Structural insights into xenobiotic and inhibitor binding to human aldehyde oxidase

Transfer learning enhanced graph neural network for aldehyde oxidase metabolism prediction and its experimental application

Aldehyde oxidase (AOX) is a molybdoenzyme that is primarily expressed in the liver and is involved in the metabolism of drugs and other xenobiotics. AOX-mediated metabolism can result in unexpected outcomes, such as the production of toxic metabolites and high metabolic clearance, which can lead to the clinical failure of novel therapeutic agents. Computational models can assist medicinal chemists in rapidly evaluating the AOX metabolic risk of compounds during the early phases of drug discovery and provide valuable clues for manipulating AOX-mediated metabolism liability. In this study, we developed a novel graph neural network called AOMP for predicting AOX-mediated metabolism. AOMP integrated the tasks of metabolic substrate/non-substrate classification and metabolic site prediction, while utilizing transfer learning from 13C nuclear magnetic resonance data to enhance its performance on both tasks. AOMP significantly outperformed the benchmark methods in both cross-validation and external testing. Using AOMP, we systematically assessed the AOX-mediated metabolism of common fragments in kinase inhibitors and successfully identified four new scaffolds with AOX metabolism liability, which were validated through in vitro experiments. Furthermore, for the convenience of the community, we established the first online service for AOX metabolism prediction based on AOMP, which is freely available at https://aomp.alphama.com.cn.

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.

Dissecting Parameters Contributing to the Underprediction of Aldehyde Oxidase-Mediated Metabolic Clearance of Drugs

We investigated the effect of variability and instability in aldehyde oxidase (AO) content and activity on the scaling of in vitro metabolism data. AO content and activity in human liver cytosol (HLC) and five recombinant human AO preparations (rAO) were determined using targeted proteomics and carbazeran oxidation assay, respectively. AO content was highly variable as indicated by the relative expression factor (REF; i.e., HLC to rAO content) ranging from 0.001 to 1.7 across different in vitro systems. The activity of AO in HLC degrades at a 10-fold higher rate in the presence of the substrate as compared with the activity performed after preincubation without substrate. To scale the metabolic activity from rAO to HLC, a protein-normalized activity factor (pnAF) was proposed wherein the activity was corrected by AO content, which revealed up to sixfold higher AO activity in HLC versus rAO systems. A similar value of pnAF was observed for another substrate, ripasudil. Physiologically based pharmacokinetic (PBPK) modeling revealed a significant additional clearance (CL; 66%), which allowed for the successful prediction of in vivo CL of four other substrates, i.e., O-benzyl guanine, BIBX1382, zaleplon, and zoniporide. For carbazeran, the metabolite identification study showed that the direct glucuronidation may be contributing to around 12% elimination. Taken together, this study identified differential protein content, instability of in vitro activity, role of additional AO clearance, and unaccounted metabolic pathways as plausible reasons for the underprediction of AO-mediated drug metabolism. Consideration of these factors and integration of REF and pnAF in PBPK models will allow better prediction of AO metabolism. SIGNIFICANCE STATEMENT: This study elucidated the plausible reasons for the underprediction of aldehyde oxidase (AO)-mediated drug metabolism and provided recommendations to address them. It demonstrated that integrating protein content and activity differences and accounting for the loss of AO activity, as well as consideration of extrahepatic clearance and additional pathways, would improve the in vitro to in vivo extrapolation of AO-mediated drug metabolism using physiologically based pharmacokinetic modeling.

Discriminating Susceptibility of Xanthine Oxidoreductase Family to Metals

The xanthine oxidoreductase (XOR) family are metal-containing enzymes that use the molybdenum cofactor (Moco), 2Fe-2S clusters, and flavin adenine dinucleotide (FAD) for their catalytic activity. This large molybdoenzyme family includes xanthine, aldehyde, and CO dehydrogenases. XORs are widely distributed from bacteria to humans due to their key roles in the catabolism of purines, aldehydes, drugs, and xenobiotics, as well as interconversions between CO and CO2. Assessing the effect of excess metals on the Rubrivivax gelatinosus bacterium, we found that exposure to copper (Cu) or cadmium (Cd) caused a dramatic decrease in the activity of a high-molecular-weight soluble complex exhibiting nitroblue tetrazolium reductase activity. Mass spectrometry and genetic analyses showed that the complex corresponds to a putative CO dehydrogenase (pCOD). Using mutants that accumulate either Cu+ or Cd2+ in the cytoplasm, we show that Cu+ or Cd2+ is a potent inhibitor of XORs (pCOD and the xanthine dehydrogenase [XDH]) in vivo. This is the first in vivo demonstration that Cu+ affects Moco-containing enzymes. The specific inhibitory effect of these compounds on the XOR activity is further supported in vitro by direct addition of competing metals to protein extracts. Moreover, emphasis is given on the inhibitory effect of Cu on bovine XOR, showing that the XOR family could be a common target of Cu. Given the conservation of XOR structure and function across the tree of life, we anticipate that our findings could be transferable to other XORs and organisms. IMPORTANCE The high toxicity of Cu, Cd, Pb, As, and other metals arises from their ability to cross membranes and target metalloenzymes in the cytoplasm. Identifying these targets provides insights into the toxicity mechanisms. The vulnerability of metalloenzymes arises from the accessibility of their cofactors to ions. Accordingly, many enzymes whose cofactors are solvent exposed are likely to be targets of competing metals. Here, we describe for the first time, with in vivo and in vitro experiments, a direct effect of excess Cu on the xanthine oxidoreductase family (XOR/XDH/pCOD). We show that toxic metal affects these Moco enzymes, and we suggest that access to the Moco center by Cu ions could explain the Cu inhibition of XORs in living organisms. Human XOR activity is associated with hyperuricemia, xanthinuria, gout arthritis, and other diseases. Our findings in vivo highlight XOR as a Cu target and thus support the potential use of Cu in metal-based therapeutics against these diseases.

High-resolution structural-omics of human liver enzymes

We applied raw human liver microsome lysate to a holey carbon grid and used cryo-electron microscopy (cryo-EM) to define its composition. From this sample we identified and simultaneously determined high-resolution structural information for ten unique human liver enzymes involved in diverse cellular processes. Notably, we determined the structure of the endoplasmic bifunctional protein H6PD, where the N- and C-terminal domains independently possess glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase enzymatic activity, respectively. We also obtained the structure of heterodimeric human GANAB, an ER glycoprotein quality-control machinery that contains a catalytic α subunit and a noncatalytic β subunit. In addition, we observed a decameric peroxidase, PRDX4, which directly contacts a disulfide isomerase-related protein, ERp46. Structural data suggest that several glycosylations, bound endogenous compounds, and ions associate with these human liver enzymes. These results highlight the importance of cryo-EM in facilitating the elucidation of human organ proteomics at the atomic level.

The pyridazine heterocycle in molecular recognition and drug discovery

The pyridazine ring is endowed with unique physicochemical properties, characterized by weak basicity, a high dipole moment that subtends π-π stacking interactions and robust, dual hydrogen-bonding capacity that can be of importance in drug-target interactions. These properties contribute to unique applications in molecular recognition while the inherent polarity, low cytochrome P450 inhibitory effects and potential to reduce interaction of a molecule with the cardiac hERG potassium channel add additional value in drug discovery and development. The recent approvals of the gonadotropin-releasing hormone receptor antagonist relugolix (24) and the allosteric tyrosine kinase 2 inhibitor deucravacitinib (25) represent the first examples of FDA-approved drugs that incorporate a pyridazine ring. In this review, the properties of the pyridazine ring are summarized in comparison to the other azines and its potential in drug discovery is illustrated through vignettes that explore applications that take advantage of the inherent physicochemical properties as an approach to solving challenges associated with candidate optimization.

Graphical Abstract

Aldehyde oxidase mediated drug metabolism: an underpredicted obstacle in drug discovery and development

Aldehyde oxidase (AO) has garnered curiosity as a non-CYP metabolizing enzyme in drug development due to unexpected consequences such as toxic metabolite generation and high metabolic clearance resulting in the clinical failure of new drugs. Therefore, poor AO mediated clearance prediction in preclinical nonhuman species remains a significant obstacle in developing novel drugs. Various isoforms of AO, such as AOX1, AOX3, AOX3L1, and AOX4 exist across species, and different AO activity among humans influences the AO mediated drug metabolism. Therefore, carefully considering the unique challenges is essential in developing successful AO substrate drugs. The in vitro to in vivo extrapolation underpredicts AO mediated drug clearance due to the lack of reliable representative animal models, substrate-specific activity, and the discrepancy between absolute concentration and activity. An in vitro tool to extrapolate in vivo clearance using a yard-stick approach is provided to address the underprediction of AO mediated drug clearance. This approach uses a range of well-known AO drug substrates as calibrators for qualitative scaling new drugs into low, medium, or high clearance category drugs. So far, in vivo investigations on chimeric mice with humanized livers (humanized mice) have predicted AO mediated metabolism to the best extent. This review addresses the critical aspects of the drug discovery stage for AO metabolism studies, challenges faced in drug development, approaches to tackle AO mediated drug clearance's underprediction, and strategies to decrease the AO metabolism of drugs.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, “general chemicals,” natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10–15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Bioinformatics-Guided Analysis Uncovers AOX1 as an Osteogenic Differentiation-Relevant Gene of Human Mesenchymal Stem Cells

Background: The available therapeutic options of bone defects, fracture nonunion, and osteoporosis remain limited, which are closely related to the osteogenic differentiation of bone marrow–derived mesenchymal stem cells (BMSCs). Thus, there remains an urgent demand to develop a prediction method to infer osteogenic differentiation–related genes in BMSCs.

Method: We performed differential expression analysis between hBMSCs and osteogenically induced samples. Association analysis, co-expression analysis, and PPI analysis are then carried out to identify potential osteogenesis-related regulators. GO enrichment analysis and GSEA are performed to identify significantly enriched pathways associated with AOX1. qRT-PCR and Western blotting were employed to investigate the expression of genes on osteogenic differentiation, and plasmid transfection was used to overexpress the gene AOX1 in hBMSCs.

Result: We identified 25 upregulated genes and 17 downregulated genes. Association analysis and PPI network analysis among these differentially expressed genes show that AOX1 is a potential regulator of osteogenic differentiation. GO enrichment analysis and GSEA show that AOX1 is significantly associated with osteoblast-related pathways. The experiments revealed that AOX1 level was higher and increased gradually in differentiated BMSCs compared with undifferentiated BMSCs, and AOX1 overexpression significantly increased the expression of osteo-specific genes, thereby clearly indicating that AOX1 plays an important role in osteogenic differentiation. Moreover, our method has ability in discriminating genes with osteogenic differentiation properties and can facilitate the process of discovery of new osteogenic differentiation–related genes.

Conclusion: These findings collectively demonstrate that AOX1 is an osteogenic differentiation-relevant gene and provide a novel method established with a good performance for osteogenic differentiation-relevant genes prediction.

Investigation of Janus Kinase (JAK) Inhibitors for Lung Delivery and the Importance of Aldehyde Oxidase Metabolism

The Janus family of tyrosine kinases (JAK1, JAK2, JAK3, and TYK2) play an essential role in the receptor signaling of cytokines that have been implicated in the pathogenesis of severe asthma, and there is emerging interest in the development of small-molecule-inhaled JAK inhibitors as treatments. Here, we describe the optimization of a quinazoline series of JAK inhibitors and the results of mouse lung pharmacokinetic (PK) studies where only low concentrations of parent compound were observed. Subsequent investigations revealed that the low exposure was due to metabolism by aldehyde oxidase (AO), so we sought to identify quinazolines that were not metabolized by AO. We found that specific substituents at the quinazoline 2-position prevented AO metabolism and this was rationalized through computational docking studies in the AO binding site, but they compromised kinome selectivity. Results presented here highlight that AO metabolism is a potential issue in the lung.

Non-cytochrome P450 enzymes involved in the oxidative metabolism of xenobiotics: Focus on the regulation of gene expression and enzyme activity

Oxidative metabolism is one of the major biotransformation reactions that regulates the exposure of xenobiotics and their metabolites in the circulatory system and local tissues and organs, and influences their efficacy and toxicity. Although cytochrome (CY)P450s play critical roles in the oxidative reaction, extensive CYP450-independent oxidative metabolism also occurs in some xenobiotics, such as aldehyde oxidase, xanthine oxidoreductase, flavin-containing monooxygenase, monoamine oxidase, alcohol dehydrogenase, or aldehyde dehydrogenase-dependent oxidative metabolism. Drugs form a large portion of xenobiotics and are the primary target of this review. The common reaction mechanisms and roles of non-CYP450 enzymes in metabolism, factors affecting the expression and activity of non-CYP450 enzymes in terms of inhibition, induction, regulation, and species differences in pharmaceutical research and development have been summarized. These non-CYP450 enzymes are detoxifying enzymes, although sometimes they mediate severe toxicity. Synthetic or natural chemicals serve as inhibitors for these non-CYP450 enzymes. However, pharmacokinetic-based drug interactions through these inhibitors have rarely been reported in vivo. Although multiple mechanisms participate in the basal expression and regulation of non-CYP450 enzymes, only a limited number of inducers upregulate their expression. Therefore, these enzymes are considered non-inducible or less inducible. Overall, this review focuses on the potential xenobiotic factors that contribute to variations in gene expression levels and the activities of non-CYP450 enzymes.

Interrogating the Inhibition Mechanisms of Human Aldehyde Oxidase by X-ray Crystallography and NMR Spectroscopy: The Raloxifene Case

Human aldehyde oxidase (hAOX1) is mainly present in the liver and has an emerging role in drug metabolism, since it accepts a wide range of molecules as substrates and inhibitors. Herein, we employed an integrative approach by combining NMR, X-ray crystallography, and enzyme inhibition kinetics to understand the inhibition modes of three hAOX1 inhibitors-thioridazine, benzamidine, and raloxifene. These integrative data indicate that thioridazine is a noncompetitive inhibitor, while benzamidine presents a mixed type of inhibition. Additionally, we describe the first crystal structure of hAOX1 in complex with raloxifene. Raloxifene binds tightly at the entrance of the substrate tunnel, stabilizing the flexible entrance gates and elucidating an unusual substrate-dependent mechanism of inhibition with potential impact on drug-drug interactions. This study can be considered as a proof-of-concept for an efficient experimental screening of prospective substrates and inhibitors of hAOX1 relevant in drug discovery.

Oxidative metabolism and pharmacokinetics of the EGFR inhibitor BIBX1382 in chimeric NOG-TKm30 mice transplanted with human hepatocytes

The epidermal growth factor receptor inhibitor BIBX1382 has failed in drug development because of poor oral exposure and low bioavailability associated with its extensive metabolism by aldehyde oxidase (AOX) in humans. In this study, we investigated the metabolic profiles and pharmacokinetics of BIBX1382 in chimeric NOG-TKm30 mice with humanized liver (humanized liver mice). After intravenous and oral BIBX1382 administration, increased plasma clearance and decreased oral exposure together with high production of the predominant oxidative metabolite (M1, BIBU1476) and secondary oxidized metabolite (M2) were observed in humanized liver mice. Extensive oxidation rates of BIBX1382 were observed in hepatocytes from humanized liver mice and were suppressed by the typical human AOX1 inhibitors raloxifene and hydralazine. Liver cytosolic fractions from humans, humanized liver mice, cynomolgus monkeys, minipigs, and guinea pigs, but not fractions from dogs, rabbits, rats, and mice, displayed high BIBX1382 clearance and resulted in oxidative metabolite production. These results indicate that humanized liver mice have human-type AOX activity based on the transplanted human liver AOX1 function. Humanized liver mice can be considered an important animal model for understanding the metabolism and pharmacokinetics of AOX drug substrates.

Classical Xanthinuria in Nine Israeli Families and Two Isolated Cases from Germany: Molecular, Biochemical and Population Genetics Aspects

Classical xanthinuria is a rare autosomal recessive metabolic disorder caused by variants in the XDH (type I) or MOCOS (type II) genes. Thirteen Israeli kindred (five Jewish and eight Arab) and two isolated cases from Germany were studied between the years 1997 and 2013. Four and a branch of a fifth of these families were previously described. Here, we reported the demographic, clinical, molecular and biochemical characterizations of the remaining cases. Seven out of 20 affected individuals (35%) presented with xanthinuria-related symptoms of varied severity. Among the 10 distinct variants identified, six were novel: c.449G>T (p.(Cys150Phe)), c.1434G>A (p.(Trp478*)), c.1871C>G (p.(Ser624*)) and c.913del (p.(Leu305fs*1)) in the XDH gene and c.1046C>T (p.(Thr349Ileu)) and c.1771C>T (p.(Pro591Ser)) in the MOCOS gene. Heterologous protein expression studies revealed that the p.Cys150Phe variant within the Fe/S-I cluster-binding site impairs XDH biogenesis, the p.Thr349Ileu variant in the NifS-like domain of MOCOS affects protein stability and cysteine desulfurase activity, while the p.Pro591Ser and a previously described p.Arg776Cys variant in the C-terminal domain affect Molybdenum cofactor binding. Based on the results of haplotype analyses and historical genealogy findings, the potential dispersion of the identified variants is discussed. As far as we are aware, this is the largest cohort of xanthinuria cases described so far, substantially expanding the repertoire of pathogenic variants, characterizing structurally and functionally essential amino acid residues in the XDH and MOCOS proteins and addressing the population genetic aspects of classical xanthinuria.

The Inactivation of Human Aldehyde Oxidase 1 by Hydrogen Peroxide and Superoxide

Mammalian aldehyde oxidases (AOX) are molybdo-flavoenzymes of pharmacological and pathophysiologic relevance that are involved in phase I drug metabolism and, as a product of their enzymatic activity, are also involved in the generation of reactive oxygen species. So far, the physiologic role of aldehyde oxidase 1 in the human body remains unknown. The human enzyme hAOX1 is characterized by a broad substrate specificity, oxidizing aromatic/aliphatic aldehydes into their corresponding carboxylic acids, and hydroxylating various heteroaromatic rings. The enzyme uses oxygen as terminal electron acceptor to produce hydrogen peroxide and superoxide during turnover. Since hAOX1 and, in particular, some natural variants produce not only H₂O₂ but also high amounts of superoxide, we investigated the effect of both ROS molecules on the enzymatic activity of hAOX1 in more detail. We compared hAOX1 to the high-O₂ ^.--producing natural variant L438V for their time-dependent inactivation with H₂O₂/O₂ ^.- during substrate turnover. We show that the inactivation of the hAOX1 wild-type enzyme is mainly based on the production of hydrogen peroxide, whereas for the variant L438V, both hydrogen peroxide and superoxide contribute to the time-dependent inactivation of the enzyme during turnover. Further, the level of inactivation was revealed to be substrate-dependent: using substrates with higher turnover numbers resulted in a faster inactivation of the enzymes. Analysis of the inactivation site of the enzyme identified a loss of the terminal sulfido ligand at the molybdenum active site by the produced ROS during turnover. SIGNIFICANCE STATEMENT: This work characterizes the substrate-dependent inactivation of human aldehyde oxidase 1 under turnover by reactive oxygen species and identifies the site of inactivation. The role of ROS in the inhibition of human aldehyde oxidase 1 will have a high impact on future studies.

A comprehensive review of in silico approaches for the prediction and modulation of aldehyde oxidase-mediated drug metabolism: The current features, challenges and future perspectives

The importance of aldehyde oxidase (AOX) in drug metabolism necessitates the development and application of the in silico rational drug design methods as an integral part of drug discovery projects for the early prediction and modulation of AOX-mediated metabolism. The current study represents an up-to-date and thorough review of in silico studies of AOX-mediated metabolism and modulation methods. In addition, the challenges and the knowledge gap that should be covered have been discussed. The importance of aldehyde oxidase (AOX) in drug metabolism is a hot topic in drug discovery. Different strategies are available for the modulation of the AOX-mediated metabolism of drugs. Application of the rational drug design methods as an integral part of drug discovery projects is necessary for the early prediction of AOX-mediated metabolism. The current study represents a comprehensive review of AOX molecular structure, AOX-mediated reactions, AOX substrates, AOX inhibition, approaches to modify AOX-mediated metabolism, prediction of AOX metabolism/substrates/inhibitors, and the AOX related structure-activity relationship (SAR) studies. Furthermore, an up-to-date and thorough review of in silico studies of AOX metabolism has been carried out. In addition, the challenges and the knowledge gap that should be covered in the scientific literature have been discussed in the current review.

Recent developments in predicting CYP-independent metabolism

As lead optimization efforts have successfully reduced metabolic liabilities due to cytochrome P450 (CYP)-mediated metabolism, there has been an increase in the frequency of involvement of non-CYP enzymes in the metabolism of investigational compounds. Although there have been numerous notable advancements in the characterization of non-CYP enzymes with respect to their localization, reaction mechanisms, species differences and identification of typical substrates, accurate prediction of non-CYP-mediated clearance, with a particular emphasis with the difficulties in accounting for any extrahepatic contributions, remains a challenge. The current manuscript comprehensively summarizes the recent advancements in the prediction of drug metabolism and the in vitro to in vitro extrapolation of clearance for substrates of non-CYP drug metabolizing enzymes.

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.

Site-Directed Mutagenesis at the Molybdenum Pterin Cofactor Site of the Human Aldehyde Oxidase: Interrogating the Kinetic Differences Between Human and Cynomolgus Monkey

The estimation of the drug clearance by aldehyde oxidase (AO) has been complicated because of this enzyme's atypical kinetics and species and substrate specificity. Since human AO (hAO) and cynomolgus monkey AO (mAO) have a 95.1% sequence identity, cynomolgus monkeys may be the best species for estimating AO clearance in humans. Here, O⁶-benzylguanine (O6BG) and dantrolene were used under anaerobic conditions, as oxidative and reductive substrates of AO, respectively, to compare and contrast the kinetics of these two species through numerical modeling. Whereas dantrolene reduction followed the same linear kinetics in both species, the oxidation rate of O6BG was also linear in mAO and did not follow the already established biphasic kinetics of hAO. In an attempt to determine why hAO and mAO are kinetically distinct, we have altered the hAO V811 and F885 amino acids at the oxidation site adjacent to the molybdenum pterin cofactor to the corresponding alanine and leucine in mAO, respectively. Although some shift to a more monkey-like kinetics was observed for the V811A mutant, five more mutations around the AO cofactors still need to be investigated for this purpose. In comparing the oxidative and reductive rates of metabolism under anaerobic conditions, we have come to the conclusion that despite having similar rates of reduction (4-fold difference), the oxidation rate in mAO is more than 50-fold slower than hAO. This finding implies that the presence of nonlinearity in AO kinetics is dependent upon the degree of imbalance between the rates of oxidation and reduction in this enzyme. SIGNIFICANCE STATEMENT: Although they have as much as 95.1% sequence identity, human and cynomolgus monkey aldehyde oxidase are kinetically distinct. Therefore, monkeys may not be good estimators of drug clearance in humans.

Production of active recombinant human aldehyde oxidase (AOX) in the baculovirus expression vector system (BEVS) and deployment in a pre-clinical fraction-of-control AOX compound exposure assay

Human aldehyde oxidase (AOX) has emerged as a key enzyme activity for consideration in modern drug discovery. The enzyme catalyzes the oxidation of a wide variety of compounds, most notably azaheterocyclics that often form the building blocks of small molecule therapeutics. Failure to consider and assess AOX drug exposure early in the drug development cycle can have catastrophic consequences for novel compounds entering the clinic. AOX is a complex molybdopterin-containing iron-sulfur flavoprotein comprised of two identical 150 kDa subunits that has proven difficult to produce in recombinant form, and a commercial source of the purified human enzyme is currently unavailable. Thus, the potential exposure of novel drug development candidates to human AOX metabolism is usually assessed by using extracts of pooled human liver cytosol as a source of the enzyme. This can complicate the assignment of AOX-specific compound exposure due to its low activity and the presence of contaminating enzymes that may have overlapping substrate specificities. Herein is described a two-step process for the isolation of recombinant human AOX dimers to near homogeneity following production in the baculovirus expression vector system (BEVS). The deployment of this BEVS-produced recombinant human AOX as a substitute for human liver extracts in a fraction-of-control AOX compound-exposure screening assay is described. The ability to generate this key enzyme activity readily in a purified recombinant form provides for a more accurate and convenient approach to the assessment of new compound exposure to bona fide AOX drug metabolism.

Toxicoproteomic Profiling of hPXR Transgenic Mice Treated with Rifampicin and Isoniazid

Tuberculosis is a global health threat that affects millions of people every year, and treatment-limiting toxicity remains a considerable source of treatment failure. Recent reports have characterized the nature of hPXR-mediated hepatotoxicity and the systemic toxicity of antitubercular drugs. The antitubercular drug isoniazid plays a role in such pathologic states as acute intermittent porphyria, anemia, hepatotoxicity, hypercoagulable states (deep vein thrombosis, pulmonary embolism, or ischemic stroke), pellagra (vitamin B₃ deficiency), peripheral neuropathy, and vitamin B₆ deficiency. However, the mechanisms by which isoniazid administration leads to these states are unclear. To elucidate the mechanism of rifampicin- and isoniazid-induced liver and systemic injury, we performed tandem mass tag mass spectrometry-based proteomic screening of mPxr^-/- and hPXR mice treated with combinations of rifampicin and isoniazid. Proteomic profiling analysis suggested that the hPXR liver proteome is affected by antitubercular therapy to disrupt [Fe-S] cluster assembly machinery, [2Fe-2S] cluster-containing proteins, cytochrome P450 enzymes, heme biosynthesis, homocysteine catabolism, oxidative stress responses, vitamin B₃ metabolism, and vitamin B₆ metabolism. These novel findings provide insight into the etiology of some of these processes and potential targets for subsequent investigations. Data are available via ProteomeXchange with identifier PXD019505.

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.

Mechanisms of promiscuity among drug metabolizing enzymes and drug transporters

Detoxication, or 'drug-metabolizing', enzymes and drug transporters exhibit remarkable substrate promiscuity and catalytic promiscuity. In contrast to substrate-specific enzymes that participate in defined metabolic pathways, individual detoxication enzymes must cope with substrates of vast structural diversity, including previously unencountered environmental toxins. Presumably, evolution selects for a balance of 'adequate' kcat /KM values for a wide range of substrates, rather than optimizing kcat /KM for any individual substrate. However, the structural, energetic, and metabolic properties that achieve this balance, and hence optimize detoxication, are not well understood. Two features of detoxication enzymes that are frequently cited as contributions to promiscuity include the exploitation of highly reactive versatile cofactors, or cosubstrates, and a high degree of flexibility within the protein structure. This review examines these intuitive mechanisms in detail and clarifies the contributions of the classic ligand binding models 'induced fit' (IF) and 'conformational selection' (CS) to substrate promiscuity. The available literature data for drug metabolizing enzymes and transporters suggest that IF is exploited by these promiscuous detoxication enzymes, as it is with substrate-specific enzymes, but the detoxication enzymes uniquely exploit 'IFs' to retain a wide range of substrates at their active sites. In contrast, whereas CS provides no catalytic advantage to substrate-specific enzymes, promiscuous enzymes may uniquely exploit it to recruit a wide range of substrates. The combination of CS and IF, for recruitment and retention of substrates, can potentially optimize the promiscuity of drug metabolizing enzymes and drug transporters.

Metabolism by Aldehyde Oxidase: Drug Design and Complementary Approaches to Challenges in Drug Discovery

Aldehyde oxidase (AO) catalyzes oxidations of azaheterocycles and aldehydes, amide hydrolysis, and diverse reductions. AO substrates are rare among marketed drugs, and many candidates failed due to poor pharmacokinetics, interspecies differences, and adverse effects. As most issues arise from complex and poorly understood AO biology, an effective solution is to stop or decrease AO metabolism. This perspective focuses on rational drug design approaches to modulate AO-mediated metabolism in drug discovery. AO biological aspects are also covered, as they are complementary to chemical design and important when selecting the experimental system for risk assessment. The authors' recommendation is an early consideration of AO-mediated metabolism supported by computational and in vitro experimental methods but not an automatic avoidance of AO structural flags, many of which are versatile and valuable building blocks. Preferably, consideration of AO-mediated metabolism should be part of the multiparametric drug optimization process, with the goal to improve overall drug-like properties.

Inhibition of vertebrate aldehyde oxidase as a therapeutic treatment for cancer, obesity, aging and amyotrophic lateral sclerosis

The aldehyde oxidases (AOXs) are a small sub-family of cytosolic molybdo-flavoenzymes, which are structurally conserved proteins and broadly distributed from plants to animals. AOXs play multiple roles in both physiological and pathological processes and AOX inhibition is of increasing significance in the development of novel drugs and therapeutic strategies. This review provides an overview of the evolution and the action mechanism of AOX and the role of each domain. The review provides an update of the polymorphisms in the human AOX. This review also summarises the physiology of AOX in different organs and its role in drug metabolism. The inhibition of AOX is a promising therapeutic treatment for cancer, obesity, aging and amyotrophic lateral sclerosis.

Systematic exploration of predicted destabilizing nonsynonymous single nucleotide polymorphisms (nsSNPs) of human aldehyde oxidase: A Bio-informatics study

Aldehyde Oxidase (hAOX1) is a cytosolic enzyme involved in the metabolism of drugs and xenobiotic compounds. The enzyme belongs to the xanthine oxidase (XO) family of Mo containing enzyme and is a homo-dimer of two 150 kDa monomers. Nonsynonymous Single Nucleotide Polymorphisms (nsSNPs) of hAOX1 have been reported as affecting the ability of the enzyme to metabolize different substrates. Some of these nsSNPs have been biochemically and structurally characterized but the lack of a systematic and comprehensive study regarding all described and validated nsSNPs is urgent, due to the increasing importance of the enzyme in drug development, personalized medicine and therapy, as well as in pharmacogenetic studies. The objective of the present work was to collect all described nsSNPs of hAOX1 and utilize a series of bioinformatics tools to predict their effect on protein structure stability with putative implications on phenotypic functional consequences. Of 526 nsSNPs reported in NCBI-dbSNP, 119 are identified as deleterious whereas 92 are identified as nondeleterious variants. The stability analysis was performed for 119 deleterious variants and the results suggest that 104 nsSNPs may be responsible for destabilizing the protein structure, whereas five variants may increase the protein stability. Four nsSNPs do not have any impact on protein structure (neutral nsSNPs) of hAOX1. The prediction results of the remaining six nsSNPs are nonconclusive. The in silico results were compared with available experimental data. This methodology can also be used to identify and prioritize the stabilizing and destabilizing variants in other enzymes involved in drug metabolism.

Aldehyde oxidase and its role as a drug metabolizing enzyme

Aldehyde oxidase (AO) is a cytosolic enzyme that belongs to the family of structurally related molybdoflavoproteins like xanthine oxidase (XO). The enzyme is characterized by broad substrate specificity and marked species differences. It catalyzes the oxidation of aromatic and aliphatic aldehydes and various heteroaromatic rings as well as reduction of several functional groups. The references to AO and its role in metabolism date back to the 1950s, but the importance of this enzyme in the metabolism of drugs has emerged in the past fifteen years. Several reviews on the role of AO in drug metabolism have been published in the past decade indicative of the growing interest in the enzyme and its influence in drug metabolism. Here, we present a comprehensive monograph of AO as a drug metabolizing enzyme with emphasis on marketed drugs as well as other xenobiotics, as substrates and inhibitors. Although the number of drugs that are primarily metabolized by AO are few, the impact of AO on drug development has been extensive. We also discuss the effect of AO on the systemic exposure and clearance these clinical candidates. The review provides a comprehensive analysis of drug discovery compounds involving AO with the focus on developmental candidates that were reported in the past five years with regards to pharmacokinetics and toxicity. While there is only one known report of AO-mediated clinically relevant drug-drug interaction (DDI), a detailed description of inhibitors and inducers of AO known to date has been presented here and the potential risks associated with DDI. The increasing recognition of the importance of AO has led to significant progress in predicting the site of AO-mediated metabolism using computational methods. Additionally, marked species difference in expression of AO makes it is difficult to predict human clearance with high confidence. The progress made towards developing in vivo, in vitro and in silico approaches for predicting AO metabolism and estimating human clearance of compounds that are metabolized by AO have also been discussed.

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.

Human aldehyde oxidase (hAOX1): structure determination of the Moco-free form of the natural variant G1269R and biophysical studies of single nucleotide polymorphisms

Human aldehyde oxidase (hAOX1) is a molybdenum enzyme with high toxicological importance, but its physiological role is still unknown. hAOX1 metabolizes different classes of xenobiotics and is one of the main drug‐metabolizing enzymes in the liver, along with cytochrome P450. hAOX1 oxidizes and inactivates a large number of drug molecules and has been responsible for the failure of several phase I clinical trials. The interindividual variability of drug‐metabolizing enzymes caused by single nucleotide polymorphisms (SNPs) is highly relevant in pharmaceutical treatments. In this study, we present the crystal structure of the inactive variant G1269R, revealing the first structure of a molybdenum cofactor (Moco)‐free form of hAOX1. These data allowed to model, for the first time, the flexible Gate 1 that controls access to the active site. Furthermore, we inspected the thermostability of wild‐type hAOX1 and hAOX1 with various SNPs (L438V, R1231H, G1269R or S1271L) by CD spectroscopy and ThermoFAD, revealing that amino acid exchanges close to the Moco site can impact protein stability up to 10 °C. These results correlated with biochemical and structural data and enhance our understanding of hAOX1 and the effect of SNPs in the gene encoding this enzyme in the human population.

Enzymes

Aldehyde oxidase (EC1.2.3.1); xanthine dehydrogenase (EC1.17.1.4); xanthine oxidase (EC1.1.3.2).

Databases

Structural data are available in the Protein Data Bank under the accession number 6Q6Q.

Fast Methods for Prediction of Aldehyde Oxidase-Mediated Site-of-Metabolism

Aldehyde Oxidase (AO) is an enzyme involved in the metabolism of aldehydes and N-containing heterocyclic compounds. Many drug compounds contain heterocyclic moieties, and AO metabolism has lead to failure of several late-stage drug candidates. Therefore, it is important to take AO-mediated metabolism into account early in the drug discovery process, and thus, to have fast and reliable models to predict the site of metabolism (SOM). We have collected a dataset of 78 substrates of human AO with a total of 89 SOMs and 347 non-SOMs and determined atomic descriptors for each compound. The descriptors comprise NMR shielding and ESP charges from density functional theory (DFT), NMR chemical shift from ChemBioDraw, and Gasteiger charges from RDKit. Additionally, atomic accessibility was considered using 2D-SASA and relative span descriptors from SMARTCyp. Finally, stability of the product, the metabolite, was determined with DFT and also used as a descriptor. All descriptors have AUC larger than 0.75. In particular, descriptors related to the chemical shielding and chemical shift (AUC = 0.96) and ESP charges (AUC = 0.96) proved to be good descriptors. We recommend two simple methods to identify the SOM for a given molecule: 1) use ChemBioDraw to calculate the chemical shift or 2) calculate ESP charges or chemical shift using DFT. The first approach is fast but somewhat difficult to automate, while the second is more time-consuming, but can easily be automated. The two methods predict correctly 93% and 91%, respectively, of the 89 experimentally observed SOMs.

Gypsy moth genome provides insights into flight capability and virus-host interactions

Since its accidental introduction to Massachusetts in the late 1800s, the European gypsy moth (EGM; Lymantria dispar dispar) has become a major defoliator in North American forests. However, in part because females are flightless, the spread of the EGM across the United States and Canada has been relatively slow over the past 150 years. In contrast, females of the Asian gypsy moth (AGM; Lymantria dispar asiatica) subspecies have fully developed wings and can fly, thereby posing a serious economic threat if populations are established in North America. To explore the genetic determinants of these phenotypic differences, we sequenced and annotated a draft genome of L. dispar and used it to identify genetic variation between EGM and AGM populations. The 865-Mb gypsy moth genome is the largest Lepidoptera genome sequenced to date and encodes ∼13,300 proteins. Gene ontology analyses of EGM and AGM samples revealed divergence between these populations in genes enriched for several gene ontology categories related to muscle adaptation, chemosensory communication, detoxification of food plant foliage, and immunity. These genetic differences likely contribute to variations in flight ability, chemical sensing, and pathogen interactions among EGM and AGM populations. Finally, we use our new genomic and transcriptomic tools to provide insights into genome-wide gene-expression changes of the gypsy moth after viral infection. Characterizing the immunological response of gypsy moths to virus infection may aid in the improvement of virus-based bioinsecticides currently used to control larval populations.

New insights about the monomer and homodimer structures of the human AOX1

We conducted MD simulations to provide a comprehensive study on the human aldehyde oxidase and on the impact that the allosteric inhibitor thioridazine and malonate ions have on its structure, particularly on the catalytic tunnel.

Carbon Monoxide Dehydrogenases

Carbon monoxide dehydrogenases (CODHs) catalyze the reversible oxidation of CO with water to CO₂, two electrons, and two protons. Two classes of CODHs exist, having evolved from different scaffolds featuring active sites built from different transition metals. The basic properties of both classes are described in this overview chapter.

Arabidopsis molybdenum cofactor sulfurase ABA3 contributes to anthocyanin accumulation and oxidative stress tolerance in ABA-dependent and independent ways

Arabidopsis ABA3 is an enzyme involved in the synthesis of the sulfurated form of the molybdenum (Mo) cofactor (MoCo), which is required for the enzymatic activity of so-called Mo enzymes such as aldehyde oxidase (AO) and xanthine dehydrogenase (XDH). It has been reported that AO and XDH are essential for the biosynthesis of the bioactive compounds, ABA and allantoin, respectively. However, aba3 mutants often exhibit pleiotropic phenotypes that are not explained by defects in ABA and/or allantoin biosynthesis, leading us to hypothesize that ABA3 regulates additional metabolic pathways. To reveal the currently unidentified functions of ABA3 we compared transcriptome and metabolome of the Arabidopsis aba3 mutant with those of wild type and a typical ABA-deficient mutant aba2. We found that endogenous levels of anthocyanins, members of the flavonoid group, were significantly lower in the aba3 mutant than in the wild type or the aba2 mutant under oxidative stress. In contrast, mutants defective in the AO and XDH holoenzymes accumulated significantly higher levels of anthocyanins when compared with aba3 mutant under the same conditions. Our findings shed light on a key role of ABA3 in the ABA- and allantoin-independent accumulation of anthocyanins during stress responses.

Metabolism of c-Met Kinase Inhibitors Containing Quinoline by Aldehyde Oxidase, Electron Donating, and Steric Hindrance Effect

Some quinoline-containing c-Met kinase inhibitors are aldehyde oxidase (AO) substrates. 3-Substituted quinoline triazolopyridine analogs were synthesized to understand the electron-donating and steric hindrance effects on AO-mediated metabolism. Metabolic stability studies for these quinoline analogs were carried out in liver cytosol from mice, rats, cynomolgus monkeys, and humans. Several 3-N-substituted analogs were found to be unstable in monkey liver cytosolic incubations (half-life, <10 minutes), and five of them (63, 53, 51, 11, and 71) were chosen for additional mechanistic studies. Mono-oxygenation on the quinoline ring was identified by liquid chromatography tandem mass spectrometry. Metabolite formation was inhibited by the AO inhibitors menadione and raloxifene, but not by the xanthine oxidase inhibitor allopurinol. It was found that small electron-donating groups at the 3-quinoline moiety made the analogs more susceptible to AO metabolism, whereas large 3-substituents could reverse the trend. Although species differences were observed, this trend was applicable to all species tested. Small electron-donating substituents at the 3-quinoline moiety increased both affinity (decreased Michaelis constant) and V _max maximum velocity toward AO in kinetic studies, whereas large substituents decreased both parameters probably as a result of steric hindrance. Based on our analysis, a common structural feature with high AO liability was proposed. Our finding could provide useful information for chemists to minimize potential AO liability when designing quinoline analogs.

X-ray-Structure-Based Identification of Compounds with Activity against Targets from Different Families and Generation of Templates for Multitarget Ligand Design

Compounds with multitarget activity (promiscuity) are increasingly sought in drug discovery. However, promiscuous compounds are often viewed controversially in light of potential assay artifacts that may give rise to false-positive activity annotations. We have reasoned that the strongest evidence for true multitarget activity of small molecules would be provided by experimentally determined structures of ligand-target complexes. Therefore, we have carried out a systematic search of currently available X-ray structures for compounds forming complexes with different targets. Rather unexpectedly, 1418 such crystallographic ligands were identified, including 702 that formed complexes with targets from different protein families (multifamily ligands). About half of these multifamily ligands originated from the medicinal chemistry literature, making it possible to consider additional target annotations and search for analogues. From 168 distinct series of analogues containing one or more multifamily ligands, 133 unique analogue-series-based scaffolds were isolated that can serve as templates for the design of new compounds with multitarget activity. As a part of our study, all of the multifamily ligands we have identified and the analogue-series-based scaffolds are made freely available.

Direct comparison of the four aldehyde oxidase enzymes present in mouse gives insight into their substrate specificities

Mammalian aldehyde oxidases (AOXs) are molybdo-flavoenzymes which are present in many tissues in various mammalian species, including humans and rodents. Different species contain a different number of AOX isoforms. In particular, the reasons why mammals other than humans express a multiplicity of tissue-specific AOX enzymes is unknown. In mouse, the isoforms mAOX1, mAOX3, mAOX4 and mAOX2 are present. We previously established a codon-optimized heterologous expression systems for the mAOX1-4 isoforms in Escherichia coli that gives yield to sufficient amounts of active protein for kinetic characterizations and sets the basis in this study for site-directed mutagenesis and structure-function studies. A direct and simultaneous comparison of the enzymatic properties and characteristics of the four enzymes on a larger number of substrates has never been performed. Here, thirty different structurally related aromatic, aliphatic and N-heterocyclic compounds were used as substrates, and the kinetic parameters of all four mAOX enzymes were directly compared. The results show that especially mAOX4 displays a higher substrate selectivity, while no major differences between mAOX1, mAOX2 and mAOX3 were identified. Generally, mAOX1 was the enzyme with the highest catalytic turnover for most substrates. To understand the factors that contribute to the substrate specificity of mAOX4, site-directed mutagenesis was applied to substitute amino acids in the substrate-binding funnel by the ones present in mAOX1, mAOX3, and mAOX2. An increase in activity was obtained by the amino acid exchange M1088V in the active site identified to be specific for mAOX4, to the amino acid identified in mAOX3.