Mechanism of human aldehyde reductase: characterization of the active site pocket

Exploring aldose reductase inhibitors as promising therapeutic targets for diabetes-linked disabilities

Diabetes mellitus significantly increases mortality and morbidity rates due to complications like neuropathy and nephropathy. It also leads to retinopathy and cataract formation, which is a leading cause of vision disability. The polyol pathway emerges as a promising therapeutic target among the various pathways associated with diabetic complications. This review focuses on the development of natural and synthetic aldose reductase inhibitors (ARIs), along with recent discoveries in diabetic complication treatment. AR, pivotal in the polyol pathway converting glucose to sorbitol, plays a key role in secondary diabetes complications' pathophysiology. Understanding AR's function and structure lays the groundwork for improving ARIs to mitigate diabetic complications. New developments in ARIs open up exciting possibilities for treating diabetes-related complications. However, it is still challenging to get preclinical successes to clinical effectiveness because of things like differences in how the disease starts, drug specificity, and the complexity of the AR's structure. Addressing these challenges is crucial for developing targeted and efficient ARIs. Continued research into AR's structural features and specific ARIs is essential. Overcoming these challenges could revolutionize diabetic complication treatment, enhance patient outcomes, and reduce the global burden of diabetes-related mortality and morbidity.

Esterase-Responsive Fluorogenic Prodrugs of Aldose Reductase Inhibitor Epalrestat: An Innovative Strategy toward Enhanced Anticancer Activity

In addition to the conventional chemotherapeutic drugs, potent inhibitors of key enzymes that are differentially overexpressed in cancer cells and associated with its progression are often considered as the drugs of choice for treating cancer. Aldose reductase (AR), which is primarily associated with complications of diabetes, is known to be closely related to the development of cancer and drug resistance. Epalrestat (EPA), an FDA-approved drug, is a potent inhibitor of AR and exhibits anticancer activity. However, its poor pharmacokinetic properties limit its bioavailability and therapeutic benefits. We report herein the first examples of esterase-responsive turn-on fluorogenic prodrugs for the sustained release of EPA to cancer cells with a turn-on fluorescence readout. Carboxylesterases are known to be overexpressed in several organ-specific cancer cells and help in selective uncaging of drug from the prodrugs. The prodrugs were synthesized using a multistep organic synthesis and successfully characterized. Absorption and emission spectroscopic studies indicated successful activation of the prodrugs in the presence of porcine liver esterase (PLE) under physiological condition. HPLC studies revealed a simultaneous release of both the drug and the fluorophore from the prodrugs over time with mechanistic insights. While the inhibitory potential of EPA released from the prodrugs toward the enzyme AR was validated in the aqueous medium, the anticancer activity of the prodrugs was studied in a representative cervical cancer cell line. Interestingly, our results revealed that the development of the prodrugs can significantly enhance the anticancer potential of EPA. Finally, the drug uncaging process from the prodrugs by the intracellular esterases was studied in the cellular medium by measuring the turn-on fluorescence using fluorescence microscopy. Therefore, the present study highlights the rational development of the fluorogenic prodrugs of EPA, which will help enhance its anticancer potential with better therapeutic potential.

Development of non-acidic 4-methylbenzenesulfonate-based aldose reductase inhibitors; Design, Synthesis, Biological evaluation and in-silicostudies

Design and virtual screening of a set of non-acidic 4-methyl-4-phenyl-benzenesulfonate-based aldose reductase 2 inhibitors had been developed followed by chemical synthesis. Based on the results, the synthesized compounds 2, 4a,b, 7a-c, 9a-c, 10a-c, 11b,c and 14a-c inhibited the ALR2 enzymatic activity in a submicromolar range (99.29-417 nM) and among them, the derivatives 2, 9b, 10a and 14b were able to inhibit ALR2 by IC50 of 160.40, 165.20, 99.29 and 120.6 nM, respectively. Moreover, kinetic analyses using Lineweaver-Burk plot revealed that the most active candidate 10a inhibited ALR2 potently via a non-competitive mechanism. In vivo studies showed that 10 mg/kg of compound 10a significantly lowered blood glucose levels in alloxan-induced diabetic mice by 46.10 %. Moreover, compound 10a showed no toxicity up to a concentration of 50 mg/kg and had no adverse effects on liver and kidney functions. It significantly increased levels of GSH and SOD while decreasing MDA levels, thereby mitigating oxidative stress associated with diabetes and potentially attenuating diabetic complications. Furthermore, the binding mode of compound 10a was confirmed through MD simulation. Noteworthy, compounds 2 and 14b showed moderate antimicrobial activity against the two fungi Aspergillus fumigatus and Aspergillus niger. Finally, we report the thiazole derivative 10a as a new promising non-acidic aldose reductase inhibitor that may be beneficial in treating diabetic complications.

Identification and functional characterization of a novel aldo-keto reductase from Aloe vera

MAIN CONCLUSION

The present investigation profoundly asserted the catalytic potential of plant-based aldo-ketoreductase, postulating its role in polyketide biosynthesis and providing new insights for tailored biosynthesis of vital plant polyketides for therapeutics. Plants hold great potential as a future source of innovative biocatalysts, expanding the possibilities within chemical reactions and generating a variety of benefits. The aldo-keto reductase (AKR) superfamily includes a huge collection of NAD(P)H-dependent oxidoreductases that carry out a variety of redox reactions essential for biosynthesis, detoxification, and intermediary metabolism. The present study involved the isolation, cloning, and purification of a novel aldo-ketoreductase (AvAKR) from the leaves of Aloe vera (Aloe barbadensis Miller) by heterologous gene expression in Escherichia coli based on the unigene sequences of putative ketoreductase and cDNA library screening by oligonucleotide hybridization. The in-silico structural analysis, phylogenetic relationship, and molecular modeling were outranged to approach the novelty of the sequence. Additionally, agroinfiltration of the candidate gene tagged with a green fluorescent protein (GFP) was employed for transient expression in the Nicotiana benthamiana to evaluate the sub-cellular localization of the candidate gene. The AvAKR preferred cytoplasmic localization and shared similarities with the known plant AKRs, keeping the majority of the conserved active-site residues in the AKR superfamily enzymes. The enzyme facilitated the NADPH-dependent reduction of various carbonyl substrates, including benzaldehyde and sugars, proclaiming a broad spectrum range. Our study successfully isolated and characterized a novel aldo-ketoreductase (AvAKR) from Aloe vera, highlighting its versatile NADPH-dependent carbonyl reduction proficiency therewith showcasing its potential as a versatile biocatalyst in diverse redox reactions.

Various concentrations of hesperetin induce different types of programmed cell death in human breast cancerous and normal cell lines in a ROS-dependent manner

The polyphenolic component of citrus fruits, hesperetin (Hst), is a metabolite of hesperidin. In this study, we examined the effect of varying doses and exposure times of hesperetin on MCF-7 and MDA-MB-231 cancer cells, as well as MCF-10A normal cells. By using MTT assay, real-time PCR, western blot, and flow cytometry, we determined the effects of Hst on cell viability, ROS levels, and markers of cell death. Furthermore, molecular docking was used to identify Hst targets that might be involved in ROS-dependent cell death. According to the results, different concentrations of Hst induced different modes of cell death at specific ROS levels. Paraptosis occurred in all cell lines at concentration ranges of IC35 to IC60, and apoptosis occurred at concentrations greater than IC65. In addition, MDA-MB-231 cells were subjected to senescence at sub-toxic doses when treated for a long period of time. When Hst levels were higher, N-acetylcysteine (NAC)'s effect on neutralizing ROS was more pronounced. According to the docking results, Hst may interact with several proteins involved in the regulation of ROS. As an example, the interaction of CCS (Copper chaperone for superoxide dismutase) with Hst might interfere with its chaperone function in folding SOD-1 (superoxide dismutase enzyme), contributing to an increase in cytoplasmic ROS levels. Finally, depending on the ROS level, Hst induces various modes of cell death.

Aldose reductase and cancer metabolism: The master regulator in the limelight

It is strongly established that metabolic reprogramming mediates the initiation, progression, and metastasis of a variety of cancers. However, there is no common biomarker identified to link the dysregulated metabolism and cancer progression. Recent studies strongly advise the involvement of aldose reductase (AR) in cancer metabolism. AR-mediated glucose metabolism creates a Warburg-like effect and an acidic tumour microenvironment in cancer cells. Moreover, AR overexpression is associated with the impairment of mitochondria and the accumulation of free fatty acids in cancer cells. Further, AR-mediated reduction of lipid aldehydes and chemotherapeutics are involved in the activation of factors promoting proliferation and chemo-resistance. In this review, we have delineated the possible mechanisms by which AR modulates cellular metabolism for cancer proliferation and survival. An in-depth understanding of cancer metabolism and the role of AR might lead to the use of AR inhibitors as metabolic modulating agents for the therapy of cancer.

Cytosolic aldose metabolism contributes to progression from cirrhosis to hepatocarcinogenesis

Oxidative stress modulates carcinogenesis in the liver; however, direct evidence for metabolic control of oxidative stress during pathogenesis, particularly, of progression from cirrhosis to hepatocellular carcinoma (HCC), has been lacking. Deficiency of transaldolase (TAL), a rate-limiting enzyme of the non-oxidative branch of the pentose phosphate pathway (PPP), restricts growth and predisposes to cirrhosis and HCC in mice and humans. Here, we show that mitochondrial oxidative stress and progression from cirrhosis to HCC and acetaminophen-induced liver necrosis are critically dependent on NADPH depletion and polyol buildup by aldose reductase (AR), while this enzyme protects from carbon trapping in the PPP and growth restriction in TAL deficiency. Both TAL and AR are confined to the cytosol; however, their inactivation distorts mitochondrial redox homeostasis in opposite directions. The results suggest that AR acts as a rheostat of carbon recycling and NADPH output of the PPP with broad implications for disease progression from cirrhosis to HCC.

A multienzyme S-nitrosylation cascade regulates cholesterol homeostasis

Accumulating evidence suggests that protein S-nitrosylation is enzymatically regulated and that specificity in S-nitrosylation derives from dedicated S-nitrosylases and denitrosylases that conjugate and remove S-nitrosothiols, respectively. Here, we report that mice deficient in the protein denitrosylase SCoR2 (S-nitroso-Coenzyme A Reductase 2; AKR1A1) exhibit marked reductions in serum cholesterol due to reduced secretion of the cholesterol-regulating protein PCSK9. SCoR2 associates with endoplasmic reticulum (ER) secretory machinery to control an S-nitrosylation cascade involving ER cargo-selection proteins SAR1 and SURF4, which moonlight as S-nitrosylases. SAR1 acts as a SURF4 nitrosylase and SURF4 as a PCSK9 nitrosylase to inhibit PCSK9 secretion, while SCoR2 counteracts nitrosylase activity by promoting PCSK9 denitrosylation. Inhibition of PCSK9 by an NO-based drug requires nitrosylase activity, and small-molecule inhibition of SCoR2 phenocopies the PCSK9-mediated reductions in cholesterol observed in SCoR2-deficient mice. Our results reveal enzymatic machinery controlling cholesterol levels through S-nitrosylation and suggest a distinct treatment paradigm for cardiovascular disease.

Natural Compounds as Source of Aldose Reductase (AR) Inhibitors for the Treatment of Diabetic Complications: A Mini Review

BACKGROUND

Aldol reductase (AR) is the polyol pathway's main enzyme that portrays a crucial part in developing 'complications of diabetes' involving cataract, retinopathy, nephropathy, and neuropathy. These diabetic abnormalities are triggered tremendously via aggregation of sorbitol formation (catalyzed by AR) in the polyol pathway. Consequently, it represents an admirable therapeutic target and vast research was done for the discovery of novel molecules as potential AR inhibitors for diabetic complications.

OBJECTIVE

This review article has been planned to discuss an outline of diabetic complications, AR and its role in diabetic complications, natural compounds reported as AR inhibitors, and benefits of natural/plant derived AR inhibitors for the management of diabetic abnormalities.

RESULTS

The goal of AR inhibition remedy is to stabilize the increased flux of blood glucose and sorbitol via the 'polyol pathway' in the affected tissues. A variety of synthetic inhibitors of AR have been established such as tolrestat and sorbinil, but both of these face limitations including low permeability and health problems. Pharmaceutical industries and other scientists were also undertaking work to develop newer, active, and 'safe' AR inhibitors from natural sources. Therefore, several naturally found molecules were documented to possess a potent inhibitory action on AR activity.

CONCLUSION

Natural inhibitors of AR appeared as harmless pharmacological agents for controlling diabetic complications. The detailed literature throughout this article shows the significance of herbal extracts and phytochemicals as prospective useful AR inhibitors in treating diabetic complications.

Characterization and Catalytic-Site-Analysis of an Aldo-Keto Reductase with Excellent Solvent Tolerance

Aldo-keto reductases (AKRs) mediated stereoselective reduction of prochiral carbonyl compounds is an efficient way of preparing single enantiomers of chiral alcohols due to their high chemo-, enantio-, and regio-selectivity. To date, the application of AKRs in the asymmetric synthesis of chiral alcohols has been limited, due to the challenges of cloning and purifying. In this work, the aldo-keto reductase (AKR3-2-9) from Bacillus sp. was obtained, purified and proved to be NADPH-dependent. It exhibits good bioactivity and stability at 37 °C, pH 6.0. AKR3-2-9 is catalytically active on 11 pairs of substrates such as 3-methylcyclohexanone and methyl pyruvate, among which it showed the highest catalytic activity for acetylacetone. In addition, AKR3-2-9 was able to be resistant to five common organic solvents such as methanol and ethanol, it retained high catalytic activity even in a reaction system containing 10% v/v organic solvent for 6 h, which indicates its broad substrate spectrum and exceptional organic solvent tolerance. Furthermore, its three-dimensional structure was constructed and catalytic-site-analysis of the enzyme was conducted. Notably, it was capable of catalyzing the reaction of the key intermediates of duloxetine. The extensive substrate spectrum and predominant organic solvents resistance makes AK3-2-9 a promising enzyme which can be potentially applied in medicine synthesis.

Understanding Aldose Reductase-Inhibitors interactions with free energy simulation

Aldose Reductase (AR) reduces a variety of substrates, such as aldehydes, aldoses and corticosteroids. It is the first and rate-limiting enzyme of the polyol pathway and is an important target enzyme for diabetes-associated complications, including retinopathy, neuropathy, and nephropathy. Inhibitors targeting this enzyme are structurally different and some of them have side effects. In existing publications, computational techniques are applied to investigate the binding affinities of existing inhibitors and predicting the affinities of newly designed ligands. However, these calculations only employ coarse and approximated methods such as docking and MM/PBSA. Brute force simulations are employed to study the dynamics of the system but no converged statistics is obtained. As a result, these computations provide results not consistent with experimental values and large discrepancies exist. In the current work, we employ the enhanced sampling technique of alchemical free energy simulation to calculate the binding affinities of several ligands targeting AR. The statistical error is defined with care and the correlation in the time-series data is fully considered. The statistically optimal estimators are used to extract the free energy estimates and the predicted results are in agreement with the experimental values. Less computationally demanding end-point free energy methods are also performed to compare their efficiency with the alchemical methods. As is expected, the end-point methods are of less accuracy and reliability compared with the alchemical free energy methods. The decomposition of the free energy difference in each alchemical transformation into the enthalpic and entropic components gives further insights on the thermodynamics. The enthalpy-entropy compensation is observed in this case. As the structural data obtained from experiments are only snapshots and more details are needed to understand the dynamics of the protein-ligand system, the conformational ensemble is analyzed. We identify important residues involved in the protein-ligand binding case and short-lived interactions formed due to fluctuations in the conformational ensemble. The current work shed light on the atomic detailed understanding of the dynamics of AR-inhibitors interactions.

Recombinant expression and molecular insights into the catalytic mechanism of an NADPH-dependent conjugated polyketone reductase for the asymmetric synthesis of (R)-pantolactone

(R)-pantolactone is a key chiral intermediate for synthesizing calcium (R)-pantothenate. The commercial synthesis of (R)-pantolactone is performed through the resolution of racemic pantolactone using lactonase-catalyzed enantioselective hydrolysis. The process needs highly toxic hydrogen cyanide and a tedious dynamic kinetic resolution. In this study, we investigated an alternative method to prepare (R)-pantolactone through asymmetric reduction of ketopantolactone (KPL). An NADPH-dependent conjugated polyketone reductase gene from Candida dubliniensis CD36 (CduCPR) was functionally overexpressed in Escherichia coli BL21 (DE3). Recombinant CduCPR belonged to the aldo-keto reductase superfamily, and showed high catalytic activity and stereoselectivity using KPL as the substrate. In a continuous feeding reaction, 200 mM ketopantolactone was reduced to (R)-pantolactone with 98% conversion and 99% enantiomeric excess (e.e.) within 2.0 h. The catalytic mechanism was further investigated. Tyr66 functions as a proton donor following hydrogen transfer from NADPH. Thr30 and His128 are critical residues to bind and orient KPL. Therefore, the recombinant CduCPR from C. dubliniensis exhibited potential application in the asymmetric synthesis of (R)-pantolactone.

Protein S-Nitrosylation: Determinants of Specificity and Enzymatic Regulation of S-Nitrosothiol-Based Signaling

SIGNIFICANCE

Protein S-nitrosylation, the oxidative modification of cysteine by nitric oxide (NO) to form protein S-nitrosothiols (SNOs), mediates redox-based signaling that conveys, in large part, the ubiquitous influence of NO on cellular function. S-nitrosylation regulates protein activity, stability, localization, and protein-protein interactions across myriad physiological processes, and aberrant S-nitrosylation is associated with diverse pathophysiologies. Recent Advances: It is recently recognized that S-nitrosylation endows S-nitroso-protein (SNO-proteins) with S-nitrosylase activity, that is, the potential to trans-S-nitrosylate additional proteins, thereby propagating SNO-based signals, analogous to kinase-mediated signaling cascades. In addition, it is increasingly appreciated that cellular S-nitrosylation is governed by dynamically coupled equilibria between SNO-proteins and low-molecular-weight SNOs, which are controlled by a growing set of enzymatic denitrosylases comprising two main classes (high and low molecular weight). S-nitrosylases and denitrosylases, which together control steady-state SNO levels, may be identified with distinct physiology and pathophysiology ranging from cardiovascular and respiratory disorders to neurodegeneration and cancer.

CRITICAL ISSUES

The target specificity of protein S-nitrosylation and the stability and reactivity of protein SNOs are determined substantially by enzymatic machinery comprising highly conserved transnitrosylases and denitrosylases. Understanding the differential functionality of SNO-regulatory enzymes is essential, and is amenable to genetic and pharmacological analyses, read out as perturbation of specific equilibria within the SNO circuitry.

FUTURE DIRECTIONS

The emerging picture of NO biology entails equilibria among potentially thousands of different SNOs, governed by denitrosylases and nitrosylases. Thus, to elucidate the operation and consequences of S-nitrosylation in cellular contexts, studies should consider the roles of SNO-proteins as both targets and transducers of S-nitrosylation, functioning according to enzymatically governed equilibria.

Molecular recognition of S-nitrosothiol substrate by its cognate protein denitrosylase

Protein S-nitrosylation mediates a large part of nitric oxide's influence on cellular function by providing a fundamental mechanism to control protein function across different species and cell types. At steady state, cellular S-nitrosylation reflects dynamic equilibria between S-nitrosothiols (SNOs) in proteins and small molecules (low-molecular-weight SNOs) whose levels are regulated by dedicated S-nitrosylases and denitrosylases. S-Nitroso-CoA (SNO-CoA) and its cognate denitrosylases, SNO-CoA reductases (SCoRs), are newly identified determinants of protein S-nitrosylation in both yeast and mammals. Because SNO-CoA is a minority species among potentially thousands of cellular SNOs, SCoRs must preferentially recognize this SNO substrate. However, little is known about the molecular mechanism by which cellular SNOs are recognized by their cognate enzymes. Using mammalian cells, molecular modeling, substrate-capture assays, and mutagenic analyses, we identified a single conserved surface Lys (Lys-127) residue as well as active-site interactions of the SNO group that mediate recognition of SNO-CoA by SCoR. Comparing SCoR^K127A versus SCoR^WT HEK293 cells, we identified a SNO-CoA-dependent nitrosoproteome, including numerous metabolic protein substrates. Finally, we discovered that the SNO-CoA/SCoR system has a role in mitochondrial metabolism. Collectively, our findings provide molecular insights into the basis of specificity in SNO-CoA-mediated metabolic signaling and suggest a role for SCoR-regulated S-nitrosylation in multiple metabolic processes.

Structural Determinants of the Selectivity of 3-Benzyluracil-1-acetic Acids toward Human Enzymes Aldose Reductase and AKR1B10

The human enzymes aldose reductase (AR) and AKR1B10 have been thoroughly explored in terms of their roles in diabetes, inflammatory disorders, and cancer. In this study we identified two new lead compounds, 2-(3-(4-chloro-3-nitrobenzyl)-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)acetic acid (JF0048, 3) and 2-(2,4-dioxo-3-(2,3,4,5-tetrabromo-6-methoxybenzyl)-3,4-dihydropyrimidin-1(2H)-yl)acetic acid (JF0049, 4), which selectively target these enzymes. Although 3 and 4 share the 3-benzyluracil-1-acetic acid scaffold, they have different substituents in their aryl moieties. Inhibition studies along with thermodynamic and structural characterizations of both enzymes revealed that the chloronitrobenzyl moiety of compound 3 can open the AR specificity pocket but not that of the AKR1B10 cognate. In contrast, the larger atoms at the ortho and/or meta positions of compound 4 prevent the AR specificity pocket from opening due to steric hindrance and provide a tighter fit to the AKR1B10 inhibitor binding pocket, probably enhanced by the displacement of a disordered water molecule trapped in a hydrophobic subpocket, creating an enthalpic signature. Furthermore, this selectivity also occurs in the cell, which enables the development of a more efficient drug design strategy: compound 3 prevents sorbitol accumulation in human retinal ARPE-19 cells, whereas 4 stops proliferation in human lung cancer NCI-H460 cells.

Cloning and characterization of a NADH-dependent aldo-keto reductase from a newly isolated Kluyveromyces lactis XP1461

An aldo-keto reductase gene (klakr) from Kluyveromyces lactis XP1461 was cloned and heterologously expressed in Escherichia coli. The aldo-keto reductase KlAKR was purified and found to be NADH-dependent with a molecular weight of approximately 36 kDa. It is active and stable at 30 °C and pH 7.0. The maximal reaction rate (vmax), apparent Michaelis-Menten constant (Km) for NADH and t-butyl 6-cyano-(5R)-hydroxy-3-oxohexanoate (1a) and catalytic number (kcat) were calculated as 7.63 U mg(-1), 0.204 mM, 4.42 mM and 697.4 min(-1), respectively. Moreover, the KlAKR has broad substrate specificity to a range of aldehydes, ketones and keto-esters, producing chiral alcohol with e.e. or d.e. >99% for the majority of test substrates.

Aldo-keto Reductase 1B15 (AKR1B15): a mitochondrial human aldo-keto reductase with activity toward steroids and 3-keto-acyl-CoA conjugates

Aldo-keto reductases (AKRs) comprise a superfamily of proteins involved in the reduction and oxidation of biogenic and xenobiotic carbonyls. In humans, at least 15 AKR superfamily members have been identified so far. One of these is a newly identified gene locus, AKR1B15, which clusters on chromosome 7 with the other human AKR1B subfamily members (i.e. AKR1B1 and AKR1B10). We show that alternative splicing of the AKR1B15 gene transcript gives rise to two protein isoforms with different N termini: AKR1B15.1 is a 316-amino acid protein with 91% amino acid identity to AKR1B10; AKR1B15.2 has a prolonged N terminus and consists of 344 amino acid residues. The two gene products differ in their expression level, subcellular localization, and activity. In contrast with other AKR enzymes, which are mostly cytosolic, AKR1B15.1 co-localizes with the mitochondria. Kinetic studies show that AKR1B15.1 is predominantly a reductive enzyme that catalyzes the reduction of androgens and estrogens with high positional selectivity (17β-hydroxysteroid dehydrogenase activity) as well as 3-keto-acyl-CoA conjugates and exhibits strong cofactor selectivity toward NADP(H). In accordance with its substrate spectrum, the enzyme is expressed at the highest levels in steroid-sensitive tissues, namely placenta, testis, and adipose tissue. Placental and adipose expression could be reproduced in the BeWo and SGBS cell lines, respectively. In contrast, AKR1B15.2 localizes to the cytosol and displays no enzymatic activity with the substrates tested. Collectively, these results demonstrate the existence of a novel catalytically active AKR, which is associated with mitochondria and expressed mainly in steroid-sensitive tissues.

Structural and mutational studies on an aldo-keto reductase AKR5C3 from Gluconobacter oxydans

An aldo-keto reductase AKR5C3 from Gluconobacter oxydans (designated as Gox0644) is a useful enzyme with various substrates, including aldehydes, diacetyl, keto esters, and α-ketocarbonyl compounds. The crystal structures of AKR5C3 in apoform in complex with NADPH and the D53A mutant (AKR5C3(-D53A) ) in complex with NADPH are presented herein. Structure comparison and site-directed mutagenesis combined with biochemical kinetics analysis reveal that the conserved Asp53 in the AKR5C3 catalytic tetrad has a crucial role in securing active pocket conformation. The gain-of-function Asp53 to Ala mutation triggers conformational changes on the Trp30 and Trp191 side chains, improving NADPH affinity to AKR5C3, which helps increase catalytic efficiency. The highly conserved Trp30 and Trp191 residues interact with the nicotinamide moiety of NADPH and help form the NADPH-binding pocket. The AKR5C3(-W30A) and AKR5C3(-W191Y) mutants show decreased activities, confirming that both residues facilitate catalysis. Residue Trp191 is in the loop structure, and the AKR5C3(-W191Y) mutant does not react with benzaldehyde, which might also determine substrate recognition. Arg192, which is involved in the substrate binding, is another important residue. The introduction of R192G increases substrate-binding affinity by improving hydrophobicity in the substrate-binding pocket. These results not only supplement the AKRs superfamily with crystal structures but also provide useful information for understanding the catalytic properties of AKR5C3 and guiding further engineering of this enzyme.

Identification of a novel polyfluorinated compound as a lead to inhibit the human enzymes aldose reductase and AKR1B10: structure determination of both ternary complexes and implications for drug design

Aldo-keto reductases (AKRs) are mostly monomeric enzymes which fold into a highly conserved (α/β)8 barrel, while their substrate specificity and inhibitor selectivity are determined by interaction with residues located in three highly variable external loops. The closely related human enzymes aldose reductase (AR or AKR1B1) and AKR1B10 are of biomedical interest because of their involvement in secondary diabetic complications (AR) and in cancer, e.g. hepatocellular carcinoma and smoking-related lung cancer (AKR1B10). After characterization of the IC50 values of both AKRs with a series of polyhalogenated compounds, 2,2',3,3',5,5',6,6'-octafluoro-4,4'-biphenyldiol (JF0064) was identified as a lead inhibitor of both enzymes with a new scaffold (a 1,1'-biphenyl-4,4'-diol). An ultrahigh-resolution X-ray structure of the AR-NADP(+)-JF0064 complex has been determined at 0.85 Å resolution, allowing it to be observed that JF0064 interacts with the catalytic residue Tyr48 through a negatively charged hydroxyl group (i.e. the acidic phenol). The non-competitive inhibition pattern observed for JF0064 with both enzymes suggests that this acidic hydroxyl group is also present in the case of AKR1B10. Moreover, the combination of surface lysine methylation and the introduction of K125R and V301L mutations enabled the determination of the X-ray crystallographic structure of the corresponding AKR1B10-NADP(+)-JF0064 complex. Comparison of the two structures has unveiled some important hints for subsequent structure-based drug-design efforts.

Discovery of new selective human aldose reductase inhibitors through virtual screening multiple binding pocket conformations

Aldose reductase reduces glucose to sorbitol. It plays a key role in many of the complications arising from diabetes. Thus, aldose reductase inhibitors (ARI) have been identified as promising therapeutic agents for treating such complications of diabetes, as neuropathy, nephropathy, retinopathy, and cataracts. In this paper, a virtual screening protocol applied to a library of compounds in house has been utilized to discover novel ARIs. IC50's were determined for 15 hits that inhibited ALR2 to greater than 50% at 50 μM, and ten of these have an IC50 of 10 μM or less, corresponding to a rather substantial hit rate of 14% at this level. The specificity of these compounds relative to their cross-reactivity with human ALR1 was also assessed by inhibition assays. This resulted in identification of novel inhibitors with IC50's comparable to the commercially available drug, epalrestat, and greater than an order of magnitude better selectivity.

Imitation and modification of bioactive lead structures via integration of boron clusters

In medicinal chemistry, carbaboranes can be employed either as boron carriers for boron neutron capture therapy (BNCT) or as scaffolds for radiodiagnostic or therapeutic agents. We have developed a suitable synthesis employing the phosphoramidite method to connect meta-carbaboranyl bis-phosphonites with the 6'-OH group of isopropylidene-protected galactose, followed by oxidation or sulfurization to give the corresponding bis-phosphonates. Deprotection yielded water-soluble compounds. The corresponding disodium salts exhibit especially low cytotoxicity. Preliminary results on the in vivo toxicity and biodistribution of two compounds in mice indicated a lack of selectivity for the cotton rat lung (CRL) tumor chosen for the experiment. For the incorporation of carbaboranes into breast tumor-selective modified neuropeptide Y, [F7, P34]-NPY, a synthesis of a carbaborane-modified lysine derivative was developed. Linkage of the lysine to the boron cluster was achieved by using a propionic acid spacer. Incorporation of the amino acid derivatives into NPY and [F7, P34]-NPY by solid-phase peptide synthesis was successful. Preliminary studies showed that the receptor binding affinity and signal transduction of the boron-modified peptides were very well retained. Asborin, the carbaborane analogue of aspirin, is a rather weak inhibitor of cyclooxygenase-1 (COX-1) and COX-2, but a highly potent aldo/keto reductase 1A1 (AKR1A1) inhibitor. Modification either at the carboxyl group or at the chlorophenyl ring in indomethacin with ortho- and meta-carbaboranyl derivatives gave active derivatives only for the ortho-carbaborane directly attached to the carboxyl group, while the corresponding adamantyl and meta-carbaboranyl derivatives were inactive.

Asborin inhibits Aldo/Keto reductase 1A1

Asborin is the carbaborane analogue of aspirin. Replacement of the phenyl ring in aspirin by ortho-carbaborane was found to change the pharmacological profile of the compound remarkably. Unlike aspirin, asborin cannot selectively acetylate a single serine residue in the active site of cyclooxygenase, and as a result inhibitory potency is reduced. Activation of the acetyl group and the presence of the hydrophobic and bulky cluster therefore did not meet the requirements for cyclooxygenase inhibition. Both features, however, match perfectly for inhibition of the aldo/keto reductase family. Herein, we describe the identification of aldo/keto reductase (AKR) 1A1 as an enzymatic target of asborin, which is inhibited in the low micromolar range. The detailed mode of inhibition was studied and is discussed with respect to the cluster properties. The results shed light on how ortho-carbaborane can be used as a drug synthon, as well as on the development of carbaborane-based inhibitors of other aldo/keto reductases.

Genetic deficiency of aldose reductase counteracts the development of diabetic nephropathy in C57BL/6 mice

AIMS/HYPOTHESIS

The aim of the study was to investigate the effects of genetic deficiency of aldose reductase in mice on the development of key endpoints of diabetic nephropathy.

METHODS

A line of Ar (also known as Akr1b3)-knockout (KO) mice, a line of Ar-bitransgenic mice and control C57BL/6 mice were used in the study. The KO and bitransgenic mice were deficient for Ar in the renal glomeruli and all other tissues, with the exception of, in the bitransgenic mice, a human AR cDNA knockin-transgene that directed collecting-tubule epithelial-cell-specific AR expression. Diabetes was induced in 8-week-old male mice with streptozotocin. Mice were further maintained for 17 weeks then killed. A number of serum and urinary variables were determined for these 25-week-old mice. Periodic acid-Schiff staining, western blots, immunohistochemistry and protein kinase C (PKC) activity assays were performed for histological analyses, and to determine the levels of collagen IV and TGF-β1 and PKC activities in renal cortical tissues.

RESULTS

Diabetes-induced extracellular matrix accumulation and collagen IV overproduction were completely prevented in diabetic Ar-KO and bitransgenic mice. Ar deficiency also completely or partially prevented diabetes-induced activation of renal cortical PKC, TGF-β1 and glomerular hypertrophy. Loss of Ar results in a 43% reduction in urine albumin excretion in the diabetic Ar-KO mice and a 48% reduction in the diabetic bitransgenic mice (p < 0.01).

CONCLUSIONS/INTERPRETATION

Genetic deficiency of Ar significantly ameliorated development of key endpoints linked with early diabetic nephropathy in vivo. Robust and specific inhibition of aldose reductase might be an effective strategy for the prevention and treatment of diabetic nephropathy.

The role of aldehyde reductase AKR1A1 in the metabolism of γ-hydroxybutyrate in 1321N1 human astrocytoma cells

The role of the aldehyde reductase AKR1A1 in the biosynthesis of gamma-hydroxybutyrate (GHB) has been investigated in cell lines using a specific double stranded siRNA designed to knock down expression of the enzyme. This enzyme, along with the aldo-keto reductase AKR7A2, has been proposed previously to be one of the major succinic semialdehyde reductases in brain. The AKR1A1 siRNA was introduced into the human astrocytoma cell line (1321N1) and AKR1A1 expression was monitored using quantitative reverse-transcriptase PCR and Western blots. Results show an 88% reduction in mRNA levels and a 94% reduction in AKR1A1 protein expression 72 h after transfection with the siRNA. Aldehyde reductase activity was examined in silenced cells by following the aldehyde-dependent conversion of NADPH to NADP at 340 nm. This revealed a 30% decrease in pNBA reductase activity in cell extracts after AKR1A1 silencing. Succinic semialdehyde reductase activity was significantly lower in silenced cells when measured using high concentrations (1mM) of succinic semialdehyde, but not with low concentrations (10 μM). The effect of silencing on intracellular and extracellular GHB levels was measured using gas chromatography-mass spectrometry. Results show that AKR1A1 has little effect on the production of GHB, indicating that in this cell line alternative enzymes such as the AKR7A2 are likely to play a more significant role in GHB biosynthesis.

Selective inhibition of the tumor marker AKR1B10 by antiinflammatory N-phenylanthranilic acids and glycyrrhetic acid

A human aldose reductase-like protein, AKR1B10 in the aldo-keto reductase (AKR) superfamily, was recently identified as a tumor marker of several types of cancer. Tolrestat, an aldose reductase inhibitor (ARI), is known to be the most potent inhibitor of the enzyme. In this study, we compared the inhibitory effects of other ARIs including flavonoids on AKR1B10 and aldose reductase to evaluate their specificity. However, ARIs showed lower inhibitory potency for AKR1B10 than for aldose reductase. In the search for potent and selective inhibitors of AKR1B10 from other drugs used clinically, we found that non-steroidal antiinflammatory N-phenylanthranilic acids, diclofenac and glycyrrhetic acid competitively inhibited AKR1B10, showing K(i) values of 0.35-2.9 microM and high selectivity to this enzyme (43-57 fold versus aldose reductase). Molecular docking studies of mefenamic acid and glycyrrhetic acid in the AKR1B10-nicotinamide adenine dinucleotide phosphate (NADP(+)) complex and site-directed mutagenesis of the putative binding residues suggest that the side chain of Val301 and a hydrogen-bonding network among residues Val301, Gln114 and Ser304 are important for determining the inhibitory potency and selectivity of the non-steroidal antiinflammatory drugs. Thus, the potent and selective inhibition may be related to the cancer chemopreventive roles of the drugs, and their structural features may facilitate the design of new anti-cancer agents targeting AKR1B10.

Computational design of Candida boidinii xylose reductase for altered cofactor specificity

In this study we introduce a computationally-driven enzyme redesign workflow for altering cofactor specificity from NADPH to NADH. By compiling and comparing data from previous studies involving cofactor switching mutations, we show that their effect cannot be explained as straightforward changes in volume, hydrophobicity, charge, or BLOSUM62 scores of the residues populating the cofactor binding site. Instead, we find that the use of a detailed cofactor binding energy approximation is needed to adequately capture the relative affinity towards different cofactors. The implicit solvation models Generalized Born with molecular volume integration and Generalized Born with simple switching were integrated in the iterative protein redesign and optimization (IPRO) framework to drive the redesign of Candida boidinii xylose reductase (CbXR) to function using the non-native cofactor NADH. We identified 10 variants, out of the 8,000 possible combinations of mutations, that improve the computationally assessed binding affinity for NADH by introducing mutations in the CbXR binding pocket. Experimental testing revealed that seven out of ten possessed significant xylose reductase activity utilizing NADH, with the best experimental design (CbXR-GGD) being 27-fold more active on NADH. The NADPH-dependent activity for eight out of ten predicted designs was either completely abolished or significantly diminished by at least 90%, yielding a greater than 10(4)-fold change in specificity to NADH (CbXR-REG). The remaining two variants (CbXR-RTT and CBXR-EQR) had dual cofactor specificity for both nicotinamide cofactors.

Doxorubicinolone formation and efflux: a salvage pathway against epirubicin accumulation in human heart

Secondary alcohol metabolites and reactive oxygen species mediate cardiomyopathy induced by cumulative doses of antitumor anthracyclines, such as doxorubicin and epirubicin. Epirubicin exhibits a defective conversion to both toxic species, thereby inducing cardiotoxicity at doses higher than equiactive to doxorubicin; however, the gain in cardiac tolerability seems to be marginal compared with the magnitude of the metabolic defects of epirubicin. Cardiomyopathy may occur independent of toxic metabolites if a given anthracycline tends to accumulate in the heart; therefore, we characterized whether epirubicin showed an unusual accumulation in human myocardial strips incubated in plasma. Epirubicin exhibited a higher uptake and reached myocardial levels 2 times higher than those of doxorubicin. Epirubicin also showed a unique metabolization to doxorubicinolone, the product of epirubicin deglycosidation and carbonyl reduction. In diffusing from the strips to plasma, doxorubicinolone caused membrane permeation effects that augmented epirubicin elimination. Experiments with purified doxorubicinolone showed that the efflux of 1 mol doxorubicinolone promoted the concomitant elimination of as many as approximately 40 mol epirubicin. Doxorubicinolone could also diffuse from plasma back to the strips, causing a permeation effect that promoted epirubicin reuptake; however, this reverse process was slower and less potent. On balance, doxorubicinolone efflux diminished the epirubicin to doxorubicin accumulation ratio to approximately 1.5. These results suggest that the cardiac tolerability of epirubicin is limited by its accumulation in the heart and that such accumulation would be even higher in the absence of doxorubicinolone formation and efflux. These results may also serve guidelines for developing noncardiotoxic anthracyclines.

Identification of the proteome composition occurring during the course of embryonic development of bees (Apis mellifera)

To investigate the proteome during embryonic development of honeybees, Apis mellifera, proteins were identified by two-dimensional gel electrophoresis, mass spectrometry and protein engine identification tools that were applied to MASCOT and Xproteo search engines. 312, 320, 315 proteins were detected in 24, 48 and 72 h embryos. Thirty-eight highly abundant proteins were identified at the three time points by MS fingerprinting. All 21 proteins could be identified as products of annotated genes of the honeybee. Identified proteins included six proteins related to the metabolism of carbohydrates and energy production, six proteins belonging to the heat shock protein family, three cytoskeletal proteins, four proteins related to the antioxidant system of the embryo and two proteins related to growth regulation of the embryo. Quantitative proteomics was applied to analyze differences in amounts of these proteins during the three above mentioned developmental stages. Our data present an initial molecular picture of honeybee embryos, and will hopefully pave the way for future research on this animal.

Tyr-51 is the proton donor-acceptor for NAD(H)-dependent interconversion of xylose and xylitol by Candida tenuis xylose reductase (AKR2B5)

Substitution of active-site Tyr-51 by Ala (Y51A) disrupted the activity of Candida tenuis xylose reductase by six orders of magnitude. External bromide brought about unidirectional rate enhancement ( approximately 2x10(3)-fold at 300mM) for NAD(+)-dependent xylitol oxidation by Y51A. Activity of the wild-type reductase was dependent on a single ionizable protein group exhibiting a pK of 9.2+/-0.1 and 7.3+/-0.3 in the holo-enzyme bound with NADH and NAD(+), respectively. This group which had to be protonated for xylose reduction and unprotonated for xylitol oxidation was eliminated in Y51A, consistent with a catalytic acid-base function of Tyr-51. Bromide may complement the xylitol dehydrogenase activity of Y51A by partly restoring the original hydrogen bond between the reactive alcohol and the phenolate of Tyr-51.

Kinetics of nucleotide binding to the beta-subunit (AKR6A2) of the voltage-gated potassium (Kv) channel

The beta-subunits of the voltage-gated potassium (Kv) channels modulate the kinetics and the gating of Kv channels and assists in channel trafficking and membrane localization. These proteins are members of the AKR6 family. They share a common (alpha/beta)(8) barrel structural fold and avidly bind pyridine nucleotides. Low catalytic activity has been reported for these proteins. Kinetic studies with rat Kvbeta2 revealed that the chemical step is largely responsible for the rate-limitation but nucleotide exchange could also contribute to the overall rate. Herein we report our investigations on the kinetics of cofactor exchange using nucleotide-free preparations of Kvbeta2. Kinetic traces measuring quenching of Kvbeta2 fluorescence by NADP(+) were consistent with a two-step binding mechanism which includes rapid formation of a loose enzyme:cofactor complex followed by a slow conformational rearrangement to form a tight final complex. Closing of the nucleotide enfolding loop, which in the crystal structure folds over the bound cofactor, provides the structural basis for this rearrangement. The rate of the loop opening required to release the cofactor is similar for NADPH and NADP(+) (0.9 min(-1)) and is of the same order of magnitude as the rate of the chemical step estimated previously from kinetic studies with 4-nitrobenzaldehyde (0.3-0.8 min(-1), [S.M. Tipparaju, O.A. Barski, S. Srivastava, A. Bhatnagar, Catalytic mechanism and substrate specificity of the beta-subunit of the voltage-gated potassium channel, Biochemistry 47 (2008) 8840-8854]). Binding of NADPH is accompanied by a second conformational change that might be responsible for a 4-fold higher affinity observed with the reduced cofactor and the resulting difficulty in removing bound NADPH from the protein. These data provide evidence that nucleotide exchange occurs on a seconds-to-minutes time scale and set the upper limit for the maximal possible rate of catalysis by Kvbeta2. Slow cofactor exchange is consistent with the role of the beta-subunit as a metabolic sensor implicated in tonic regulation of potassium currents.

Polyol pathway mediates iron-induced oxidative injury in ischemic-reperfused rat heart

Recent studies have shown that the polyol pathway is involved in ischemia-reperfusion (I/R)-induced myocardial infarction, but the mechanism is unclear. We previously found that lack of aldose reductase (AR), the first enzyme of the polyol pathway, attenuated the increase in transferrin (Tf) level in I/R brain, suggesting that AR contributes to iron-catalyzed free radical-induced damage. We therefore investigated if this mechanism occurs in I/R hearts. We found that inhibition of AR or sorbitol dehydrogenase (SDH), the second enzyme of the polyol pathway, both attenuated the I/R-mediated increases in HIF-1alpha, Tf, TfR, and intracellular iron content and reduced the I/R-induced infarct area of the heart. Further, administration of niacin, which replenishes NAD+, the cofactor for SDH, also normalized TfR and HIF-1alpha levels in I/R hearts. These results suggest that during I/R polyol pathway activity increases the cytosolic NADH/NAD+ ratio. This activates HIF-1alpha that induces the expression of TfR, which in turn increases Tf uptake and iron accumulation and exacerbates oxidative damage that increases the lipid peroxidation. This was confirmed by the fact that administration of the iron chelator deferoxamine attenuated the I/R-induced myocardial infarction.

Catalytic mechanism and substrate specificity of the beta-subunit of the voltage-gated potassium channel

The beta-subunits of voltage-gated potassium (Kv) channels are members of the aldo-keto reductase (AKR) superfamily. These proteins regulate inactivation and membrane localization of Kv1 and Kv4 channels. The Kvbeta proteins bind to pyridine nucleotides with high affinity; however, their catalytic properties remain unclear. Here we report that recombinant rat Kvbeta2 catalyzes the reduction of a wide range of aldehydes and ketones. The rate of catalysis was slower (0.06-0.2 min(-1)) than those of most other AKRs but displayed the expected hyperbolic dependence on substrate concentration, with no evidence of allosteric cooperativity. Catalysis was prevented by site-directed substitution of Tyr-90 with phenylalanine, indicating that the acid-base catalytic residue, identified in other AKRs, has a conserved function in Kvbeta2. The protein catalyzed the reduction of a broad range of carbonyls, including aromatic carbonyls, electrophilic aldehydes and prostaglandins, phospholipids, and sugar aldehydes. Little or no activity was detected with carbonyl steroids. Initial velocity profiles were consistent with an ordered bi-bi rapid equilibrium mechanism in which NADPH binding precedes carbonyl binding. Significant primary kinetic isotope effects (2.0-3.1) were observed under single- and multiple-turnover conditions, indicating that the bond-breaking chemical step is rate-limiting. Structure-activity relationships with a series of para-substituted benzaldehydes indicated that the electronic interactions predominate during substrate binding and that no significant charge develops during the transition state. These data strengthen the view that Kvbeta proteins are catalytically active AKRs that impart redox sensitivity to Kv channels.

Merging the binding sites of aldose and aldehyde reductase for detection of inhibitor selectivity-determining features

Inhibition of human aldose reductase (ALR2) evolved as a promising therapeutic concept to prevent late complications of diabetes. As well as appropriate affinity and bioavailability, putative inhibitors should possess a high level of selectivity for ALR2 over the related aldehyde reductase (ALR1). We investigated the selectivity-determining features by gradually mapping the residues deviating between the binding pockets of ALR1 and ALR2 into the ALR2 binding pocket. The resulting mutational constructs of ALR2 (eight point mutations and one double mutant) were probed for their influence towards ligand selectivity by X-ray structure analysis of the corresponding complexes and isothermal titration calorimetry (ITC). The binding properties of these mutants were evaluated using a ligand set of zopolrestat, a related uracil derivative, IDD388, IDD393, sorbinil, fidarestat and tolrestat. Our study revealed induced-fit adaptations within the mutated binding site as an essential prerequisite for ligand accommodation related to the selectivity discrimination of the ligands. However, our study also highlights the limits of the present understanding of protein-ligand interactions. Interestingly, binding site mutations not involved in any direct interaction to the ligands in various cases show significant effects towards their binding thermodynamics. Furthermore, our results suggest the binding site residues deviating between ALR1 and ALR2 influence ligand affinity in a complex interplay, presumably involving changes of dynamic properties and differences of the solvation/desolvation balance upon ligand binding.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Validation of an automated procedure for the prediction of relative free energies of binding on a set of aldose reductase inhibitors

Among the available methods for predicting free energies of binding of ligands to a protein, the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) and molecular mechanics generalized Born surface area (MM-GBSA) approaches have been validated for a relatively limited number of targets and compounds in the training set. Here, we report the results of an extensive study on a series of 28 inhibitors of aldose reductase with experimentally determined crystal structures and inhibitory activities, in which we evaluate the ability of MM-PBSA and MM-GBSA methods in predicting binding free energies using a number of different simulation conditions. While none of the methods proved able to predict absolute free energies of binding in quantitative agreement with the experimental values, calculated and experimental free energies of binding were significantly correlated. Comparing the predicted and experimental DeltaG of binding, MM-PBSA proved to perform better than MM-GBSA, and within the MM-PBSA methods, the PBSA of Amber performed similarly to Delphi. In particular, significant relationships between experimental and computed free energies of binding were obtained using Amber PBSA and structures minimized with a distance-dependent dielectric function. Importantly, while free energy predictions are usually made on large collections of equilibrated structures sampled during molecular dynamics in water, we have found that a single minimized structure is a reasonable approximation if relative free energies of binding are to be calculated. This finding is particularly relevant, considering that the generation of equilibrated MD ensembles and the subsequent free energy analysis on multiple snapshots is computationally intensive, while the generation and analysis of a single minimized structure of a protein-ligand complex is relatively fast, and therefore suited for high-throughput virtual screening studies. At this aim, we have developed an automated workflow that integrates all the necessary steps required to generate structures and calculate free energies of binding. The procedure is relatively fast and able to screen automatically and iteratively molecules contained in databases and libraries of compounds. Taken altogether, our results suggest that the workflow can be a valuable tool for ligand identification and optimization, being able to automatically and efficiently refine docking poses, which sometimes may not be accurate, and rank the compounds based on more accurate scoring functions.

Substrate specificity and catalytic efficiency of aldo-keto reductases with phospholipid aldehydes

Phospholipid oxidation generates several bioactive aldehydes that remain esterified to the glycerol backbone ('core' aldehydes). These aldehydes induce endothelial cells to produce monocyte chemotactic factors and enhance monocyte-endothelium adhesion. They also serve as ligands of scavenger receptors for the uptake of oxidized lipoproteins or apoptotic cells. The biochemical pathways involved in phospholipid aldehyde metabolism, however, remain largely unknown. In the present study, we have examined the efficacy of the three mammalian AKR (aldo-keto reductase) families in catalysing the reduction of phospholipid aldehydes. The model phospholipid aldehyde POVPC [1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine] was efficiently reduced by members of the AKR1, but not by the AKR6 or the ARK7 family. In the AKR1 family, POVPC reductase activity was limited to AKR1A and B. No significant activity was observed with AKR1C enzymes. Among the active proteins, human AR (aldose reductase) (AKR1B1) showed the highest catalytic activity. The catalytic efficiency of human small intestinal AR (AKR1B10) was comparable with the murine AKR1B proteins 1B3 and 1B8. Among the murine proteins AKR1A4 and AKR1B7 showed appreciably lower catalytic activity as compared with 1B3 and 1B8. The human AKRs, 1B1 and 1B10, and the murine proteins, 1B3 and 1B8, also reduced C-7 and C-9 sn-2 aldehydes as well as POVPE [1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphoethanolamine]. AKR1A4, B1, B7 and B8 catalysed the reduction of aldehydes generated in oxidized C(16:0-20:4) phosphatidylcholine with acyl, plasmenyl or alkyl linkage at the sn-1 position or C(16:0-20:4) phosphatidylglycerol or phosphatidic acid. AKR1B1 displayed the highest activity with phosphatidic acids; AKR1A4 was more efficient with long-chain aldehydes such as 5-hydroxy-8-oxo-6-octenoyl derivatives, whereas AKR1B8 preferred phosphatidylglycerol. These results suggest that proteins of the AKR1A and B families are efficient phospholipid aldehyde reductases, with non-overlapping substrate specificity, and may be involved in tissue-specific metabolism of endogenous or dietary phospholipid aldehydes.

Defective taxane stimulation of epirubicinol formation in the human heart: insight into the cardiac tolerability of epirubicin-taxane chemotherapies

The antitumor anthracycline doxorubicin induces a dose-related cardiotoxicity that correlates with the myocardial levels of its secondary alcohol metabolite doxorubicinol. Combining doxorubicin with taxanes such as paclitaxel or docetaxel may aggravate cardiotoxicity, presumably because the taxanes cause an allosteric-like stimulation of cytoplasmic aldehyde reductases that convert doxorubicin to doxorubicinol in the heart. A less severe aggravation of cardiotoxicity was observed on combining taxanes with epirubicin, a closely related analog of doxorubicin; therefore, we characterized whether the cardiac tolerability of epirubicin-taxane therapies could be due to a defective taxane stimulation of the conversion of epirubicin to its secondary alcohol metabolite epirubicinol. Comparisons between doxorubicin and epirubicin in isolated human heart cytosol showed that epirubicin exhibited a lower V(max)/K(m) value for reaction with aldehyde reductases and a defective stimulation of epirubicinol formation by paclitaxel or docetaxel. A similar pattern occurred in the soluble fraction of human myocardial strips incubated in plasma with anthracyclines and paclitaxel or docetaxel, formulated in their clinical vehicles Cremophor EL or polysorbate 80. Doxorubicin, but not epirubicin, was also able to generate reactive oxygen species in the membrane fraction of myocardial strips; however, the levels of doxorubicin-derived reactive oxygen species were not further augmented by paclitaxel. These results support the notion that taxanes might aggravate the cardiotoxicity of doxorubicin through a specific stimulation of doxorubicinol formation. The failure of paclitaxel or docetaxel to stimulate epirubicinol formation therefore uncovers an important determinant of the improved cardiac tolerability of epirubicin-taxane combinations.