Carbonyl Reductases and Pluripotent Hydroxysteroid Dehydrogenases of the Short-chain Dehydrogenase/reductase Superfamily

AS1DHRS4, a head-to-head natural antisense transcript, silences the DHRS4 gene cluster in cis and trans

The human genome, like other mammalian genomes, encodes numerous natural antisense transcripts (NATs) that have been classified into head-to-head, tail-to-tail, or fully overlapped categories in reference to their sense transcripts. Evidence for NAT-mediated epigenetic silencing of sense transcription remains scanty. The DHRS4 gene encodes a metabolic enzyme and forms a gene cluster with its two immediately downstream homologous genes, DHRS4L2 and DHRS4L1, generated by gene duplication. We identified a head-to-head NAT of DHRS4, designated AS1DHRS4, which markedly regulates the expression of these three genes in the DHRS4 gene cluster. By pairing with ongoing sense transcripts, AS1DHRS4 not only mediates deacetylation of histone H3 and demethylation of H3K4 in cis for the DHRS4 gene, but also interacts physically in trans with the epigenetic modifiers H3K9- and H3K27-specific histone methyltransferases G9a and EZH2, targeting the promoters of the downstream DHRS4L2 and DHRS4L1 genes to induce local repressive H3K9me2 and H3K27me3 histone modifications. Furthermore, AS1DHRS4 induces DNA methylation in the promoter regions of DHRS4L2 by recruiting DNA methyltransferases. This study demonstrates that AS1DHRS4, as a long noncoding RNA, simultaneously controls the chromatin state of each gene within the DHRS4 gene cluster in a discriminative manner. This finding provides an example of transcriptional control over the multiple and highly homologous genes in a tight gene cluster, and may help explain the role of antisense RNAs in the regulation of duplicated genes as the result of genomic evolution.

Crystallization of an atypical short-chain dehydrogenase from Vibrio vulnificus lacking the conserved catalytic tetrad

Short-chain dehydrogenases/reductases (SDRs) are a rapidly expanding superfamily of enzymes that are found in all kingdoms of life. Hallmarked by a highly conserved Asn-Ser-Tyr-Lys catalytic tetrad, SDRs have a broad substrate spectrum and play diverse roles in key metabolic processes. Locus tag VVA1599 in Vibrio vulnificus encodes a short-chain dehydrogenase (hereafter referred to as SDRvv) which lacks the signature catalytic tetrad of SDR members. Structure-based protein sequence alignments have suggested that SDRvv may harbour a unique binding site for its nicotinamide cofactor. To date, structural studies of SDRs with altered catalytic centres are underrepresented in the scientific literature, thus limiting understanding of their spectrum of substrate and cofactor preferences. Here, the expression, purification and crystallization of recombinant SDRvv are presented. Two well diffracting crystal forms could be obtained by cocrystallization in the presence of the reduced form of the phosphorylated nicotinamide cofactor NADPH. The collected data were of sufficient quality for successful structure determination by molecular replacement and subsequent refinement. This work sets the stage for deriving the identity of the natural substrate of SDRvv and the structure-function landscape of typical and atypical SDRs.

Induction of carbonyl reductase 1 (CBR1) expression in human lung tissues and lung cancer cells by the cigarette smoke constituent benzo[a]pyrene

Carbonyl reductase 1 (CBR1) reduces various xenobiotic carbonyl substrates to corresponding alcohol metabolites. Here we demonstrated that benzo[a]pyrene (B[a]P), a potent pro-carcinogen and predominant polycyclic aromatic hydrocarbon (PAH) compound in cigarette smoke and air pollutants, upregulates CBR1 gene expression in vitro and in vivo, and that a proximal xenobiotic response element (XRE) motif (₋₁₂₂XRE) mediates the induction effect of B[a]P. First, we observed 46% and 50% increases in CBR1 mRNA and CBR1 protein levels, respectively, in human lung tissue samples from smokers compared to never-smokers. Second, we detected 3.0-fold (p<0.0001) induction of CBR1 mRNA and 1.5-fold (p<0.01) induction of CBR1 protein levels in cells of the human lung cancer cell line A549 incubated with 2.5 μM B[a]P for 24h. Third, results from experiments with CBR1 promoter constructs indicated that a proximal XRE motif ₋₁₂₂XRE) mediates induction of reporter activity in response to B[a]P. Furthermore, we detected enhanced nuclear translocation of aryl hydrocarbon receptor (AhR) following B[a]P exposure in A549 cells. Finally, we demonstrated increased binding of specific protein complexes to ₋₁₂₂XRE in nuclear extracts from B[a]P-treated cells and the presence of the AhR/Arnt complex in the specific nuclear protein ₋₁₂₂XRE complexes.

Carbonyl reductase 1 offers a novel therapeutic target to enhance leukemia treatment by arsenic trioxide

Arsenic trioxide (As2O3) is used, in current practice, as an effective chemotherapeutic agent for acute promyelocytic leukemia (APL). However, the side effects and relatively low efficacy of As2O3 in treating other leukemias have limited its wider use in therapeutic applications. In the present study, we found that the expression of carbonyl reductase 1 (CBR1) affects the resistance to As2O3 in leukemias, including APL; As2O3 upregulated CBR1 expression at the transcriptional level by stimulating the activity of the transcription factor activator protein-1. Moreover, CBR1 overexpression was sufficient to protect cells against As2O3 through modulation of the generation of reactive oxygen species, whereas the attenuation of CBR1 was sufficient to sensitize cells to As2O3. A combination treatment with the specific CBR1 inhibitor hydroxy-PP-Me remarkably increased As2O3-induced apoptotic cell death compared with As2O3 alone, both in vitro and in vivo. These results were confirmed in primary cultured human acute and chronic myeloid leukemia cells, with no significant cell death observed in normal leukocytes. Taken together, our findings indicate that CBR1 contributes to the low efficacy of As2O3 and, therefore, is a rational target for the development of combination chemotherapy with As2O3 in diverse leukemias including APL.

Selective inhibition of human type-5 17β-hydroxysteroid dehydrogenase (AKR1C3) by baccharin, a component of Brazilian propolis

The human aldo-keto reductase (AKR) 1C3, also known as type-5 17β-hydroxysteroid dehydrogenase and prostaglandin F synthase, has been suggested as a therapeutic target in the treatment of prostate and breast cancers. In this study, AKR1C3 inhibition was examined by Brazilian propolis-derived cinnamic acid derivatives that show potential antitumor activity, and it was found that baccharin (1) is a potent competitive inhibitor (K(i) 56 nM) with high selectivity, showing no significant inhibition toward other AKR1C isoforms (AKR1C1, AKR1C2, and AKR1C4). Molecular docking and site-directed mutagenesis studies suggested that the nonconserved residues Ser118, Met120, and Phe311 in AKR1C3 are important for determining the inhibitory potency and selectivity of 1. The AKR1C3-mediated metabolism of 17-ketosteroid and farnesal in cancer cells was inhibited by 1, which was effective from 0.2 μM with an IC(50) value of about 30 μM. Additionally, 1 suppressed the proliferation of PC3 prostatic cancer cells stimulated by AKR1C3 overexpression. This study is the first demonstration that 1 is a highly selective inhibitor of AKR1C3.

The role of Ala231 and Trp227 in the substrate specificities of fungal 17β-hydroxysteroid dehydrogenase and trihydroxynaphthalene reductase: Steroids versus smaller substrates

17β-Hydroxysteroid dehydrogenase and trihydroxynaphthalene reductase from the fungus Curvularia lunata (teleomorph: Cochliobolus lunatus; 17β-HSDcl and 3HNR, respectively) are two homologous short-chain dehydrogenase/reductase proteins that are 58% identical and have 86% similar amino acids. The minor differences in their substrate-binding regions are believed to be crucial for their substrate specificities. 3HNR shows high affinity for substrates with two rings, like trihydroxynaphthalene and 2,3-dihydro-2,5-dihydroxy-4H-benzopyran-4-one (DDBO), while 17β-HSDcl can accommodate ligands with four rings, like steroids. In the present study, we examined the role of Ala231 in 17β-HSDcl and Trp227 in 3HNR, as the potential key amino acids in the determination of substrate recognition based on size. We constructed Ala231Trp 17β-HSDcl and Trp227Ala 3HNR mutant proteins and used spectrophotometric analyses to compare their catalytic activities with those of the wild-type enzymes, for oxidation of 4-estrene-17β-ol-3-one and DDBO and for reduction of 4-estrene-3,17-dione and 9,10-phenanthrenequinone (PQ). The Ala231Trp side-chain substitution in 17β-HSDcl abolished and decreased (by 14.6-fold) the initial rates for steroid oxidation and reduction, respectively, while the initial rate for PQ reduction was increased 5.6-fold. The bulky Trp227Ala side-chain substitution in 3HNR enabled oxidation of 4-estrene-17β-ol-3-one, increased the initial rates for reduction of 4-estrene-3,17-dione and PQ by 4.5-fold and 1.5-fold, respectively, while the initial rate for DDBO oxidation was decreased 4.1-fold. Our TLC analysis and docking simulations also support these findings. Our study thus confirms the important roles of Ala231 in 17β-HSDcl and Trp227 in 3HNR, for the selection between larger and smaller substrates. Article from a special issue on steroids and microorganisms.

Short-chain dehydrogenases/reductases in cyanobacteria

The short-chain dehydrogenases/reductases (SDRs) represent a large superfamily of enzymes, most of which are NAD(H)-dependent or NADP(H)-dependent oxidoreductases. They display a wide substrate spectrum, including steroids, alcohols, sugars, aromatic compounds, and xenobiotics. On the basis of characteristic sequence motifs, the SDRs are subdivided into two main (classical and extended) and three smaller (divergent, intermediate, and complex) families. Despite low residue identities in pairwise comparisons, the three-dimensional structure among the SDRs is conserved and shows a typical Rossmann fold. Here, we used a bioinformatics approach to determine whether and which SDRs are present in cyanobacteria, microorganisms that played an important role in our ecosystem as the first oxygen producers. Cyanobacterial SDRs could indeed be identified, and were clustered according to the SDR classification system. Furthermore, because of the early availability of its genome sequence and the easy application of transformation methods, Synechocystis sp. PCC 6803, one of the most important cyanobacterial strains, was chosen as the model organism for this phylum. Synechocystis sp. SDRs were further analysed with bioinformatics tools, such as hidden Markov models (HMMs). It became evident that several cyanobacterial SDRs show remarkable sequence identities with SDRs in other organisms. These so-called 'homologous' proteins exist in plants, model organisms such as Drosophila melanogaster and Caenorhabditis  elegans, and even in humans. As sequence identities of up to 60% were found between Synechocystis and humans, it was concluded that SDRs seemed to have been well conserved during evolution, even after dramatic terrestrial changes such as the conversion of the early reducing atmosphere to an oxidizing one by cyanobacteria.

Mouse 11β-hydroxysteroid dehydrogenase type 2 for human application: homology modeling, structural analysis and ligand-receptor interaction

Mouse (m) 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) was homology-modeled, and its structure and ligand-receptor interaction were analyzed. The modeled m11βHSD2 showed significant 3D similarities to the human (h) 11βHSD1 and 2 structures. The contact energy profiles of the m11βHSD2 model were in good agreement with those of the h11βHSD1 and 2 structures. The secondary structure of the m11βHSD2 model exhibited a central 6-stranded all-parallel β-sheet sandwich-like structure, flanked on both sides by 3-helices. Ramachandran plots revealed that only 1.1% of the amino acid residues were in the disfavored region for m11βHSD2. Further, the molecular surfaces and electrostatic analyses of the m11βHSD2 model at the ligand-binding site exhibited that the model was almost identical to the h11βHSD2 model. Furthermore, docking simulation and ligand-receptor interaction analyses revealed the similarity of the ligand-receptor bound conformation between the m11βHSD2 and h11βHSD2 models. These results indicate that the m11βHSD2 model was successfully evaluated and analyzed. To the best of our knowledge, this is the first report of a m11βHSD2 model with detailed analyses, and our data verify that the mouse model can be utilized for application to the human model to target 11βHSD2 for the development of anticancer drugs.

A conserved antioxidant response element (ARE) in the promoter of human carbonyl reductase 3 (CBR3) mediates induction by the master redox switch Nrf2

Carbonyl reductase activity catalyzes the two electron reduction of several endogenous and exogenous carbonyl substrates. Recent data indicate that the expression of human carbonyl reductase 3 (CBR3) is regulated by the master redox switch Nrf2. Nrf2 binds to conserved antioxidant response elements (AREs) in the promoters of target genes. The presence of functional AREs in the CBR3 promoter has not yet been reported. In this study, experiments with reporter constructs showed that the prototypical Nrf2 activator tert-butyl hydroquinone (t-BHQ) induces CBR3 promoter activity in cultures of HepG2 (2.7-fold; p<0.05) and MCF-7 cells (22-fold; p<0.01). Computational searches identified a conserved ARE in the distal CBR3 promoter region ((-2698)ARE). Deletion of this ARE from a 4212-bp CBR3 promoter construct impacted basal promoter activity and induction of promoter activity in response to treatment with t-BHQ. Deletion of (-2698)ARE also impacted the induction of CBR3 promoter activity in cells overexpressing Nrf2. Electrophoretic mobility shift assays (EMSA) demonstrated increased binding of specific protein complexes to (-2698)ARE in nuclear extracts from t-BHQ treated cells. The presence of Nrf2 in the specific nuclear protein-(-2698)ARE complexes was evidenced in EMSA experiments with anti-Nrf2 antibodies. These data suggest that the distal (-2698)ARE mediates the induction of human CBR3 in response to prototypical activators of Nrf2.

The catalytic promiscuity of a microbial 7α-hydroxysteroid dehydrogenase. Reduction of non-steroidal carbonyl compounds

A thermostable 7α-hydroxysteroid dehydrogenase from Bacteroides fragilis ATCC 25285 was found to catalyze the reduction of various benzaldehyde analogues to their corresponding benzyl alcohols. The enzyme activity was dependent upon the substituent on the benzene ring of the substrates. Benzaldehydes with electron-withdrawing substituent usually showed higher activity than those with electron-donating groups. Furthermore, this enzyme was tolerant to some organic solvents. These results together with previous studies suggested that 7α-hydroxysteroid dehydrogenase from B. fragilis might play multiple functional roles in biosynthesis and metabolism of bile acids, and in the detoxification of xenobiotics containing carbonyl groups in the large intestine. In addition, its broad substrate spectrum offers great potential for finding applications not only in the synthesis of steroidal compounds of pharmaceutical importance, but also for the production of other high-value fine chemicals.

Molecular docking and structural analysis of cofactor-protein interaction between NAD⁺ and 11β-hydroxysteroid dehydrogenase type 2

Molecular docking and structural analysis of the cofactor-protein interaction between NAD(+) and human (h) or mouse (m) 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) were performed with the molecular operating environment (MOE). 11βHSD1 (PDB code: 3HFG) was selected as a template for the 3D structure modeling of 11βHSD2. The MOE docking (MOE-dock) and the alpha sphere and excluded volume-based ligand-protein docking (ASE-dock) showed that both NAD(+)-h11βHSD2 and NAD(+)-m11βHSD2 models have a similar binding orientation to the template cofactor-protein model. Our present study also revealed that Asp91, Phe94, Tyr232 and Thr267 could be of importance in the interaction between NAD(+) and 11βHSD2. NADP(+) was incapable of entering into the cofactor-binding site of the 11βHSD2 models. The present study proposes the latest models for 11βHSD2 and its cofactor NAD(+), and to the best of our knowledge, this is the first report of a m11βHSD2 model with NAD(+).

In vitro metabolism of dexamethasone cipecilate, a novel synthetic corticosteroid, in human liver and nasal mucosa

Dexamethasone cipecilate (DX-CP, 9-fluoro-11β,17,21-trihydroxy-16α-methylpregna-1,4-diene-3,20-dione 21-cyclohexanecarboxylate 17-cyclopropanecarboxylate) is a novel synthetic corticosteroid used to treat allergic rhinitis. The pharmacological effect of DX-CP is considered to be mainly due to its active de-esterified metabolite (DX-17-CPC). To investigate the in vitro metabolism of DX-CP in human liver, DX-CP was incubated with human liver microsomes and S9. In addition, a metabolism study of DX-CP with human nasal mucosa was carried out in order to elucidate whether DX-17-CPC is formed in nasal mucosa, the site of action of DX-CP. DX-17-CPC was the major metabolite in both liver microsomes and S9. Two new epoxide metabolites, UK1 and UK2, were detected in liver S9, while only UK1 was detected in liver microsomes. This suggests that cytosol enzymes are responsible for the formation of UK2. In human nasal mucosa, DX-CP was mainly transformed into DX-17-CPC. By using recombinant human carboxylesterases (CESs), the reaction was shown to be catalyzed by CES2. These results provide the evidence that the active metabolite DX-17-CPC is the main contributor to the pharmacological action after the intranasal administration of DX-CP to humans.

17β-Hydroxysteroid dehydrogenases (17β-HSDs) as therapeutic targets: protein structures, functions, and recent progress in inhibitor development

17β-Hydroxysteroid dehydrogenases (17β-HSDs) are oxidoreductases, which play a key role in estrogen and androgen steroid metabolism by catalyzing final steps of the steroid biosynthesis. Up to now, 14 different subtypes have been identified in mammals, which catalyze NAD(P)H or NAD(P)(+) dependent reductions/oxidations at the 17-position of the steroid. Depending on their reductive or oxidative activities, they modulate the intracellular concentration of inactive and active steroids. As the genomic mechanism of steroid action involves binding to a steroid nuclear receptor, 17β-HSDs act like pre-receptor molecular switches. 17β-HSDs are thus key enzymes implicated in the different functions of the reproductive tissues in both males and females. The crucial role of estrogens and androgens in the genesis and development of hormone dependent diseases is well recognized. Considering the pivotal role of 17β-HSDs in steroid hormone modulation and their substrate specificity, these proteins are promising therapeutic targets for diseases like breast cancer, endometriosis, osteoporosis, and prostate cancer. The selective inhibition of the concerned enzymes might provide an effective treatment and a good alternative to the existing endocrine therapies. Herein, we give an overview of functional and structural aspects for the different 17β-HSDs. We focus on steroidal and non-steroidal inhibitors recently published for each subtype and report on existing animal models for the different 17β-HSDs and the respective diseases. Article from the Special issue on Targeted Inhibitors.

Homology modeling and structural analysis of 11β-hydroxysteroid dehydrogenase type 2

11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) was homology-modeled by a Boltzmann-weighted randomized modeling procedure, using the X-ray crystal structure of 11βHSD1 (PDB code: 3HFG) as a template. The model exhibited significant 3D similarities to 11βHSD1. The contact energy profiles of the 11βHSD2 model were in good agreement with that of the X-ray structure of 11βHSD1. The secondary structure of the 11βHSD2 model exhibited a central 6-stranded all-parallel β-sheet sandwich-like structure, flanked on both sides by 3-helices. Ramachandran plots revealed that only 1.9% of the amino acid residues were in the disfavored region for 11βHSD2. Furthermore, the ligand-binding site (LBS) volume was calculated to be 845 Å(3), which suggests that the LBS of 11βHSD2 is sufficiently large to contain cofactors and substrates (ligands), such as NAD(+) and cortisol. The electrostatic analysis revealed that the 11βHSD2 model had a positive potential at the LBS, which indicates that 11βHSD2 possibly attracts negatively charged ligands at the LBS. These results indicate that the model was successfully evaluated and analyzed. Consequently, it is proposed that the 11βHSD2 model in the present study will be suitable for further in silico structure-based de novo antitumor drug designing. To the best of our knowledge, this is the latest report of an accurate 11βHSD2 model to target 11βHSD2 for the development of anticancer drugs.

Human carbonyl reductase 1 upregulated by hypoxia renders resistance to apoptosis in hepatocellular carcinoma cells

BACKGROUND & AIMS

Human carbonyl reductase1 (CBR1) has been reported to protect cells against lipid peroxidation. Since human hepatocellular carcinoma (HCC) cells are under oxidative stress in hypoxic conditions, we tested if CBR1 is upregulated by hypoxia inducible factor (HIF)-1α, helps tumor growth under hypoxia, and renders chemoresistance to cisplatin and doxorubicin in HCC.

METHODS

Luciferase, EMSA, and chromatin immunoprecipitation (ChIP) assays were performed to analyze whether HIF-1α transactivates CBR1 promoter. CBR1 overexpression, siRNA, and inhibitors were used to study the role of CBR1 in tumor survival under hypoxia and chemoresistance to cisplatin and doxorubicin in HCC. FACS and Western blot analysis for oxidative stress markers were performed to measure ROS. Immunohistochemistry (IHC) was performed to analyze CBR1 expression in 78 cases of HCC and 123 cases of colon cancer tissues.

RESULTS

The CBR1 promoter was activated by HIF-1α. CBR1 overexpression enhanced cell survival by decreasing oxidative stress under hypoxia, cisplatin, and doxorubicin treatment. CBR1-siRNA increased apoptosis via increasing oxidative stress. Combinational therapy of CBR1 inhibitors with cisplatin or doxorubicin enhanced cell death in HCC cells. IHC showed CBR1 overexpression in 56 (72%) out of 78 HCC and 88 (72%) out of 123 colon cancer cases.

CONCLUSIONS

Overexpressed CBR1 by HIF-1α plays important roles in tumor growth under hypoxia and chemoresistance to anticancer drugs. The inhibition of CBR1 by specific inhibitors enhances anticancer drug efficacy in HCC. Therefore, we concluded that CBR1 is a good molecular target for the development of anticancer drugs for HCC patients.

Studies on reduction of S-nitrosoglutathione by human carbonyl reductases 1 and 3

Human carbonyl reductases 1 and 3 (CBR1 and CBR3) are monomeric NADPH-dependent enzymes of the short-chain dehydrogenase/reductase superfamily. Despite 72% identity in primary structure they exhibit substantial differences in substrate specificity. Recently, the endogenous low molecular weight S-nitrosothiol S-nitrosoglutathione (GSNO) has been added to the broad substrate spectrum of CBR1. The current study initially addressed whether CBR3 could equally reduce GSNO which was not the case. Neither the introduction of residues which contribute to glutathione binding in CBR1, i.e. K106Q and S97V/D98A, nor the exchange C143S, which prevents a theoretical disulfide bond with C227 in CBR3, could engender activity towards GSNO. However, exchanging amino acids 236-244 in CBR3 to correspond to CBR1 was sufficient to engender catalytic activity towards GSNO. Catalytic efficiency was further improved by the exchanges Q142M, C143S, P230W and H270S. Hence, the same residues previously reported as important for reduction of carbonyl compounds appear to be key to CBR1-mediated reduction of GSNO. Furthermore, for CBR1-mediated reduction of GSNO, considerable substrate inhibition at concentrations >5 K(m) was observed. Treatment of CBR1 with GSNO followed by removal of low molecular weight compounds decreased the GSNO reducing activity, suggesting a covalent modification. Treatment with dithiothreitol, but not with ascorbic acid, could rescue the activity, indicating S-glutathionylation rather than S-nitrosation as the underlying mechanism. As C227 has previously been identified as the reactive cysteine in CBR1, the variant CBR1 C227S was generated, which, in comparison to the wild-type protein, displayed a similar k(cat), but a 30-fold higher K(m), and did not show substrate inhibition. Collectively, the results clearly argue for a physiological role of CBR1, but not for CBR3, in GSNO reduction and thus ultimately in regulation of NO signaling. Furthermore, at higher concentrations, GSNO appears to work as a suicide inhibitor for CBR1, probably through glutathionylation of C227.

The Drosophila carbonyl reductase sniffer is an efficient 4-oxonon-2-enal (4ONE) reductase

Studies with the fruit-fly Drosophila melanogaster demonstrated that the enzyme sniffer prevented oxidative stress-induced neurodegeneration. Mutant flies overexpressing sniffer had significantly extended life spans in a 99.5% oxygen atmosphere compared to wild-type flies. However, the molecular mechanism of this protection remained unclear. Sequence analysis and database searches identified sniffer as a member of the short-chain dehydrogenase/reductase superfamily with a 27.4% identity to the human enzyme carbonyl reductase type I (CBR1). As CBR1 catalyzes the reduction of the lipid peroxidation products 4HNE and 4ONE, we tested whether sniffer is able to metabolize these lipid derived aldehydes by carbonyl reduction. To produce recombinant enzyme, the coding sequence of sniffer was amplified from a cDNA-library, cloned into a bacterial expression vector and the His-tagged protein was purified by Ni-chelate chromatography. We found that sniffer catalyzed the NADPH-dependent carbonyl reduction of 4ONE (K(m)=24±2 μM, k(cat)=500±10 min(-1), k(cat)/K(m)=350 s(-1) mM(-1)) but not that of 4HNE. The reaction product of 4ONE reduction by sniffer was mainly 4HNE as shown by HPLC- and GC/MS analysis. Since 4HNE, though still a potent electrophile, is less neurotoxic and protein reactive than 4ONE, one mechanism by which sniffer exerts its neuroprotective effects in Drosophila after oxidative stress may be enzymatic reduction of 4ONE.

Naturally occurring variants of human aldo-keto reductases with reduced in vitro metabolism of daunorubicin and doxorubicin

Doxorubicin (DOX) and daunorubicin (DAUN) are effective anticancer drugs; however, considerable interpatient variability exists in their pharmacokinetics. This may be caused by altered metabolism by nonsynonymous single-nucleotide polymorphisms (ns-SNPs) in genes encoding aldo-keto reductases (AKRs) and carbonyl reductases. This study examined the effect of 27 ns-SNPs, in eight human genes, on the in vitro metabolism of both drugs to their major metabolites, doxorubicinol and daunorubicinol. Kinetic assays measured metabolite levels by high-performance liquid chromatography separation with fluorescence detection using purified, histidine-tagged, human wild-type, and variant enzymes. Maximal rate of activity (V(max)), substrate affinity (K(m)), turnover rate (k(cat)), and catalytic efficiency (k(cat)/K(m)) were determined. With DAUN as substrate, variants for three genes exhibited significant differences in these parameters compared with their wild-type counterparts: the A106T, R170C, and P180S variants significantly reduced metabolism compared with the AKR1C3 wild-type (V(max), 23-47% decrease; k(cat), 22-47%; k(cat)/K(m), 38-44%); the L311V variant of AKR1C4 significantly decreased V(max) (47% lower) and k(cat) and k(cat)/K(m) (both 43% lower); and the A142T variant of AKR7A2 significantly affected all kinetic parameters (V(max) and k(cat), 61% decrease; K(m), 156% increase; k(cat)/K(m), 85% decrease). With DOX, the R170C and P180S variants of AKR1C3 showed significantly reduced V(max) (41-44% decrease), k(cat) (39-45%), and k(cat)/K(m) (52-69%), whereas the A142T variant significantly altered all kinetic parameters for AKR7A2 (V(max), 41% decrease; k(cat), 44% decrease; K(m), 47% increase; k(cat)/K(m), 60% decrease). These findings suggest that ns-SNPs in human AKR1C3, AKR1C4, and AKR7A2 significantly decrease the in vitro metabolism of DOX and DAUN.

Bacterial degradation of bile salts

Bile salts are surface-active steroid compounds. Their main physiological function is aiding the digestion of lipophilic nutrients in intestinal tracts of vertebrates. Many bacteria are capable of transforming and degrading bile salts in the digestive tract and in the environment. Bacterial bile salt transformation and degradation is of high ecological relevance and also essential for the biotechnological production of steroid drugs. While biotechnological aspects have been reviewed many times, the physiological, biochemical and genetic aspects of bacterial bile salt transformation have been neglected. This review provides an overview of the reaction sequence of bile salt degradation and on the respective enzymes and genes exemplified with the degradation pathway of the bile salt cholate. The physiological adaptations for coping with the toxic effects of bile salts, recent biotechnological applications and ecological aspects of bacterial bile salt metabolism are also addressed. As the pathway for bile salt degradation merges with metabolic pathways for bacterial transformation of other steroids, such as testosterone and cholesterol, this review provides helpful background information for metabolic engineering of steroid-transforming bacteria in general.

Physiologic roles of hepatic lipid droplets and involvement of peroxisome proliferator-activated receptor alpha in their dynamism

The liver is not a storage site of excess energy as triacylglycerides but a major site of carbohydrate storage, playing a vital role in glucose homeostasis, and the hepatic lipid droplets (LDs) should have a distinct physiologic role from those in lipid-storing tissues. Most studies so far have been limited to characterization of the LDs in cultured cells or of the liver of animals maintained on a normal laboratory diet, and little is known about the properties of the LDs in the liver responding to dietary excess, irregular fats, and potentially toxic compounds contained in a natural food diet. We started to characterize the hepatic LDs in wild-type and peroxisome proliferator-activated receptor alpha (PPARalpha)-null mice fed various natural diets by identifying the liver-enriched LD-associated proteins and the changes in lipid compositions. Based on the currently available data, we propose the hypothesis that hepatic LDs play vital protective roles against diet-derived excess fatty acids and potentially toxic hydrophobic compounds by temporarily storing them as neutral lipids or compounds until completion of the remodeling of fatty acids and detoxification of the compounds in a PPARalpha-dependent manner.

Redox control of endoplasmic reticulum function

The lumen of the endoplasmic reticulum constitutes a separate intracellular compartment with a special proteome and metabolome. The redox conditions of the organelle are also characteristically different from those of the other subcellular compartments. The luminal environment has been considered more oxidizing than the cytosol due to the presence of oxidative protein folding. However, recent observations suggest that redox systems in reduced and oxidized states are present simultaneously. The concerted action of membrane transporters and oxidoreductase enzymes maintains the oxidized state of the thiol-disulfide and the reduced state of the pyridine nucleotide redox systems, which are prerequisites for the normal redox reactions localized in the organelle. The powerful thiol-oxidizing machinery of oxidative protein folding continuously challenges the local antioxidant defense. Alterations of the luminal redox conditions, either in oxidizing or reducing direction, affect protein processing, are sensed by the accumulation of misfolded/unfolded proteins, and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce programmed cell death if these attempts fail. Recent findings strongly support the involvement of redox-based endoplasmic reticulum stress in a plethora of human diseases, either as causative agents or as complications.

Selenoprotein T deficiency alters cell adhesion and elevates selenoprotein W expression in murine fibroblast cells

Mammalian selenoproteins have diverse functions, cellular locations, and evolutionary histories, but all use the amino acid selenocysteine (Sec), often present in the enzyme's active site. Only about half of mammalian selenoproteins have been functionally characterized, with most being oxidoreductases. The cellular role of selenoprotein T (SelT), manifesting a CxxU motif in a thioredoxin-like fold and localized to Golgi and the endoplasmic reticulum, is not known. To examine its biological function, we knocked down SelT expression in mouse fibroblast cells and found that SelT deficiency alters cell adhesion and enhances the expression of several oxidoreductase genes, while decreasing the expression of genes involved in cell structure organization, suggesting the involvement of SelT in redox regulation and cell anchorage. Furthermore, we found that the loss of SelT elevates expression of another selenoprotein, selenoprotein W (SepW1). SelT and SepW1 belong to the same protein family, suggesting that SepW1 may functionally compensate for SelT.

The glucocorticoid-activating enzyme 11beta-hydroxysteroid dehydrogenase type 1 has broad substrate specificity: Physiological and toxicological considerations

The primary function of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) is to catalyze the conversion of inactive to active glucocorticoid hormones and to modulate local glucocorticoid-dependent gene expression. Thereby 11beta-HSD1 plays a key role in the regulation of metabolic functions and in the adaptation of the organism to energy requiring situations. Importantly, elevated 11beta-HSD1 activity has been associated with metabolic disorders, and recent investigations with rodent models of obesity and type 2 diabetes provided evidence for beneficial effects of 11beta-HSD1 inhibitors, making this enzyme a promising therapeutic target. Several earlier and recent studies, mainly performed in vitro, revealed a relatively broad substrate spectrum of 11beta-HSD1 and suggested that this enzyme has additional functions in the metabolism of some neurosteroids (7-oxy- and 11-oxyandrogens and -progestins) and 7-oxysterols, as well as in the detoxification of various xenobiotics that contain reactive carbonyl groups. While there are many studies on the effect of inhibitors on cortisone reduction and circulating glucocorticoid levels and on the transcriptional regulation of 11beta-HSD1 in obesity and diabetes, only few address the so-called alternative functions of this enzyme. We review recent progress on the biochemical characterization of 11beta-HSD1, with a focus on cofactor and substrate specificity and on possible alternative functions of this enzyme.

Naturally occurring variants of human CBR3 alter anthracycline in vitro metabolism

Doxorubicin (DOX) and daunorubicin (DAUN) are anthracycline anticancer agents; however, considerable interpatient variability exists in their pharmacokinetics. This interpatient variability is attributed in part to altered metabolism by nonsynonymous single-nucleotide polymorphisms (ns-SNPs) in genes encoding the carbonyl reductases. This study examines the effect of seven naturally occurring ns-SNPs in the CBR3 gene on in vitro metabolism of anthracyclines to doxorubicinol and daunorubicinol. Kinetic assays measure metabolite levels by high-performance liquid chromatography separation with fluorescence detection by use of purified, histidine-tagged, human CBR3 wild type and variant enzymes. The V224M, C4Y, and V93I variants resulted in significantly reduced maximal reaction velocity (V(max)) for both anthracyclines compared with the wild-type enzyme, whereas the M235L variant had significantly reduced V(max) for DOX only. Significant increases in substrate affinity were found for the V244M variant with DAUN, as well as the C4Y and V93I variants with DOX. The catalytic efficiency values for the V244M, C4Y, and V93I variants were significantly lower than the wild type for DAUN and DOX. Furthermore, DOX was observed to be a better substrate than DAUN for the wild-type enzyme and its variants. HapMap analysis indicated that a haplotype carrying the C4Y and V244M mutations may occur in some individuals in the 11 ethnic populations studied in the HapMap project. Our preparation of the double mutant indicated a significant reduction in activity compared with the wild-type enzyme and single-mutant preparations. These findings suggest that commonly occurring ns-SNPs in human CBR3 significantly alter the in vitro metabolism of DOX and DAUN.

Stereospecific reduction of a potent kinesin spindle protein (KSP) inhibitor in human tissues

Compound A, 1-{(3R,3aR)-3-[3-(4-acetylpiperazin-1-yl)propyl]-7-fluoro-3-phenyl-3a,4-dihydro-3H-pyrazolo[5,1-c][1,4]benzoxazin-2-yl}ethanone, is a novel and potent inhibitor of the mitotic kinesin spindle protein. Metabolism studies with human hepatocytes showed that Compound A underwent significant ketone reduction to its biologically active metabolite M1. Here, we describe the studies that characterized the metabolic interconversion between Compound A and M1 in vitro in human tissues. LC-MS/MS analysis showed that the ketone reduction was stereospecific for M1 with no diastereomer of M1 detected in incubations with human hepatocytes. Interestingly, such stereospecific ketone reduction was not observed with Compound B, the enantiomer of Compound A. No reductive products were observed when Compound B was incubated with human hepatocytes. Studies with human liver subcellular fractions showed that Compound A was reduced to M1 primarily by human liver cytosol with little contribution from human liver microsomes and mitochondria. NADPH was the preferred cofactor for the reduction reaction. Reverse oxidation of M1 back to Compound A was also observed, preferentially in human liver cytosol with NADP(+) as the cofactor. The interconversion between Compound A and M1 in human liver cytosol was inhibited significantly by flufenamic acid and phenolphthalein (potent inhibitors for aldo-keto reductase 1Cs, p<0.05), but not by menadione, a selective inhibitor for carbonyl reductase. In addition to the liver, S9 from human lung and kidney was also capable of catalyzing this interconversion. Collectively, the results implicated the aldo-keto reductase 1Cs as the most likely enzymes responsible for the metabolic interconversion of Compound A and its active metabolite M1.

Molecular modeling of the reductase domain to elucidate the reaction mechanism of reduction of peptidyl thioester into its corresponding alcohol in non-ribosomal peptide synthetases

Background

Nonribosomal peptide synthetases (NRPSs) are multienzymatic, multidomain megasynthases involved in the biosynthesis of pharmaceutically important nonribosomal peptides. The peptaibol synthetase from Trichoderma virens (TPS) is an important member of the NRPS family that exhibits antifungal properties. The majority of the NRPSs terminate peptide synthesis with the thioesterase (TE) domain, which either hydrolyzes the thioester linkage, releasing the free peptic acid, or catalyzes the intramolecular macrocyclization to produce a macrolactone product. TPS is an important NRPS that does not encompass a TE domain, but rather a reductase domain (R domain) to release the mature peptide product reductively with the aid of a NADPH cofactor. However, the catalytic mechanism of the reductase domain has not yet been elucidated.

Results

We present here a three-dimensional (3D) model of the reductase domain based on the crystal structure of vestitone reductase (VR). VR belongs to the short-chain dehydrogenase/reductase (SDR) superfamily and is responsible for the nicotinamide dinucleotide phosphate (NADPH)-dependent reduction of the substrate into its corresponding secondary alcohol product. The binding sites of the probable linear substrates, alamethicin, trichotoxin, antiamoebin I, chrysopermin C and gramicidin, were identified within the modeled R domain using multiple docking approaches. The docking results of the ligand in the active site of the R domain showed that reductase side chains have a high affinity towards ligand binding, while the thioester oxygen of each substrate forms a hydrogen bond with the OH group of Tyr176 and the thiol group of the substrate is closer to the Glu220. The modeling and docking studies revealed the reaction mechanism of reduction of thioester into a primary alcohol.

Conclusion

Peptaibol biosynthesis incorporates a single R domain, which appears to catalyze the four-electron reduction reaction of a peptidyl carrier protein (PCP)-bound peptide to its corresponding primary alcohol. Analysis of R domains present in the non-redundant (nr) database of the NCBI showed that the R domain always resides in the last NRPS module and is involved in either a two or four-electron reduction reaction.

Structural basis for substrate specificity in human monomeric carbonyl reductases

Carbonyl reduction constitutes a phase I reaction for many xenobiotics and is carried out in mammals mainly by members of two protein families, namely aldo-keto reductases and short-chain dehydrogenases/reductases. In addition to their capacity to reduce xenobiotics, several of the enzymes act on endogenous compounds such as steroids or eicosanoids. One of the major carbonyl reducing enzymes found in humans is carbonyl reductase 1 (CBR1) with a very broad substrate spectrum. A paralog, carbonyl reductase 3 (CBR3) has about 70% sequence identity and has not been sufficiently characterized to date. Screening of a focused xenobiotic compound library revealed that CBR3 has narrower substrate specificity and acts on several orthoquinones, as well as isatin or the anticancer drug oracin. To further investigate structure-activity relationships between these enzymes we crystallized CBR3, performed substrate docking, site-directed mutagenesis and compared its kinetic features to CBR1. Despite high sequence similarities, the active sites differ in shape and surface properties. The data reveal that the differences in substrate specificity are largely due to a short segment of a substrate binding loop comprising critical residues Trp229/Pro230, Ala235/Asp236 as well as part of the active site formed by Met141/Gln142 in CBR1 and CBR3, respectively. The data suggest a minor role in xenobiotic metabolism for CBR3.

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Diazepane-acetamide derivatives as selective 11beta-hydroxysteroid dehydrogenase type 1 inhibitors

WO2008052638 describes the identification and synthesis of diazepane- acetamide derivatives as a novel class of selective small molecule inhibitors of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) for the treatment of metabolic syndrome. The generic structure of the disclosed diazepane-acetamide derivatives offers considerable possibilities for modifications that allow optimizing compound properties. Further studies to assess target selectivity, species-specificity, modulation of tissue-specific functions of 11beta-HSD1 as well as interference with alternative functions of this enzyme are needed to explore the therapeutic potential of these chemicals.

Nitric oxide synthase variants and disease-free survival among treated and untreated breast cancer patients in a Southwest Oncology Group clinical trial

PURPOSE

Numerous chemotherapeutic agents are cytotoxic through generation of reactive species, and variability in genes related to oxidative stress may influence disease-free survival (DFS). We examined relationships between DFS and variants in NOS3, as well as NQO1, NQO2, and CBR3, among treated and untreated breast cancer patients in a Southwest Oncology Group clinical trial (S8897).

EXPERIMENTAL DESIGN

In the parent trial, women were assigned according to prognostic features; the high-risk group was randomized to cyclophosphamide, i.v. methotrexate, and 5-fluorouracil or to cyclophosphamide, i.v. doxorubicin, and 5-fluorouracil +/- tamoxifen, and the low-risk group did not receive adjuvant therapy. We extracted DNA from normal lymph node tissue and examined functional polymorphisms in NOS3, NQO1, NQO2, and CBR3, in relation to DFS, using Cox proportional hazard model.

RESULTS

There were significant interactions between DFS, adjuvant therapy, and NOS3 Glu298Asp and -786 polymorphisms, alone and in combination (P for interaction = 0.008). When NOS3 genotypes were combined, women with genotypes encoding for lower nitric oxide who received chemotherapy had a >2-fold increase in hazard of progression (hazard ratio, 2.32; 95% confidence interval, 1.26-4.25), whereas there was reduced risk for those who did not receive adjuvant therapy (hazard ratio, 0.42; 95% confidence interval, 0.19-0.95). There were no associations between the other genotypes and DFS in either group.

CONCLUSION

Variants encoding lower activity of NOS3 may affect outcomes in breast cancer patients, with the direction of risk differing depending on chemotherapy status. These results may mirror the known dual functions of nitric oxide and nitric oxide synthase, depending on oxidative environment.

Two nonsynonymous single nucleotide polymorphisms of human carbonyl reductase 1 demonstrate reduced in vitro metabolism of daunorubicin and doxorubicin

Carbonyl reductases (CBRs) are a group of metabolic enzymes belonging to the short-chain dehydrogenase family with NADPH-dependent oxidoreductase activity. These enzymes are known to metabolize the anthracyclines doxorubicin (DOX) and daunorubicin (DAUN). Both DOX and DAUN are highly effective in cancer therapy; however, there is considerable interpatient variability in adverse effects seen in patients undergoing treatment with these drugs. This may be attributed to altered metabolism associated with nonsynonymous single nucleotide polymorphisms (ns-SNPs) in the genes encoding for CBRs. In this study, we examine the effect of the V88I and P131S mutations in the human CBR1 gene on the metabolism of anthracyclines to their respective major metabolites, doxorubicinol and daunorubicinol. Kinetic studies using purified, histidine-tagged, recombinant enzymes in a high-performance liquid chromatography-fluorescence assay demonstrated that the V88I mutation leads to a significantly reduced maximal rate of activity (V(max)) (2090 +/- 112 and 257 +/- 11 nmol/min x mg of purified protein for DAUN and DOX, respectively) compared with that for the wild-type (3430 +/- 241 and 364 +/- 37 nmol/min x mg of purified protein for DAUN and DOX, respectively). In the case of the P131S mutation, a significant increase in substrate affinity (K(m)) was observed for DAUN only (89 +/- 13 microM) compared with that for the wild-type (51 +/- 13 microM). In the presence of either anthracycline, both variants exhibited a 20 to 40% decrease in catalytic efficiency (k(cat)/K(m)) compared with that for the wild-type enzyme. Therefore, the ns-SNPs generating both these mutations may alter bioavailability of these anthracyclines in cancer patients and should be examined in clinical studies as potential biomarkers for DAUN- and DOX-induced adverse effects.

Expression profiles of human 11beta-hydroxysteroid dehydrogenases type 1 and type 2 in inflammatory bowel diseases

Inflammatory bowel diseases such as Crohn's disease (CD) and ulcerative colitis (UC) are characterized by an increase in pro-inflammatory cytokines. On the other hand, endogenous cortisol is regarded as physiological compound to combat inflammation. The local activation of glucocorticoids is mediated by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) which increases cortisol, and 11beta-HSD2 which decreases cortisol concentrations. We hypothesized that in inflamed tissues of patients suffering from inflammatory bowel diseases 11beta-HSD1 is upregulated whereas 11beta-HSD2 is downregulated. By using quantitative real-time PCR, we investigated the transcription levels of 11beta-HSD1 and 11beta-HSD2 in patients diagnosed with CD or UC. Expression of 11beta-HSD1 was significantly elevated in inflamed tissue compared to non-inflamed colonic tissue in both, CD (2.7-fold) and UC (3.8-fold), whereas 11beta-HSD2 expression was decreased in the same samples. In both diseases, male patients showed a more pronounced upregulation of 11beta-HSD1 (CD: 4.8-fold, UC: 6.5-fold) compared to females (CD: 1.8-fold, UC: 1.8-fold), a fact which might be due to the higher levels of circulating anti-inflammatory estrogens in women. Our data support the hypothesis that both enzymes play a crucial role in inflammation by affecting local tissue ratios between active and inactive glucocorticoids.

The SDR (short-chain dehydrogenase/reductase and related enzymes) nomenclature initiative

Short-chain dehydrogenases/reductases (SDR) constitute one of the largest enzyme superfamilies with presently over 46,000 members. In phylogenetic comparisons, members of this superfamily show early divergence where the majority have only low pairwise sequence identity, although sharing common structural properties. The SDR enzymes are present in virtually all genomes investigated, and in humans over 70 SDR genes have been identified. In humans, these enzymes are involved in the metabolism of a large variety of compounds, including steroid hormones, prostaglandins, retinoids, lipids and xenobiotics. It is now clear that SDRs represent one of the oldest protein families and contribute to essential functions and interactions of all forms of life. As this field continues to grow rapidly, a systematic nomenclature is essential for future annotation and reference purposes. A functional subdivision of the SDR superfamily into at least 200 SDR families based upon hidden Markov models forms a suitable foundation for such a nomenclature system, which we present in this paper using human SDRs as examples.

Increased age reduces DAF-16 and SKN-1 signaling and the hormetic response of Caenorhabditis elegans to the xenobiotic juglone

Cells adapt to stressors by activating mechanisms that repair damage and protect them from further injury. Stress-induced damage accumulates with age and contributes to age associated diseases. Increased age attenuates the ability to mount a stress response, but little is known about the mechanisms by which this occurs. To begin addressing this problem, we studied hormesis in the nematode Caenorhabditis elegans. When exposed to a low concentration of the xenobiotic juglone, young worms mount a robust hormetic stress response and survive a subsequent exposure to a higher concentration of juglone that is normally lethal to naïve animals. Old worms are unable to mount this adaptive response. Microarray and RNAi analyses demonstrate that an altered transcriptional response to juglone is responsible in part for the reduced adaptation of old worms. Many genes differentially regulated in young versus old animals are known or postulated to be regulated by the FOXO homologue DAF-16 and the Nrf2 homologue SKN-1. Activation of these pathways is greatly reduced in juglone stressed old worms. DAF-16- and SKN-1-like transcription factors play highly conserved roles in regulating stress resistance and longevity genes. Our studies provide a foundation for developing a molecular understanding of how age affects cytoprotective transcriptional pathways.

Detoxification reactions: relevance to aging

It is widely (although not universally) accepted that organismal aging is the result of two opposing forces: (i) processes that destabilize the organism and increase the probability of death, and (ii) longevity assurance mechanisms that prevent, repair, or contain damage. Processes of the first group are often chemical and physico-chemical in nature, and are either inevitable or only under marginal biological control. In contrast, protective mechanisms are genetically determined and are subject to natural selection. Life span is therefore largely dependent on the investment into protective mechanisms which evolve to optimize reproductive fitness. Recent data indicate that toxicants, both environmental and generated endogenously by metabolism, are major contributors to macromolecular damage and physiological dysregulation that contribute to aging; electrophilic carbonyl compounds derived from lipid peroxidation appear to be particularly important. As a consequence, detoxification mechanisms, including the removal of electrophiles by glutathione transferase-catalyzed conjugation, are major longevity assurance mechanisms. The expression of multiple detoxification enzymes, each with a significant but relatively modest effect on longevity, is coordinately regulated by signaling pathways such as insulin/insulin-like signaling, explaining the large effect of such pathways on life span. The major aging-related toxicants and their cognate detoxification systems are discussed in this review.

Molecular determinants for the stereospecific reduction of 3-ketosteroids and reactivity towards all-trans-retinal of a short-chain dehydrogenase/reductase (DHRS4)

DHRS4, a member of the short-chain dehydrogenase/reductase superfamily, reduces all-trans-retinal and xenobiotic carbonyl compounds. Human DHRS4 differs from other animal enzymes in kinetic constants for the substrates, particularly in its low reactivity to retinoids. We have found that pig, rabbit and dog DHRS4s reduce benzil and 3-ketosteroids into S-benzoin and 3alpha-hydroxysteroids, respectively, in contrast to the stereoselectivity of human DHRS4 which produces R-benzoin and 3beta-hydroxysteroids. Among substrate-binding residues predicted from the crystal structure of pig DHRS4, F158 and L161 in the animal DHRS4 are serine and phenylalanine, respectively, in the human enzyme. Double mutation (F158S/L161F) of pig DHRS4 led to an effective switch of its substrate affinity and stereochemistry into those similar to human DHRS4. The roles of the two residues in determining the stereospecificity in 3-ketosteroid reduction were confirmed by reverse mutation (S158F/F161L) in the human enzyme. The stereochemical control was evaluated by comparison of the 3D models of pig wild-type and mutant DHRS4s with the modeled substrates. Additional mutation of T177N into the human S158F/F161L mutant resulted in almost complete kinetic conversion into a pig DHRS4-type form, suggesting a role of N177 in forming the substrate-binding cavity through an intersubunit interaction in pig and other animal DHRS4s, and explaining why the human enzyme shows low reactivity towards retinoids.

Analysis of the substrate-binding site of human carbonyl reductases CBR1 and CBR3 by site-directed mutagenesis

Human carbonyl reductase is a member of the short-chain dehydrogenase/reductase (SDR) protein superfamily and is known to play an important role in the detoxification of xenobiotics bearing a carbonyl group. The two monomeric NADPH-dependent human isoforms of cytosolic carbonyl reductase CBR1 and CBR3 show a sequence similarity of 85% on the amino acid level, which is definitely high if compared to the low similarities usually observed among other members of the SDR superfamily (15-30%). Despite the sequence similarity and the similar features found in the available crystal structures of the two enzymes, CBR3 shows only low or no activity towards substrates that are metabolised by CBR1. This surprising substrate specificity is still not fully understood. In the present study, we introduced several point mutations and changed sequences of up to 17 amino acids of CBR3 to the corresponding amino acids of CBR1, to gather insight into the catalytic mechanism of both enzymes. Proteins were expressed in Escherichia coli and purified by Ni-affinity chromatography. Their catalytic properties were then compared using isatin and 9,10-phenanthrenequinone as model substrates. Towards isatin, wild-type CBR3 showed a catalytic efficiency of 0.018 microM(-1)min(-1), whereas wild-type CBR1 showed a catalytic efficiency of 13.5 microM(-1)min(-1). In particular, when nine residues (236-244) in the vicinity of the catalytic center and a proline (P230) in CBR3 were mutated to the corresponding residues of CBR1 a much higher k(cat)/K(m) value (5.7 microM(-1)min(-1)) towards isatin was observed. To gain further insight into the protein-ligand binding process, docking simulations were perfomed on this mutant and on both wild-type enzymes (CBR1 and CBR3). The theoretical model of the mutant was ad hoc built by means of standard comparative modelling.