Inhibition of xanthine oxidase by the aldehyde oxidase inhibitor raloxifene: implications for identifying molybdopterin nitrite reductases

Sources of nitric oxide alternative to nitric oxide synthases are gaining significant traction as crucial mediators of vessel function under hypoxic inflammatory conditions. For example, capacity to catalyze the one electron reduction of nitrite (NO2-) to ·NO has been reported for hemoglobin, myoglobin and molybdopterin-containing enzymes including xanthine oxidoreductase (XOR) and aldehyde oxidase (AO). For XOR and AO, use of selective inhibition strategies is therefore crucial when attempting to assign relative contributions to nitrite-mediated ·NO formation in cells and tissue. To this end, XOR inhibition has been accomplished with application of classic pyrazolopyrimidine-based inhibitors allo/oxypurinol or the newly FDA-approved XOR-specific inhibitor, Uloric® (febuxostat). Likewise, raloxifene, an estrogen receptor antagonist, has been identified as a potent (Ki=1.0 nM) inhibitor of AO. Herein, we characterize the inhibition kinetics of raloxifene for XOR and describe the resultant effects on inhibiting XO-catalyzed ·NO formation. Exposure of purified XO to raloxifene (PBS, pH 7.4) resulted in a dose-dependent (12.5-100 μM) inhibition of xanthine oxidation to uric acid. Dixon plot analysis revealed a competitive inhibition process with a Ki=13 μM. This inhibitory process was more effective under acidic pH; similar to values encountered under hypoxic/inflammatory conditions. In addition, raloxifene also inhibited anoxic XO-catalyzed reduction of NO2- to NO (EC50=64 μM). In contrast to having no effect on XO-catalyzed uric acid production, the AO inhibitor menadione demonstrated potent inhibition of XO-catalyzed NO2- reduction (EC50=60 nM); somewhat similar to the XO-specific inhibitor, febuxostat (EC50=4 nM). Importantly, febuxostat was found to be a very poor inhibitor of human AO (EC50=613 μM) suggesting its usefulness for validating XO-dependent contributions to NO2- reduction in biological systems. Combined, these data indicate care should be taken when choosing inhibition strategies as well as inhibitor concentrations when assigning relative NO2- reductase activity of AO and XOR.

Enzyme kinetics, inhibition, and regioselectivity of aldehyde oxidase

The aldehyde oxidase (AO) enzyme family plays an increasing role in drug development. However, a number of compounds that are AO substrates have failed in the clinic because the clearance or toxicity is underestimated by preclinical species. Human AO is much more active than rodent AO, and dogs do not have functional AO. While AOs normally make non-reactive metabolites such as lactams, the metabolic products often have much lower solubility that can lead to renal failure. While an endogenous substrate for the oxidation reaction is not known, electron acceptors for the reductive part of the reaction include oxygen and nitrites. Reduction of oxygen leads to the reactive oxygen species (ROS) superoxide radical anion, and hydrogen peroxide. Reduction of nitrite leads to the formation of nitric oxide with potential pharmacological implications. To date, no clinically important drug-drug interactions (DDIs) have been observed for AOs. However, the inhibition kinetics are complex, and multiple probe substrates should be used when assessing the potential for DDIs. Finally, AO appears to be amenable to computational predictions of both regioselectivity and rates of reaction, which holds promise for virtual screening.

Absolute quantification of aldehyde oxidase protein in human liver using liquid chromatography-tandem mass spectrometry

The function of the enzyme human aldehyde oxidase (AOX1) is uncertain; however, recent studies have implicated significant biochemical involvement in humans. AOX1 has also rapidly become an important drug-metabolizing enzyme. Until now, quantitation of AOX1 in complex matrices such as tissue has not been achieved. Herein, we developed and employed a trypsin digest and subsequent liquid chromatography-tandem mass spectrometry analysis to determine absolute amounts of AOX1 in human liver. E. coli expressed human purified AOX1 was used to validate the linearity, sensitivity, and selectivity of the method. Overall, the method is highly efficient and sensitive for determination of AOX1 in cytosolic liver fractions. Using this method, we observed substantial batch-to-batch variation in AOX1 content (21-40 pmol AOX1/mg total protein) between various pooled human liver cytosol preparations. We also observed interbatch variation in Vmax (3.3-4.9 nmol min(-1) mg(-1)) and a modest correlation between enzyme concentration and activity. In addition, we measured a large difference in kcat/Km, between purified (kcat/Km of 1.4) and human liver cytosol (kcat/Km of 15-20) indicating cytosol to be 11-14 times more efficient in the turnover of DACA than the E. coli expressed purified enzyme. Finally, we discussed the future impact of this method for the development of drug metabolism models and understanding the biochemical role of this enzyme.

Activity, inhibition, and induction of cytochrome P450 2J2 in adult human primary cardiomyocytes

Cytochrome P450 2J2 plays a significant role in the epoxidation of arachidonic acid to signaling molecules important in cardiovascular events. CYP2J2 also contributes to drug metabolism and is responsible for the intestinal clearance of ebastine. However, the interaction between arachidonic acid metabolism and drug metabolism in cardiac tissue, the main expression site of CYP2J2, has not been examined. Here we investigate an adult-derived human primary cardiac cell line as a suitable model to study metabolic drug interactions (inhibition and induction) of CYP2J2 in cardiac tissue. The primary human cardiomyocyte cell line demonstrated similar mRNA-expression profiles of P450 enzymes to adult human ventricular tissue. CYP2J2 was the dominant isozyme with minor contributions from CYP2D6 and CYP2E1. Both terfenadine and astemizole oxidation were observed in this cell line, whereas midazolam was not metabolized suggesting lack of CYP3A activity. Compared with recombinant CYP2J2, terfenadine was hydroxylated in cardiomyocytes at a similar K(m) value of 1.5 μM. The V(max) of terfenadine hydroxylation in recombinant enzyme was found to be 29.4 pmol/pmol P450 per minute and in the cells 6.0 pmol/pmol P450 per minute. CYP2J2 activity in the cell line was inhibited by danazol, astemizole, and ketoconazole in submicromolar range, but also by xenobiotics known to cause cardiac adverse effects. Of the 14 compounds tested for CYP2J2 induction, only rosiglitazone increased mRNA expression, by 1.8-fold. This cell model can be a useful in vitro model to investigate the role of CYP2J2-mediated drug metabolism, arachidonic acid metabolism, and their association to drug induced cardiotoxicity.

Evaluation of rhesus monkey and guinea pig hepatic cytosol fractions as models for human aldehyde oxidase

Aldehyde oxidase (AOX) is a cytosolic enzyme expressed across a wide range of species, including guinea pig and rhesus monkey. These species are believed to be the best preclinical models for studying human AOX-mediated metabolism. We compared AOX activity in rhesus monkeys, guinea pigs, and humans using phthalazine and N-[2-(dimethylamino)ethyl]acridone-4-carboxamide (DACA) as substrates and raloxifene as an inhibitor. Michaelis-Menten kinetics was observed for phthalazine oxidation in rhesus monkey, guinea pig, and human liver cytosol, whereas substrate inhibition was seen with DACA oxidase activity in all three livers. Raloxifene inhibited phthalazine and DACA oxidase activity uncompetitively in guinea pig, whereas mixed-mode inhibition was seen in rhesus monkey. Our analysis of the primary sequence alignment of rhesus monkey, guinea pig, and human aldehyde oxidase isoform 1 (AOX1) along with homology modeling has led to the identification of several amino acid residue differences within the active site and substrate entrance channel of AOX1. We speculate that some of these residues might be responsible for the differences observed in activity. Overall, our data indicate that rhesus monkeys and guinea pigs would overestimate intrinsic clearance in humans and would be unsuitable to use as animal models. Our study also showed that AOX metabolism in species is substrate-dependent and no single animal model can be reliably used to predict every drug response in humans.

Predicting intrinsic clearance for drugs and drug candidates metabolized by aldehyde oxidase

Metabolism by aldehyde oxidase (AO) has been responsible for a number of drug failures in clinical trials. The main reason is the clearance values for drugs metabolized by AO are underestimated by allometric scaling from preclinical species. Furthermore, in vitro human data also underestimates clearance. We have developed the first in silico models to predict both in vitro and in vivo human intrinsic clearance for 8 drugs with just two chemical descriptors. These models explain a large amount of the variance in the data using two computational estimates of the electronic and steric features of the reaction. The in vivo computational models for human metabolism are better than in vitro preclinical animal testing at predicting human intrinsic clearance. Thus, it appears that AO is amenable to computational prediction of rates, which may be used to guide drug discovery, and predict pharmacokinetics for clinical trials.

A guide to tracking single transmembrane proteins in supported lipid bilayers

The purpose of this chapter is to serve as a guide for those who wish to carry out experiments tracking single transmembrane proteins in planar supported membrane biomimetics. This chapter describes, in detail, the construction of a simple single-molecule microscope, which includes (1) a parts list, (2) an alignment procedure, (3) a calibration procedure, and (4) a procedure for measuring the mechanical stability of the instrument. It also gives procedures for making planar supported POPC bilayers on hydrophilically treated borosilicate and quartz, POPC/PEG-PE cushioned bilayers on hydrophilically treated surfaces, and POPC/PEG-PE cushioned bilayers on BSA passivated substrates. The procedure for the detergent-mediated incorporation of the transmembrane protein 5HT(3A) (a serotonin receptor) is also described and can be used as a starting point for other large non-self-inserting transmembrane proteins. The final experimental section of this chapter details different procedures for data analysis including (1) a quantitative analysis of mean displacements from individually tracked particles, (2) a Gaussian analysis of step-size distributions, (3) the Gaussian analysis of diffusion coefficients from ensembles of transmembrane proteins, and (4) a perspective associated with the interpretation of single-particle tracking data.

Evidence for substrate-dependent inhibition profiles for human liver aldehyde oxidase

The goal of this study was to provide a reasonable assessment of how probe substrate selection may impact the results of in vitro aldehyde oxidase (AO) inhibition experiments. Here, we used a previously studied set of seven known AO inhibitors to probe the inhibition profile of a pharmacologically relevant substrate N-[(2-dimethylamino)ethyl]acridine-4-carboxamide (DACA). DACA oxidation in human liver cytosol was characterized with a measured V(max) of 2.3 ± 0.08 nmol product · min(-1) · mg(-1) and a K(m) of 6.3 ± 0.8 µM. The K(ii) and K(is) values describing the inhibition of DACA oxidation by the panel of seven inhibitors were tabulated and compared with previous findings with phthalazine as the substrate. In every case, the inhibition profile shifted to a much less uncompetitive mode of inhibition for DACA relative to phthalazine. With the exception of one inhibitor, raloxifene, this change in inhibition profile seems to be a result of a decrease in the uncompetitive mode of inhibition (an affected K(ii) value), whereas the competitive mode (K(is)) seems to be relatively consistent between substrates. Raloxifene was found to inhibit competitively when using DACA as a probe, and a previous report showed that raloxifene inhibited uncompetitively with other substrates. The relevance of these data to the mechanistic understanding of aldehyde oxidase inhibition and potential implications on drug-drug interactions is discussed. Overall, it appears that the choice in substrate may be critical when conducting mechanistic inhibition or in vitro drug-drug interactions prediction studies with AO.

The impact of single nucleotide polymorphisms on human aldehyde oxidase

Aldehyde oxidase (AO) is a complex molybdo-flavoprotein that belongs to the xanthine oxidase family. AO is active as a homodimer, and each 150-kDa monomer binds two distinct [2Fe2S] clusters, FAD, and the molybdenum cofactor. AO has an important role in the metabolism of drugs based on its broad substrate specificity oxidizing aromatic aza-heterocycles, for example, N(1)-methylnicotinamide and N-methylphthalazinium, or aldehydes, such as benzaldehyde, retinal, and vanillin. Sequencing the 35 coding exons of the human AOX1 gene in a sample of 180 Italian individuals led to the identification of relatively frequent, synonymous, missense and nonsense single-nucleotide polymorphisms (SNPs). Human aldehyde oxidase (hAOX1) was purified after heterologous expression in Escherichia coli. The recombinant protein was obtained with a purity of 95% and a yield of 50 μg/l E. coli culture. Site-directed mutagenesis of the hAOX1 cDNA allowed the purification of protein variants bearing the amino acid changes R802C, R921H, N1135S, and H1297R, which correspond to some of the identified SNPs. The hAOX1 variants were purified and compared with the wild-type protein relative to activity, oligomerization state, and metal content. Our data show that the mutation of each amino acid residue has a variable impact on the ability of hAOX1 to metabolize selected substrates. Thus, the human population is characterized by the presence of functionally inactive hAOX1 allelic variants as well as variants encoding enzymes with different catalytic activities. Our results indicate that the presence of these allelic variants should be considered for the design of future drugs.

Identifying a selective substrate and inhibitor pair for the evaluation of CYP2J2 activity

CYP2J2, an arachidonic acid epoxygenase, is recognized for its role in the first-pass metabolism of astemizole and ebastine. To fully assess the role of CYP2J2 in drug metabolism, a selective substrate and potent specific chemical inhibitor are essential. In this study, we report amiodarone 4-hydoxylation as a specific CYP2J2-catalyzed reaction with no CYP3A4, or other drug-metabolizing enzyme, involvement. Amiodarone 4-hydroxylation enabled the determination of liver relative activity factor and intersystem extrapolation factor for CYP2J2. Amiodarone 4-hydroxylation correlated with astemizole O-demethylation but not with CYP2J2 protein content in a sample of human liver microsomes. To identify a specific CYP2J2 inhibitor, 138 drugs were screened using terfenadine and astemizole as probe substrates with recombinant CYP2J2. Forty-two drugs inhibited CYP2J2 activity by ≥50% at 30 μM, but inhibition was substrate-dependent. Of these, danazol was a potent inhibitor of both hydroxylation of terfenadine (IC(50) = 77 nM) and O-demethylation of astemizole (K(i) = 20 nM), and inhibition was mostly competitive. Danazol inhibited CYP2C9, CYP2C8, and CYP2D6 with IC(50) values of 1.44, 1.95, and 2.74 μM, respectively. Amiodarone or astemizole were included in a seven-probe cocktail for cytochrome P450 (P450) drug-interaction screening potential, and astemizole demonstrated a better profile because it did not appreciably interact with other P450 probes. Thus, danazol, amiodarone, and astemizole will facilitate the ability to determine the metabolic role of CYP2J2 in hepatic and extrahepatic tissues.

The impact of single nucleotide polymorphisms on human aldehyde oxidase

Aldehyde oxidase (AO) is a complex molybdo-flavoprotein that belongs to the xanthine oxidase family. AO is active as a homodimer, and each 150-kDa monomer binds two distinct [2Fe2S] clusters, FAD, and the molybdenum cofactor. AO has an important role in the metabolism of drugs based on its broad substrate specificity oxidizing aromatic aza-heterocycles, for example, N(1)-methylnicotinamide and N-methylphthalazinium, or aldehydes, such as benzaldehyde, retinal, and vanillin. Sequencing the 35 coding exons of the human AOX1 gene in a sample of 180 Italian individuals led to the identification of relatively frequent, synonymous, missense and nonsense single-nucleotide polymorphisms (SNPs). Human aldehyde oxidase (hAOX1) was purified after heterologous expression in Escherichia coli. The recombinant protein was obtained with a purity of 95% and a yield of 50 μg/l E. coli culture. Site-directed mutagenesis of the hAOX1 cDNA allowed the purification of protein variants bearing the amino acid changes R802C, R921H, N1135S, and H1297R, which correspond to some of the identified SNPs. The hAOX1 variants were purified and compared with the wild-type protein relative to activity, oligomerization state, and metal content. Our data show that the mutation of each amino acid residue has a variable impact on the ability of hAOX1 to metabolize selected substrates. Thus, the human population is characterized by the presence of functionally inactive hAOX1 allelic variants as well as variants encoding enzymes with different catalytic activities. Our results indicate that the presence of these allelic variants should be considered for the design of future drugs.

A non-acidic sulfaphenazole analog demonstrating high intrinsic clearance and selectivity by canine CYP2C21

In contrast to human CYP2C9, non-human CYP2C enzymes do not appear to preferentially bind and metabolize anionic drugs. Using analogs of sulfaphenazole, the effect of an acidic sulfonamide group on apparent affinity and turnover rates was characterized with canine CYP2C21. Blocking the sulfonamide with a methyl group increased the intrinsic clearance by CYP2C21 > 100-fold and decreased K(m). Furthermore, CYP2C21 demonstrated selectivity for formation of the benzylic hydroxylation product and a high estimated f(m,CYP) value. The findings suggest that canine CYP2C21, unlike human CYP2C9, does not derive ligand binding affinity from an anion binding interaction with sulfaphenazole analogs.

Comparative study of the affinity and metabolism of type I and type II binding quinoline carboxamide analogues by cytochrome P450 3A4

Compounds that coordinate to the heme-iron of cytochrome P450 (CYP) enzymes are assumed to increase metabolic stability. However, recently we observed that the type II binding quinoline carboxamide (QCA) compounds were metabolically less stable. To test if the higher intrinsic clearance of type II binding compounds relative to type I binding compounds is general for other metabolic transformations, we synthesized a library of QCA compounds that could undergo N-dealkylation, O-dealkylation, benzylic hydroxylation, and aromatic hydroxylation. The results demonstrated that type II binding QCA analogues were metabolically less stable (2- to 12-fold) at subsaturating concentration compared to type I binding counterparts for all the transformations. When the rates of different metabolic transformations between type I and type II binding compounds were compared, they were found to be in the order of N-demethylation > benzylic hydroxylation> O-demethylation > aromatic hydroxylation. Finally, for the QCA analogues with aza-heteroaromatic rings, we did not detect metabolism in aza-aromatic rings (pyridine, pyrazine, pyrimidine), indicating that electronegativity of the nitrogen can change regioselectivity in CYP metabolism.

Inhibition of human liver aldehyde oxidase: implications for potential drug-drug interactions

During the course of our research efforts to understand the kinetics of human aldehyde oxidase as a xenobiotic-clearing enzyme, we investigated the effect of eight different inhibitors on the oxidation of the probe substrate phthalazine. Saturation kinetic parameters for phthalazine oxidation in human liver cytosol were found to be the following: K(m) = 8.0 ± 0.4 μM and V(max) = 4.3 ± 0.1 nmol · min(-1) · mg protein(-1). Inhibitory potency of the inhibitors tested ranged from 0.1 to 5 μM. Of the eight different inhibitor compounds tested, seven were observed to inhibit through a mixed mode and one through a strictly competitive mode. A ratio of the K(ii) and K(is) values was used to assess the relative competitiveness of each inhibitor. For the mixed inhibitors, the mode of inhibition varied from mostly uncompetitive to predominantly competitive (K(ii)/K(is) values ranging from 0.1 to 15). The implications for potential drug-drug interactions and inhibition mechanism are discussed. We found two inhibitors, clozapine and chlorpromazine, that have a moderate predicted risk of drug-drug interactions based on the K(i) value relative to the inhibitor concentration in human plasma, having a calculated [I]/K(i) value of 0.4 and 0.8, respectively.

Small molecule quantification by liquid chromatography-mass spectrometry for metabolites of drugs and drug candidates

Identification and quantification of the metabolites of drugs and drug candidates are routinely performed using liquid chromatography-mass spectrometry (LC-MS). The best practice is to generate a standard curve with the metabolite versus the internal standard. However, to avoid the difficulties in metabolite synthesis, standard curves are sometimes prepared using the substrate, assuming that the signal for substrate and the metabolite will be equivalent. We have tested the errors associated with this assumption using a series of very similar compounds that undergo common metabolic reactions using both conventional flow electrospray ionization LC-MS and low-flow captive spray ionization (CSI) LC-MS. The differences in standard curves for four different types of transformations (O-demethylation, N-demethylation, aromatic hydroxylation, and benzylic hydroxylation) are presented. The results demonstrate that the signals of the substrates compared with those of the metabolites are statistically different in 18 of the 20 substrate-metabolite combinations for both methods. The ratio of the slopes of the standard curves varied up to 4-fold but was slightly less for the CSI method.

Performance and macrokinetic analysis of biofiltration of toluene and p-xylene mixtures in a conventional biofilter packed with inert material

Interactions of toluene and p-xylene in air treatment biofilters packed with an inert filter media were studied. The effect of the inlet load of toluene, p-xylene and mixtures of both compounds on the biodegradation rate was analyzed in three lab-scale biofilters. A maximum elimination capacity (EC) of 26.5 and 40.3 gCm(-3)h(-1) for an inlet load (IL) of 65.6 and 57.8 gCm(-3)h(-1) was obtained for p-xylene and toluene biofilters, respectively. Inhibition of p-xylene biodegradation by the presence of toluene took place when the mixture was treated, whereas the presence of p-xylene had an enhancing effect on the toluene removal efficiency. Specific growth rates (μ) from 0.019 to 0.068 h(-1) were calculated in the mixed biofilter, where the highest values were similar to mixtures with lower p-xylene levels (IL(p-Xyl) 8.84 ± 0.29 gCm(-3)h(-1)). Michaelis-Menten and Haldane type models were fitted to experimental EC for p-xylene and toluene biofilters, respectively.

The effects of nitrogen-heme-iron coordination on substrate affinities for cytochrome P450 2E1

A descriptor based computational model was developed for cytochrome P450 2E1 (CYP2E1) based on inhibition constants determined for inhibition of chlorzoxazone, or 4-nitrophenol, metabolism. An empirical descriptor for type II binding was developed and tested for a series of CYP2E1 inhibitors. Inhibition constants where measured for 51 different compounds. A fast 2-dimensional predictive model was developed based on 40 compounds, and tested on 8 compounds of diverse structure. The trained model (n=40) had an r(2) value of 0.76 and an RMSE of 0.48. The correlation between the predicted and actual pK(i) values of the test set of compounds not included in the model gives an r(2) value of 0.78. The features that described binding include heme coordination (type II binding), molecular volume, octanol/water partition coefficient, solvent accessible surface area, and the sum of the atomic polarizabilities. The heme coordination parameter assigns an integer between 0 and 6 depending on structure, and is a new descriptor, based on simple quantum chemical calculations with correction for steric effects. The type II binding parameter was found to be important in obtaining a good correlation between predicted and experimental inhibition constants increasing the r(2) value from 0.38 to 0.77.

The kinetic mechanism for cytochrome P450 metabolism of type II binding compounds: evidence supporting direct reduction

The metabolic stability of a drug is an important property that should be optimized during drug design and development. Nitrogen incorporation is hypothesized to increase the stability by coordination of nitrogen to the heme iron of cytochrome P450, a binding mode that is referred to as type II binding. However, we noticed that the type II binding compound 1 has less metabolic stability at sub-saturating conditions than a closely related type I binding compound 3. Three kinetic models will be presented for type II binder metabolism; (1) Dead-end type II binding, (2) a rapid equilibrium between type I and II binding modes before reduction, and (3) a direct reduction of the type II coordinated heme. Data will be presented on reduction rates of iron, the off rates of substrate (using surface plasmon resonance) and the catalytic rate constants. These data argue against the dead-end, and rapid equilibrium models, leaving the direct reduction kinetic mechanism for metabolism of the type II binding compound 1.

Studies on the mechanism of aldehyde oxidase and xanthine oxidase

DFT calculations support a concerted mechanism for xanthine oxidase and aldehyde oxidase hydride displacement from the sp(2) carbon of 6-substituted 4-quinazolinones. The variations in transition state structure show that C-O bond formation is nearly complete in the transition state and the transition state changes are anti-Hammond with the C-H and C-O bond lengths being more product-like for the faster reactions. The C-O bond length in the transition state is around 90% formed. However, the C-H bond is only about 80% broken. This leads to a very tetrahedral transition state with an O-C-N angle of 109 degrees. Thus, while the mechanism is concerted, the antibonding orbital of the C-H bond that is broken is not directly attacked by the nucleophile and instead hydride displacement occurs after almost complete tetrahedral transition state formation. In support of this the C=N bond is lengthened in the transition state indicating that attack on the electrophilic carbon occurs by addition to the C=N bond with negative charge increasing on the nitrogen. Differences in experimental reaction rates are accurately reproduced by these calculations and tend to support this mechanism.

The effects of type II binding on metabolic stability and binding affinity in cytochrome P450 CYP3A4

One goal in drug design is to decrease clearance due to metabolism. It has been suggested that a compound's metabolic stability can be increased by incorporation of a sp(2) nitrogen into an aromatic ring. Nitrogen incorporation is hypothesized to increase metabolic stability by coordination of nitrogen to the heme-iron (termed type II binding). However, questions regarding binding affinity, metabolic stability, and how metabolism of type II binders occurs remain unanswered. Herein, we use pyridinyl quinoline-4-carboxamide analogs to answer these questions. We show that type II binding can have a profound influence on binding affinity for CYP3A4, and the difference in binding affinity can be as high as 1200-fold. We also find that type II binding compounds can be extensively metabolized, which is not consistent with the dead-end complex kinetic model assumed for type II binders. Two alternate kinetic mechanisms are presented to explain the results. The first involves a rapid equilibrium between the type II bound substrate and a metabolically oriented binding mode. The second involves direct reduction of the nitrogen-coordinated heme followed by oxygen binding.

Identification of novel substrates for human cytochrome P450 2J2

Several antihistamine drugs including terfenadine, ebastine, and astemizole have been identified as substrates for CYP2J2. The overall importance of this enzyme in drug metabolism has not been fully explored. In this study, 139 marketed therapeutic agents and compounds were screened as potential CYP2J2 substrates. Eight novel substrates were identified that vary in size and overall topology from relatively rigid structures (amiodarone) to larger complex structures (cyclosporine). The substrates displayed in vitro intrinsic clearance values ranging from 0.06 to 3.98 mul/min/pmol CYP2J2. Substrates identified for CYP2J2 are also metabolized by CYP3A4. Extracted ion chromatograms of metabolites observed for albendazole, amiodarone, astemizole, thioridazine, mesoridazine, and danazol showed marked differences in the regioselectivity of CYP2J2 and CYP3A4. CYP3A4 commonly metabolized compounds at multiple sites, whereas CYP2J2 metabolism was more restrictive and limited, in general, to a single site for large compounds. Although the CYP2J2 active site can accommodate large substrates, it may be more narrow than CYP3A4, limiting metabolism to moieties that can extend closer toward the active heme iron. For albendazole, CYP2J2 forms a unique metabolite compared with CYP3A4. Albendazole and amiodarone were evaluated in various in vitro systems including recombinant CYP2J2 and CYP3A4, pooled human liver microsomes (HLM), and human intestinal microsomes (HIM). The Michaelis-Menten-derived intrinsic clearance of N-desethyl amiodarone was 4.6 greater in HLM than in HIM and 17-fold greater in recombinant CYP3A4 than in recombinant CYP2J2. The resulting data suggest that CYP2J2 may be an unrecognized participant in first-pass metabolism, but its contribution is minor relative to that of CYP3A4.

Purification and mechanism of human aldehyde oxidase expressed in Escherichia coli

Human aldehyde oxidase 1 (AOX1) has been subcloned into a vector suitable for expression in Escherichia coli, and the protein has been expressed. The resulting protein is active, with sulfur being incorporated in the molybdopterin cofactor. Expression levels are modest, but 1 liter of cells supplies enough protein for both biochemical and kinetic characterization. Partial purification is achieved by nickel affinity chromatography through the addition of six histidines to the amino-terminal end of the protein. Kinetic analysis, including kinetic isotope effects and comparison with xanthine oxidase, reveal similar mechanisms, with some subtle differences. This expression system will allow for the interrogation of human aldehyde oxidase structure/function relationships by site-directed mutagenesis and provide protein for characterizing the role of AOX1 in drug metabolism.

Cytochrome P450 2C9 type II binding studies on quinoline-4-carboxamide analogues

CYP2C9 is a significant P450 protein responsible for drug metabolism. With the increased use of heterocyclic compounds in drug design, a rapid and efficient predrug screening of these potential type II binding compounds is essential to avoid adverse drug reactions. To understand binding modes, we use quinoline-4-carboxamide analogues to study the factors that determine the structure-activity relationships. The results of this study suggest that the more accessible pyridine with the nitrogen para to the linkage can coordinate directly with the ferric heme iron, but this is not seen for the meta or ortho isomers. The pi-cation interaction of the naphthalene moiety and Arg 108 residue may also assist in stabilizing substrate binding within the active-site cavity. The type II substrate binding affinity is determined by the combination of steric, electrostatic, and hydrophobicity factors; meanwhile, it is enhanced by the strength of lone pair electrons coordination with the heme iron.