Characterization of Two Isoforms of Mouse 3(17).ALPHA.-Hydroxysteroid Dehydrogenases of the Aldo-Keto Reductase Family

Microbiota modulates the steroid response to acute immune stress in male mice

Microbiota plays a role in shaping the HPA-axis response to psychological stressors. To examine the role of microbiota in response to acute immune stressor, we stimulated the adaptive immune system by anti-CD3 antibody injection and investigated the expression of adrenal steroidogenic enzymes and profiling of plasma corticosteroids and their metabolites in specific pathogen-free (SPF) and germ-free (GF) mice. Using UHPLC-MS/MS, we showed that 4 hours after immune challenge the plasma levels of pregnenolone, progesterone, 11-deoxycorticosterone, corticosterone (CORT), 11-dehydroCORT and their 3α/β-, 5α-, and 20α-reduced metabolites were increased in SPF mice, but in their GF counterparts, only CORT was increased. Neither immune stress nor microbiota changed the mRNA and protein levels of enzymes of adrenal steroidogenesis. In contrast, immune stress resulted in downregulated expression of steroidogenic genes (Star, Cyp11a1, Hsd3b1, Hsd3b6) and upregulated expression of genes of the 3α-hydroxysteroid oxidoreductase pathway (Akr1c21, Dhrs9) in the testes of SPF mice. In the liver, immune stress downregulated the expression of genes encoding enzymes with 3β-hydroxysteroid dehydrogenase (HSD) (Hsd3b2, Hsd3b3, Hsd3b4, Hsd3b5), 3α-HSD (Akr1c14), 20α-HSD (Akr1c6, Hsd17b1, Hsd17b2) and 5α-reductase (Srd5a1) activities, except for Dhrs9, which was upregulated. In the colon, microbiota downregulated Cyp11a1 and modulated the response of Hsd11b1 and Hsd11b2 expression to immune stress. These data underline the role of microbiota in shaping the response to immune stressor. Microbiota modulates the stress-induced increase in C21 steroids, including those that are neuroactive that could play a role in alteration of HPA axis response to stress in GF animals.

Structure-function characterization of an aldo-keto reductase involved in detoxification of the mycotoxin, deoxynivalenol

Deoxynivalenol (DON) is a mycotoxin, produced by filamentous fungi such as Fusarium graminearum, that causes significant yield losses of cereal grain crops worldwide. One of the most promising methods to detoxify this mycotoxin involves its enzymatic epimerization to 3-epi-DON. DepB plays a critical role in this process by reducing 3-keto-DON, an intermediate in the epimerization process, to 3-epi-DON. DepB_Rleg from Rhizobium leguminosarum is a member of the new aldo-keto reductase family, AKR18, and it has the unusual ability to utilize both NADH and NADPH as coenzymes, albeit with a 40-fold higher catalytic efficiency with NADPH compared to NADH. Structural analysis of DepB_Rleg revealed the putative roles of Lys-217, Arg-290, and Gln-294 in NADPH specificity. Replacement of these residues by site-specific mutagenesis to negatively charged amino acids compromised NADPH binding with minimal effects on NADH binding. The substrate-binding site of DepB_Rleg is larger than its closest structural homolog, AKR6A2, likely contributing to its ability to utilize a wide range of aldehydes and ketones, including the mycotoxin, patulin, as substrates. The structure of DepB_Rleg also suggests that 3-keto-DON can adopt two binding modes to facilitate 4-pro-R hydride transfer to either the re- or si-face of the C3 ketone providing a possible explanation for the enzyme's ability to convert 3-keto-DON to 3-epi-DON and DON in diastereomeric ratios of 67.2% and 32.8% respectively.

Structure-based rational design of hydroxysteroid dehydrogenases for improving and diversifying steroid synthesis

A group of steroidogenic enzymes, hydroxysteroid dehydrogenases are involved in steroid metabolism which is very important in the cell: signaling, growth, reproduction, and energy homeostasis. The enzymes show an inherent function in the interconversion of ketosteroids and hydroxysteroids in a position- and stereospecific manner on the steroid nucleus and side-chains. However, the biocatalysis of steroids reaction is a vital and demanding, yet challenging, task to produce the desired enantiopure products with non-natural substrates or non-natural cofactors, and/or in non-physiological conditions. This has driven the use of protein design strategies to improve their inherent biosynthetic efficiency or activate their silent catalytic ability. In this review, the innate features and catalytic characteristics of enzymes based on sequence-structure-function relationships of steroidogenic enzymes are reviewed. Combining structure information and catalytic mechanisms, progress in protein redesign to stimulate potential function, for example, substrate specificity, cofactor dependence, and catalytic stability are discussed.

Characterization of aldo-keto reductase 1C subfamily members encoded in two rat genes (akr1c19 and RGD1564865). Relationship to 9-hydroxyprostaglandin dehydrogenase

Rat genes, akr1c19 and RGD1564865, encode members (R1C19 and 20HSDL, respectively) of the aldo-keto reductase (AKR) 1C subfamily, whose functions, however, remain unknown. Here, we show that recombinant R1C19 and 20HSDL exhibit NAD⁺-dependent dehydrogenase activity for prostaglandins (PGs) with 9α-hydroxy group (PGF_2α, its 13,14-dihydro- and 15-keto derivatives, 9α,11β-PGF₂ and PGD₂). 20HSDL oxidized the PGs with much lower K_m (0.3-14 μM) and higher k_cat/K_m values (0.064-2.6 min^-1μM^-1) than those of R1C19. They also differed in other properties: R1C19, but not 20HSDL, oxidized some 17β-hydroxysteroids (5β-androstane-3α,17β-diol and 5β-androstan-17β-ol-3-one). 20HSDL was specifically inhibited by zomepirac, but not by R1C19-selective inhibitors (hexestrol, flavonoids, ibuprofen and flufenamic acid), although the two enzymes were sensitive to indomethacin and cis-unsaturated fatty acids. The mRNA for 20HSDL was expressed abundantly in rat kidney and at low levels in the liver, testis, brain, heart and colon, in contrast to ubiquitous expression of R1C19 mRNA. The comparison of enzymic features of R1C19 and 20HSDL with rat PG dehydrogenases and other AKRs suggests not only a close relationship of 20HSDL with 9-hydroxy-PG dehydrogenase in rat kidney, but also roles of R1C19 and rat AKRs (1C16 and 1C24) in the metabolism of PGF_2α, PGD₂ and 9α,11β-PGF₂ in other tissues.

Human dehydrogenase/reductase (SDR family) member 11 is a novel type of 17β-hydroxysteroid dehydrogenase

We report characterization of a member of the short-chain dehydrogenase/reductase superfamily encoded in a human gene, DHRS11. The recombinant protein (DHRS11) efficiently catalyzed the conversion of the 17-keto group of estrone, 4- and 5-androstenes and 5α-androstanes into their 17β-hydroxyl metabolites with NADPH as a coenzyme. In contrast, it exhibited reductive 3β-hydroxysteroid dehydrogenase activity toward 5β-androstanes, 5β-pregnanes, 4-pregnenes and bile acids. Additionally, DHRS11 reduced α-dicarbonyls (such as diacetyl and methylglyoxal) and alicyclic ketones (such as 1-indanone and loxoprofen). The enzyme activity was inhibited in a mixed-type manner by flavonoids, and competitively by carbenoxolone, glycyrrhetinic acid, zearalenone, curcumin and flufenamic acid. The expression of DHRS11 mRNA was observed widely in human tissues, most abundantly in testis, small intestine, colon, kidney and cancer cell lines. Thus, DHRS11 represents a novel type of 17β-hydroxysteroid dehydrogenase with unique catalytic properties and tissue distribution.

Characterization of hamster NAD+-dependent 3(17)β-hydroxysteroid dehydrogenase belonging to the aldo-keto reductase 1C subfamily

The cDNAs for morphine 6-dehydrogenase (AKR1C34) and its homologous aldo-keto reductase (AKR1C35) were cloned from golden hamster liver, and their enzymatic properties and tissue distribution were compared. AKR1C34 and AKR1C35 similarly oxidized various xenobiotic alicyclic alcohols using NAD(+), but differed in their substrate specificity for hydroxysteroids and inhibitor sensitivity. While AKR1C34 showed 3α/17β/20α-hydroxysteroid dehydrogenase activities, AKR1C35 efficiently oxidized various 3β- and 17β-hydroxysteroids, including biologically active 3β-hydroxy-5α/β-dihydro-C19/C21-steroids, dehydroepiandrosterone and 17β-estradiol. AKR1C35 also differed from AKR1C34 in its high sensitivity to flavonoids, which inhibited competitively with respect to 17β-estradiol (Ki 0.11-0.69 μM). The mRNA for AKR1C35 was expressed liver-specific in male hamsters and ubiquitously in female hamsters, whereas the expression of the mRNA for AKR1C34 displayed opposite sexual dimorphism. Because AKR1C35 is the first 317Β-HYDROXYSTEROID DEHYDROGENASE IN THE AKR SUPERFAMILY: , we also investigated the molecular determinants for the 3β-hydroxysteroid dehydrogenase activity by replacement of Val54 and Cys310 in AKR1C35 with the corresponding residues in AKR1C34, Ala and Phe, respectively. The mutation of Val54Ala, but not Cys310Phe, significantly impaired this activity, suggesting that Val54 plays a critical role in recognition of the steroidal substrate.

Revisiting the roles of progesterone and allopregnanolone in the nervous system: resurgence of the progesterone receptors

Progesterone is commonly considered as a female reproductive hormone and is well-known for its role in pregnancy. It is less well appreciated that progesterone and its metabolite allopregnanolone are also male hormones, as they are produced in both sexes by the adrenal glands. In addition, they are synthesized within the nervous system. Progesterone and allopregnanolone are associated with adaptation to stress, and increased production of progesterone within the brain may be part of the response of neural cells to injury. Progesterone receptors (PR) are widely distributed throughout the brain, but their study has been mainly limited to the hypothalamus and reproductive functions, and the extra-hypothalamic receptors have been neglected. This lack of information about brain functions of PR is unexpected, as the protective and trophic effects of progesterone are much investigated, and as the therapeutic potential of progesterone as a neuroprotective and promyelinating agent is currently being assessed in clinical trials. The little attention devoted to the brain functions of PR may relate to the widely accepted assumption that non-reproductive actions of progesterone may be mainly mediated by allopregnanolone, which does not bind to PR, but acts as a potent positive modulator of γ-aminobutyric acid type A (GABA(A) receptors. The aim of this review is to critically discuss effects of progesterone on the nervous system via PR, and of allopregnanolone via its modulation of GABA(A) receptors, with main focus on the brain.

Tissue distribution, ontogeny, and chemical induction of aldo-keto reductases in mice

Aldo-keto reductases (Akrs) are a conserved group of NADPH-dependent oxido-reductase enzymes. This study provides a comprehensive examination of the tissue distribution of the 16 substrate-metabolizing Akrs in mice, their expression during development, and whether they are altered by chemicals that activate distinct transcriptional factor pathways. Akr1c6, 1c14, 1c20, and 1c22 are primarily present in liver; Akr1a4, 1c18, 1c21, and 7a5 in kidney; Akr1d1 in liver and kidney; Akr1b7 in small intestine; Akr1b3 and Akr1e1 in brain; Akr1b8 in testes; Akr1c14 in ovaries; and Akrs1c12, 1c13, and 1c19 are expressed in numerous tissues. Liver expression of Akr1d1 and Akr1c is lowest during prenatal and postnatal development. However, by 20 days of age, liver Akr1d1 increases 120-fold, and Akr1c mRNAs increase as much as 5-fold (Akr1c19) to 1000-fold (Akr1c6). Treatment of mice with chemical activators of transcription factors constitutive androgen receptor (CAR), pregnane X receptor (PXR), and the nuclear factor-erythroid-2 (Nrf2) transcription factor alters liver mRNAs of Akrs. Specifically, CAR activation by 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) increases mRNAs of Akr1b7, Akr1c6, Akr1c19, and Akr1d1, whereas PXR activation by 5-pregnenolone-16α-carbonitrile (PCN) increases the mRNA of Akr1b7 and suppresses mRNAs of Akr1c13 and Akr1c20. The Nrf2 activator 2-cyano-3,12-dioxooleana-1,9-dien-28-imidazolide (CDDO-Im) induces mRNAs of Akr1c6 and Akr1c19. Moreover, Nrf2-null and Nrf2 overexpressing mice demonstrate that this induction is Nrf2-dependent.

Characterization of rabbit morphine 6-dehydrogenase and two NAD(+)-dependent 3α(17β)-hydroxysteroid dehydrogenases

Mammalian morphine 6-dehydrogenase (M6DH)(1) converts morphine into a reactive electrophile, morphinone. M6DH belongs to the aldo-keto reductase (AKR) superfamily, but its endogenous substrates and entire amino acid sequence remain unknown. A recent rabbit genomic sequencing predicts three genes for novel AKRs (1C26, 1C27 and 1C28) that share >87% amino acid sequence identity and are similar to the partial sequence of rabbit liver M6DH. We isolated cDNAs for the three AKRs, and compared the properties of their recombinant enzymes. Like M6DH, only AKR1C26 that shares the highest sequence identity with hepatic M6DH oxidized morphine. The three AKRs showed NAD(+)-dependent dehydrogenase activity towards other non-steroidal alicyclic alcohols and 3α/17β-hydroxy-C(18)/C(19)/C(21)-steroids, and their mRNAs were ubiquitously expressed in rabbit tissues. The kinetic constants for the substrates suggest that at least AKR1C26 and AKR1C28 act as NAD(+)-dependent 3α/17β-hydroxysteroid dehydrogenases. AKR1C27 differed from AKR1C28 in its high K(m) values for the substrates and low sensitivity towards competitive inhibitors (ikarisoside A, hinokitiol, hexestrol and zearalenone), despite their 95% sequence identity. The site-directed mutagenesis of Tyr118 and Phe310 in AKR1C27 to the corresponding residues (Phe and Ile, respectively) in AKR1C28 produced an enzyme that was similar to AKR1C28, suggesting their key roles in ligand binding.

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.

Progesterone synthesis in the nervous system: implications for myelination and myelin repair

Progesterone is well known as a female reproductive hormone and in particular for its role in uterine receptivity, implantation, and the maintenance of pregnancy. However, neuroendocrine research over the past decades has established that progesterone has multiple functions beyond reproduction. Within the nervous system, its neuromodulatory and neuroprotective effects are much studied. Although progesterone has been shown to also promote myelin repair, its influence and that of other steroids on myelination and remyelination is relatively neglected. Reasons for this are that hormonal influences are still not considered as a central problem by most myelin biologists, and that neuroendocrinologists are not sufficiently concerned with the importance of myelin in neuron functions and viability. The effects of progesterone in the nervous system involve a variety of signaling mechanisms. The identification of the classical intracellular progesterone receptors as therapeutic targets for myelin repair suggests new health benefits for synthetic progestins, specifically designed for contraceptive use and hormone replacement therapies. There are also major advantages to use natural progesterone in neuroprotective and myelin repair strategies, because progesterone is converted to biologically active metabolites in nervous tissues and interacts with multiple target proteins. The delivery of progesterone however represents a challenge because of its first-pass metabolism in digestive tract and liver. Recently, the intranasal route of progesterone administration has received attention for easy and efficient targeting of the brain. Progesterone in the brain is derived from the steroidogenic endocrine glands or from local synthesis by neural cells. Stimulating the formation of endogenous progesterone is currently explored as an alternative strategy for neuroprotection, axonal regeneration, and myelin repair.

Studies on a Tyr residue critical for the binding of coenzyme and substrate in mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21): structure of the Y224D mutant enzyme

Mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) is the only aldo-keto reductase that catalyzes the stereospecific reduction of 3- and 17-ketosteroids to the corresponding 3(17)alpha-hydroxysteroids. The Y224D mutation of AKR1C21 reduced the K(m) value for NADP(H) by up to 80-fold and completely reversed the 17alpha stereospecificity of the enzyme. The crystal structure of the Y224D mutant at 2.3 A resolution revealed that the mutation resulted in a change in the conformation of the flexible loop B, including the V-shaped groove, which is a unique feature of the active-site architecture of wild-type AKR1C21 and is formed by the side chains of Tyr224 and Trp227. Furthermore, mutations (Y224F and Q222N) of residues involved in forming the safety belt for binding of the coenzyme showed similar alterations in kinetic constants for 3alpha-hydroxy/3-ketosteroids and 17-hydroxy/ketosteroids compared with the wild type.

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.

Structure of 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) holoenzyme from an orthorhombic crystal form: an insight into the bifunctionality of the enzyme

Mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) is a bifunctional enzyme that catalyses the oxidoreduction of the 3- and 17-hydroxy/keto groups of steroid substrates such as oestrogens, androgens and neurosteroids. The structure of the AKR1C21-NADPH binary complex was determined from an orthorhombic crystal belonging to space group P2(1)2(1)2(1) at a resolution of 1.8 A. In order to identify the factors responsible for the bifunctionality of AKR1C21, three steroid substrates including a 17-keto steroid, a 3-keto steroid and a 3alpha-hydroxysteroid were docked into the substrate-binding cavity. Models of the enzyme-coenzyme-substrate complexes suggest that Lys31, Gly225 and Gly226 are important for ligand recognition and orientation in the active site.

Rat NAD+-dependent 3α-hydroxysteroid dehydrogenase (AKR1C17): A member of the aldo-keto reductase family highly expressed in kidney cytosol

Mammalian 3alpha-hydroxysteroid dehydrogenases (3alpha-HSDs) have been divided into two types: Cytosolic NADP(H)-dependent 3alpha-HSDs belonging to the aldo-keto reductase family, and mitochondrial and microsomal NAD(+)-dependent 3alpha-HSDs belonging to the short-chain dehydrogenase/reductase family. In this study, we characterized a rat aldo-keto reductase (AKR1C17), whose functions are unknown. The recombinant AKR1C17 efficiently oxidized 3alpha-hydroxysteroids and bile acids using NAD(+) as the preferred coenzyme at an optimal pH of 7.4-9.5, and was inhibited by ketamine and organic anions. The mRNA for AKR1C17 was detected specifically in rat kidney, where the enzyme was more highly expressed as a cytosolic protein than NADP(H)-dependent 3alpha-HSD (AKR1C9). Thus, AKR1C17 represents a novel NAD(+)-dependent type of cytosolic 3alpha-HSD with unique inhibitor sensitivity and tissue distribution. In addition, the replacement of Gln270 and Glu276 of AKR1C17 with the corresponding residues of NADP(H)-dependent 3alpha-HSD resulted in a switch in favor of NADP(+) specificity, suggesting their key roles in coenzyme specificity.

Enzymatic characteristics of an aldo-keto reductase family protein (AKR1C15) and its localization in rat tissues

A member of the aldo-keto reductase superfamily, AKR1C15, was isolated via cDNA cloning, but its physiological function remains unknown. Here, we show that recombinant AKR1C15 is an NADPH-dependent reductase with broad substrate specificity for aromatic, alicyclic and aliphatic carbonyl compounds, including acetoin, 2,5-hexanedione, methylglyoxal, farnesal, retinals, 17-ketosteroids and monosaccharides. Especially, all-trans-retinal, alpha-diketones and lipid-derived aldehydes including 4-hydroxynonenal were excellent substrates showing low K(m) values (0.3-5.5 microM). Immunohistochemical and reverse transcription-PCR analyses revealed that AKR1C15 is highly expressed in rat bronchiolar Clara cells, type II alveolar cells, gastric parietal cells, the epithelial cells of the stomach and colon, and the brown adipocytes. The enzyme was not detected in cells of other rat tissues, but is consistently expressed in the vascular endothelial cells. These results suggest that AKR1C15 plays a role in retinoid, steroid, isoprenoid and carbohydrate metabolism, as well as a defense system, protecting against reactive carbonyl compounds.

Substrate specificity of a mouse aldo-keto reductase (AKR1C12)

AKR1C12, a mouse member of the aldo-keto reductase (AKR) superfamily, is highly expressed in the stomach and is identical to a protein encoded in an interleukin-3-regulated gene in mouse myeloid cells, but its function remains unknown. In this study, the recombinant AKR1C12 was purified to homogeneity and the specificity for coenzymes and substrates was examined at a physiological pH of 7.4. The enzyme reduced various alpha-dicarbonyl compounds, several ketosteroids, aldehydes and some ketones using NADH as the preferred coenzyme. In the reverse reaction, the enzyme showed coenzyme preference for NAD+, and oxidized 3alpha-, 17beta- and 20alpha-hydroxysteroids, and non-steroidal aliphatic and alicyclic alcohols, of which many hydroxysteroids and geranylgeraniol were good substrates, exhibiting low Km and high kcat/Km values. The results, together with the intracellular high ratio of NAD+/NADH, suggest that AKR1C12 functions as a dehydrogenase for the endogenous hydroxysteroids and geranylgeraniol in mouse stomach and myeloid cells.

Crystal structures of mouse 17alpha-hydroxysteroid dehydrogenase (apoenzyme and enzyme-NADP(H) binary complex): identification of molecular determinants responsible for the unique 17alpha-reductive activity of this enzyme

Very recently, the mouse 17alpha-hydroxysteroid dehydrogenase (m17alpha-HSD), a member of the aldo-keto reductase (AKR) superfamily, has been characterized and identified as the unique enzyme able to catalyze efficiently and in a stereospecific manner the conversion of androstenedione (Delta4) into epitestosterone (epi-T), the 17alpha-epimer of testosterone. Indeed, the other AKR enzymes that significantly reduce keto groups situated at position C17 of the steroid nucleus, the human type 3 3alpha-HSD (h3alpha-HSD3), the human and mouse type 5 17beta-HSD, and the rabbit 20alpha-HSD, produce only 17beta-hydroxy derivatives, although they possess more than 70% amino acid identity with m17alpha-HSD. Structural comparisons of these highly homologous enzymes thus offer an excellent opportunity of identifying the molecular determinants responsible for their 17alpha/17beta-stereospecificity. Here, we report the crystal structure of the m17alpha-HSD enzyme in its apo-form (1.9 A resolution) as well as those of two different forms of this enzyme in binary complex with NADP(H) (2.9 A and 1.35 A resolution). Interestingly, one of these binary complex structures could represent a conformational intermediate between the apoenzyme and the active binary complex. These structures provide a complete picture of the NADP(H)-enzyme interactions involving the flexible loop B, which can adopt two different conformations upon cofactor binding. Structural comparison with binary complexes of other AKR1C enzymes has also revealed particularities of the interaction between m17alpha-HSD and NADP(H), which explain why it has been possible to crystallize this enzyme in its apo form. Close inspection of the m17alpha-HSD steroid-binding cavity formed upon cofactor binding leads us to hypothesize that the residue at position 24 is of paramount importance for the stereospecificity of the reduction reaction. Mutagenic studies have showed that the m17alpha-HSD(A24Y) mutant exhibited a completely reversed stereospecificity, producing testosterone only from Delta4, whereas the h3alpha-HSD3(Y24A) mutant acquires the capacity to metabolize Delta4 into epi-T.

Molecular Cloning of a Novel Type of Rat Cytoplasmic 17β-Hydroxysteroid Dehydrogenase Distinct from the Type 5 Isozyme

Rat liver contains two cytosolic enzymes (TBER1 and TBER2) that reduce 6-tert-butyl-2,3-epoxy-5-cyclohexene-1,4-dione into its 4R- and 4S-hydroxy metabolites. In this study, we cloned the cDNA for TBER1 and examined endogenous substrates using the homogenous recombinant enzyme. The cDNA encoded a protein composed of 323 amino acids belonging to the aldo-keto reductase family. The recombinant TBER1 efficiently oxidized 17beta-hydroxysteroids and xenobiotic alicyclic alcohols using NAD+ as the preferred coenzyme at pH 7.4, and showed low activity towards 20alpha- and 3alpha-hydroxysteroids, and 9-hydroxyprostaglandins. The enzyme was potently inhibited by diethylstilbestrol, hexestrol and zearalenone. The coenzyme specificity, broad substrate specificity and inhibitor sensitivity of the enzyme differed from those of rat NADPH-dependent 17beta-hydroxysteroid dehydrogenase type 5, which was cloned from the liver and characterized using the recombinant enzyme. The mRNA for TBER1 was highly expressed in rat liver, gastrointestinal tract and ovary, in contrast to specific expression of 17beta-hydroxysteroid dehydrogenase type 5 mRNA in the liver and kidney. Thus, TBER1 represents a novel type of 17beta-hydroxysteroid dehydrogenase with unique catalytic properties and tissue distribution. In addition, TBER2 was identified as 3alpha-hydroxysteroid dehydrogenase on chromatographic analysis of the enzyme activities in rat liver cytosol and characterization of the recombinant 3alpha-hydroxysteroid dehydrogenase.

Multiplicity of mammalian reductases for xenobiotic carbonyl compounds

A variety of carbonyl compounds are present in foods, environmental pollutants, and drugs. These xenobiotic carbonyl compounds are metabolized into the corresponding alcohols by many mammalian NAD(P)H-dependent reductases, which belong to the short-chain dehydrogenase/reductase (SDR) and aldo-keto reductase superfamilies. Recent genomic analysis, cDNA isolation and characterization of the recombinant enzymes suggested that, in humans, the six members of each of the two superfamilies, i.e., total of 12 enzymes, are involved in the reductive metabolism of xenobiotic carbonyl compounds. They comprise three types of carbonyl reductase, dehydrogenase/reductase (SDR family) member 4, 11beta-hydroxysteroid dehydrogenase type 1, L-xylulose reductase, two types of aflatoxin B1 aldehyde reductase, 20alpha-hydroxysteroid dehydrogenase, and three types of 3alpha-hydroxysteroid dehydrogenase. Accumulating data on the human enzymes provide new insights into their roles in cellular and molecular reactions including xenobiotic metabolism. On the other hand, mice and rats lack the gene for a protein corresponding to human 3alpha-hydroxysteroid dehydrogenase type 3, but instead possess additional five or six genes encoding proteins that are structurally related to human hydroxysteroid dehydrogenases. Characterization of the additional enzymes suggested their involvement in species-specific biological events and species differences in the metabolism of xenobiotic carbonyl compounds.

Enzymatic Properties of a Member (AKR1C20) of the Aldo-Keto Reductase Family

AKR1C20, a member of the aldo-keto reductase (AKR) superfamily, found by mouse genomic analysis, exhibits the highest sequence identity (89%) with mouse liver 17beta-hydroxysteroid dehydrogenase (HSD) type 5, but its function remains unknown. In this report, we have expressed the recombinant AKR1C20 from its cDNA, and examined its properties. The purified enzyme was a 36-kDa monomer, and showed both 17beta-HSD and 3alpha-HSD activities in the presence of NADP(H) as the coenzymes. While the Km values for testosterone and 5alpha-dihydrotestosterone were high (>0.2 mM), those for 3alpha-hydroxy- and 3-keto-steroids were low (0.3-5 microM), resulting in high catalytic efficiency for the substrates. Although no significant dehydrogenase activity towards non-steroidal alcohols was observed, the enzyme highly reduced alpha-dicarbonyl compounds such as 16-ketoestrone, 9,10-phenanthrenequinone, acenaphthenequinone, 1-phenylisatin and camphorquinone. The pH optima of the dehydrogenase and reductase activities were 10.5 and 6.5-7.5, respectively. The enzyme was inhibited by sulfobromophthalein, hexestrol, indomethacin and flufenamic acid. The properties of AKR1C20 are distinct from those of previously known mouse 17beta-HSD type 5 (AKR1C6), 3alpha-HSD (AKR1C14) and other members of the AKR1C subfamily. Thus, AKR1C20 is a novel 3alpha(17beta)-HSD, which may also function as a reductase for xenobiotic alpha-dicarbonyl compounds.

Crystallization and preliminary X-ray diffraction analysis of mouse 3(17)alpha-hydroxysteroid dehydrogenase

The 3(17)alpha-hydroxysteroid dehydrogenase from mouse is involved in the metabolism of oestrogens, androgens, neurosteroids and xenobiotic compounds. The enzyme was crystallized by the hanging-drop vapour-diffusion method in space group P222(1), with unit-cell parameters a = 84.91, b = 84.90, c = 95.83 A. The Matthews coefficient (VM) and the solvent content were 2.21 A3 Da(-1) and 44.6%, respectively, assuming the presence of two molecules in the asymmetric unit. Diffraction data were collected to a resolution of 1.8 A at the Swiss Light Source beamline X06SA using a MAR CCD area detector and gave a data set with an overall Rmerge of 6.8% and a completeness of 91.1%.

Enzymatic Properties of a Member (AKR1C19) of the Aldo-Keto Reductase Family

A member (AKR1C19) of the aldo-keto reductase (AKR) superfamily, found by mouse genomic analysis, was shown to be highly expressed in the liver and gastrointestinal tract, but its function remains unknown. In this study, the recombinant AKR1C19 was expressed and purified to homogeneity. The enzyme was a 36-kDa monomer, and reduced alpha-dicarbonyl compounds such as camphorquinone and isatin using both NADH and NADPH as the coenzymes. Although apparent kinetic constants for the two coenzymes were similar, the NADPH-linked activity was potently inhibited by submillimolar concentrations of NAD+, but the inhibition of the NADH-linked activity was not significant, suggesting that the enzyme exhibits the NADH-linked reductase activity in vivo. AKR1C19 slowly oxidized 3-hydroxyhexobarbital, S-indan-1-ol and cis-benzene dihydrodiol, but was inactive towards steroids, prostaglandins, monosaccharides, and other xenobiotic alcohols. In addition, the enzyme was inhibited only by dicumarol, lithocholic acid and genistein of various compounds tested. Thus, AKR1C19 possesses properties distinct from other members of the AKR superfamily, and may function as a reductase for endogenous isatin and xenobiotic alpha-dicarbonyl compounds in the liver and gastrointestinal tract.