochrome P-450. The disappearance of spectrally detectable cytochrome P-450 was accompanied by an increase in cytochrome P-420. The formation of cytochrome P-420 was both time and temperature dependent, and it also occurred to a greater extent in microsomal preparations containing cytochrome P-448 than in microsomes containing the phenobarbital-induced form of cytochrome P-450. In all cases, the decreases in spectrally detectable cytochrome P-450 produced by the organotin were not accompanied by decreases in microsomal heme or cytochrome b5 content. The findings provide evidence for the direct interaction followed by conversion of cytochrome P-450 to cytochrome P-420 produced by a trialkyltin compound in vitro, and indicate that different susceptibilities to degradation exist within the various subspecies of this hemeprotein.

The interaction of arylalkybenzimidazoles and related compounds with microsomal oxidation

A series of 2-arylalkyl- and 2-(4'-alkyl)phenoxymethylbenzimidazoles was synthesized and evaluated as inhibitors of mixed-function oxidase activity in phenobarbitone- and beta-naphthoflavone-induced rat liver microsomes. Higher homologues of the 2-arylalkyl series were more potent inhibitors than lower homologues against all mono-oxygenase activities except aniline p-hydroxylation. Smaller 2-substituents were associated with relatively low-affinity reverse type I spectral binding behaviour, whereas larger substituents were associated with type I binding of high affinity.

Differential effects of phenobarbitone and 3-methylcholanthrene induction on the hepatic microsomal metabolism and cytochrome P-450-binding of phenoxazone and a homologous series of its n-alkyl ethers (alkoxyresorufins)

The metabolism and cytochrome P-450-binding of phenoxazone and a homologous series of its n-alkyl ethers (1-8C) was studied in hepatic microsomes of control, phenobarbitone-pretreated (PB) and 3-methylcholanthrene-pretreated (3MC) C57/BL10 mice. Phenoxazone and its ethers were hydroxylated and O-dealkylated respectively to a common metabolite, resorufin. The three categories of microsomes differed greatly in activity for the metabolism and binding of the various substrate homologues. The most rapidly metabolised substrates for control microsomes were phenoxazone and its shortest-chain ethers, for PB microsomes phenoxazone and the pentyl ether, and for 3MC microsomes the ethyl and propyl ethers. The variations in activity occurred in Vmax rather than in the apparent Km-value. All the ethers gave Type I cytochrome P-450-binding spectra. The substrates giving the largest Type I spectra were the same for all microsomes--the ethyl, propyl and butyl ethers--but the magnitudes of the spectra differed in the order 3MC- greater than PB- greater than control microsomes. Phenoxazone and resorufin gave Modified Type II cytochrome P-450-binding spectra. PB-induction was most marked for the depentylation reaction (increased 101-fold), whereas 3MC-induction was most marked for depropylation and debutylation (88- and 96-fold). The intermicrosomal differences were interpreted as reflecting the different metabolic specificities of variant forms of cytochrome P-450. Substrate lipophilicity increased with increasing ether chain length and was not a major influence on specificity. The main substrate influence on specificity was steric, due to the presence and length of the ether side chain. The preeminent effect of ether chain length was considered to be on the rate of substrate transformation rather than on substrate interaction with cytochrome P-450.

The relationship between chemical structure and the in vivo metabolism of an homologous series of n-alkyl carbamates

The metabolism and pharmacokinetics of carbonyl [14C] labelled ethyl, n-butyl, n-hexyl and n-octyl carbamates has been examined in rats after oral and intravenous administration. Hydrolysis of the carbamate group was a major metabolic fate, particularly of the more water soluble carbamates. Conversely, with increasing lipophilicity increasing amounts of omega-1 oxidation products were found both in plasma and urine. The plasma pharmacokinetic data could not be explained by a simple bi-exponential model, ethyl carbamate in particular showing unexpectedly persistent blood levels. A model has been proposed to explain the pharmacokinetic data for ethyl, n-butyl, n-hexyl and n-octyl carbamates. The essential features of this model are that carbamate is thought not to be in equilibrium between the peripheral and central compartment and that hydrolytic metabolism takes place in the peripheral compartment while oxidative metabolism to urinary metabolites occurs in the central compartment.

The relationship between the binding of 2-n-alkylbenzimidazoles to rat hepatic microsomal cytochrome P-450 and the inhibition of monooxygenation

The binding of a homologous series of 2-n-alkylbenzimidazoles to rat hepatic microsomal cytochrome P-450 has been examined. Type I, Type RI and mixed Type I/RI spectra were observed with control, phenobarbitone or 20-methylcholanthrene-induced microsomal preparations. In general short chain (C1-C4) substituted compounds elicited Type RI spectra, whereas C5-C9 substituted benzimidazoles gave rise to Type RI/I or Type I spectra. The type of binding spectrum observed was dependent upon the substrate concentration, the source of microsomes and the length of the substituent alkyl chain. As the lipophilic character of the substituent was increased a corresponding increase in Type I nature was noted. However, an optimal chain length of C7-C8 carbon atoms was observed for Type I binding; compounds with longer side chains showed a decreased affinity for the Type I site. The apparent spectral binding constants (Ks values) for the Type I site (but not the Type RI site) were closely associated with the Ki and I50 values for the inhibition of cytochrome P-450-dependent monooxygenation. From their inhibition properties it seems that even the short chain (C1-C4) substituted benzimidazoles also bind toi the Type I site and thus compete for the substrate binding site of cytochrome P-450.

The relationship between binding to cytochrome P-450 and metabolism of n-alkyl carbamates in isolated rat hepatocytes

The binding constants of an homologous series of n-alkyl (C2-C10) carbamates (see formula in text) to the cytochrome P-450 of suspensions of isolated, viable rat hepatocytes have been measured. All the carbamates except ethyl and propyl carbamate produced type I difference spectra and their binding affinities (1/Ks) were found to be directly dependent upon their lipophilicity. These binding affinities were similar to those determined in rat liver microsomes. Maximum development of the binding spectrum in hepatocytes was always within one second of the addition of each carbamate, indicating that for these carbamates membrane permeability was not rate limiting for access to, and metabolism by, cytochrome P-450 and that much of the cells' cytochrome P-450 was unoccupied by endogenous substrates. Th major metabolites of C4-C8 carbamates were unconjugated omega-1 oxidation products. Below hexyl carbamate only the omega-1 hydroxylated metabolite was observed but for the more lipophilic carbamates the keto metabolite was also a major product. The same products were found in blood after i.p. dosing of rats with hexyl carbamate. A direct relationship was observed between the affinity constant of the carbamate for cytochrome P-450 and the total rate of oxidative metabolism in the omega-1 position. Hydrolysis of the carbamate group was a minor metabolic pathway in contrast to the in vivo situation.

Metabolism of lofepramine by rat liver microsomes

1. The metabolism of lofepramine in vitro by rat liver microsomes was studied. The enzyme system involved was dependent on cytochrome P-450 and the main metabolites were desmethylimipramine (DMI), formaldehyde and p-chlorobenzoic acid. 2. Microsomes from rats pretreated with phenobarbital or 3-methylcholanthrene showed enhanced lofepramine metabolizing activity. The induction was reflected in increased Vmax values, whereas Km values were not changed. 3. Pretreatment of rats with lofepramine or DMI did not alter the rate of aniline hydroxylation, aminopyrine demethylation or lofepramine metabolism per mg microsomal protein. Nor was the amount of cytochrome P-450 changed. 4. Lofepramine, imipramine and DMI inhibited competitively the microsomal hydroxylation of aniline in vitro. Lofepramine was the most potent inhibitor, which probably reflects the higher lipophilicity of this compound.

Structural parameters in the microsomal hydrolysis of 3-acyloxy-1, 4-benzodiazepines and the multiplicity of the esterases involved

The biotransformation of several prodrug-type esters of centrally acting 1, 4-benzodiazepines was studied. Their rates of hydrolysis catalyzed by the hepatic microsomal fraction of mice were measured by pH-stat. The heterogeneity of the microsomal esterases was investigated with induction by phenobarbital and with inhibition by DFP. The resulting changes in esterase activity indicated that the phenyl-substituted esters separate from the homogenous sets of oxazepam and lorazepam esters. Regression analysis of the relative hydrolysis rates of the homogenous ester sets revealed a similar dependence on the steric ES the polar sigma* and hydrophobic deltaRM terms of the acyl moiety. The role of the polar term shows that a nucleophilic attack of the acyl moiety determines the hydrolysis. The role of hydrophobicity can be attributed to its interrelation with the steric parameter. The common equations for the aliphatic sters of oxazepam and larazepam suggest the similar nature of the esterases in question and the same catalytic mechanism. Different 3-acetoxy-1, 4-benzodiazepines were also synthetised and their maximal hydrolysis rates were quite different. This excludes the possibility that the deacylation step of the enzymes is rate-determining. Instead, our data suggest that acylation of the microsomal esterases is rate-limiting for the hydrolysis of the aliphatic esters of 3-OH-benzodiazepines.