Non-cytochrome P450 enzyme aldehyde oxidase is involved in the oxidative metabolic pathway of diquat and its detoxification effect

Diquat (DQ) poisoning has garnered attention in recent years, primarily due to the rising incidence of cases worldwide, coupled with the absence of a viable antidote for its treatment. Despite the fact that diquat monopyridone (DQ-M) has been identified as a significant metabolite of DQ, the enzyme responsible for its formation remains unknown. In this study, we have identified aldehyde oxidase (AOX) as a vital enzyme involved in DQ oxidative metabolism. The metabolism of DQ to DQ-M was significantly inhibited by AOX inhibitors including raloxifene and hydralazine. The source of oxygen incorporated into DQ-M was proved to be from water through a H218O incubation experiment which further corroborated DQ-M formation via AOX metabolism. The product of DQ-M in vitro generated by fresh rat tissues co-incubation was consistent with its AOX expression. The result of the molecular docking analysis of DQ and AOX protein showed that DQ is capable of binding to AOX. Furthermore, the cytotoxicity of DQ was significantly higher than DQ-M at the same concentration tested in six cell types. This work is the first to uncover the involvement of aldehyde oxidase, a non-cytochrome P450 enzyme, in the oxidative metabolic pathway of diquat, thus providing a potential target for the development of detoxification treatment.

Large Computational Survey of Intrinsic Reactivity of Aromatic Carbon Atoms with Respect to a Model Aldehyde Oxidase

Aldehyde oxidase (AOX) and other related molybdenum-containing enzymes are known to oxidize the C-H bonds of aromatic rings. This process contributes to the metabolism of pharmaceutical compounds and, therefore, is of vital importance to drug pharmacokinetics. The present work describes an automated computational workflow and its use for the prediction of intrinsic reactivity of small aromatic molecules toward a minimal model of the active site of AOX. The workflow is based on quantum chemical transition state searches for the underlying single-step oxidation reaction, where the automated protocol includes identification of unique aromatic C-H bonds, creation of three-dimensional reactant and product complex geometries via a templating approach, search for a transition state, and validation of reaction end points. Conformational search on the reactants, products, and the transition states is performed. The automated procedure has been validated on previously reported transition state barriers and was used to evaluate the intrinsic reactivity of nearly three hundred heterocycles commonly found in approved drug molecules. The intrinsic reactivity of more than 1000 individual aromatic carbon sites is reported. Stereochemical and conformational aspects of the oxidation reaction, which have not been discussed in previous studies, are shown to play important roles in accurate modeling of the oxidation reaction. Observations on structural trends that determine the reactivity are provided and rationalized.

Aldehyde oxidase 1 activity and protein expression in human, rabbit, and pig ocular tissues

Aldehyde oxidase (AOX) is a cytosolic drug-metabolizing enzyme which has attracted increasing attention in drug development due to its high hepatic expression, broad substrate profile and species differences. In contrast, there is limited information on the presence and activity of AOX in extrahepatic tissues including ocular tissues. Because several ocular drugs are potential substrates for AOX, we performed a comprehensive analysis of the AOX1 expression and activity profile in seven ocular tissues from humans, rabbits, and pigs. AOX activities were determined using optimized assays for the established human AOX1 probe substrates 4-dimethylamino-cinnamaldehyde (DMAC) and phthalazine. Inhibition studies were undertaken in conjunctival and retinal homogenates using well-established human AOX1 inhibitors menadione and chlorpromazine. AOX1 protein contents were quantitated with targeted proteomics and confirmed by immunoblotting. Overall, DMAC oxidation rates varied over 10-fold between species (human ˃˃ rabbit ˃ pig) and showed 2- to 6-fold differences between tissues from the same species. Menadione seemed a more potent inhibitor of DMAC oxidation across species than chlorpromazine. Human AOX1 protein levels were highest in the conjunctiva, followed by most posterior tissues, whereas anterior tissues showed low levels. The rabbit AOX1 expression was high in the conjunctiva, retinal pigment epithelial (RPE), and choroid while lower in the anterior tissues. Quantification of pig AOX1 was not successful but immunoblotting confirmed the presence of AOX1 in all species. DMAC oxidation rates and AOX1 contents correlated quite well in humans and rabbits. This study provides, for the first time, insights into the ocular expression and activity of AOX1 among multiple species.

Enzyme Kinetics, Pharmacokinetics, and Inhibition of Aldehyde Oxidase

Aldehyde oxidase (AO) has emerged as an important drug metabolizing enzyme over the last decade. Several compounds have failed in the clinic because the clearance or toxicity was underestimated by preclinical species. Human AO is much more active than rodent AO, and dogs do not have functional AO. Metabolic products from AO-catalyzed oxidation are generally nonreactive and often they have much lower solubility. AO metabolism is not limited to oxidation as AO can also catalyze reduction of oxygen and nitrite. 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. AO is also reported to catalyze the reductive metabolism of nitro-compounds, N-oxides, sulfoxides, isoxazoles, isothiazoles, nitrite, and hydroxamic acids. These reductive transformations may cause toxicity due to the formation of reactive metabolites. Moreover, 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.

Identification and Characterization of Aldehyde Oxidase 5 in the Pheromone Gland of the Silkworm (Lepidoptera: Bombycidae)

Aldehyde oxidases (AOXs) are a subfamily of cytosolic molybdo-flavoenzymes that play critical roles in the detoxification and degradation of chemicals. Active AOXs, such as AOX1 and AOX2, have been identified and functionally analyzed in insect antennae but are rarely reported in other tissues. This is the first study to isolate and characterize the cDNA that encodes aldehyde oxidase 5 (BmAOX5) in the pheromone gland (PG) of the silkworm, Bombyx mori. The size of BmAOX5 cDNA is 3,741 nucleotides and includes an open reading frame, which encodes a protein of 1,246 amino acid residues. The theoretical molecular weight and isoelectric point of BmAOX5 are approximately 138 kDa and 5.58, respectively. BmAOX5 shares a similar primary structure with BmAOX1 and BmAOX2, containing two [2Fe-2S] redox centers, a FAD-binding domain, and a molybdenum cofactor (MoCo)-binding domain. RT-PCR revealed BmAOX5 to be particularly highly expressed in the PG (including ovipositor) of the female silkworm moth, and the expression was further confirmed by in situ hybridization, AOX activity staining, and anti-BmAOX5 western blotting. Further, BmAOX5 was shown to metabolize aromatic aldehydes, such as benzaldehyde, salicylaldehyde, and vanillic aldehyde, and fatty aldehydes, such as heptaldehyde and propionaldehyde. The maximum reaction rate (Vmax) of benzaldehyde as substrate was 21 mU and Km was 1.745 mmol/liter. These results suggested that BmAOX5 in the PG could metabolize aldehydes in the cytoplasm for detoxification or participate in the degradation of aldehyde pheromone substances and odorant compounds to identify mating partners and locate suitable spawning sites.

Evolution, expression, and substrate specificities of aldehyde oxidase enzymes in eukaryotes

Aldehyde oxidases (AOXs) are a small group of enzymes belonging to the larger family of molybdo-flavoenzymes, along with the well-characterized xanthine oxidoreductase. The two major types of reactions that are catalyzed by AOXs are the hydroxylation of heterocycles and the oxidation of aldehydes to their corresponding carboxylic acids. Different animal species have different complements of AOX genes. The two extremes are represented in humans and rodents; whereas the human genome contains a single active gene (AOX1), those of rodents, such as mice, are endowed with four genes (Aox1-4), clustering on the same chromosome, each encoding a functionally distinct AOX enzyme. It still remains enigmatic why some species have numerous AOX enzymes, whereas others harbor only one functional enzyme. At present, little is known about the physiological relevance of AOX enzymes in humans and their additional forms in other mammals. These enzymes are expressed in the liver and play an important role in the metabolisms of drugs and other xenobiotics. In this review, we discuss the expression, tissue-specific roles, and substrate specificities of the different mammalian AOX enzymes and highlight insights into their physiological roles.

Effects of Phenothiazines on Aldehyde Oxidase Activity Towards Aldehydes and N-Heterocycles: an In Vitro and In Silico Study

BACKGROUND

Aldehyde oxidase (AOX) is an important molybdenum-containing enzyme with high similarity with xanthine oxidase (XO). AOX involved in the metabolism of a large array of aldehydes and N-heterocyclic compounds and its activity is highly substrate-dependent.

OBJECTIVES

The aim of this work was to study the effect of five important phenothiazine drugs on AOX activity using benzaldehyde and phenanthridine as aldehyde and N-heterocyclic substrates, respectively.

METHODS

The effect of trifluperazine, chlorpromazine, perphenazine, thioridazine and promethazine on rat liver AOX was measured spectrophotometrically. To predict the mode of interactions between the studied compounds and AOX, a combination of homology modeling and a molecular docking study was performed.

RESULTS

All phenothiazines could inhibit AOX activity measured either by phenanthridine or benzaldehyde with almost no effect on XO activity. In the case of benzaldehyde oxidation, the lowest and highest half-maximal inhibitory concentration (IC₅₀) values were obtained for promethazine (IC₅₀ = 0.9 µM), and trifluoperazine (IC₅₀ = 3.9 µM), respectively; whereas perphenazine (IC₅₀ = 4.3 µM), and trifluoperazine (IC₅₀ = 49.6 µM) showed the strongest and weakest inhibitory activity against AOX-catalyzed phenanthridine oxidation, respectively. The in silico findings revealed that the binding site of thioridazine is near the dimer interference, and that hydrophobic interactions are of great importance in all the tested phenothiazines.

CONCLUSION

The five studied phenothiazine drugs showed dual inhibitory effects on AOX activity towards aldehydes and N-heterocycles as two major classes of enzyme substrates. Most of the interactions between the phenothiazine-related drugs and AOX in the binding pocket showed a hydrophobic nature.

A single nucleotide polymorphism causes enhanced radical oxygen species production by human aldehyde oxidase

Aldehyde oxidases (AOXs) are molybdo-flavoenzymes characterized by broad substrate specificity, oxidizing aromatic/aliphatic aldehydes into the corresponding carboxylic acids and hydroxylating various heteroaromatic rings. The enzymes use oxygen as the terminal electron acceptor and produce reduced oxygen species during turnover. The physiological function of mammalian AOX isoenzymes is still unclear, however, human AOX (hAOX1) is an emerging enzyme in phase-I drug metabolism. Indeed, the number of xenobiotics acting as hAOX1 substrates is increasing. Further, numerous single-nucleotide polymorphisms (SNPs) have been identified within the hAOX1 gene. SNPs are a major source of inter-individual variability in the human population, and SNP-based amino acid exchanges in hAOX1 reportedly modulate the catalytic function of the enzyme in either a positive or negative fashion. In this report we selected ten novel SNPs resulting in amino acid exchanges in proximity to the FAD site of hAOX1 and characterized the purified enzymes after heterologous expression in Escherichia coli. The hAOX1 variants were characterized carefully by quantitative differences in their ability to produce superoxide radical. ROS represent prominent key molecules in physiological and pathological conditions in the cell. Our data reveal significant alterations in superoxide anion production among the variants. In particular the SNP-based amino acid exchange L438V in proximity to the isoalloxanzine ring of the FAD cofactor resulted in increased rate of superoxide radical production of 75%. Considering the high toxicity of the superoxide in the cell, the hAOX1-L438V SNP variant is an eventual candidate for critical or pathological roles of this natural variant within the human population.

Insights into the structural determinants of substrate specificity and activity in mouse aldehyde oxidases

In this work, a combination of homology modeling and molecular dynamics (MD) simulations was used to investigate the factors that modulate substrate specificity and activity of the mouse AOX isoforms: mAOX1, mAOX2 (previously mAOX3l1), mAOX3 and mAOX4. The results indicate that the AOX isoform structures are highly preserved and even more conserved than the corresponding amino acid sequences. The only differences are at the protein surface and substrate-binding site region. The substrate-binding site of all isoforms consists of two regions: the active site, which is highly conserved among all isoforms, and a isoform-specific region located above. We predict that mAOX1 accepts a broader range of substrates of different shape, size and nature relative to the other isoforms. In contrast, mAOX4 appears to accept a more restricted range of substrates. Its narrow and hydrophobic binding site indicates that it only accepts small hydrophobic substrates. Although mAOX2 and mAOX3 are very similar to each other, we propose the following pairs of overlapping substrate specificities: mAOX2/mAOX4 and mAOX3/mAXO1. Based on these considerations, we propose that the catalytic activity between all isoforms should be similar but the differences observed in the binding site might influence the substrate specificity of each enzyme. These results also suggest that the presence of several AOX isoforms in mouse allows them to oxidize more efficiently a wider range of substrates. This contrasts with the same or other organisms that only express one isoform and are less efficient or incapable of oxidizing the same type of substrates.

[Advances in the study of aldehyde oxidases]

Aldehyde oxidase (AOX), a highly conserved molybdoflavoenzyme in mammal cytoplasm, has broad substrate specificity and ability to catalyze the oxidation of aldehydes and nitrogen, oxygen-containing heterocyclic rings. AOX was found to widely distribute with the individual differences in vivo and plays an important role in phase I metabolism of drugs and xenobiotics. The biological characteristics of AOX and its contributions in drug metabolism are introduced briefly in this review.

Identification and characterization of an antennae-specific aldehyde oxidase from the navel orangeworm

Antennae-specific odorant-degrading enzymes (ODEs) are postulated to inactivate odorant molecules after they convey their signal. Different classes of insect ODEs are specific to esters, alcohols, and aldehydes – the major functional groups of female-produced, hydrophobic sex pheromones from moth species. Esterases that rapidly inactive acetate and other esters have been well-studied, but less is known about aldehyde oxidases (AOXs). Here we report cloning of an aldehyde oxidase, AtraAOX2, from the antennae of the navel orangeworm (NOW), Amyelois transitella, and the first activity characterization of a recombinant insect AOX. AtraAOX2 gene spans 3,813 bp and encodes a protein with 1,270 amino acid residues. AtraAOX2 cDNA was expressed in baculovirus-infected insect Sf21 cells as a ≈280 kDa homodimer with 140 kDa subunits. Recombinant AtraAOX2 degraded Z11Z13–16Ald and plant volatile aldehydes as substrates. However, as expected for aldehyde oxidases, recombinant AtraAOX2 did not show specificity for Z11Z13–16Ald, the main constituent of the sex pheromone, but showed high activity for plant volatile aldehydes. Our data suggest AtraAOX2 might be involved in degradation of a diversity of aldehydes including sex pheromones, plant-derived semiochemicals, and chemical cues for oviposition sites. Additionally, AtraAOX2 could protect the insect's olfactory system from xenobiotics, including pesticides that might reach the sensillar lymph surrounding the olfactory receptor neurons.

Mechanistic evaluation of substrate inhibition kinetics observed from aldehyde oxidase-catalyzed reactions

While most enzyme-catalyzed reactions are adequately described by Michaelis-Menten kinetics, Aldehyde Oxidase (AOX) metabolism might exhibit atypical kinetics due to possible substrate inhibition. Ignoring this phenomenon may lead to erroneous estimates of kinetic parameters and over simplification of the enzyme mechanism. In this study, in vitro metabolism data for 3 AOX substrates exhibiting varying degrees of substrate inhibition were analyzed with the following kinetic models: A) Michaelis-Menten (naïve) model; B) Substrate inhibition (empirical) model; and C) Twobinding site (mechanistic) model. The application of this mechanistic model is a novel interpretation for kinetic analysis of AOX metabolism whereby substrate can presumably bind to two enzymes' active site(s). Unlike the other models, this mechanistic model quantitatively captures the degree of substrate inhibition observed. Analysis by this model showed: A) All tested substrates have simultaneous access to the metabolic and inhibitory site of the enzyme with Ks (binding affinity for inhibitory site) greater (1.3- to 28-fold) than Km (binding affinity for metabolic site); B) Dissociation constants for binding of a second substrate in either the productive and nonproductive enzyme conformations decreased with factor α ranging from 2.58 to 15.6 between compounds; and C) In addition, a drastic decrease (from 64%-98%) in the metabolism rates between compounds was exhibited by factor β (ranging from 0.02-0.36). Overall, the mechanistic two-binding site model best fitted the experimental data. Moreover, the observed differences between kinetic parameters generated by these models highlight the importance of appropriate model selection to adequately fit the substrate inhibition kinetics of AOX metabolism.

A highly sensitive RP-HPLC-fluorescence method to study aldehyde oxidase activity

Aldehyde oxidase is a widely distributed enzyme that is involved in the metabolism of an extensive range of aldehydes and N-heterocyclic compounds with physiological, pharmacological, and toxicological relevance. In the present study, a highly sensitive RP-HPLC-fluorescence method based on the oxidation of phenanthridine to phenanthridinone has been developed and validated to assay aldehyde oxidase activity in biological samples. Determination of phenanthridinone was achieved on a C18 column using 10 mmol/L phosphate buffer (pH 5.0) containing 0.1 mmol/L EDTA-acetonitrile (40 + 60, v/v) as the mobile phase. The fluorescence intensity of phenanthridinone was measured at 364 nm with excitation at 236 nm. The proposed method was precise, accurate, specific and rapid (analysis time, approximately 8 min) with a mean RSD of 2.54%. Peak responses were linear from 0.5 to 100 nmol/L, with an LOD of 0.125 nmol/L. The applicability of the method was demonstrated by measurement of aldehyde oxidase activity in rat liver, kidney, ovary, and heart fractions.

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.

Site directed mutagenesis of amino acid residues at the active site of mouse aldehyde oxidase AOX1

Mouse aldehyde oxidase (mAOX1) forms a homodimer and belongs to the xanthine oxidase family of molybdoenzymes which are characterized by an essential equatorial sulfur ligand coordinated to the molybdenum atom. In general, mammalian AOs are characterized by broad substrate specificity and an yet obscure physiological function. To define the physiological substrates and the enzymatic characteristics of mAOX1, we established a system for the heterologous expression of the enzyme in Eschericia coli. The recombinant protein showed spectral features and a range of substrate specificity similar to the native protein purified from mouse liver. The EPR data of recombinant mAOX1 were similar to those of AO from rabbit liver, but differed from the homologous xanthine oxidoreductase enzymes. Site-directed mutagenesis of amino acids Val806, Met884 and Glu1265 at the active site resulted in a drastic decrease in the oxidation of aldehydes with no increase in the oxidation of purine substrates. The double mutant V806E/M884R and the single mutant E1265Q were catalytically inactive enzymes regardless of the aldehyde or purine substrates tested. Our results show that only Glu1265 is essential for the catalytic activity by initiating the base-catalyzed mechanism of substrate oxidation. In addition, it is concluded that the substrate specificity of molybdo-flavoenzymes is more complex and not only defined by the three characterized amino acids in the active site.

Cloning and expression analysis of an aldehyde oxidase gene in Arachis hygogaea L

Aldehyde oxidase (AO) plays important role in plant hormone biosynthetic pathways, such as abscisic acid (ABA) and indole-3-acetic acid (IAA). The enzyme catalyzes the last step of the pathways. In this study a full-length cDNA encoding an aldyhyde oxidase was cloned and sequenced from leaves of peanut by RT-PCR, RACE-PCR and genomic DNA walking methods. The full-length cDNA, designated as Arachis hygogaea L. aldehyde oxidase 1 (AhAO1), consists of an open reading frame of 4131 bp, a 326 bp 5' untranslated region and a 128 bp 3' untranslated region including a poly (A) tail of 21 nucleotides. The gene encodes a polypeptide of 1377 amino acids with a calculated molecular weight of 150 kDa and an isoelectric point (pl) of 6.99. Analysis of amino acid sequence of AhAO1 shows that it had 61%, 59% and 55% identity with the AOs from tomato, Arabidopsis and maize, respectively The peanut AO polypeptide contains consensus sequences for iron-sulfur centers and a molybdenum cofactor (MoCo)-binding domain. Semi-quantitative RT-PCR analysis showed that AhAO1 expression was higher in leaves than in roots of peanut.

Aldehyde oxidase and its contribution to the metabolism of a structurally novel, functionally selective GABAAα5-subtype inverse agonist

(3-Tert-butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)pyrazolo[1,5-d] [1,2,4]triazine was recently identified as a functionally selective, inverse agonist at the benzodiazepine site of GABA(A) alpha5-containing receptors, which enhances performance in animal models of cognition. The routes of metabolism of this compound in rat, dog, rhesus monkey and human in vitro systems, and in vivo in rat, dog and rhesus monkey have been characterized. The current study demonstrates that both a cytosolic oxidative reaction and cytochrome P450 play important roles in the metabolism of the compound. Chemical inhibition studies showed the oxidation in human cytosol to be catalysed predominantly by aldehyde oxidase rather than the related enzyme, xanthine oxidase. The aldehyde oxidase-mediated metabolites were present in vitro and in vivo in both rat and rhesus monkey, and also in vitro in man. They were absent both in vitro and in vivo in dog.

Lack of dimer formation ability in rat strains with low aldehyde oxidase activity

Aldehyde oxidase (AO) is a homodimer with a molecular weight of 300 kDa. To clarify the reasons for the well-known differences in rat strains, we set out to study the relationship between AO activity and the expression levels of its dimer. AO-catalyzed 2-oxidation activity of (S)-RS-8359 was measured in liver cytosols from ten rat strains. The expression levels of AO dimeric protein were evaluated by the native-PAGE/Western blot. Rat strains with low AO activity showed only a monomer, whereas strains with high activity overwhelmingly exhibited a dimer. Exceptionally, one strain in the high AO activity group displayed complex mixed expression patterns of low and high AO activity groups. However, there was a good relationship between AO activity and the expression levels of a dimer, but not of a monomer. The results suggest that rat strains with low AO activity lack the ability to produce a dimer necessary for catalytic activity, and AO differences in rat strains should be discussed in terms of the expression levels of the dimer itself.

Identification of candidate aldehyde oxidases from the silkworm Bombyx mori potentially involved in antennal pheromone degradation

Signal inactivation is a crucial step in the dynamic of olfactory process and involves various Odorant-Degrading Enzymes. In the silkworm Bombyx mori, one of the best models for studying olfaction in insects, the involvement of an antennal-specific aldehyde oxidase in the degradation of the sex pheromone component bombykal has been demonstrated over the three past decades by biochemical studies. However, the corresponding enzyme has never been characterized at the molecular level. Bioinformatic screening of B. mori genome and molecular approaches have been used to isolate several candidate sequences of aldehyde oxidases. Two interesting antennal-expressed genes have been further characterized and their putative functions are discussed in regard to their respective expression pattern and to our knowledge on aldehyde oxidase properties. Interestingly, one gene appeared as specifically expressed in the antennae of B. mori and associated in males with the bombykal-sensitive sensilla, strongly suggesting that it could encode for the previously biochemically characterized enzyme.

Use of Density Functional Calculations To Predict the Regioselectivity of Drugs and Molecules Metabolized by Aldehyde Oxidase

Aldehyde oxidase is a molybdenum hydroxylase that catalyzes the oxidation of aldehydes and nitrogen-containing heterocycles. The enzyme plays a dual role in the metabolism of physiologically important endogenous compounds and the biotransformation of xenobiotics. Using density functional theory methods, geometry optimization of tetrahedral intermediates of drugs and druglike compounds was examined to predict the likely metabolites of aldehyde oxidase. The calculations suggest that the lowest energy tetrahedral intermediate resulting from the initial substrate corresponds to the observed metabolite >or=90% of the time. Additional calculations were performed on a series of heterocyclic compounds where the products resulting from metabolism by xanthine oxidase and aldehyde oxidase differ in many instances. Again, the lowest energy tetrahedral intermediate corresponded to the observed product of aldehyde oxidase metabolism >or=90% for the compounds examined, while the observed products of xanthine oxidase were not well predicted.

Lack of formation of aldehyde oxidase dimer possibly due to 377G>A nucleotide substitution

In addition to the many articles reporting on the marked differences in species and large differences in rat strains in response to aldehyde oxidase (AO), individual differences in some rat strains have also been reported. However, little has been clarified about any related molecular biological mechanisms. We previously revealed that nucleotide substitutions of 377G>A and 2604C>T in the AO gene might be responsible for individual differences in AO activity in Donryu strain rats. By using native polyacrylamide gel electrophoresis/Western blotting in this study, the lack of formation of the AO dimer protein, which is essential for catalytic activity, was shown in poor metabolizer Donryu rats, and this could be a major reason for the individual differences. Rat strain differences were also verified from the same perspectives of nucleotide substitutions and expression levels of a dimer protein. Rat strains with high AO activity showed nucleotide sequences of (377G, 2604C) and a dimer protein. In the case of those with low AO activity, the nucleotide at position 2604 was fixed at T, but varied at position 377, such as G, G/A, and A. An AO dimer was detected in the liver cytosols of rat strains with (377G, 2604T), whereas a monomer was observed in those with (377A, 2604T). These results suggest that the lack of formation of a dimer protein leading to loss of catalytic activity might be due to 377G>A nucleotide substitution. Individual and strain differences in AO activity in rats could be explained by this 377G>A substitution, at least in the rat strains used in this study.

Human Xanthine Oxidase Changes its Substrate Specificity to Aldehyde Oxidase Type upon Mutation of Amino Acid Residues in the Active Site: Roles of Active Site Residues in Binding and Activation of Purine Substrate

Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition, but differ in substrate preference. Here we show that mutation of two amino acid residues in the active site of human XOR for purine substrates results in conversion of the substrate preference to AO type. Human XOR and its Glu803-to-valine (E803V) and Arg881-to-methionine (R881M) mutants were expressed in an Escherichia coli system. The E803V mutation almost completely abrogated the activity towards hypoxanthine as a substrate, but very weak activity towards xanthine remained. On the other hand, the R881M mutant lacked activity towards xanthine, but retained slight activity towards hypoxanthine. Both mutants, however, exhibited significant aldehyde oxidase activity. The crystal structure of E803V mutant of human XOR was determined at 2.6 A resolution. The overall molybdopterin domain structure of this mutant closely resembles that of bovine milk XOR; amino acid residues in the active centre pocket are situated at very similar positions and in similar orientations, except that Glu803 was replaced by valine, indicating that the decrease in activity towards purine substrate is not due to large conformational change in the mutant enzyme. Unlike wild-type XOR, the mutants were not subject to time-dependent inhibition by allopurinol.

Characterization of superoxide production from aldehyde oxidase: an important source of oxidants in biological tissues

Aldehyde oxidase, a molybdoflavoenzyme that plays an important role in aldehyde biotransformation, requires oxygen as substrate and produces reduced oxygen species. However, little information is available regarding its importance in cellular redox stress. Therefore, studies were undertaken to characterize its superoxide and hydrogen peroxide production. Aldehyde oxidase was purified to >98% purity and exhibited a single band at approximately 290 kDa on native polyacrylamide gradient gel electrophoresis. Superoxide generation was measured and quantitated by cytochrome c reduction and EPR spin trapping with p-dimethyl aminocinnamaldehyde as reducing substrate. Prominent superoxide generation was observed with an initial rate of 295 nmol min(-1) mg(-1). Electrochemical measurements of oxygen consumption and hydrogen peroxide formation yielded values of 650 and 355 nmol min(-1) mg(-1). In view of the ubiquitous distribution of aldehydes in tissues, aldehyde oxidase can be an important basal source of superoxide that would be enhanced in disease settings where cellular aldehyde levels are increased.

Cloning, Expression, and Characterization of Male Cynomolgus Monkey Liver Aldehyde Oxidase

In this study, we investigated the properties of monkey liver aldehyde oxidase directed toward the clarification of species differences. The aldehyde oxidase preparation purified from male cynomolgus monkey liver cytosol showed a major 150 kDa Coomassie brilliant blue (CBB)-stained band together with a minor 130 kDa band using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Both bands were identified as being aldehyde oxidase by a database search of the MS data obtained with nano-liquid chromatography, quardrupole time of flight, mass spectrometry (nano-LC Q/TOF MS). Based on the sequence coverage, the 130 kDa protein was presumed to be deficient in 20-30 kDa mass from the N-terminus. Full male cynomolgus monkey aldehyde oxidase cDNA was cloned and sequenced with the four degenerate primers designed by considering the peptide sequences containing the amino acids specific for monkey aldehyde oxidase. The deduced amino acid sequences had 96% amino acid identity with those of human enzyme. The aldehyde oxidase expressed in Escherichia coli also exhibited two immunoreactive bands on SDS-PAGE/Western blot analysis. Further, the biphasic pattern was observed for Eadie-Hofstee plots of the (S)-enantiospecific 2-oxidation activity of RS-8359 with the expressed and cytosolic monkey liver aldehyde oxidase. The results suggested that two forms of aldehyde oxidase in monkey were the expression products by a single gene. In contrast, the similarly expressed rat aldehyde oxidase showed only one immunoreactive protein and monophasic pattern. The biphasic phenomenon could be caused by the existence of two aldehyde oxidase isoforms or two active sites in a single enzyme or some other reasons. Further studies on the problems of the biphasic pattern and species differences in aldehyde oxidase are needed.

Substrate Specificity of Rabbit Aldehyde Oxidase for Nitroguanidine and Nitromethylene Neonicotinoid Insecticides

The nitroguanidine or nitromethylene moiety of the newest major class of insecticides, the neonicotinoids, is important for potency at insect nicotinic receptors and selectivity relative to mammalian receptors. Aldehyde oxidase (AOX) was recently identified as the imidacloprid nitroreductase of mammalian liver, producing both nitrosoguanidine and aminoguanidine metabolites. The present study considers the ability of AOX, partially purified from rabbit liver, to reduce five commercial nitroguanidine (i.e., imidacloprid, thiamethoxam, clothianidin, and dinotefuran) and nitromethylene (i.e., nitenpyram) neonicotinoid insecticides and three derivatives thereof (i.e., the N-methyl and nitromethylene analogues of imidacloprid and desmethylthiamethoxam). LC/MS/MS was used to demonstrate that AOX reduces nitroguanidines to both nitroso- and aminoguanidines, while nitromethylenes are reduced only to the corresponding nitroso metabolites. Additionally, nitrosonitenpyram was found to spontaneously dehydrate to form a 2-cyanoamidine metabolite, mimicking a predominant photoreaction. The substrate specificity of AOX was characterized as follows: Neonicotinoids with a tertiary nitrogen (N-methylimidacloprid and thiamethoxam) are poor substrates; nitroguanidines are metabolized faster than nitromethylenes; and clothianidin is the most rapidly reduced. Kinetic constants were measured for reduction of three nitroguanidines at two concentrations of AOX. At 2 mg protein/mL, only nitroso metabolites were detected, with Km values of 1.03, 2.99, and 2.41 mM and Vmax values of 5.13, 2.54, and 0.98 nmol/min/mg protein measured for clothianidin, imidacloprid, and dinotefuran, respectively. At 5 mg protein/mL, both amino and nitroso metabolites were detected. However, with each nitroguanidine, the formation of nitroso metabolites did not saturate at substrate levels up to 4 mM, whereas amino metabolite formation exhibited Km values of 0.052, 0.16, and 0.084 mM with corresponding Vmax values of 0.80, 1.24, and 0.79 nmol/min/mg protein for clothianidin, imidacloprid, and dinotefuran, respectively. These in vitro observations show large structural differences in the rates of AOX-catalyzed reduction and help to interpret the extensive studies on in vivo metabolism of neonicotinoid insecticides.

A new aldehyde oxidase selectively expressed in chemosensory organs of insects

Signal termination is a crucial step in the dynamic of the olfactory process. It involves different classes of odorant-degrading enzymes. Whereas aldehyde oxidase enzymatic activities have been demonstrated in insect antennae by previous biochemical studies, the corresponding enzymes have never been characterized at the molecular level. In the cabbage armyworm Mamestra brassicae, we isolated for the first time an aldehyde oxidase partial cDNA specifically expressed in chemosensory organs, with the strongest expression in antennae of both sexes. In these organs, expression was restricted to the olfactory sensilla. Our results suggest that the corresponding enzyme could degrade aldehyde odorant compounds, such as pheromones or plant's volatiles.

Geometrical control of the active site electronic structure of pyranopterin enzymes by metal-dithiolate folding: aldehyde oxidase

Density functional calculations on geometry-optimized oxidized (Mo(VI)) and reduced (Mo(IV)) analogues of the isolated active site of aldehyde oxidase (MOP), a member of the xanthine oxidase family of pyranopterindithiolate enzymes, show that fold angle changes of the dithiolate ligand modulate the relative metal and dithiolate contributions to the frontier redox orbitals. Proton abstraction from the equatorial aqua ligand of the oxidized Mo(VI) site also flattens the metal dithiolate fold angle. It is proposed that static and/or dynamic changes in the structure of the protein surrounding the active site can induce changes in the dithiolate fold angle and thereby provide a mechanism for electronic buffering of the redox orbital, for fine-tuning the nucleophilicity of the equatorial aqua/hydroxide ligand, and for modulating the electron-transfer regeneration of the active sites of molybdenum and tungsten enzymes via a "dithiolate folding effect".

Identification of Aldehyde Oxidase as the Neonicotinoid Nitroreductase

Imidacloprid (IMI), the prototypical neonicotinoid insecticide, is used worldwide for crop protection and flea control on pets. It is both oxidatively metabolized by cytochrome P450 enzymes and reduced at the nitroguanidine moiety by a previously unidentified cytosolic "neonicotinoid nitroreductase", the subject of this investigation. Two major metabolites are detected on incubation of IMI with rabbit liver cytosol: the nitrosoguanidine (IMI-NO) and the aminoguanidine (IMI-NH2). Three lines of evidence identify the molybdo-flavoenzyme aldehyde oxidase (AOX, EC 1.2.3.1) as the neonicotinoid nitroreductase. First, classical AOX electron donor substrates (benzaldehyde, 2-hydroxypyrimidine, and N-methylnicotinamide) dramatically increase the rate of formation of IMI metabolites. Allopurinol and diquat are also effective electron donors, while NADPH and xanthine are not. Second, AOX inhibitors (potassium cyanide, menadione, and promethazine) inhibit metabolite formation when N-methylnicotinamide is utilized as an electron donor. Without the addition of an electron donor, rabbit liver cytosol reduces IMI only to IMI-NO at a slow rate. This reduction is also inhibited by potassium cyanide, menadione, and promethazine, as well as by additional AOX inhibitors, cimetidine and chlorpromazine. Finally, IMI nitroreduction by AOX is sensitive to an aerobic atmosphere, but to a much lesser extent than cytochrome P450 2D6. Large species differences are observed in the IMI nitroreductive activity of liver cytosol. While rabbit and monkey (Cynomolgus) give the highest levels of total metabolite formation, human, mouse, cow, and rat also metabolize IMI rapidly. In contrast, dog, cat, and chicken liver cytosols do not reduce IMI at appreciable rates. AOX, as a neonicotinoid nitroreductase, may limit the persistence of IMI, and possibly other neonicotinoids, in mammals.

Potent inhibition of human liver aldehyde oxidase by raloxifene

The selective estrogen receptor modulator, raloxifene, has been demonstrated as a potent uncompetitive inhibitor of human liver aldehyde oxidase-catalyzed oxidation of phthalazine, vanillin, and nicotine-Delta1'(5')-iminium ion, with K(i) values of 0.87 to 1.4 nM. Inhibition was not time-dependent. Raloxifene has also been shown to be a noncompetitive inhibitor of an aldehyde oxidase-catalyzed reduction reaction of a hydroxamic acid-containing compound, with a K(i) of 51 nM. However, raloxifene had only small effects on xanthine oxidase, an enzyme related to aldehyde oxidase. In addition, several other compounds of the same therapeutic class as raloxifene were examined for their potential to inhibit aldehyde oxidase. However, none were as potent as raloxifene, since IC(50) values were orders of magnitude higher and ranged from 0.29 to 57 micro M. In an examination of analogs of raloxifene, it was shown that the bisphenol structure with a hydrophobic group on the 3-position of the benzthiophene ring system was the most important element that imparts inhibitory potency. The relevance of these data to the mechanistic understanding of aldehyde oxidase catalysis, as well as to the potential for raloxifene to cause drug interactions with agents for which aldehyde oxidase-mediated metabolism is important, such as zaleplon or famciclovir, is discussed.

Purification and characterization of an aldehyde oxidase fromPseudomonassp. KY 4690

An aldehyde oxidase, which oxidizes various aliphatic and aromatic aldehydes using O(2) as an electron acceptor, was purified from the cell-free extracts of Pseudomonas sp. KY 4690, a soil isolate, to an electrophoretically homogeneous state. The purified enzyme had a molecular mass of 132 kDa and consisted of three non-identical subunits with molecular masses of 88, 39, and 18 kDa. The absorption spectrum of the purified enzyme showed characteristics of an enzyme belonging to the xanthine oxidase family. The enzyme contained 0.89 mol of flavin adenine dinucleotide, 1.0 mol of molybdenum, 3.6 mol of acid-labile sulfur, and 0.90 mol of 5'-CMP per mol of enzyme protein, on the basis of its molecular mass of 145 kDa. Molecular oxygen served as the sole electron acceptor. These results suggest that aldehyde oxidase from Pseudomonas sp. KY 4690 is a new member of the xanthine oxidase family and might contain 1 mol of molybdenum-molybdpterin-cytosine dinucleotide, 1 mol of flavin adenine dinucleotide, and 2 mol of [2Fe-2S] clusters per mol of enzyme protein. The enzyme showed high reaction rates toward various aliphatic and aromatic aldehydes and high thermostability.