Overexpression and mutagenesis of the cDNA for rat liver 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase. Role of cysteines and tyrosines in catalysis

A Positive Feedback Loop of AKR1C3-Mediated Activation of NF-κB and STAT3 Facilitates Proliferation and Metastasis in Hepatocellular Carcinoma

AKR1C3 is an enzyme belonging to the aldo-ketoreductase family, the members of which catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. AKR1C3 plays an important role in tumor progression and metastasis, however, little is known about the function and the molecular mechanism underlying the role of AKR1C3 in hepatocellular carcinoma (HCC). In this study, we report that AKR1C3 is significantly upregulated in HCC and that increased AKR1C3 is associated with poor survival. AKR1C3 positively regulated HCC cell proliferation and metastasis in vitro and in vivo. AKR1C3 promoted tumor proliferation and metastasis by activating NF-κB signaling. Furthermore, AKR1C3 regulated NF-κB activity by modulating TRAF6 and inducing its autoubiquitination in HCC cells. Activation of NF-κB released proinflammatory factors that facilitated the phosphorylation of STAT3 and increased tumor cell proliferation and invasion. Gain- and loss-of-function experiments showed that AKR1C3 promoted tumor proliferation and invasion via the IL6/STAT3 pathway. STAT3 also directly bound the AKR1C3 promoter and increased transcription of AKR1C3, thereby establishing a positive regulatory feedback loop. Treatment with the AKR1C3 inhibitors indocin and medroxyprogesterone acetate inhibited tumor growth and invasion and promoted apoptosis in HCC cells. Collectively, these results indicate that a AKR1C3/NF-κB/STAT3 signaling loop results in HCC cell proliferation and metastasis and could be a promising therapeutic target in HCC. SIGNIFICANCE: These findings elucidate a novel AKR1C3-driven signaling loop that regulates proliferation and metastasis in HCC, providing potential prognostic and therapeutic targets in this disease.

The Steroidogenic Enzyme AKR1C3 Regulates Stability of the Ubiquitin Ligase Siah2 in Prostate Cancer Cells

Re-activation of androgen receptor (AR) activity is the main driver for development of castration-resistant prostate cancer. We previously reported that the ubiquitin ligase Siah2 enhanced AR transcriptional activity and prostate cancer cell growth. Among the genes we found to be regulated by Siah2 was AKR1C3, which encodes a key androgen biosynthetic enzyme implicated in castration-resistant prostate cancer development. Here, we found that Siah2 inhibition in CWR22Rv1 prostate cancer cells decreased AKR1C3 expression as well as intracellular androgen levels, concomitant with inhibition of cell growth in vitro and in orthotopic prostate tumors. Re-expression of either wild-type or catalytically inactive forms of AKR1C3 partially rescued AR activity and growth defects in Siah2 knockdown cells, suggesting a nonenzymatic role for AKR1C3 in these outcomes. Unexpectedly, AKR1C3 re-expression in Siah2 knockdown cells elevated Siah2 protein levels, whereas AKR1C3 knockdown had the opposite effect. We further found that AKR1C3 can bind Siah2 and inhibit its self-ubiquitination and degradation, thereby increasing Siah2 protein levels. We observed parallel expression of Siah2 and AKR1C3 in human prostate cancer tissues. Collectively, our findings identify a new role for AKR1C3 in regulating Siah2 stability and thus enhancing Siah2-dependent regulation of AR activity in prostate cancer cells.

Specificity of human aldo-keto reductases, NAD(P)H:quinone oxidoreductase, and carbonyl reductases to redox-cycle polycyclic aromatic hydrocarbon diones and 4-hydroxyequilenin-o-quinone

Polycyclic aromatic hydrocarbons (PAHs) are suspect human lung carcinogens and can be metabolically activated to remote quinones, for example, benzo[a]pyrene-1,6-dione (B[a]P-1,6-dione) and B[a]P-3,6-dione by the action of either P450 monooxygenase or peroxidases, and to non-K region o-quinones, for example B[a]P-7,8-dione, by the action of aldo keto reductases (AKRs). B[a]P-7,8-dione also structurally resembles 4-hydroxyequilenin o-quinone. These three classes of quinones can redox cycle, generate reactive oxygen species (ROS), and produce the mutagenic lesion 8-oxo-dGuo and may contribute to PAH- and estrogen-induced carcinogenesis. We compared the ability of a complete panel of human recombinant AKRs to catalyze the reduction of PAH o-quinones in the phenanthrene, chrysene, pyrene, and anthracene series. The specific activities for NADPH-dependent quinone reduction were often 100-1000 times greater than the ability of the same AKR isoform to oxidize the cognate PAH-trans-dihydrodiol. However, the AKR with the highest quinone reductase activity for a particular PAH o-quinone was not always identical to the AKR isoform with the highest dihydrodiol dehydrogenase activity for the respective PAH-trans-dihydrodiol. Discrete AKRs also catalyzed the reduction of B[a]P-1,6-dione, B[a]P-3,6-dione, and 4-hydroxyequilenin o-quinone. Concurrent measurements of oxygen consumption, superoxide anion, and hydrogen peroxide formation established that ROS were produced as a result of the redox cycling. When compared with human recombinant NAD(P)H:quinone oxidoreductase (NQO1) and carbonyl reductases (CBR1 and CBR3), NQO1 was a superior catalyst of these reactions followed by AKRs and last CBR1 and CBR3. In A549 cells, two-electron reduction of PAH o-quinones causes intracellular ROS formation. ROS formation was unaffected by the addition of dicumarol, suggesting that NQO1 is not responsible for the two-electron reduction observed and does not offer protection against ROS formation from PAH o-quinones.

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.

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.

Tissue distribution of human AKR1C3 and rat homolog in the adult genitourinary system

Human aldo-keto reductase (AKR) 1C3 (type 2 3alpha-hydroxysteroid dehydrogenase/type 5 17beta-hydroxysteroid dehydrogenase) catalyzes androgen, estrogen, and prostaglandin metabolism. AKR1C3 is therefore implicated in regulating ligand access to the androgen receptor, estrogen receptor, and peroxisome proliferator activating receptor gamma in hormone target tissues. Recent reports on close relationships between ARK1C3 and various cancers including breast and prostate cancers implicate the involvement of AKR1C3 in cancer development or progression. We previously described the characterization of an isoform-specific monoclonal antibody against AKR1C3 that does not cross-react with related, >86% sequence identity, human AKR1C1, AKR1C2, or AKR1C4, human aldehyde reductase AKR1A1, or rat 3alpha-hydroxysteroid dehydrogenase (AKR1C9). In this study, a clone of murine monoclonal antibody raised against AKR1C3 was identified and characterized for its recognition of rat homolog. Tissue distribution of human AKR1C3 and its rat homolog in adult genitourinary systems including kidney, bladder, prostate, and testis was studied by IHC. A strong immunoreactivity was detected not only in classically hormone-associated tissues such as prostate and testis but also in non-hormone-associated tissues such as kidney and bladder in humans and rats. The distribution of these two enzymes was comparable but not identical between the two species. These features warrant future studies of AKR1C3 in both hormone- and non-hormone-associated tissues and identification of the rodent homolog for establishing animal models.

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.

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.

Structural and Mutational Analysis of Trypanosoma brucei Prostaglandin H2 Reductase Provides Insight into the Catalytic Mechanism of Aldo-ketoreductases

Trypanosoma brucei prostaglandin F2alpha synthase is an aldo-ketoreductase that catalyzes the reduction of prostaglandin H2 to PGF2alpha in addition to that of 9,10-phenanthrenequinone. We report the crystal structure of TbPGFS.NADP+.citrate at 2.1 angstroms resolution. TbPGFS adopts a parallel (alpha/beta)8-barrel fold lacking the protrudent loops and possesses a hydrophobic core active site that contains a catalytic tetrad of tyrosine, lysine, histidine, and aspartate, which is highly conserved among AKRs. Site-directed mutagenesis of the catalytic tetrad residues revealed that a dyad of Lys77 and His110, and a triad of Tyr52, Lys77, and His110 are essential for the reduction of PGH2 and 9,10-PQ, respectively. Structural and kinetic analysis revealed that His110, acts as the general acid catalyst for PGH2 reduction and that Lys77 facilitates His110 protonation through a water molecule, while exerting an electrostatic repulsion against His110 that maintains the spatial arrangement which allows the formation of a hydrogen bond between His110 and C11 that carbonyl of PGH2. We also show Tyr52 acts as the general acid catalyst for 9,10-PQ reduction, and thus we not only elucidate the catalytic mechanism of a PGH2 reductase but also provide an insight into the catalytic specificity of AKRs.

Characterization of a monoclonal antibody for human aldo-keto reductase AKR1C3 (type 2 3alpha-hydroxysteroid dehydrogenase/type 5 17beta-hydroxysteroid dehydrogenase); immunohistochemical detection in breast and prostate

Human aldo-keto reductase AKR1C3 (type 2 3alpha-hydroxysteroid dehydrogenase/type 5 17beta-hydroxysteroid dehydrogenase) catalyzes the reduction of Delta(4)-androstene-3,17-dione to yield testosterone, the reduction of 5alpha-dihydrotestosterone to yield 3alpha- and 3beta-androstanediol, and the reduction of estrone to yield 17beta-estradiol. Relatively, high mRNA expression of AKR1C3 was found in human prostate and mammary gland where it is implicated in regulating ligand access to the androgen and estrogen receptor, respectively. AKR1C3 shares high sequence identity >86% with related plastic human 20alpha-hydroxysteroid dehydrogenases (AKR1C1), type 3 3alpha-hydroxysteroid dehydrogenase (AKR1C2) and type 1 3alpha-hydroxysteroid dehydrogenase (AKR1C4), and reagents are urgently needed to discriminate between these enzymes at the mRNA, protein and functional level. We describe the characterization of a high-titer isoform specific monoclonal antibody (Ab) for AKR1C3. It does not cross react with human AKR1C1, AKR1C2 or AKR1C4, human aldehyde reductase AKR1A1 or rat 3alpha-hydroxysteroid dehydrogenase (AKR1C9) on immunoblot analysis. The monoclonal Ab can be used to detect AKR1C3 expression by immunohistochemistry in sections of paraffin-embedded mammary gland and prostate. In the breast enzyme staining was detected in ductal carcinoma in situ where the cancerous cells were strongly immunoreactive. In normal prostate immunoreactivity was limited to stromal cells with only faint staining in the epithelial cells. In adenocarcinoma of the prostate elevated staining was observed in the endothelial cells and carcinoma cells. The reagent thus has utility to access the localized expression of AKR1C3 in hormonal dependent malignancies of the breast and prostate.

Loop relaxation, a mechanism that explains the reduced specificity of rabbit 20alpha-hydroxysteroid dehydrogenase, a member of the aldo-keto reductase superfamily

The aldo-keto reductase rabbit 20alpha-hydroxysteroid dehydrogenase (rb20alpha-HSD; AKR1C5) is less selective than other HSDs, since it exerts its activity both on androgens (C19 steroids) and progestins (C21 steroids). In order to identify the molecular determinants responsible for this reduced selectivity, binary (NADPH) and ternary (NADP(+)/testosterone) complex structures were solved to 1.32A and 2.08A resolution, respectively. Inspection of the cofactor-binding cavity led to the identification of a new interaction between side-chains of residues His222 and Lys270, which cover the central phosphate chain of the cofactor, reminiscent of the "safety-belt" found in other aldo-keto reductases. Testosterone is stabilized by a phenol/benzene tunnel composed of side-chains of numerous residues, among which Phe54, which forces the steroid to take up an orientation markedly contrasting with that found in HSD ternary complexes reported. Combining structural, site-directed mutagenesis, kinetic and fluorescence titration studies, we found that the selectivity of rb20alpha-HSD is mediated by (i) the relaxation of loop B (residues 223-230), partly controlled by the nature of residue 230, (ii) the nature of the residue found at position 54, and (iii) the residues found in the C-terminal tail of the protein especially the side-chain of the amino acid 306.

Transient-phase kinetic studies on the nucleotide binding to 3alpha-hydroxysteroid dehydrogenase from Pseudomonas sp. B-0831 using fluorescence stopped-flow procedures

The dual nucleotide cofactor-specific enzyme, 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) from Pseudomonas sp. B-0831, is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. Transient-phase kinetic studies using the fluorescence stopped-flow method were conducted with 3alpha-HSD to characterize the nucleotide binding mechanism. The binding of oxidized nucleotides, NAD(+), NADP(+) and nicotinic acid adenine dinucleotide (NAAD(+)), agreed well with a one-step mechanism, while that of reduced nucleotide, NADH, showed a two-step mechanism. This difference draws attention to previous characteristic findings on rat liver 3alpha-HSD, which is a member of the aldo-keto reductase (AKR) superfamily. Although functionally similar, AKRs are structurally different from SDRs. The dissociation rate constants associated with the enzyme-nucleotide complex formation were larger than the k(cat) values for either oxidation or reduction of substrates, indicating that the release of cofactors is not rate-limiting overall. It should also be noted that k(cat) for a substrate, cholic acid, with NADP(+) was only 6% of that with NAD(+), and no catalytic activity was detectable with NAAD(+), despite the similar binding affinities of nucleotides. These results suggest that a certain type of nucleotide can modulate nucleotide-binding mode and further the catalytic function of the enzyme.

Structure-function of human 3 alpha-hydroxysteroid dehydrogenases: genes and proteins

Four soluble human 3 alpha-hydroxysteroid dehydrogenase (HSD) isoforms exist which are aldo-keto reductase (AKR) superfamily members. They share 86% sequence identity and correspond to: AKR1C1 (20 alpha(3 alpha)-HSD); AKR1C2 (type 3 3 alpha-HSD and bile-acid binding protein); AKR1C3 (type 2 3 alpha-HSD and type 5 17 beta-HSD); and AKR1C4 (type 1 3 alpha-HSD). Each of the homogeneous recombinant enzymes are plastic and display 3-, 17- and 20-ketosteroid reductase and 3 alpha- 17 beta- and 20 alpha-hydroxysteroid oxidase activities with different k(cat)/K(m) ratios in vitro. The crystal structure of the AKR1C2.NADP(+).ursodeoxycholate complex provides an explanation for this functional plasticity. Ursodeoxycholate is bound backwards (D-ring in the A-ring position) and upside down (beta-face of steroid inverted) relative to the position of 3-ketosteroids in the related rat liver 3 alpha-HSD (AKR1C9) structure. Transient transfection indicates that in COS-1 cells, AKR1C enzymes function as ketosteroid reductases due to potent inhibition of their oxidase activity by NADPH. By acting as ketosteroid reductases they may regulate the occupancy of the androgen, estrogen and progesterone receptors. RT-PCR showed that AKRs are discretely localized. AKR1C4 is virtually liver specific, while AKR1C2 and AKR1C3 are dominantly expressed in prostate and mammary gland. AKR1C genes are highly conserved in structure and may be transcriptionally regulated by steroid hormones and stress.

Human cytosolic 3alpha-hydroxysteroid dehydrogenases of the aldo-keto reductase superfamily display significant 3beta-hydroxysteroid dehydrogenase activity: implications for steroid hormone metabolism and action

The source of NADPH-dependent cytosolic 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity is unknown to date. This important reaction leads e.g. to the reduction of the potent androgen 5alpha-dihydrotestosterone (DHT) into inactive 3beta-androstanediol (3beta-Diol). Four human cytosolic aldo-keto reductases (AKR1C1-AKR1C4) are known to act as non-positional-specific 3alpha-/17beta-/20alpha-HSDs. We now demonstrate that AKR1Cs catalyze the reduction of DHT into both 3alpha- and 3beta-Diol (established by (1)H NMR spectroscopy). The rates of 3alpha- versus 3beta-Diol formation varied significantly among the isoforms, but with each enzyme both activities were equally inhibited by the nonsteroidal anti-inflammatory drug flufenamic acid. In vitro, AKR1Cs also expressed substantial 3alpha[17beta]-hydroxysteroid oxidase activity with 3alpha-Diol as the substrate. However, in contrast to the 3-ketosteroid reductase activity of the enzymes, their hydroxysteroid oxidase activity was potently inhibited by low micromolar concentrations of the opposing cofactor (NADPH). This indicates that in vivo all AKR1Cs will preferentially work as reductases. Human hepatoma (HepG2) cells (which lack 3beta-HSD/Delta(5-4) ketosteroid isomerase mRNA expression, but express AKR1C1-AKR1C3) were able to convert DHT into 3alpha- and 3beta-Diol. This conversion was inhibited by flufenamic acid establishing the in vivo significance of the 3alpha/3beta-HSD activities of the AKR1C enzymes. Molecular docking simulations using available crystal structures of AKR1C1 and AKR1C2 demonstrated how 3alpha/3beta-HSD activities are achieved. The observation that AKR1Cs are a source of 3beta-tetrahydrosteroids is of physiological significance because: (i) the formation of 3beta-Diol (in contrast to 3alpha-Diol) is virtually irreversible, (ii) 3beta-Diol is a pro-apoptotic ligand for estrogen receptor beta, and (iii) 3beta-tetrahydrosteroids act as gamma-aminobutyric acid type A receptor antagonists.

Examination of the differences in structure-function of human and rat 3alpha-hydroxysteroid dehydrogenase

Human 3alpha-hydroxysteroid dehydrogenases (HSDs) are potential drug targets since they regulate the occupancy and trans-activation of steroid hormone receptors by interconverting potent hormones with their cognate inactive metabolites. The human isoforms (AKR1C1-4) are all members of the aldo-keto reductase superfamily and display distinctive differences in steroid specificity and catalytic efficiency when compared with the closely related and more extensively studied rat 3alpha-HSD (AKR1C9). Specifically, AKR1C1-4 display 3alpha-, 17beta- and 20alpha-HSD activities to varying degrees whereas AKR1C9 is positional- and stereo-specific for the 3alpha-HSD reaction. In addition, AKR1C1-4 isoforms have significantly lower catalytic efficiencies (k(cat)/K(m)) than AKR1C9 and this is largely due to a lower k(cat). To understand these functional differences, human type 3 3alpha-HSD (AKR1C2) was studied as a representative human 3alpha-HSD. Comparison of the crystal structure of AKR1C2-NADP(+)-ursodeoxycholate ternary complex (3.0 A) with that of the AKR1C9-NADP(+)-testosterone ternary complex (2.8 A) demonstrates the expected conservancy in overall structure and active site topology. More interestingly, it reveals striking differences in the structure of the steroid binding pockets of the two enzymes and shows how ursodeoxycholate binds 'backwards' and 'upside-down' with respect to testosterone. This difference in steroid binding provides a structural basis for the broad positional specificity of AKR1C2 and the exquisite stereospecificity of AKR1C9. To determine why AKR1C2 has a much lower k(cat) than AKR1C9, the events associated with the binding of cofactor to both enzymes were studied by steady state fluorescence titration and stopped-flow experiments. Comparable K(d) values for E-NADP(H) and k(obs) values for the fluorescence transients were obtained for the two enzymes. These data are consistent with both enzymes binding NADP(H) in a conserved manner which is supported by the available crystal structures. The results suggest that cofactor binding or release for the human and rat 3alpha-HSDs are similar and do not account for the observed differences in k(cat).

Characterization of Ypr1p from Saccharomyces cerevisiae as a 2-methylbutyraldehyde reductase

The metabolism of aldehydes and ketones in yeast is important for biosynthetic, catabolic and detoxication processes. Aldo-keto reductases are a family of enzymes that are able to reduce aldehydes and ketones. The roles of individual aldo-keto reductases in yeast has been difficult to determine because of overlapping substrate specificities of these enzymes. In this study, we have cloned, expressed and characterized the aldo-keto reductase Ypr1p from the yeast Saccharomyces cerevisiae and we describe its substrate specificity. The enzyme displays high specific activity towards 2-methylbutyraldehyde, as well as other aldehydes such as hexanal. It exhibits extremely low activity as a glycerol dehydrogenase. The enzyme functions over a wide pH range and uses NADPH as co-factor. In comparison to other mammalian and yeast aldo-keto reductases, Ypr1p has relatively high affinity for D,L-glyceraldehyde (1.08 mM) and hexanal (0.39 mM), but relatively low affinity for 4-nitrobenzaldehyde (1.07 mM). It displays higher specific activity for 2-methylbutyraldehyde than does yeast alcohol dehydrogenase and has a K(m) for 2-methyl butyraldehyde of 1.09 mM. The enzyme is expressed during growth on glucose, but its levels are rapidly induced by osmotic and oxidative stress. Yeast in which the YPR1 gene has been deleted possess 50% lower 2-methylbutyraldehyde reductase activity than the wild-type strain. This suggests that the enzyme may contribute to 2-methyl butyraldehyde reduction in vivo. It may therefore play a role in isoleucine catabolism and fusel alcohol formation and may influence flavour formation by strains of brewing yeast.

Genetic analysis of the mammalian K+ channel beta subunit Kvbeta 2 (Kcnab2)

Kvbeta2 binds to K(+) channel alpha subunits from at least two different families (Kv1 and Kv4) and is a member of the aldo-ketoreductase (AKR) superfamily. Proposed functions for this protein in vivo include a chaperone-like role in Kv1 alpha subunit biogenesis and catalytic activity as an AKR oxidoreductase. To investigate the in vivo function of Kvbeta2, Kvbeta2-null and point mutant (Y90F) mice were generated through gene targeting in embryonic stem cells. In Kvbeta2-null mice, Kv1.1 and Kv1.2 localize normally in cerebellar basket cell terminals and the juxtaparanodal region of myelinated nerves. Moreover, normal glycosylation patterns are observed for Kv1.1 and Kv1.2 in whole brain lysates. Thus, loss of the chaperone-like activity does not appear to account for the phenotype of Kvbeta2-null mice, which include reduced life spans, occasional seizures, and cold swim-induced tremors similar to that observed in Kv1.1-null mice. Mice expressing Kvbeta2, mutated at a site (Y90F) that abolishes AKR-like catalytic activity in other family members, have no overt phenotype. We conclude that Kvbeta2 contributes to regulation of excitability in vivo, although not directly through either chaperone-like or typical AKR catalytic activity. Rather, Kvbeta2 relies upon as yet unidentified mechanisms in the regulation of K(+) channel and/or oxidoreductive functions.

Exploring the active site of yeast xylose reductase by site-directed mutagenesis of sequence motifs characteristic of two dehydrogenase/reductase family types

Starting from a common tyrosine, yeast xylose reductases (XRs) contain two conserved sequence motifs corresponding to the catalytic signatures of single-domain reductases/epimerases/dehydrogenases (Tyr(n)-(X)3-Lys(n+4)) and aldo/keto reductases (AKRs) (Tyr(n)-(X)28-Lys(n+29)). Tyr(51), Lys(55) and Lys(80) of XR from Candida tenuis were replaced by site-directed mutagenesis. The purified Tyr(51)--> Phe and Lys(80)-->Ala mutants showed turnover numbers and catalytic efficiencies for NADH-dependent reduction of D-xylose between 2500- and 5000-fold below wild-type levels, suggesting a catalytic role of both residues. Replacing Lys(55) by Asn, a substitution found in other AKRs, did not detectably affect binding of coenzymes, and enzymatic catalysis to carbonyl/alcohol interconversion. The contribution of Tyr(51) to rate enhancement of aldehyde reduction conforms with expectations for the general acid catalyst of the enzymatic reaction.

Stable expression of rat dihydrodiol dehydrogenase (AKR1C9) in human breast MCF-7 cells results in the formation of PAH-o-quinones and enzyme mediated cell death

Dihydrodiol dehydrogenase members of the aldo-keto reductase (AKR) superfamily have been implicated in the metabolic activation of PAH trans-dihydrodiols because they convert these proximate carcinogens to reactive and redox-active o-quinones. In this study, rat liver 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (AKR1C9) was stably transfected into human breast carcinoma (MCF-7) cells, which represent a null-environment for AKR expression, to detect the formation of PAH o-quinones in a cellular context and the cellular consequences of o-quinone formation. The heterologous transfected cells expressed AKR1C9 mRNA and protein. Immunotitration of the enzyme activity indicated that the expressed protein constituted 1.0% of the soluble protein. The specific activity of the expressed enzyme was also comparable to that observed in rat liver cytosol. The transfectants were found to convert (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-diol) to benzo[a]pyrene-7,8-dione (BPQ). The identity of this metabolite was confirmed by co-chromatography and by UV-Vis diode-array spectrometry. This conversion was not evident in mock-transfected cells. The cytotoxic consequences of BPQ formation was also examined. Transfectants exposed to 1 microM B[a]P-7,8-diol revealed that cytotoxicity, as measured by lactate dehydrogenase (LDH) release, occurred over the time course of o-quinone formation leading to 77% of the cellular LDH being released by 16 h. AKR1C9 inhibitors blocked the B[a]P-7,8-diol dependent cytotoxicity indicating that it was mediated by the enzymatically formed BPQ. These data indicate that high stable constitutive expression of AKR1C9 will result in B[a]P-7,8-diol mediated cytotoxicity due to the formation of unconjugated BPQ.

Coupling of voltage-dependent potassium channel inactivation and oxidoreductase active site of Kvbeta subunits

The accessory beta subunits of voltage-dependent potassium (Kv) channels form tetramers arranged with 4-fold rotational symmetry like the membrane-integral and pore-forming alpha subunits (Gulbis, J. M., Mann, S., and MacKinnon, R. (1999) Cell. 90, 943-952). The crystal structure of the Kvbeta2 subunit shows that Kvbeta subunits are oxidoreductase enzymes containing an active site composed of conserved catalytic residues, a nicotinamide (NADPH)-cofactor, and a substrate binding site. Also, Kvbeta subunits with an N-terminal inactivating domain like Kvbeta1.1 (Rettig, J., Heinemann, S. H., Wunder, F., Lorra, C., Parcej, D. N., Dolly, O., and Pongs, O. (1994) Nature 369, 289-294) and Kvbeta3.1 (Heinemann, S. H., Rettig, J., Graack, H. R., and Pongs, O. (1996) J. Physiol. (Lond.) 493, 625-633) confer rapid N-type inactivation to otherwise non-inactivating channels. Here we show by a combination of structural modeling and electrophysiological characterization of structure-based mutations that changes in Kvbeta oxidoreductase activity may markedly influence the gating mode of Kv channels. Amino acid substitutions of the putative catalytic residues in the Kvbeta1.1 oxidoreductase active site attenuate the inactivating activity of Kvbeta1.1 in Xenopus oocytes. Conversely, mutating the substrate binding domain and/or the cofactor binding domain rescues the failure of Kvbeta3.1 to confer rapid inactivation to Kv1.5 channels in Xenopus oocytes. We propose that Kvbeta oxidoreductase activity couples Kv channel inactivation to cellular redox regulation.

Steroid 5alpha-reductases and 3alpha-hydroxysteroid dehydrogenases: key enzymes in androgen metabolism

Androgen action in mammals can be regulated at the pre-receptor level by the intracellular formation and degradation of potent androgens, such as 5alpha-dihydrotestosterone (5alpha-DHT). In androgen target tissues (e.g. prostate), 5alpha-DHT is formed from circulating testosterone by the action of the type 2 steroid 5alpha-reductase (5alpha-R) and its action is terminated by the action of a reductive 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) which forms the weak androgen 3alpha-androstanediol. Oxidative 3alpha-HSD isoforms, however, can provide an alternative source of potent androgens by converting 3alpha-androstanediol to 5alpha-DHT. Working in concert, 5alpha-Rs and 3alpha-HSDs determine the amount and the type of androgen available for the androgen receptor and hence affect transcription of genes under androgen control. In peripheral tissues (e.g. liver), type 1 5alpha-R and reductive 3alpha-HSD isoforms work consecutively to eliminate androgens and protect against hormone excess. Thus, different 5alpha-R and 3alpha-HSD isoforms participate in distinct anabolic and catabolic processes and their important roles in androgen action render them drug targets for the treatment of androgen-dependent diseases.

The crystal structure of an aldehyde reductase Y50F mutant-NADP complex and its implications for substrate binding

In order to understand more fully the structural features of aldo-keto reductases (AKRs) that determine their substrate specificities it would be desirable to obtain crystal structures of an AKR with a substrate at the active site. Unfortunately the reaction mechanism does not allow a binary complex between enzyme and substrate and to date ternary complexes of enzyme, NADP(H) and substrate or product have not been achieved. Previous crystal structures, in conjunction with numerous kinetic and theoretical analyses, have led to the general acceptance of the active site tyrosine as the general acid-base catalytic residue in the enzyme. This view is supported by the generation of an enzymatically inactive site-directed mutant (tyrosine-48 to phenylalanine) in human aldose reductase [AKR1B1]. However, crystallization of this mutant was unsuccessful. We have attempted to generate a trapped cofactor/substrate complex in pig aldehyde reductase [AKR1A2] using a tyrosine 50 to phenylalanine site-directed mutant. We have been successful in the generation of the first high resolution binary AKR-Y50F:NADP(H) crystal structure, but we were unable to generate any ternary complexes. The binary complex was refined to 2.2A and shows a clear lack of density due to the missing hydroxyl group. Other residues in the active site are not significantly perturbed when compared to other available reductase structures. The mutant binds cofactor (both oxidized and reduced) more tightly but shows a complete lack of binding of the aldehyde reductase inhibitor barbitone as determined by fluorescence titrations. Attempts at substrate addition to the active site, either by cocrystallization or by soaking, were all unsuccessful using pyridine-3-aldehyde, 4-carboxybenzaldehyde, succinic semialdehyde, methylglyoxal, and other substrates. The lack of ternary complex formation, combined with the significant differences in the binding of barbitone provides some experimental proof of the proposal that the hydroxyl group on the active site tyrosine is essential for substrate binding in addition to its major role in catalysis. We propose that the initial event in catalysis is the binding of the oxygen moiety of the carbonyl-group of the substrate through hydrogen bonding to the tyrosine hydroxyl group.

Functional genomic studies of aldo-keto reductases

Aldose reductase (AR) is considered a potential mediator of diabetic complications and is a drug target for inhibitors of diabetic retinopathy and neuropathy in clinical trials. However, the physiological role of this enzyme still has not been established. Since effective inhibition of diabetic complications will require early intervention, it is important to delineate whether AR fulfills a physiological role that cannot be compensated by an alternate aldo-keto reductase. Functional genomics provides a variety of powerful new tools to probe the physiological roles of individual genes, especially those comprising gene families. Several eucaryotic genomes have been sequenced and annotated, including yeast, nematode and fly. To probe the function of AR, we have chosen to utilize the budding yeast Saccharomyces cerevisiae as a potential model system. Unlike Caenorhabditis elegans and D. melanogaster, yeast provides a more desirable system for our studies because its genome is manipulated more readily and is able to sustain multiple gene deletions in the presence of either drug or auxotrophic selectable markers. Using BLAST searches against the human AR gene sequence, we identified six genes in the complete S. cerevisiae genome with strong homology to AR. In all cases, amino acids thought to play important catalytic roles in human AR are conserved in the yeast AR-like genes. All six yeast AR-like open reading frames (ORFs) have been cloned into plasmid expression vectors. Substrate and AR inhibitor specificities have been surveyed on four of the enzyme forms to identify, which are the most functionally similar to human AR. Our data reveal that two of the enzymes (YDR368Wp and YHR104Wp) are notable for their similarity to human AR in terms of activity with aldoses and substituted aromatic aldehydes. Ongoing studies are aimed at characterizing the phenotypes of yeast strains containing single and multiple knockouts of the AR-like genes.

Engineering steroid hormone specificity into aldo-keto reductases

Steroid hormone transforming aldo-keto reductases (AKRs) include virtually all mammalian 3alpha-hydroxysteroid dehydrogenases (3alpha-HSDs), 20alpha-HSDs, as well as the 5beta-reductases. To elucidate the molecular determinants of steroid hormone recognition we used rat liver 3alpha-HSD (AKR1C9) as a starting structure to engineer either 5beta-reductase or 20alpha-HSD activity. 5beta-Reductase activity was introduced by a single point mutation in which the conserved catalytic His (H117) was mutated to Glu117. The H117E mutant had a k(cat) comparable to that for homogeneous rat and human liver 5beta-reductases. pH versus k(cat) profiles show that this mutation increases the acidity of the catalytic general acid Tyr55. It is proposed that the increased TyrOH(2)(+) character facilitates enolization of the Delta(4)-3-ketosteroid and subsequent hydride transfer to C5. Since 5beta-reductase precedes 3alpha-HSD in steroid hormone metabolism it is likely that this metabolic pathway arose by gene duplication and point mutation. 3alpha-HSD is positional and stereospecific for 3-ketosteroids and inactivates androgens. The enzyme was converted to a robust 20alpha-HSD, which is positional and stereospecific for 20-ketosteroids and inactivates progesterone, by the generation of loop-chimeras. The shift in log(10)(k(cat)/K(m)) from androgens to progestins was of the order of 10(11). This represents a rare example of how steroid hormone specificity can be changed at the enzyme level. Protein engineering with predicted outcomes demonstrates that the molecular determinants of steroid hormone recognition in AKRs will be ultimately rationalized.

Structural and functional properties of aldose xylose reductase from the D-xylose-metabolizing yeast Candida tenuis

The primary structure of the aldose xylose reductase from Candida tenuis (CtAR) is shown to be 39% identical to that of human aldose reductase (hAR). The catalytic tetrad of hAR is completely conserved in CtAR (Tyr51, Lys80, Asp46, His113). The amino acid residues involved in binding of NADPH by hAR (D.K. Wilson, et al., Science 257 (1992) 81-84) are 64% identical in CtAR. Like hAR the yeast enzyme is specific for transferring the 4-pro-R hydrogen of the coenzyme. These properties suggest that CtAR is a member of the aldo/keto reductase superfamily. Unlike hAR the enzyme from C. tenuis has a dual coenzyme specificity and shows similar specificity constants for NADPH and NADH. It binds NADP(+) approximately 250 times less tightly than hAR. Typical turnover numbers for aldehyde reduction by CtAR (15-20 s(-1)) are up to 100-fold higher than corresponding values for hAR, probably reflecting an overall faster dissociation of NAD(P)(+) in the reaction catalyzed by the yeast enzyme.

The reactive oxygen species--and Michael acceptor-inducible human aldo-keto reductase AKR1C1 reduces the alpha,beta-unsaturated aldehyde 4-hydroxy-2-nonenal to 1,4-dihydroxy-2-nonene

The human aldo-keto reductase AKR1C1 (20alpha(3alpha)-hydroxysteroid dehydrogenase) is induced by electrophilic Michael acceptors and reactive oxygen species (ROS) via a presumptive antioxidant response element (Burczynski, M. E., Lin, H. K., and Penning, T. M. (1999) Cancer Res. 59, 607-614). Physiologically, AKR1C1 regulates progesterone action by converting the hormone into its inactive metabolite 20alpha-hydroxyprogesterone, and toxicologically this enzyme activates polycyclic aromatic hydrocarbon trans-dihydrodiols to redox-cycling o-quinones. However, the significance of its potent induction by Michael acceptors and oxidative stress is unknown. 4-Hydroxy-2-nonenal (HNE) and other alpha,beta-unsaturated aldehydes produced during lipid peroxidation were reduced by AKR1C1 with high catalytic efficiency. Kinetic studies revealed that AKR1C1 reduced HNE (K(m) = 34 microm, k(cat) = 8.8 min(-1)) with a k(cat)/K(m) similar to that for 20alpha-hydroxysteroids. Six other homogeneous recombinant AKRs were examined for their ability to reduce HNE. Of these, AKR1C1 possessed one of the highest specific activities and was the only isoform induced by oxidative stress and by agents that deplete glutathione (ethacrynic acid). Several hydroxysteroid dehydrogenases of the AKR1C subfamily catalyzed the reduction of HNE with higher activity than aldehyde reductase (AKR1A1). NMR spectroscopy identified the product of the NADPH-dependent reduction of HNE as 1,4-dihydroxy-2-nonene. The K(m) of recombinant AKR1C1 for nicotinamide cofactors (K(m) NADPH approximately 6 microm, K(m)(app) NADH >6 mm) suggested that it is primed for reductive metabolism of HNE. Isoform-specific reverse transcription-polymerase chain reaction showed that exposure of HepG2 cells to HNE resulted in elevated levels of AKR1C1 mRNA. Thus, HNE induces its own metabolism via AKR1C1, and this enzyme may play a hitherto unrecognized role in a response mounted to counter oxidative stress. AKRs represent alternative GSH-independent/NADPH-dependent routes for the reductive elimination of HNE. Of these, AKR1C1 provides an inducible cytosolic barrier to HNE following ROS exposure.

Structure-function aspects and inhibitor design of type 5 17beta-hydroxysteroid dehydrogenase (AKR1C3)

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) type 5 has been cloned from human prostate and is identical to type 2 3alpha-HSD and is a member of the aldo-keto reductase (AKR) superfamily; it is formally AKR1C3. In vitro the homogeneous recombinant enzyme expressed in Escherichia coli functions as a 3-keto-, 17-keto- and 20-ketosteroid reductase and as a 3alpha-, 17beta- and 20alpha-hydroxysteroid oxidase. The enzyme will reduce 5alpha-DHT, Delta(4)-androstene-3,17-dione, estrone and progesterone to produce 3alpha-androstanediol, testosterone, 17beta-estradiol and 20alpha-hydroxprogesterone, respectively. It will also oxidize 3alpha-androstanediol, testosterone, 17beta-estradiol and 20alpha-hydroxyprogesterone to produce 5alpha-androstane-3,17-dione, Delta(4)-androstene-3,17-dione, and progesterone, respectively. Many of these properties are shared by the related AKR1C1, AKR1C2 and AKR1C4 isoforms. RT-PCR shows that AKR1C3 is dominantly expressed in the human prostate and mammary gland. Examination of k(cat)/K(m) for these reactions indicates that as a reductase it prefers 5alpha-dihydrotestosterone and 5alpha-androstane-3,17-dione as substrates to Delta(4)-androstene-3,17-dione, suggesting that in the prostate it favors the formation of inactive androgens. Its concerted reductase activity may, however, lead to a pro-estrogenic state in the breast since it will convert estrone to 17beta-estradiol; convert Delta(4)-androstene-3,17-dione to testosterone (which can be aromatized to 17beta-estradiol); and it will reduce progesterone to its inactive metabolite 20alpha-hydroxyprogesterone. Drawing on detailed structure-function analysis of the related rat 3alpha-HSD (AKR1C9), which shares 69% sequence identity with AKR1C3, it is predicted that AKR1C3 catalyzes an ordered bi bi mechanism, that the rate determining step is k(chem), and that an oxyanion prevails in the transition state. Based on these relationships steroidal-based inhibitors that compete with the steroid product would be desirable since they would act as uncompetitive inhibitors. With regards to transition state analogs steroid carboxylates and pyrazoles may be preferred while 3alpha, 17beta or 20alpha-spiro-oxiranes may act as mechanism-based inactivators.

Role of quinones in toxicology

Quinones represent a class of toxicological intermediates which can create a variety of hazardous effects in vivo, including acute cytotoxicity, immunotoxicity, and carcinogenesis. The mechanisms by which quinones cause these effects can be quite complex. Quinones are Michael acceptors, and cellular damage can occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radicals, leading to formation of reactive oxygen species (ROS), including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can cause severe oxidative stress within cells through the formation of oxidized cellular macromolecules, including lipids, proteins, and DNA. Formation of oxidatively damaged bases such as 8-oxodeoxyguanosine has been associated with aging and carcinogenesis. Furthermore, ROS can activate a number of signaling pathways, including protein kinase C and RAS. This review explores the varied cytotoxic effects of quinones using specific examples, including quinones produced from benzene, polycyclic aromatic hydrocarbons, estrogens, and catecholamines. The evidence strongly suggests that the numerous mechanisms of quinone toxicity (i.e., alkylation vs oxidative stress) can be correlated with the known pathology of the parent compound(s).

Mutation of nicotinamide pocket residues in rat liver 3 alpha-hydroxysteroid dehydrogenase reveals different modes of cofactor binding

Rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), an aldo-keto reductase, binds NADP(+) in an extended anti-conformation across an (alpha/beta)(8)-barrel. The orientation of the nicotinamide ring, which permits stereospecific transfer of the 4-pro-R hydride from NAD(P)H to substrate, is achieved by hydrogen bonds formed between the C3-carboxamide of the nicotinamide ring and Ser 166, Asn 167, and Gln 190 and by pi-stacking between this ring and Tyr 216. These residues were mutated to yield S166A, N167A, Q190A, and Y216S. In these mutants, K(d)(NADP(H)) increased by 2-11-fold but without a significant change in K(d)(NAD(H)). Steady-state kinetic parameters showed that K(m)(NADP)()+ increased 13-151-fold, and this was accompanied by comparable decreases in k(cat)/K(m)(NADP)()+. By contrast, K(m)(NAD)()+ increased 4-8-fold, but changes in k(cat)/K(m)(NAD)()+ were more dramatic and ranged from 23- to 930-fold. Corresponding changes in binding energies indicated that each residue contributed equally to the binding of NADP(H) in the ground and transition states. However, the same residues stabilized the binding of NAD(H) only in the transition state. These observations suggest that different modes of binding exist for NADP(H) and NAD(H). Importantly, these modes were revealed by mutating residues in the nicotinamide pocket indicating that direct interactions with the 2'-phosphate in the adenine mononucleotide is not the sole determinant of cofactor preference. The single mutations were unable to invert or racemize the stereochemistry of hydride transfer even though the nicotinamide pocket can accommodate both anti- and syn-conformers once the necessary hydrogen bonds are eliminated. When 4-pro-R-[(3)H]NADH was used to monitor incorporation into [(14)C]-5alpha-dihydrotestosterone, a decrease in the (3)H:(14)C ratio was observed in the mutants relative to wild-type enzyme reflecting a pronounced primary kinetic isotope effect. This observation coupled with the change in the binding energy for NAD(P)(H) in the transition state suggests that these mutants have altered the reaction trajectory for hydride transfer.

Genomic structure of rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD, AKR1C9)

Rat liver 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD) is a member of the aldo-keto reductase (AKR) superfamily. It is involved in the inactivation of steroid hormones and the metabolic activation of polycyclic aromatic hydrocarbons (PAH) by converting trans-dihydrodiols into reactive and redox-active o-quinones. The structure of the 5'-flanking region of the gene and factors involved in the constitutive and regulated expression of this gene have been reported [H.-K. Lin, T.M. Penning, Cloning, sequencing, and functional analysis of the 5'-flanking region of the rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase gene, Cancer Res. 55 (1995) 4105-4113]. We now describe the complete genomic structure of the rat type 1 3alpha-HSD/DD gene. Charon 4A and P1 genomic clones contained at least three rat genes (type 1, type 2 and type 3 3alpha-HSD/DD) each of which encoded for the same open reading frame (ORF) but differed in their exon-intron organization. 5'-RACE confirmed that the type 1 3alpha-HSD/DD gene encodes for the dominant transcript in rat liver and it was the regulation of this gene that was previously studied. The rat type 1 3alpha-HSD/DD gene is 30 kb in length and consists of nine exons and eight introns. Exon 9 encodes +931 to 966 bp of the ORF and the 1292 bp 3'-UTR implicated in mRNA stability. This genomic structure is nearly identical to the homologous human genes, type 1 3alpha-HSD (chlordecone reductase/DD4, AKR1C4), type 2 3alpha-HSD (AKR1C3) and type 3 3alpha-HSD (bile-acid binding protein, AKR1C2) genes. Three different cDNA's containing identical ORFs for 3alpha-HSD have been reported suggesting that all three genes may be expressed in rat liver. Using 5' primers corresponding to the 5'-UTR's of the three different cDNA's only one PCR fragment was obtained and corresponded to the type 1 3alpha-HSD/DD gene. These data suggested that the type 2 and type 3 3alpha-HSD/DD genes are not abundantly expressed in rat liver. It is unknown whether the type 2 and type 3 3alpha-HSD/DD genes represent pseudo-genes or whether they represent genes that are differentially expressed in other rat tissues.

Conversion of mammalian 3alpha-hydroxysteroid dehydrogenase to 20alpha-hydroxysteroid dehydrogenase using loop chimeras: changing specificity from androgens to progestins

Hydroxysteroid dehydrogenases (HSDs) regulate the occupancy and activation of steroid hormone receptors by converting potent steroid hormones into their cognate inactive metabolites. 3alpha-HSD catalyzes the inactivation of androgens in the prostate by converting 5alpha-dihydrotestosterone to 3alpha-androstanediol, where excess 5alpha-dihydrotestosterone is implicated in prostate disease. By contrast, 20alpha-HSD catalyzes the inactivation of progestins in the ovary and placenta by converting progesterone to 20alpha-hydroxyprogesterone, where progesterone is essential for maintaining pregnancy. Mammalian 3alpha-HSDs and 20alpha-HSDs belong to the aldo-keto reductase superfamily and share 67% amino acid sequence identity yet show positional and stereospecificity for the formation of secondary alcohols on opposite ends of steroid hormone substrates. The crystal structure of 3alpha-HSD indicates that the mature steroid binding pocket consists of 10 residues located on five loops, including loop A and the mobile loops B and C. 3alpha-HSD was converted to 20alpha-HSD by replacing these loops with those found in 20alpha-HSD. However, when pocket residues in 3alpha-HSD were mutated to those found in 20alpha-HSD altered specificity was not achieved. Replacement of loop A created a 17beta-HSD activity that was absent in either 3alpha- or 20alpha-HSD. Once loops A and C were replaced, the chimera had both 3alpha- and 20alpha-HSD activity. When loops A, B, and C were substituted, 3alpha-HSD was converted to a stereospecific 20alpha-HSD with a resultant shift in k(cat)/K(m) for the desired reaction of 2 x 10(11). This study represents an example where sex hormone specificity can be changed at the enzyme level.

Structure of a voltage-dependent K+ channel beta subunit

The integral membrane subunits of many voltage-dependent potassium channels are associated with an additional protein known as the beta subunit. One function of beta subunits is to modify K+ channel gating. We have determined the structure of the conserved core of mammalian beta subunits by X-ray crystallography at 2.8 A resolution. Like the integral membrane component of K+ channels, beta subunits form a four-fold symmetric structure. Each subunit is an oxidoreductase enzyme complete with a nicotinamide co-factor in its active site. Several structural features of the enzyme active site, including its location with respect to the four-fold axis, imply that it may interact directly or indirectly with the K+ channel's voltage sensor. This structure suggests a mechanism for coupling membrane electrical excitability directly to chemistry of the cell.

The arginine 276 anchor for NADP(H) dictates fluorescence kinetic transients in 3 alpha-hydroxysteroid dehydrogenase, a representative aldo-keto reductase

Fluorescence stopped-flow studies were conducted with recombinant rat liver 3 alpha-HSD, an aldo-keto reductase (AKR) that plays critical roles in steroid hormone inactivation, to characterize the binding of nicotinamide cofactor, the first step in the kinetic mechanism. Binding of NADP(H) involved two events: the fast formation of a loose complex (E.NADP(H)), followed by a conformational change in enzyme structure leading to a tightly bound complex (E.NADP(H)), which was observed as a fluorescence kinetic transient. Binding of NAD(H) was not characterized by a similar kinetic transient, implying a difference in the mode of binding of the two cofactors. Unlike previously characterized AKRs, the rates associated with the formation and decay of E.NADP(H) and E.NADP(H) were much faster than kcat for the oxidoreduction of various substrates, indicating that binding and release of cofactor is not rate-limiting overall in 3 alpha-HSD. Mutation of Arg 276, a highly conserved residue in AKRs that forms a salt bridge with the adenosine 2'-phosphate of NADP(H), resulted in large changes in Km and Kd for NADP(H) that were not observed with NAD(H). The loss in free energy associated with the increase in Kd for NADP(H) is consistent with the elimination of an electrostatic link. Importantly, this mutation abolished the kinetic transient associated with NADPH binding. Thus, anchoring of the adenosine 2'-phosphate of NADPH by Arg 276 appears to be obligatory for the fluorescence kinetic transients to be observed. The removal of Trp 86, a residue involved in fluorescence energy transfer with NAD(P)H, also abolished the kinetic transient, but mutation of Trp 227, a residue on a mobile loop associated with cofactor binding, did not. It is concluded that in 3 alpha-HSD, the time dependence of the change in Trp 86 fluorescence is due to cofactor anchoring, and thus, Trp 86 is a distal reporter of this event. Further, the loop movement that accompanies cofactor binding is spectrally silent.

Mutagenesis of histidinol dehydrogenase reveals roles for conserved histidine residues

The dimeric zinc metalloenzyme L-histidinol dehydrogenase (HDH) catalyzes an unusual four-electron oxidation of the amino alcohol histidinol via the histidinaldehyde intermediate to the acid product histidine with the reduction of two molecules of NAD. An essential base, with pKa about 8, is involved in catalysis. Here we report site-directed mutagenesis studies to replace each of the five histidine residues (His-98, His-261, His-326, His-366, and His-418) in Salmonella typhimurium with either asparagine or glutamine. In all cases, the overexpressed enzymes were readily purified and behaved as dimers. Substitution of His-261 and His-326 by asparagine caused about 7000- and 500-fold decreases in kcat, respectively, with little change in KM values. Similar loss of activity was also reported for a H261N mutant Brassica HDH [Nagai, A., and Ohta, D. (1994) J. Biochem. 115, 22-25]. Kinetic isotope effects, pH profiles, substrate rescue, and stopped-flow experiments suggested that His-261 and His-326 are involved in proton transfers during catalysis. Sensitivity to metal ion chelator and decreased affinities for metal ions with substitutions at His-261 and His-418 suggested that these two residues are candidates for zinc ion ligands.

Molecular determinants of steroid recognition and catalysis in aldo-keto reductases. Lessons from 3alpha-hydroxysteroid dehydrogenase

Hydroxysteroid Dehydrogenases (HSDs) regulate the occupancy of steroid hormone receptors by converting active steroid hormones into their cognate inactive metabolites. HSDs belong to either the Short-chain Dehydrogenase/Reductases (SDRs) or the Aldo-Keto Reductases (AKRs). The AKRs include virtually all mammalian 3alpha-HSDs, Type 5 17beta-HSD, ovarian 20alpha-HSDs as well as the steroid 5beta-reductases. Selective inhibitors of 3alpha-HSD isoforms could control occupancy of the androgen and GABA(A) receptors, while broader based AKR inhibitors targeting 3alpha-HSD, 20alpha-HSD and prostaglandin F2alpha synthase could maintain pregnancy. We have determined three X-ray crystal structures of rat liver 3alpha-HSD, a representative AKR. These structures are of the apoenzyme (E), the binary-complex (E.NADP-), and the ternary complex (E.NADP+.testosterone). These structures are being used with site-directed mutagenesis to define the molecular determinants of steroid recognition and catalysis as a first step in rational inhibitor design. A conserved catalytic tetrad (Tyr55, Lys84, His117 and Asp50) participates in a 'proton-relay' in which Tyr55 acts as general acid/base catalyst. Its bifunctionality relies on contributions from His117 and Lys84 which alter the pKb and pKa, respectively of this residue. Point mutation of the tetrad results in different enzymatic activities. H117E mutants display 5beta-reductase activity while Y55F and Y55S mutants retain quinone reductase activity. Our results suggest that different transition states are involved in these reaction mechanisms. The ternary complex structure shows that the mature steroid binding pocket is comprised of ten residues recruited from five loops, and that there is significant movement of a C-terminal loop on binding ligand. Mutagenesis of pocket tryptophans shows that steroid substrates and classes of nonsteroidal inhibitors exhibit different binding modes which may reflect ligand-induced loop movement. Exploitation of these findings using steroidal and nonsteroidal mechanism based inactivators may lead to selective and broad based AKR inhibitors.

Retention of NADPH-linked quinone reductase activity in an aldo-keto reductase following mutation of the catalytic tyrosine

Aldo-keto reductases (AKR) are monomeric oxidoreductases that retain a conserved catalytic tetrad (Tyr, Lys, Asp, and His) at their active sites in which the Tyr acts as a general acid-base catalyst. In rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD, AKR1C9), a well-characterized AKR, the catalytic tyrosine is Tyr 55. This enzyme displays a high catalytic efficiency for a common AKR substrate 9,10-phenanthrenequinone (9,10-PQ). Surprisingly, Y55F and Y55S mutants of 3alpha-HSD reduced 9,10-PQ with high kcat values. This is the first report whereby the invariant catalytic tyrosine of an AKR has been mutated with retention of kcat values similar to wild-type enzyme. The Y55F and Y55S mutants displayed narrow substrate specificity and reduced select aromatic quinones and alpha-dicarbonyls. kcat versus pH profiles for steroid oxidoreduction catalyzed by wild-type 3alpha-HSD exhibited a single ionizable group with a pK= 7.0-7.5, which has been assigned to Tyr 55. This group was not evident in the kcat versus pH profiles for 9, 10-PQ reduction catalyzed by either wild-type or the Tyr 55 mutant enzymes, indicating that the protonation state of Tyr 55 is unimportant for 9,10-PQ turnover. Instead, wild-type and the active-site mutants Y55F, Y55S, H117A, D50N, K84R, and K84M showed the presence of a new titratable group with a pKb = 8.3-9.9. Thus, the group being titrated is not part of the tetrad. All the mutants decreased kcat/Km considerably more than they decreased kcat. Thus, the K84R mutant demonstrated a 30-fold decrease in the pH-independent value of kcat but 2200-fold decrease in the pH-independent value of kcat/Km. This suggests that all the tetrad residues influence quinone binding and that Lys 84 plays a dominant role in maintaining proper substrate orientation. Using wild-type enzyme, the energy of activation (Ea) for 9,10-PQ reduction was approximately 11 kcal/mol less than steroid oxidoreduction. The Ea for 9,10-PQ reduction was unchanged in the Tyr 55 mutants, suggesting that the reaction proceeds through the same low-energy barrier in the wild-type enzyme and these mutants. The retention of quinone reductase activity in this AKR in the absence of Tyr 55 with kcat versus pH rate profiles and activation energies identical to wild-type enzyme suggests that quinone reduction occurs via a mechanism that differs from 3-ketosteroid reduction. In this mechanism, the electron donor (NADPH) and acceptor (o-quinone) are bound in close proximity, which permits hydride transfer without formal protonation of the acceptor carbonyl by Tyr 55. This represents a rare example where one enzyme can catalyze the same chemical reaction (carbonyl reduction) by either acid catalysis or by a propinquity effect and where these two mechanisms can be discriminated by site-directed mutagenesis.

Engineering steroid 5 beta-reductase activity into rat liver 3 alpha-hydroxysteroid dehydrogenase

Delta 4-3-Ketosteroid-5 beta-reductase (5 beta-reductase) precedes 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) in steroid hormone metabolism. Both enzymes are members of the aldo-keto reductase (AKR) superfamily and possess catalytic tetrads differing by a single amino acid. In 3 alpha-HSD, the tetrad consists of Tyr55, Lys84, Asp50, and His117, but a glutamic acid replaces His117 in 5 beta-reductase. By introducing the H117E point mutation into 3 alpha-HSD, we engineered 5 beta-reductase activity into the dehydrogenase. Homogeneous H117E 3 alpha-HSD reduced the double bond in testosterone to form 5 beta-dihydrotestosterone with kcat = 0.25 min-1 and Km = 19.0 microM and reduced the double bond in progesterone to generate 5 beta-dihydroprogesterone with kcat = 0.97 min-1 and Km = 33.0 microM. These kinetic parameters were similar to those reported for homogeneous rat liver 5 beta-reductase [Okuda, A., and Okuda, R. (1984) J. Biol. Chem. 259, 7519-7524]. The H117E mutant also reduced 5beta-dihydrosteroids to 5 beta, 3 alpha-tetrahydrosteroids with a 600-1000-fold decrease in kcat/Km versus wild-type 3 alpha-HSD. The ratio of 5 beta-reductase:3 alpha-HSD activity in the H117E mutant was approximately 1:1. Although the H117A mutant reduced Delta 4-3-ketosteroids, the 3 alpha-HSD activity predominated because the 5 beta-dihydrosteroids were rapidly converted to the 5 beta,3 alpha-tetrahydrosteroids. The pH-rate profiles for carbon-carbon double-bond and ketone reduction catalyzed by the H117E mutant were superimposable, suggesting a common titratable group (pKb = 6.3) for both reactions. In wild-type 3 alpha-HSD, the titratable group responsible for 3-ketosteroid reduction has a pKb = 6.9 and is assignable to Tyr55. The pH-rate profiles for 3-ketosteroid reduction by the H117A mutant were pH-independent. Our data indicate that Tyr55 functions as a general acid for both 3 alpha-HSD and 5 beta-reductase activities. We suggest that a protonated Glu117 increases the acidity of Tyr55 to promote acid-catalyzed enolization of the Delta 4-3-ketosteroid substrate. Further, the identity of amino acid 117 determines whether an AKR can function as a 5 beta-reductase by reorienting the substrate relative to the nicotinamide cofactor. This study provides functional evidence that utilization of modified catalytic residues on an identical protein scaffold is important for evolution of enzymatic activities within the same metabolic pathway.

Mutagenesis of 3 alpha-hydroxysteroid dehydrogenase reveals a "push-pull" mechanism for proton transfer in aldo-keto reductases

Rat liver 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD, E.C. 1.1.1.213, AKR1C9) is a member of the aldo-keto reductase (AKR) superfamily which inactivates circulating steroid hormones. We have proposed a catalytic mechanism in which Tyr 55 acts as a general acid with its pK value being lowered by a hydrogen bond with Lys 84 which is salt-linked to Asp 50. To test this mechanism, residues at the active site were mutated and the mutant enzymes (Y55F, Y55S, K84M, K84R, D50N, D50E, and H117A) were purified to homogeneity from an Escherichia coli expression system. Spectrophotometric assays showed that mutations of Tyr 55 and Lys 84 gave enzymes that were apparently inactive for steroid oxidation and reduction. All mutants appeared inactive for steroid reduction. The catalytic efficiencies for steroid oxidation were reduced 4-10-fold for the Asp 50 mutants and 300-fold for the H117A mutant. Fluorescence titration with NADPH demonstrated that each mutant bound cofactor unimpeded. Equilibrium dialysis indicated that the competitive inhibitor testosterone formed E.NADH.testosterone complexes only with the Y55F, Y55S, and D50N mutants with Kd values 10-fold greater than those for wild-type. Therefore the loss of steroid oxidoreductase activity observed for the Tyr 55 mutants cannot be attributed simply to an inability to bind steroid. Using a highly sensitive radiometric assay in which the conversion of [14C]-5 alpha-dihydrotestosterone (DHT) to [14C]-3 alpha-androstanediol (3 alpha-Diol) was measured, the rate enhancement (kcat/knoncat) for the reaction was estimated to be 2.6 x 10(9). Using this assay, all mutants formed steroid product with decreases in an overall rate enhancement of 10(1)-10(4). It was found that Tyr 55 made the single largest contribution to rate enhancement. This is the first instance where point mutations in the conserved catalytic tetrad of an AKR yielded enzymes which were still catalytically active. This enabled the construction of pH vs kcat profiles for the reduction of [14C]-5 alpha-DHT catalyzed by the tetrad mutants. These profiles revealed that the titratable group assigned to the general acid (pK = 6.50 +/- 0.42) was eliminated in the Y55F and H117A mutants. The pH-independent value of kcat was decreased in the H117A and Y55F mutants, by 2 and 4 log units, respectively. pH vs kcat(app) profiles for the oxidation of [3H]-3 alpha-Diol showed that the same titratable group (pK = 7.50 +/- 0.30) was eliminated in both the Y55F and K84M mutants but was retained in the H117A mutant. Since only the Y55F mutant eliminated the titratable group in both the reduction and oxidation directions it is assigned as the catalytic general acid/base. The differential effects of His 117 and Lys 84 on the ionization of Tyr 55 are explained by a "push-pull" mechanism in which His 117 facilitates proton donation and Lys 84 facilitates proton removal by Tyr 55.

Catalytic mechanism of aldose reductase studied by the combined potentials of quantum mechanics and molecular mechanics

The catalytic reduction of D-glyceraldehyde to glycerol by aldose reductase has been investigated with the combined potentials of quantum mechanics (QM) and molecular mechanics (MM) to resolve the question of whether Tyr48 or His110 serves as the proton donor during catalysis. Site directed mutagenesis studies favor Tyr48 as the proton donor while the presence of a water channel linking the N delta 1 of His110 to the bulk solvent suggests that His110 is the proton donor. Utilizing the combined potentials of QM and MM, the binding mode of substrate D-glyceraldehyde was investigated by optimizing the local geometry of Asp43, Lys77, Tyr48, His110 and NADPH at the active site of aldose reductase. Reaction pathways for the reduction of D-glyceraldehyde to glycerol were then constructed by treating both Tyr48 and His110 as proton donors. Comparison of energetics obtained from the reaction pathways suggests His110 to be the proton donor. Based on these findings, a reduction mechanism of D-glyceraldehyde to glycerol is described.

Dexamethasone regulation of the rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase gene

Rat liver 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD), a member of the aldo-keto reductase superfamily, inactivates circulating steroid hormones and may contribute to the carcinogenicity of polycyclic aromatic hydrocarbons (PAHs) by oxidizing trans-dihydrodiols to reactive o-quinones with the concomitant generation of reactive oxygen species. The 3alpha-HSD/DD gene has been cloned, and its 5'-flanking region contains a negative response element (NRE; -797 to -498 bp) that may repress constitutive expression by binding to Oct transcription factors. Upstream from the NRE are three distal imperfect glucocorticoid response elements (GRE1, GRE2, and GRE3); in addition, a proximal imperfect GRE (GRE4) is adjacent to an Oct binding site in the NRE. When rat hepatocytes were cultured on Matrigel and exposed to dexamethasone (Dex), steady state levels of 3alpha-HSD/DD mRNA were increased 4-fold in a dose-dependent manner, yielding an EC50 value of 10 nM. Time to maximal response was 24 hr, and the effect was blocked with the anti-glucocorticoid RU486. Measurement of the half-life of 3alpha-HSD/DD mRNA, with and without Dex treatment, indicated that the increase in steady state mRNA levels was not due to increased mRNA stability. By contrast, nuclear run-off experiments using nuclei obtained from Dex-stimulated hepatocytes indicated that Dex increased transcription of the rat 3alpha-HSD/DD gene. Tandem repeats of the imperfect GRE1, GRE2, GRE3, and GRE4 were inserted into thymidine kinase-chloramphenicol acetyl-transferase vectors and cotransfected with the human glucocorticoid receptor into human hepatoma cells. On treatment with Dex, maximal trans-activation of the chloramphenicol acetyl-transferase reporter gene activity was mediated via the proximal GRE (GRE4). These data imply that GRE4 is a functional cis-element and that binding of the occupied glucocorticoid receptor to this element increases 3alpha-HSD/DD gene transcription. A model is proposed for the positive and negative regulation of the rat 3alpha-HSD/DD gene by the glucocorticoid receptor and Oct transcription factors, respectively.

Expression and characterization of recombinant type 2 3 alpha-hydroxysteroid dehydrogenase (HSD) from human prostate: demonstration of bifunctional 3 alpha/17 beta-HSD activity and cellular distribution

In androgen target tissues, 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) may regulate occupancy of the androgen receptor (AR) by catalyzing the interconversion of 5alpha-dihydrotestosterone (5alpha-DHT) (a potent androgen) and 3alpha-androstanediol (a weak androgen). In this study, a 3alpha-HSD cDNA (1170 bp) was isolated from a human prostate cDNA library. The human prostatic 3alpha-HSD cDNA encodes a 323-amino acid protein with 69.9%, 84.1%, 99.4%, and 87.9% sequence identity to rat liver 3alpha-HSD and human type 1, type 2, and type 3 3alpha-HSDs, respectively, and is a member of the aldo-keto reductase superfamily. The close homology with human type 2 3alpha-HSD suggests that it is either identical to this enzyme or a structural allele. Surprisingly, when the recombinant protein was expressed and purified from Escherichia coli, the enzyme did not oxidize androsterone when measured spectrophotometrically, an activity previously assigned to recombinant type 2 3alpha-HSD using this assay. Complete kinetic characterization of the purified protein using spectrophotometric, fluorometric, and radiometric assays showed that the catalytic efficiency favored 3alpha-androstanediol oxidation over 5alpha-DHT reduction. Using [14C]-5alpha-DHT as substrate, TLC analysis confirmed that the reaction product was [14C]-3alpha-androstanediol. However, in the reverse reaction, [3H]-3alpha-androstanediol was oxidized first to [3H]-androsterone and then to [3H]-androstanedione, revealing that the expressed protein possessed both 3alpha- and 17beta-HSD activities. The 17beta-HSD activity accounted for the higher catalytic efficiency observed with 3alpha-androstanediol. These findings indicate that, in the prostate, type 2 3alpha-HSD does not interconvert 5alpha-DHT and 3alpha-androstanediol but inactivates 5alpha-DHT through its 3-ketosteroid reductase activity. Levels of 3alpha-HSD mRNA were measured in primary cultures of human prostatic cells and were higher in epithelial cells than stromal cells. In addition, elevated levels of 3alpha-HSD mRNA were observed in epithelial cells derived from benign prostatic hyperplasia and prostate carcinoma tissues. Expression of 3alpha-HSD was not prostate specific, since high levels of mRNA were also found in liver, small intestine, colon, lung, and kidney. This study is the first complete characterization of recombinant type 2 3alpha-HSD demonstrating dual activity and cellular distribution in the human prostate.

Structure and function of 3 alpha-hydroxysteroid dehydrogenase

Mammalian 3 alpha-hydroxysteroid dehydrogenases (3 alpha-HSDs) inactivate circulating steroid hormones, and in target tissues regulate the occupancy of steroid hormone receptors. Molecular cloning indicates that 3 alpha-HSDs are members of the aldo-keto reductase (AKR) superfamily and display high sequence identity (> 60%). Of these, the most extensively characterized is rat liver 3 alpha-HSD. X-ray crystal structures of the apoenzyme and the E.NADP+ complex have been determined and serve as structural templates for other 3 alpha-HSDs. These structures reveal that rat liver 3 alpha-HSD adopts an (alpha/beta)8-barrel protein fold. NAD(P)(H) lies perpendicular to the barrel axis in an extended conformation, with the nicotinamide ring at the core of the barrel, and the adenine ring at the periphery of the structure. The nicotinamide ring is stabilized by interaction with Y216, S166, D167, and Q190, so that the A-face points into the vacant active site. The 4-pro-(R) hydrogen transferred in the oxidoreduction of steroids is in close proximity to a catalytic tetrad that consists of D50, Y55, K84, and H117. A water molecule is within hydrogen bond distance of H117 and Y55, and its position may mimic the position of the carbonyl of a 3-ketosteroid substrate. The catalytic tetrad is conserved in members of the AKR superfamily and resides at the base of an apolar cleft implicated in binding steroid hormone. The apolar cleft consists of a side of apolar residues (L54, W86, F128, and F129), and opposing this side is a flexible loop that contains W227. These constraints suggest that the alpha-face of the steroid would orient itself along that side of the cleft containing W86. Site-directed mutagenesis of the catalytic tetrad indicates that Y55 and K84 are essential for catalysis. Y55S and Y55F mutants are catalytically inactive, but still form binary (E.NADPH) and ternary (E.NADH.Testosterone) complexes; by contrast K84R and K84M mutants are catalytically inactive, but do not bind steroid hormone. The reliance on a Tyr/Lys pair is reminiscent of catalytic mechanisms proposed for other AKR members as well as for HSDs that belong to the short-chain dehydrogenase/reductase (SDR) family, in which Tyr is the general acid, with its pKa being lowered by Lys. Superimposition of the nicotinamide rings in the structures of 3 alpha-HSD (an AKR) and 3 alpha, 20 beta-HSD (an SDR) show that the Tyr/Lys pairs are positionally conserved, suggesting convergent evolution across protein families to a common mechanism for HSD catalysis. W86Y and W227Y mutants bind testosterone to the E.NADH complex, with effective increases in Kd of 8- and 20-fold. These data provide the first evidence that the side of the apolar cleft containing W86 and the opposing flexible loop containing W227 are parts of the steroid-binding site. Detailed mutagenesis studies of the apolar cleft and elucidation of a ternary complex structure will ultimately provide details of the determinants that govern steroid hormone recognition. These determinants could provide a rational basis for structure-based inhibitor design.

Characterization of the substrate binding site in rat liver 3alpha-hydroxysteroid/dihydrodiol dehydrogenase. The roles of tryptophans in ligand binding and protein fluorescence

Rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), a member of the aldoketoreductase superfamily, inactivates circulating steroid hormones using NAD(P)H as cofactor. Despite determination of the 3alpha-HSD.NADP+ binary complex structure, the functional elements that dictate the binding of steroids remain unclear (Bennett, M.J., Schlegel, B.P., Jez, J.M., Penning, T.M., and Lewis, M. (1996) Biochemistry 35, 10702-10711). Two tryptophans (Trp86 and Trp227) near the active site may have roles in substrate binding, and their fluorescence may be quenched upon binding of NADPH. Trp86 is located within an apolar cleft, while Trp227 is found on an opposing loop near the active site. A third tryptophan, Trp148, is on the periphery of the structure. To investigate the roles of these tryptophans in protein fluorescence and ligand binding, we generated three mutant enzymes (W86Y, W148Y, and W227Y) by site-directed mutagenesis. Spectroscopic measurements on these proteins showed that Trp148 contributed the most to the enzyme fluorescence spectra, with Trp227 adding the least. Trp86 was identified as the tryptophan quenched by bound NADPH through an energy transfer mechanism. The W86Y mutant altered binding of cofactor (a 3-fold increase in Kd for NADPH) and steroid (a 7-fold increase in Kd for testosterone). This mutation also dramatically decreased the catalytic efficiency observed with one-, two-, and three-ring substrates and decreased the binding affinity for nonsteroidal anti-inflammatory drugs but had little effect on the binding of aldose reductase inhibitors. Interestingly, mutation of Trp227 significantly impaired steroid binding (a 22-fold increase in Kd for testosterone), but did not alter binding of cofactor, smaller substrates, or inhibitors. Kinetically, the W148Y mutant was similar to wild-type enzyme. Our results demonstrate that Trp86 and the apolar cleft is part of the substrate binding pocket. In addition, we propose a role for Trp227 and its associated loop in binding steroids, but not small substrates or inhibitors, most likely through interaction with the C- and D-rings of the steroid. This work provides the first evidence that tryptophans on opposite sides of the apolar cleft are part of the steroid binding pocket and suggests how the enzyme may discriminate between nonsteroidal anti-inflammatory drugs and aldose reductase inhibitors like zopolrestat. A model of how androstanedione binds in the apolar cleft is developed. These data provide further evidence that loop structures in members of the aldoketoreductase superfamily are critical determinants of ligand binding.