Steroid dehydrogenase structures, mechanism of action, and disease

Lipid Droplets in Endosymbiotic Symbiodiniaceae spp. Associated with Corals

Symbiodiniaceae species is a dinoflagellate that plays a crucial role in maintaining the symbiotic mutualism of reef-building corals in the ocean. Reef-building corals, as hosts, provide the nutrition and habitat to endosymbiotic Symbiodiniaceae species and Symbiodiniaceae species transfer the fixed carbon to the corals for growth. Environmental stress is one of the factors impacting the physiology and metabolism of the corals-dinoflagellate association. The environmental stress triggers the metabolic changes in Symbiodiniaceae species resulting in an increase in the production of survival organelles related to storage components such as lipid droplets (LD). LDs are found as unique organelles, mainly composed of triacylglycerols surrounded by phospholipids embedded with some proteins. To date, it has been reported that investigation of lipid droplets significantly present in animals and plants led to the understanding that lipid droplets play a key role in lipid storage and transport. The major challenge of investigating endosymbiotic Symbiodiniaceae species lies in overcoming the strategies in isolating lesser lipid droplets present in its intercellular cells. Here, we review the most recent highlights of LD research in endosymbiotic Symbiodiniaceae species particularly focusing on LD biogenesis, mechanism, and major lipid droplet proteins. Moreover, to comprehend potential novel ways of energy storage in the symbiotic interaction between endosymbiotic Symbiodiniaceae species and its host, we also emphasize recent emerging environmental factors such as temperature, ocean acidification, and nutrient impacting the accumulation of lipid droplets in endosymbiotic Symbiodiniaceae species.

Employing deuterium kinetic isotope effects to uncover the mechanism of (R)-3-hydroxybutyrate dehydrogenase

Short-chain dehydrogenases/reductases (SDR) form a large enzyme superfamily playing important roles in health and disease. Furthermore, they are useful tools in biocatalysis. Unveiling the nature of the transition state for hydride transfer is a crucial undertaking toward defining the physicochemical underpinnings of catalysis by SDR enzymes, including possible contributions from quantum mechanical tunneling. Primary deuterium kinetic isotope effects can uncover the contribution from chemistry to the rate-limiting step and potentially provide detailed information on the hydride-transfer transition state in SDR-catalyzed reactions. For the latter, however, one needs to determine the intrinsic isotope effect: that which would be measured if hydride transfer were rate determining. Alas, as is the case for many other enzymatic reactions, those catalyzed by SDRs are often limited by the rate of isotope-insensitive steps, such as product release and conformational changes, which masks the expression of the intrinsic isotope effect. This can be overcome by the powerful yet underexplored method of Palfey and Fagan via which intrinsic kinetic isotope effects can be extracted from pre-steady-state kinetics data. SDRs are ideal systems to which this method can be applied. We have employed this approach to elucidate the transition states for hydride transfer catalyzed by NADH-dependent cold- and warm-adapted (R)-3-hydroxybutyrate dehydrogenase. Experimental conditions which simplify the analysis are discussed.

Carbenoxolone disodium suppresses the migration of gastric cancer by targeting HDAC6

Aim: Because of the severe morbidity and mortality of gastric cancer, discovering new candidate drugs has been an urgent issue. The close association between histone deacetylase 6 (HDAC6) and gastric cancer makes the development of HDAC6-targeted anti-gastric cancer drugs a viable idea. Methods & results: Carbenoxolone disodium was identified as a novel HDAC6 inhibitor. Cellular thermal shift assay, surface plasmon resonance assay and molecular docking confirmed its binding ability to HDAC6. Cell viability, wound healing and transwell assays as well as animal studies have demonstrated that carbenoxolone disodium could block the proliferation and migration of gastric cancer cells MGC-803 in vitro and in vivo. Conclusion: This is the first report to indicate that carbenoxolone disodium could be an HDAC6 inhibitor with potential for treatment of gastric cancer.

A Structural View on the Stereospecificity of Plant Borneol-Type Dehydrogenases

The development of sustainable processes for the valorization of byproducts and other waste streams remains an ongoing challenge in the field of catalysis. Racemic borneol, isoborneol and camphor are currently produced from α-pinene, a side product from the production of cellulose. The pure enantiomers of these monoterpenoids have numerous applications in cosmetics and act as reagents for asymmetric synthesis, making an enzymatic route for their separation into optically pure enantiomers a desirable goal. Known short-chain borneol-type dehydrogenases (BDHs) from plants and bacteria lack the required specificity, stability or activity for industrial utilization. Prompted by reports on the presence of pure (-)-borneol and (-)-camphor in essential oils from rosemary, we set out to investigate dehydrogenases from the genus Salvia and discovered a dehydrogenase with high specificity (E>120) and high specific activity (>0.02 U mg-1) for borneol and isoborneol. Compared to other specific dehydrogenases, the one reported here shows remarkably higher stability, which was exploited to obtain the first three-dimensional structure of an enantiospecific borneol-type short-chain dehydrogenase. This, together with docking studies, led to the identification of a hydrophobic pocket in the enzyme that plays a crucial role in the stereo discrimination of bornane-type monoterpenoids. The kinetic resolution of borneol and isoborneol can be easily integrated into the existing synthetic route from α-pinene to camphor thereby allowing the facile synthesis of optically pure monoterpenols from an abundant renewable source.

Contribution of placental 11β-HSD2 to the pathogenesis of preeclampsia

Preeclampsia, a major human pregnancy-specific disorder, leads to maternal and fetal morbidity and mortality. Here we reported that 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), an enzyme that degrades active glucocorticoids, is one of the key factors that contributes to preeclampsia development. In the pregnant rat model, we firstly confirmed that administration of 11β-HSD2 inhibitor carbenoxolone (CBX) subcutaneously or by placenta-targeted delivery system could lead to a decrease in placental 11β-HSD2 expression and activity and an increase in corticosterone level in placenta and maternal circulation. Then, we showed that subcutaneous administration and placenta-targeted delivery of CBX resulted in the hallmark of preeclampsia-like features including hypertension, proteinuria, renal damages as well as elevated circulatory soluble fms-like tyrosine kinase 1 (sFlt1) and increased sFlt1/placental growth factor (PlGF) ratio in pregnant rats. These animals displayed decreased trophoblast invasion in uterus, impaired spiral artery remodeling, and reduced placental blood flow. Preeclampsia-like features could also be induced by administration of dexamethasone in pregnant rats. In the cultured human trophoblast models, we found that cortisol only inhibited migration and invasion of the extravillous trophoblasts with 11β-HSD2 knockdown, and promoted sFlt1 release in the cultured syncytiotrophoblasts with 11β-HSD2 knockdown. Furthermore, we elucidated that cortisol stimulated a disintegrin and metalloprotease (ADAM)17 expression in placentas, thereby promoting sFlt1 release in placenta. Collectively, our study provided the evidence that placental 11β-HSD2 dysfunction plays a key role in the development of preeclampsia and immediately identified innovative target to counteract preeclampsia.

Inhibitors of 17β-hydroxysteroid dehydrogenase type 1, 2 and 14: Structures, biological activities and future challenges

During the past 25 years, the modulation of estrogen action by inhibition of 17β-hydroxysteroid dehydrogenase types 1 and 2 (17β-HSD1 and 17β-HSD2), respectively, has been pursued intensively. In the search for novel treatment options for estrogen-dependent diseases (EDD) and in order to explore estrogenic signaling pathways, a large number of steroidal and nonsteroidal inhibitors of these enzymes has been described in the literature. The present review gives a survey on the development of inhibitor classes as well as the structural formulas and biological properties of their most interesting representatives. In addition, rationally designed dual inhibitors of both 17β-HSD1 and steroid sulfatase (STS) as well as the first inhibitors of 17β-HSD14 are covered.

Disruption of Gap Junction-Mediated Intercellular Communication in the Spiral Ligament Causes Hearing and Outer Hair Cell Loss in the Cochlea of Mice

It is well-known that outer hair cell (OHC) loss occurs in the cochlea of animal models of permanent hearing loss induced by intense noise exposure. Our earlier studies demonstrated the production of hydroxynonenal and peroxynitrite, as well as the disruption of gap junction-mediated intercellular communication (GJIC), in the cochlear spiral ligament prior to noise-induced sudden hearing loss. The goal of the present study was to evaluate the mechanism underlying cochlear OHC loss after sudden hearing loss induced by intense noise exposure. In organ of Corti explant cultures from mice, no significant OHC loss was observed after in vitro exposure to 4-hydroxynonenal (a product of lipid peroxidation), H₂O₂, SIN-1 (peroxynitrite generator), and carbenoxolone (a gap junction inhibitor). Interestingly, in vivo intracochlear carbenoxolone injection through the posterior semicircular canal caused marked OHC and hearing loss, as well as the disruption of gap junction-mediated intercellular communication in the cochlear spiral ligament. However, no significant OHC loss was observed in vivo in animals treated with 4-hydroxynonenal and SIN-1. Taken together, our data suggest that disruption of GJIC in the cochlear lateral wall structures is an important cause of cochlear OHC loss in models of hearing loss, including those induced by noise.

Structural and Biochemical Investigation of PglF from Campylobacter jejuni Reveals a New Mechanism for a Member of the Short Chain Dehydrogenase/Reductase Superfamily

Within recent years it has become apparent that protein glycosylation is not limited to eukaryotes. Indeed, in Campylobacter jejuni, a Gram-negative bacterium, more than 60 of its proteins are known to be glycosylated. One of the sugars found in such glycosylated proteins is 2,4-diacetamido-2,4,6-trideoxy-α-d-glucopyranose, hereafter referred to as QuiNAc4NAc. The pathway for its biosynthesis, initiating with UDP-GlcNAc, requires three enzymes referred to as PglF, PglE, and PlgD. The focus of this investigation is on PglF, an NAD⁺-dependent sugar 4,6-dehydratase known to belong to the short chain dehydrogenase/reductase (SDR) superfamily. Specifically, PglF catalyzes the first step in the pathway, namely, the dehydration of UDP-GlcNAc to UDP-2-acetamido-2,6-dideoxy-α-d-xylo-hexos-4-ulose. Most members of the SDR superfamily contain a characteristic signature sequence of YXXXK where the conserved tyrosine functions as a catalytic acid or a base. Strikingly, in PglF, this residue is a methionine. Here we describe a detailed structural and functional investigation of PglF from C. jejuni. For this investigation five X-ray structures were determined to resolutions of 2.0 Å or better. In addition, kinetic analyses of the wild-type and site-directed variants were performed. On the basis of the data reported herein, a new catalytic mechanism for a SDR superfamily member is proposed that does not require the typically conserved tyrosine residue.

Rac1 GTPase regulates 11β hydroxysteroid dehydrogenase type 2 and fibrotic remodeling

The aim of the study was to characterize the role of Rac1 GTPase for the mineralocorticoid receptor (MR)-mediated pro-fibrotic remodeling. Transgenic mice with cardiac overexpression of constitutively active Rac1 (RacET) develop an age-dependent phenotype with atrial dilatation, fibrosis, and atrial fibrillation. Expression of MR was similar in RacET and WT mice. The expression of 11β hydroxysteroid dehydrogenase type 2 (11β-HSD2) was age-dependently up-regulated in the atria and the left ventricles of RacET mice on mRNA and protein levels. Statin treatment inhibiting Rac1 geranylgeranylation reduced 11β-HSD2 up-regulation. Samples of human left atrial myocardium showed a positive correlation between Rac1 activity and 11β-HSD2 expression (r = 0.7169). Immunoprecipitation showed enhanced Rac1-bound 11β-HSD2 relative to Rac1 expression in RacET mice that was diminished with statin treatment. Both basal and phorbol 12-myristate 13-acetate (PMA)-induced NADPH oxidase activity were increased in RacET and correlated positively with 11β-HSD2 expression (r = 0.788 and r = 0.843, respectively). In cultured H9c2 cardiomyocytes, Rac1 activation with l-buthionine sulfoximine increased; Rac1 inhibition with NSC23766 decreased 11β-HSD2 mRNA and protein expression. Connective tissue growth factor (CTGF) up-regulation induced by aldosterone was prevented with NSC23766. Cardiomyocyte transfection with 11β-HSD2 siRNA abolished the aldosterone-induced CTGF up-regulation. Aldosterone-stimulated MR nuclear translocation was blocked by the 11β-HSD2 inhibitor carbenoxolone. In cardiac fibroblasts, nuclear MR translocation induced by aldosterone was inhibited with NSC23766 and spironolactone. NSC23766 prevented the aldosterone-induced proliferation and migration of cardiac fibroblasts and the up-regulation of CTGF and fibronectin. In conclusion, Rac1 GTPase regulates 11β-HSD2 expression, MR activation, and MR-mediated pro-fibrotic signaling.

Steroid Hormone Signaling Is Essential for Pheromone Production and Oenocyte Survival

Many of the lipids found on the cuticles of insects function as pheromones and communicate information about age, sex, and reproductive status. In Drosophila, the composition of the information-rich lipid profile is dynamic and changes over the lifetime of an individual. However, the molecular basis of this change is not well understood. To identify genes that control cuticular lipid production in Drosophila, we performed a RNA interference screen and used Direct Analysis in Real Time and gas chromatography mass spectrometry to quantify changes in the chemical profiles. Twelve putative genes were identified whereby transcriptional silencing led to significant differences in cuticular lipid production. Amongst them, we characterized a gene which we name spidey, and which encodes a putative steroid dehydrogenase that has sex- and age-dependent effects on viability, pheromone production, and oenocyte survival. Transcriptional silencing or overexpression of spidey during embryonic development results in pupal lethality and significant changes in levels of the ecdysone metabolite 20-hydroxyecdysonic acid and 20-hydroxyecdysone. In contrast, inhibiting gene expression only during adulthood resulted in a striking loss of oenocyte cells and a concomitant reduction of cuticular hydrocarbons, desiccation resistance, and lifespan. Oenocyte loss and cuticular lipid levels were partially rescued by 20-hydroxyecdysone supplementation. Taken together, these results identify a novel regulator of pheromone synthesis and reveal that ecdysteroid signaling is essential for the maintenance of cuticular lipids and oenocytes throughout adulthood.

Pheromones are used by many animals to control social behaviors such as mate choice and kin recognition. The pheromone profile of insects is dynamic and can change depending on environmental, physiological, and social conditions. While many genes responsible for the biosynthesis of insect pheromones have been identified, much less is known about how pheromone production is systemically regulated over the lifetime of an animal. In this work, we identify 12 genes in Drosophila melanogaster that play a role in pheromone production. We characterized the function of one gene, which we name spidey, and which encodes a steroid dehydrogenase. Silencing spidey expression during the larval stage results in the rapid inactivation of an essential insect steroid, 20-hydroxyecdysone, and developmental arrest. In adults, spidey is needed for maintaining the viability of oenocytes, specialized cells that produce pheromones and also regulate energy homeostasis. Our work reveals a novel role for ecdysteroids in the adult animal and uncovers a regulatory mechanism for oenocyte activity. Potentially, ecdysteroid signaling serves as a mechanism by which environmental or social conditions shape pheromone production. Exploitation of this conserved pathway could be useful for interfering with the mating behavior and lifespan of disease-bearing insects or agricultural pests.

Intrathecal carbenoxolone inhibits neuropathic pain and spinal wide-dynamic range neuronal activity in rats after an L5 spinal nerve injury

Spinal glial gap junctions may play an important role in dorsal horn neuronal sensitization and neuropathic pain. In rats after an L5 spinal nerve ligation (SNL), we examined the effects of intrathecal injection of carbenoxolone (CBX), a gap junction decoupler, on neuropathic pain manifestations and on wide-dynamic range (WDR) neuronal activity in vivo. Intrathecal injection of CBX dose-dependently (0.1-50 μg, 10 μl) inhibited mechanical hypersensitivity in rats at 2-3 weeks post-SNL. However, the same doses of glycyrrhizic acid (an analogue of CBX that does not affect gap junctions) and mefloquine hydrochloride (a selective neuronal gap junction decoupler) were ineffective. Intrathecal CBX (5μg) also attenuated heat hypersensitivity in SNL rats. Further, rats did not develop tachyphylaxis to CBX-induced inhibition of mechanical hypersensitivity after repetitive drug treatments (25 μg/day) during days 14-16 post-SNL. Electrophysiological study in SNL rats showed that spinal topical application of CBX (100 μg, 50 μl), which mimics intrathecal drug administration, attenuated WDR neuronal responses to mechanical stimuli and to repetitive intracutaneous electrical stimuli (0.5 Hz) that induce windup, a short-form of activity-dependent neuronal sensitization. The current findings suggest that the inhibition of neuropathic pain manifestations by intrathecal injection of CBX in SNL rats may involve an inhibition of glial gap junctions and an attenuation of WDR neuronal activity in the dorsal horn.

Environmental pollutants and hydroxysteroid dehydrogenases

Hydroxysteroid dehydrogenases (HSD) are a group of steroidogenic enzymes that are involved in the steroid biosynthesis and metabolism. Four classes of HSDs, namely, 3β-, 11β-, 17β-, and 20α-HSDs, are discussed. 3β-HSDs catalyze the conversion of pregnenolone, 17α-hydroxypregnenolone, and dehydroepiandrosterone to progesterone, 17α-hydroxyprogesterone, and androstenedione, respectively. 11β-HSDs catalyze the interconversion between active cortisol and inactive cortisone. 17β-HSDs catalyze the interconversion between 17β-hydroxyl steroids and 17-ketoandrogens and estrogens. 20α-HSDs catalyze the conversion of progesterone into 20α-hydroxyprogesterone. Many environmental pollutants directly inhibit one or more enzymes of these HSDs, thus interfering with endogenous active steroid hormone levels. These chemicals include industrial materials (perfluoroalkyl compounds, phthalates, bisphenol A, and benzophenone), pesticides/biocides (methoxychlor, organotins, 1,2-dibromo-3-chloropropane, and prochloraz), and plant constituents (genistein, gossypol, and licorice). This chapter reviews these inhibitors targeting on HSDs.

Dissection of malonyl-coenzyme A reductase of Chloroflexus aurantiacus results in enzyme activity improvement

The formation of fusion protein in biosynthetic pathways usually improves metabolic efficiency either channeling intermediates and/or colocalizing enzymes. In the metabolic engineering of biochemical pathways, generating unnatural protein fusions between sequential biosynthetic enzymes is a useful method to increase system efficiency and product yield. Here, we reported a special case. The malonyl-CoA reductase (MCR) of Chloroflexus aurantiacus catalyzes the conversion of malonyl-CoA to 3-hydroxypropionate (3HP), and is a key enzyme in microbial production of 3HP, an important platform chemical. Functional domain analysis revealed that the N-terminal region of MCR (MCR-N; amino acids 1-549) and the C-terminal region of MCR (MCR-C; amino acids 550-1219) were functionally distinct. The malonyl-CoA was reduced into free intermediate malonate semialdehyde with NADPH by MCR-C fragment, and further reduced to 3HP by MCR-N fragment. In this process, the initial reduction of malonyl-CoA was rate limiting. Site-directed mutagenesis demonstrated that the TGXXXG(A)X(1-2)G and YXXXK motifs were important for enzyme activities of both MCR-N and MCR-C fragments. Moreover, the enzyme activity increased when MCR was separated into two individual fragments. Kinetic analysis showed that MCR-C fragment had higher affinity for malonyl-CoA and 4-time higher K_cat/K_m value than MCR. Dissecting MCR into MCR-N and MCR-C fragments also had a positive effect on the 3HP production in a recombinant Escherichia coli strain. Our study showed the feasibility of protein dissection as a new strategy in biosynthetic systems.

Cloning, characterization, and expression analysis of a putative 17 beta-hydroxysteroid dehydrogenase 11 in the abalone, Haliotis diversicolor supertexta

The 17-beta-hydroxysteroid dehydrogenases (17β-HSDs) are key enzymes for sex steroid biosynthesis. To date, relatively little is known about the presence and function of 17β-HSDs in marine gastropods. In the present study, a cDNA sequence encoding putative 17β-HSD type 11 (17β-HSD-11) was identified in marine abalone (Haliotis diversicolor supertexta). The full-length cDNA contains 1058bp, including an open reading frame (ORF) of 900bp that encodes a protein of 299 amino acids. Comparative structural analysis revealed that abalone 17β-HSD-11 shares relatively high homology with other 17b-HSD-11 hormologues, and a lesser degree of amino acid identity with other forms of 17b-HSD, especially in the functional domains, including the cofactor binding domain (TGxxxGxG) and catalytic site (YxxSK). Phylogenetic analysis showed that abalone 17β-HSD-11 belongs to the short-chain dehydrogenase/reductase (SDR) family. Functional analysis following transient transfection of the ORF into human embryonic kidney-293 (HEK-293) cells indicated that abalone 17β-HSD-11 has the ability to convert 5α-androstane-3α,17β-diol (3α-diol) to androsterone (A) and testosterone (T) to androstenedione (4A). Expression analysis in vivo demonstrated that abalone 17β-HSD-11 is differentially expressed during three stages (non-reproductive, reproductive, and post-reproductive). Taken together, these results indicate that ab-17β-HSD-11 is an SDR family member with a potential role in steroid regulation during the reproductive stage.

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

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

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

Carbonyl reduction of aldehydes, ketones, and quinones to their corresponding hydroxy derivatives plays an important role in the phase I metabolism of many endogenous (biogenic aldehydes, steroids, prostaglandins, reactive lipid peroxidation products) and xenobiotic (pharmacologic drugs, carcinogens, toxicants) compounds. Carbonyl-reducing enzymes are grouped into two large protein superfamilies: the aldo-keto reductases (AKR) and the short-chain dehydrogenases/reductases (SDR). Whereas aldehyde reductase and aldose reductase are AKRs, several forms of carbonyl reductase belong to the SDRs. In addition, there exist a variety of pluripotent hydroxysteroid dehydrogenases (HSDs) of both superfamilies that specifically catalyze the oxidoreduction at different positions of the steroid nucleus and also catalyze, rather nonspecifically, the reductive metabolism of a great number of nonsteroidal carbonyl compounds. The present review summarizes recent findings on carbonyl reductases and pluripotent HSDs of the SDR protein superfamily.

Cloning of chicken 11beta-hydroxysteroid dehydrogenase type 1 and its tissue distribution

11beta-Hydroxysteroid dehydrogenase type 1 (11HSD1) is an enzyme that interconverts active 11-hydroxy glucocorticoids (cortisol, corticosterone) and their inactive 11-oxo derivatives (cortisone, 11-dehydrocorticosterone). Although bidirectional, it is considered to operate in vivo as an 11-reductase that regenerates active glucocorticoids and thus amplifies their local activity in mammals. Here we report the cloning, characterization and tissue distribution of chicken 11HSD1 (ch11HSD1). Its cDNA predicts a protein of 300 amino acids that share 51-56% sequence identity with known mammalian 11HSD1 proteins, while in contrast to most mammals, ch11HSD1 contains only one N-linked glycosylation site. Analysis of the tissue distribution pattern by RT-PCR revealed that ch11HSD1 is expressed in a large variety of tissues, with high expression in the liver, kidney and intestine, and weak in the gonads, brain and heart. 11-Reductase activity has been found in the liver, kidney, intestine and gonads with low or almost zero activity in the brain and heart. These results provide evidence for a role of 11HSD1 as a tissue-specific regulator of glucocorticoid action in non-mammalian vertebrates and may serve as a suitable model for further analysis of 11HSD1 evolution in vertebrates.

Rho-family GTPases modulate Ca(2+) -dependent ATP release from astrocytes

Previously, we reported that activation of G protein-coupled receptors (GPCR) in 1321N1 human astrocytoma cells elicits a rapid release of ATP that is partially dependent on a G(q)/phophospholipase C (PLC)/Ca(2+) mobilization signaling cascade. In this study we assessed the role of Rho-family GTPase signaling as an additional pathway for the regulation of ATP release in response to activation of protease-activated receptor-1 (PAR1), lysophosphatidic acid receptor (LPAR), and M3-muscarinic (M3R) GPCRs. Thrombin (or other PAR1 peptide agonists), LPA, and carbachol triggered quantitatively similar Ca(2+) mobilization responses, but only thrombin and LPA caused rapid accumulation of active GTP-bound Rho. The ability to elicit Rho activation correlated with the markedly higher efficacy of thrombin and LPA, relative to carbachol, as ATP secretagogues. Clostridium difficile toxin B and Clostridium botulinum C3 exoenzyme, which inhibit Rho-GTPases, attenuated the thrombin- and LPA-stimulated ATP release but did not decrease carbachol-stimulated release. Thus the ability of certain G(q)-coupled receptors to additionally stimulate Rho-GTPases acts to strongly potentiate a Ca(2+)-activated ATP release pathway. However, pharmacological inhibition of Rho kinase I/II or myosin light chain kinase did not attenuate ATP release. PAR1-induced ATP release was also reduced twofold by brefeldin treatment suggesting the possible mobilization of Golgi-derived, ATP-containing secretory vesicles. ATP release was also markedly repressed by the gap junction channel inhibitor carbenoxolone in the absence of any obvious thrombin-induced change in membrane permeability indicative of hemichannel gating.

Functional analysis of Arabidopsis thaliana RHM2/MUM4, a multidomain protein involved in UDP-D-glucose to UDP-L-rhamnose conversion

UDP-L-rhamnose is required for the biosynthesis of cell wall rhamnogalacturonan-I, rhamnogalacturonan-II, and natural compounds in plants. It has been suggested that the RHM2/MUM4 gene is involved in conversion of UDP-D-glucose to UDP-L-rhamnose on the basis of its effect on rhamnogalacturonan-I-directed development in Arabidopsis thaliana. RHM2/MUM4-related genes, RHM1 and RHM3, can be found in the A. thaliana genome. Here we present direct evidence that all three RHM proteins have UDP-D-glucose 4,6-dehydratase, UDP-4-keto-6-deoxy-D-glucose 3,5-epimerase, and UDP-4-keto-L-rhamnose 4-keto-reductase activities in the cytoplasm when expressed in the yeast Saccharomyces cerevisiae. Functional domain analysis revealed that the N-terminal region of RHM2 (RHM2-N; amino acids 1-370) has the first activity and the C-terminal region of RHM2 (RHM2-C; amino acids 371-667) has the two following activities. This suggests that RHM2 converts UDP-d-glucose to UDP-L-rhamnose via an UDP-4-keto-6-deoxy-D-glucose intermediate. Site-directed mutagenesis of RHM2 revealed that mucilage defects in MUM4-1 and MUM4-2 mutant seeds of A. thaliana are caused by abolishment of RHM2 enzymatic activity in the mutant strains and furthermore, that the GXXGXX(G/A) and YXXXK motifs are important for enzymatic activity. Moreover, a kinetic analysis of purified His(6)-tagged RHM2-N protein revealed 5.9-fold higher affinity of RHM2 for UDP-D-glucose than for dTDP-D-glucose, the preferred substrate for dTDP-D-glucose 4,6-dehydratase from bacteria. RHM2-N activity is strongly inhibited by UDP-L-rhamnose, UDP-D-xylose, and UDP but not by other sugar nucleotides, suggesting that RHM2 maintains cytoplasmic levels of UDP-D-glucose and UDP-L-rhamnose via feedback inhibition by UDP-L-rhamnose and UDP-D-xylose.

Molecular basis of substrate recognition in D-3-hydroxybutyrate dehydrogenase from Pseudomonas putida

D-3-Hydroxybutyrate dehydrogenase from Pseudomonas putida (EC 1.1.1.30) belongs to the family of short-chain dehydrogenases/reductases (SDRs). It catalyzes the reversible and stereospecific oxidation of D-3-hydroxybutyrate (D-3-HB) to acetoacetate with the aid of NAD(+) as coenzyme. This study contributes to understanding the mechanism and the high specificity of this enzyme towards its negatively charged and hydrophilic substrate. Sequence comparison of 44 bacterial HBDHs shows the residues Gln91, His141, Lys149, Lys192, and Gln193 to be strictly conserved. Site-directed mutagenesis of these amino acids to alanine and subsequent kinetic characterization of the mutated enzymes provides insight into the importance of these residues for substrate recognition and catalysis. Docking studies and molecular-dynamics simulations based on a three-dimensional structure model of a complex between P. putida HBDH and its coenzyme obtained by comparative molecular modeling were performed and provided deeper insight into the binding of the ligands at the molecular level. They show the residues Gln91, His141, Gln193, and, in particular, Lys149 to be involved in a hydrogen-bonding network with the carboxylate group of the substrate.

Sequence of pig 17beta-hydroxysteroid dehydrogenase Type3 cDNA and its expression in mammalian cells

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) Type3 is an NADPH-dependent membrane-bound enzyme that is specifically expressed in testis and catalyzes the conversion of androstenedione to testosterone. To date, the sequence of Type3 enzymes has been clarified in humans, mice and rats; however, the sequence of the pig enzyme remains unknown. In this study, we determined the cDNA sequence of pig testicular 17beta-HSD Type3. PCR primers for partial pig testicular 17beta-HSD Type3 were designed from rat and human enzyme consensus sequences. Full-length cDNA was obtained by 3'- and 5'-RACE based on partial PCR products. The cDNA coding region was 933 bp in length, which is the same as the human enzyme, and shared 84.7% sequence identity with the human cDNA coding region. The monomer was estimated to have a molecular weight of 34,855 and to contain 310 amino acid residues. The predicted pig amino acid sequence showed 81.9, 75.5 and 72.9% sequence identity with the human, rat and mouse sequences, respectively. To elucidate 17beta-HSD Type3 activity, the expression vector pCMV/pig17beta-HSD3 was established and transfected into human embryo kidney 293 cells. Subsequently, 17beta-HSD activity (androstenedione conversion to testosterone) was strongly detected in cell lysates.

11beta-hydroxysteroid dehydrogenase type 2 activity in hypothalamic paraventricular nucleus modulates sympathetic excitation

Aldosterone stimulates the sympathetic nervous system by binding to a select population of brain mineralocorticoid receptors (MR). These MR have an equal affinity for corticosterone that is present in substantially higher concentrations, but are held in reserve for aldosterone by activity of the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD-2), which converts corticosterone to an inactive metabolite. Thus, colocalization of MR and 11beta-HSD-2 activity may help identify brain regions that mediate the effects of aldosterone. The present studies tested the hypothesis that 11beta-HSD-2 activity regulates MR-mediated responses in the paraventricular nucleus (PVN) of the hypothalamus, a forebrain region implicated in sympathetic regulation. Real-time-polymerase chain reaction revealed the presence of 11beta-HSD-2 mRNA in PVN. In anesthetized adult male Sprague-Dawley rats, microinjection of the 11beta-HSD-2 inhibitor carbenoxolone (CBX) into PVN increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. Intracerebroventricular injections of CBX excited PVN neurons and increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. The ability of CBX to increase sympathetic activity by inhibiting 11beta-HSD-2, thereby permitting corticosterone to activate MR, was confirmed by the following: Intracerebroventricular glycyrrhizic acid, another 11beta-HSD-2 inhibitor, mimicked the sympathoexcitatory effects of CBX; the sympathoexcitatory effects of CBX were blocked by spironolactone, a MR antagonist. Neither CBX nor glycyrrhizic acid elicited a response in adrenalectomized rats. These findings suggest that MR in PVN contribute to sympathetic regulation and may be activated by aldosterone or corticosterone (or cortisol in humans) depending on the state of 11beta-HSD-2 activity.

Structure and function of human 17beta-hydroxysteroid dehydrogenases

17Beta-hydroxysteroid dehydrogenases (17beta-HSDs) catalyze the NAD(P)(H) dependent oxidoreduction at C17 oxo/beta-hydroxyl groups of androgen and estrogen hormones. This reversible reaction constitutes an important pre-receptor control mechanism for nuclear receptor ligands, since the conversion "switches" between the 17beta-OH receptor ligands and their inactive 17-oxo metabolites. At present, 14 mammalian 17beta-HSDs are described, of which at least 11 exist within the human genome, encoded by different genes. The enzymes differ in their expression pattern, nucleotide cofactor preference, steroid substrate specificity and subcellular localization, and thus constitute a complex system ensuring cell-specific adaptation and regulation of sex steroid hormone levels. Broad and overlapping substrate specificities with enzymes involved in lipid metabolism suggest interactions of several 17beta-HSDs with other metabolic pathways. Several 17beta-HSDs enzymes constitute promising drug targets, of particular importance in cancer, metabolic diseases, neurodegeneration and possibly immunity.

Roles of FHIT and WWOX fragile genes in cancer

It was hypothesized as early as 1986, that the recently discovered common fragile sites could facilitate recombination events, such as deletions and translocations, that result in clonally expanded cancer cell populations with specific chromosome alterations in specific cancer types. A natural extension of this hypothesis is that the clonal expansion must be driven by alteration of genes at recombination breakpoints whose altered functions actually drive clonal expansion. Nevertheless, when the FHIT gene was discovered at FRA3B, the most active common chromosome fragile region, and proposed as an example of a tumor suppressor gene altered by chromosome translocations and deletions, a wave of reports suggested that the FHIT gene was altered in cancer simply because it was in a fragile region and not because it had contributed to the clonal expansion, thus turning the original hypothesis upside down. Now, after nearly ten years and more than 500 FHIT reports, it is apparent that FHIT is an important tumor suppressor gene and that there are genes at other fragile regions that contribute significantly to development of cancer. A second fragile gene with a demonstrated role in cancer development is the WWOX gene on chromosome 16q; alterations to the WWOX gene contribute to development of hormone responsive and other cancers. Results of our recent studies of these two fragile tumor suppressor genes were summarized at the first Fragilome meeting in Heidelberg, Feb. 2005.

Atomic Resolution Structures of R-specific Alcohol Dehydrogenase from Lactobacillus brevis Provide the Structural Bases of its Substrate and Cosubstrate Specificity

The R-specific alcohol dehydrogenase (RADH) from Lactobacillus brevis is an NADP-dependent, homotetrameric member of the extended enzyme family of short-chain dehydrogenases/reductases (SDR) with a high biotechnological application potential. Its preferred in vitro substrates are prochiral ketones like acetophenone with almost invariably a small methyl group as one substituent and a bulky (often aromatic) moiety as the other. On the basis of an atomic-resolution structure of wild-type RADH in complex with NADP and acetophenone, we designed the mutant RADH-G37D, which should possess an improved cosubstrate specificity profile for biotechnological purposes, namely, a preference for NAD rather than NADP. Comparative kinetic measurements with wild-type and mutant RADH showed that this aim was achieved. To characterize the successful mutant structurally, we determined several, partly atomic-resolution, crystal structures of RADH-G37D both as an apo-enzyme and as ternary complex with NAD or NADH and phenylethanol. The increased affinity of RADH-G37D for NAD(H) depends on an interaction between the adenosine ribose moiety of NAD and the inserted aspartate side-chain. A structural comparison between RADH-G37D as apo-enzyme and as a part of a ternary complex revealed significant rearrangements of Ser141, Glu144, Tyr189 and Met205 in the vicinity of the active site. This plasticity contributes to generate a small hydrophobic pocket for the methyl group typical for RADH substrates, and a hydrophobic coat for the second, more variable and often aromatic, substituent. Around Ser141 we even found alternative conformations in the backbone. A structural adaptability in this region, which we describe here for the first time for an SDR enzyme, is probably functionally important, because it concerns Ser142, a member of the highly conserved catalytic tetrad typical for SDR enzymes. Moreover, it affects an extended proton relay system that has been identified recently as a critical element for the catalytic mechanism in SDR enzymes.

Structure of the tetrameric form of human L-Xylulose reductase: Probing the inhibitor-binding site with molecular modeling and site-directed mutagenesis

L-Xylulose reductase (XR) is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. In this study we report the structure of the biological tetramer of human XR in complex with NADP(+) and a competitive inhibitor solved at 2.3 A resolution. A single subunit of human XR is formed by a centrally positioned, seven-stranded, parallel beta-sheet surrounded on either side by two arrays of three alpha-helices. Two helices located away from the main body of the protein form the variable substrate-binding cleft, while the dinucleotide coenzyme-binding motif is formed by a classical Rossmann fold. The tetrameric structure of XR, which is held together via salt bridges formed by the guanidino group of Arg203 from one monomer and the carboxylate group of the C-terminal residue Cys244 from the neighboring monomer, explains the ability of human XR to prevent the cold inactivation seen in the rodent forms of the enzyme. The orientations of Arg203 and Cys244 are maintained by a network of hydrogen bonds and main-chain interactions of Gln137, Glu238, Phe241, and Trp242. These interactions are similar to those defining the quaternary structure of the closely related carbonyl reductase from mouse lung. Molecular modeling and site-directed mutagenesis identified the active site residues His146 and Trp191 as forming essential contacts with inhibitors of XR. These results could provide a structural basis in the design of potent and specific inhibitors for human XR.

The molecular architecture of galactose mutarotase/UDP-galactose 4-epimerase from Saccharomyces cerevisiae

The metabolic pathway by which beta-D-galactose is converted to glucose 1-phosphate is known as the Leloir pathway and consists of four enzymes. In most organisms, these enzymes appear to exist as soluble entities in the cytoplasm. In yeast such as Saccharomyces cerevisiae, however, the first and last enzymes of the pathway, galactose mutarotase and UDP-galactose 4-epimerase, are contained within a single polypeptide chain referred to as Gal10p. Here we report the three-dimensional structure of Gal10p in complex with NAD(+), UDP-glucose, and beta-D-galactose determined to 1.85-A resolution. The enzyme is dimeric with dimensions of approximately 91 A x 135 A x 108 A and assumes an almost V-shaped appearance. The overall architecture of the individual subunits can be described in terms of two separate N- and C-terminal domains connected by a Type II turn formed by Leu-357 to Val-360. The first 356 residues of Gal10p fold into the classical bilobal topology observed for all other UDP-galactose 4-epimerases studied thus far. This N-terminal domain contains the binding sites for NAD(+) and UDP-glucose. The polypeptide chain extending from Glu-361 to Ser-699 adopts a beta-sandwich motif and harbors the binding site for beta-D-galactose. The two active sites of Gal10p are separated by over 50 A. This investigation represents the first structural analysis of a dual function enzyme in the Leloir pathway.

Overview of steroidogenic enzymes in the pathway from cholesterol to active steroid hormones

Significant advances have taken place in our knowledge of the enzymes involved in steroid hormone biosynthesis since the last comprehensive review in 1988. Major developments include the cloning, identification, and characterization of multiple isoforms of 3beta-hydroxysteroid dehydrogenase, which play a critical role in the biosynthesis of all steroid hormones and 17beta-hydroxysteroid dehydrogenase where specific isoforms are essential for the final step in active steroid hormone biosynthesis. Advances have taken place in our understanding of the unique manner that determines tissue-specific expression of P450aromatase through the utilization of alternative promoters. In recent years, evidence has been obtained for the expression of steroidogenic enzymes in the nervous system and in cardiac tissue, indicating that these tissues may be involved in the biosynthesis of steroid hormones acting in an autocrine or paracrine manner. This review presents a detailed description of the enzymes involved in the biosynthesis of active steroid hormones, with emphasis on the human and mouse enzymes and their expression in gonads, adrenal glands, and placenta.

11beta-hydroxysteroid dehydrogenase type 1: a tissue-specific regulator of glucocorticoid response

11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) interconverts inactive cortisone and active cortisol. Although bidirectional, in vivo it is believed to function as a reductase generating active glucocorticoid at a prereceptor level, enhancing glucocorticoid receptor activation. In this review, we discuss both the genetic and enzymatic characterization of 11beta-HSD1, as well as describing its role in physiology and pathology in a tissue-specific manner. The molecular basis of cortisone reductase deficiency, the putative "11beta-HSD1 knockout state" in humans, has been defined and is caused by intronic mutations in HSD11B1 that decrease gene transcription together with mutations in hexose-6-phosphate dehydrogenase, an endoluminal enzyme that provides reduced nicotinamide-adenine dinucleotide phosphate as cofactor to 11beta-HSD1 to permit reductase activity. We speculate that hexose-6-phosphate dehydrogenase activity and therefore reduced nicotinamide-adenine dinucleotide phosphate supply may be crucial in determining the directionality of 11beta-HSD1 activity. Therapeutic inhibition of 11beta-HSD1 reductase activity in patients with obesity and the metabolic syndrome, as well as in glaucoma and osteoporosis, remains an exciting prospect.

Human 17beta-hydroxysteroid dehydrogenases types 1, 2, and 3 catalyze bi-directional equilibrium reactions, rather than unidirectional metabolism, in HEK-293 cells

Human 17beta-hydroxysteroid dehydrogenases (17betaHSDs) catalyze the interconversion of weak and potent androgen and estrogen pairs. Although the reactions using purified enzymes can be driven in either direction, these enzymes appear to function unidirectionally in intact cells: only reductive reactions for 17betaHSD1 and 17beta HSD3 and only oxidative reactions for 17betaHSD2. We show that, after exhaustive incubations with either 17beta-hydroxy- or 17-ketosteroid, the medium for HEK-293 cells expressing 17betaHSD1 or 17betaHSD3 contains a 92:8 ratio of reduced:oxidized steroid. Similarly, 17betaHSD2 yields a >95:5 ratio of oxidized:reduced steroids for both androgens and estrogens. Dual-isotope kinetic measurements show that the rates of the forward and reverse reactions are identical at these functional equilibrium states in intact cells for all three 17betaHSD isoforms, and these rates are much faster than those estimated from single-isotope flux studies. Mutation L36D converts 17betaHSD1 to an oxidative enzyme in intact cells, reversing the equilibrium distribution of estradiol:estrone to 5:95; however, the rates of the forward and reverse reactions at equilibrium are equal and comparable to those of the wild-type enzymes. The co-expression of 17betaHSD2 paradoxically increases the potency of estrone in transactivation assays, demonstrating the physiological relevance of "backwards" metabolism to estradiol. We conclude that 17betaHSD types 1, 2, and 3 catalyze both oxidative and reductive reactions in HEK-293 cells at intrinsic rates that are much faster than those estimated from single-isotope studies. These 17betaHSD isoforms do not drive steroid flux in one direction but rather may achieve functional equilibria in intact cells, reflecting thermodynamically driven steroid distributions.

Spinal gap junctions: Potential involvement in pain facilitation

Glia are now recognized as important contributors in pathological pain creation and maintenance. Spinal cord glia exhibit extensive gap junctional connectivity, raising the possibility that glia are involved in the contralateral spread of excitation resulting in mirror image pain. In the present experiments, the gap junction decoupler carbenoxolone was administered intrathecally after induction of neuropathic pain in response to sciatic nerve inflammation (sciatic inflammatory neuropathy) or partial nerve injury (chronic constriction injury). In both neuropathic pain models, a low dose of carbenoxolone reversed mirror image mechanical allodynia, while leaving ipsilateral mechanical allodynia unaffected. Ipsilateral thermal hyperalgesia was briefly attenuated. Critically, blockade of mechanical allodynia and thermal hyperalgesia was not observed in response to intrathecal glycyrrhizic acid, a compound similar to carbenoxolone in all respects but it does not decouple gap junctions. Thus, blockade of mechanical allodynia and thermal hyperalgesia by carbenoxolone does appear to reflect an effect on gap junctions. Examination of carbenoxolone's effects on intrathecal human immunodeficiency virus type 1 gp120 showed that blockade of pain facilitation might result, at least in part, via suppression of interleukin-1 and, in turn, interleukin-6. These data provide the first suggestion that spread of excitation via gap junctions might contribute importantly to inflammatory and traumatic neuropathic pain.

PERSPECTIVE

The current studies provide evidence for involvement of gap junctions in spinal cord pain facilitation. Intrathecal carbenoxolone, a gap junction decoupler, reversed neuropathy-induced mirror image pain and intrathecal gp120-induced allodynia. In addition, it decreased gp120-induced proinflammatory cytokines. This suggests gap junction activation might lead to proinflammatory cytokine release by distantly activated glia.

Two distinct steroleosins are present in seed oil bodies

In addition to oleosin isoforms, three minor proteins, Sop1, 2 and 3 are present in sesame oil bodies. Genes encoding Sop1 and Sop2, named caleosin and steroleosin for their calcium and sterol-binding capacity, respectively, have been cloned recently. Blast sequence analysis of the first 32 N-terminal residues revealed that Sop3 was presumably a steroleosin-like protein homologous to Sop2. A putative cDNA clone of Sop3 was obtained by PCR, and subsequently confirmed by immunological recognition with antibodies against its over-expressed protein in Escherichia coli. Although Sop2 and Sop3, tentatively named steroleosin-A and -B, were found homologous, they could not be cross-recognized immunologically. Sequence comparison showed that these two steroleosins possessed a conserved NADP+ binding subdomain but a diverse sterol-binding subdomain of different size. Both steroleosins were progressively accumulated in maturing seeds but with different cumulating patterns. Dehydrogenase activity detected in their expressed proteins indicated that steroleosin-B might comparably possess a broader sterol selectivity and higher NADP+ specificity than steroleosin-A. Immunological cross-recognition implies that steroleosin-B is present in seed oil bodies of diverse species. A structural model of an oil-body was drawn with all its known essential constituents, and secondary structure organizations of the three classes of oil-body proteins were compared.

Crystal structure of human L-xylulose reductase holoenzyme: probing the role of Asn107 with site-directed mutagenesis

L-Xylulose reductase (XR), an enzyme in the uronate cycle of glucose metabolism, belongs to the short-chain dehydrogenase/reductase (SDR) superfamily. Among the SDR enzymes, XR shows the highest sequence identity (67%) with mouse lung carbonyl reductase (MLCR), but the two enzymes show different substrate specificities. The crystal structure of human XR in complex with reduced nicotinamide adenine dinucleotide phosphate (NADPH) was determined at 1.96 A resolution by using the molecular replacement method and the structure of MLCR as the search model. Features unique to human XR include electrostatic interactions between the N-terminal residues of subunits related by the P-axis, termed according to SDR convention, and an interaction between the hydroxy group of Ser185 and the pyrophosphate of NADPH. Furthermore, identification of the residues lining the active site of XR (Cys138, Val143, His146, Trp191, and Met200) together with a model structure of XR in complex with L-xylulose, revealed structural differences with other members of the SDR family, which may account for the distinct substrate specificity of XR. The residues comprising a recently proposed catalytic tetrad in the SDR enzymes are conserved in human XR (Asn107, Ser136, Tyr149, and Lys153). To examine the role of Asn107 in the catalytic mechanism of human XR, mutant forms (N107D and N107L) were prepared. The two mutations increased K(m) for the substrate (>26-fold) and K(d) for NADPH (95-fold), but only the N107L mutation significantly decreased k(cat) value. These results suggest that Asn107 plays a critical role in coenzyme binding rather than in the catalytic mechanism.

The role of 17 beta-hydroxysteroid dehydrogenases

The biological activity of steroid hormones is regulated at the pre-receptor level by several enzymes including 17 beta-hydroxysteroid dehydrogenases (17 beta -HSD). The latter are present in many microorganisms, invertebrates and vertebrates. Dysfunctions in human 17 beta-hydroxysteroid dehydrogenases result in disorders of biology of reproduction and neuronal diseases, the enzymes are also involved in the pathogenesis of various cancers. 17 beta-hydroxysteroid dehydrogenases reveal a remarkable multifunctionality being able to modulate concentrations not only of steroids but as well of fatty and bile acids. Current knowledge on genetics, biochemistry and medical implications is presented in this review.

High resolution X-ray structure of dTDP-glucose 4,6-dehydratase from Streptomyces venezuelae

Desosamine is a 3-(dimethylamino)-3,4,6-trideoxyhexose found in some macrolide antibiotics. In Streptomyces venezuelae, there are seven genes required for the biosynthesis of this unusual sugar. One of the genes, desIV, codes for a dTDP-glucose 4,6-dehydratase, which is referred to as DesIV. The reaction mechanisms for these types of dehydratases are quite complicated with proton abstraction from the sugar 4'-hydroxyl group and hydride transfer to NAD+, proton abstraction at C-5, and elimination of the hydroxyl group at C-6 of the sugar, and finally return of a proton to C-5 and a hydride from NADH to C-6. Here we describe the cloning, overexpression, and purification, and high resolution x-ray crystallographic analysis to 1.44 A of wild-type DesIV complexed with dTDP. Additionally, for this study, a double site-directed mutant protein (D128N/E129Q) was prepared, crystallized as a complex with NAD+ and the substrate dTDP-glucose and its structure determined to 1.35 A resolution. In DesIV, the phenolate group of Tyr(151) and O(gamma) of Thr(127) lie at 2.7 and 2.6 A, respectively from the 4'-hydroxyl group of the dTDP-glucose substrate. The side chain of Asp(128) is in the correct position to function as a general acid for proton donation to the 6'-hydroxyl group while the side chain of Glu(129) is ideally situated to serve as the general base for proton abstraction at C-5. This investigation provides further detailed information for understanding the exquisite chemistry that occurs in these remarkable enzymes.

Identification of a mouse short-chain dehydrogenase/reductase gene, retinol dehydrogenase-similar. Function of non-catalytic amino acid residues in enzyme activity

We report a mouse short-chain dehydrogenase/reductase (SDR), retinol dehydrogenase-similar (RDH-S), with intense mRNA expression in liver and kidney. The RDH-S gene localizes to chromosome 10D3 with the SDR subfamily that catalyzes metabolism of retinoids and 3 alpha-hydroxysteroids. RDH-S has no activity with prototypical retinoid/steroid substrates, despite 92% amino acid similarity to mouse RDH1. This afforded the opportunity to analyze for functions of non-catalytic SDR residues. We produced RDH-S Delta 3 by mutating RDH-S to remove an "additional" Asn residue relative to RDH1 in its center, to convert three residues into RDH1 residues (L121P, S122N, and Q123E), and to substitute RDH1 sequence G208FKTCVTSSD for RDH-S sequence F208-FLTGMASSA. RDH-S Delta 3 catalyzed all-trans-retinol and 5 alpha-androstane-3 alpha,17 alpha-diol (3 alpha-adiol) metabolism 60-70% as efficiently (Vm/Km) as RDH1. Conversely, substituting RDH-S sequence F208FLTGMASSA into RDH1 produced a chimera (viz. C3) that was inactive with all-trans-retinol, but was 4-fold more efficient with 3 alpha-adiol. A single RDH1 mutation in the C3 region (K210L) reduced efficiency for all-trans-retinol by >1250-fold. In contrast, the C3 area mutation C212G enhanced efficiency with all-trans-retinol by approximately 2.4-fold. This represents a >6000-fold difference in catalytic efficiency for two enzymes that differ by a single non-catalytic amino acid residue. Another chimera (viz. C5) retained efficiency with all-trans-retinol, but was not saturated and was weakly active with 3 alpha-adiol, stemming from three residue differences (K224Q, K229Q, and A230T). The residues studied contribute to the substrate-binding pocket: molecular modeling indicated that they would affect orientation of substrates with the catalytic residues. These data report a new member of the SDR gene family, provide insight into the function of non-catalytic SDR residues, and illustrate that limited changes in the multifunctional SDR yield major alterations in substrate specificity and/or catalytic efficiency.

Glutamate-115 renders specificity of human 11beta-hydroxysteroid dehydrogenase type 2 for the cofactor NAD+

The renal 11beta-hydroxysteroid dehydrogenase (11beta-HSD) type 2 catalyzes the NAD(+)-dependent oxidation of the C11-alcohol on cortisol and corticosterone to yield inactive 11-ketosteroids. The lack of purified active enzyme complicates structure-function analyses of 11beta-HSD2. Here, we constructed a 3D-structural model of 11beta-HSD2, based on known 3D-structures of other short-chain dehydrogenases/reductases (SDR), and functionally analyzed 11beta-HSD2 mutants predicted to be involved in cofactor binding. Our 3D-model explains the preference for NAD(+) over NADP(+) by the coulombic repulsion between the adenosine ribose 2'-phosphate on NADP(+) and the carboxylate on Glu(115) and to steric hindrance with the side chain on Glu(115). Indeed, replacement of Glu(115) with serine or threonine, lacking repulsive charge and unfavorable steric interactions, showed only 3-fold preference for NAD(+), compared to 40-fold for wild-type 11beta-HSD2. Mutation of both Asp(91) and Glu(115) to serine raised NADP(+)-dependent activity to that with NAD(+), but caused reduced enzymatic activity. The 3D-model predicted that this is due to a loss of stabilizing interactions of Asp(91) with Cys(90), Glu(115), Asn(117) and Gly(120). Thus, predictions using the 3D-model combined with analysis of mutants allowed the identification of residues critical for NAD(+)-dependent activity of 11beta-HSD2.

Short-chain dehydrogenases/reductases (SDR): the 2002 update

Short-chain dehydrogenases/reductases (SDR) form a large, functionally heterogeneous protein family presently with about 3000 primary and about 30 3D structures deposited in databases. Despite low sequence identities between different forms (about 15-30%), the 3D structures display highly similar alpha/beta folding patterns with a central beta-sheet, typical of the Rossmann-fold. Based on distinct sequence motifs functional assignments and classifications are possible, making it possible to build a general nomenclature system. Recent mutagenetic and structural studies considerably extend the knowledge on the general reaction mechanism, thereby establishing a catalytic tetrad of Asn-Ser-Tyr-Lys residues, which presumably form the framework for a proton relay system including the 2'-OH of the nicotinamide ribose, similar to the mechanism found in horse liver ADH. Based on their cellular functions, several SDR enzymes appear as possible and promising pharmacological targets with application areas spanning hormone-dependent cancer forms or metabolic diseases such as obesity and diabetes, and infectious diseases.

Enzymology and Molecular Biology of Glucocorticoid Metabolism in Humans

Glucocorticoids (GCs) are a vital class of steroid hormones that are secreted by the adrenal cortex and that are regulated by ACTH largely under the control of the hypothalamic-pituitary-adrenal axis. GCs mediate profound and diverse physiological effects in vertebrates, ranging from development, metabolism, neurobiology, anti-inflammation and programmed cell death to many other fuctions. Multiple factors "downstream" of GC secretion, such as glucocorticoid receptor (GR) number and the abundance of plasma binding proteins have originally been considered as modulators of GC action. However, in the last decade the role of tissue-specific GC activating and inactivating enzymes have been identified as additional determinants in GC signalling pathways. On the cellular level, they function as important pre-receptor regulators by acting as "molecular switches" for receptor-active and receptor-inactive GC hormones. According to their biologic activity to catalyze the interconversion of C11-hydroxyl and C11-oxo GCs these enzymes have been named 11beta-hydroxysteroid dehydrogenase (11beta-HSD; EC 1.1.1.146). Two isoforms of 11beta-HSD have been cloned and characterized so far. 11beta-HSD type 1 is found in a wide range of tissues, acts predominantly as a reductase in intact cells and tissues by regenerating active cortisol from cortisone, and has been described to regulate GC access to the GR. 11beta-HSD type 2 is found mainly in mineralocorticoid target tissues such as kidney and colon, acts only as a dehydrogenase by producing inactive cortisone, and has been found to protect the mineralocorticoid receptor from high levels of receptor-active cortisol. Recently, 11beta-HSD 1 has become highly topical due to the finding that 11beta-HSD 1 plays a pivotal role in the pathogenesis of central obesity and the appearance of the metabolic syndrome. This review provides an overview on the components involved in GC signalling of 11beta-HSD type 1 as an important pre-receptor control enzyme that modulates activation of the GR.

Crystal structure of formaldehyde dehydrogenase from Pseudomonas putida: the structural origin of the tightly bound cofactor in nicotinoprotein dehydrogenases

Formaldehyde dehydrogenase from Pseudomonas putida (PFDH) is a member of the zinc-containing medium-chain alcohol dehydrogenase family. The pyridine nucleotide NAD(H) in PFDH, which is distinct from the coenzyme (as cosubstrate) in typical alcohol dehydrogenases (ADHs), is tightly but not covalently bound to the protein and acts as a cofactor. PFDH can catalyze aldehyde dismutations without an external addition of NAD(H). The structural basis of the tightly bound cofactor of PFDH is unknown. The crystal structure of PFDH has been solved by the multiwavelength anomalous diffraction method using intrinsic zinc ions and has been refined at a 1.65 A resolution. The 170-kDa homotetrameric PFDH molecule shows 222 point group symmetry. Although the secondary structure arrangement and the binding mode of catalytic and structural zinc ions in PFDH are similar to those of typical ADHs, a number of loop structures that differ between PFDH and ADHs in their lengths and conformations are observed. A comparison of the present structure of PFDH with that of horse liver ADH, a typical example of an ADH, reveals that a long insertion loop of PFDH shields the adenine part of the bound NAD(+) molecule from the solvent, and a tight hydrogen bond network exists between the insertion loop and the adenine part of the cofactor, which is unique to PFDH. This insertion loop is conserved completely among the aldehyde-dismutating formaldehyde dehydrogenases, whereas it is replaced by a short turn among typical ADHs. Thus, the insertion loop specifically found among the aldehyde-dismutating formaldehyde dehydrogenases is responsible for the tight cofactor binding of these enzymes and explains why PFDH can effectively catalyze alternate oxidation and reduction of aldehydes without the release of cofactor molecule from the enzyme.

Structural analysis of the Y299C mutant of Escherichia coli UDP-galactose 4-epimerase. Teaching an old dog new tricks

UDP-galactose 4-epimerase catalyzes the interconversion of UDP-Gal and UDP-Glc during normal galactose metabolism. The mammalian form of the enzyme, unlike its Escherichia coli counterpart, can also interconvert UDP-GalNAc and UDP-GlcNAc. One key feature of the epimerase reaction mechanism is the rotation of a 4-ketopyranose intermediate in the active site. By comparing the high resolution x-ray structures of both the bacterial and human forms of the enzyme, it was previously postulated that the additional activity in the human epimerase was due to replacement of the structural equivalent of Tyr-299 in the E. coli enzyme with a cysteine residue, thereby leading to a larger active site volume. To test this hypothesis, the Y299C mutant form of the E. coli enzyme was prepared and its three-dimensional structure solved as described here. Additionally, the Y299C mutant protein was assayed for activity against both UDP-Gal and UDP-GalNAc. These studies have revealed that, indeed, this simple mutation did confer UDP-GalNAc/UDP-GlcNAc converting activity to the bacterial enzyme with minimal changes in its three-dimensional structure. Specifically, although the Y299C mutation in the bacterial enzyme resulted in a loss of epimerase activity with regard to UDP-Gal by almost 5-fold, it resulted in a gain of activity against UDP-GalNAc by more than 230-fold.

Variation on a theme of SDR. dTDP-6-deoxy-L- lyxo-4-hexulose reductase (RmlD) shows a new Mg2+-dependent dimerization mode

dTDP-6-deoxy-L-lyxo-4-hexulose reductase (RmlD) catalyzes the final step in the conversion of dTDP-D-glucose to dTDP-L-rhamnose in an NAD(P)H- and Mg2+-dependent reaction. L-rhamnose biosynthesis is an antibacterial target. The structure of RmlD from Salmonella enterica serovar Typhimurium has been determined, and complexes with NADH, NADPH, and dTDP-L-rhamnose are reported. RmlD differs from other short chain dehydrogenases in that it has a novel dimer interface that contains Mg2+. Enzyme catalysis involves hydride transfer from the nicotinamide ring of the cofactor to the C4'-carbonyl group of the substrate. The substrate is activated through protonation by a conserved tyrosine. NAD(P)H is bound in a solvent-exposed cleft, allowing facile replacement. We suggest a novel role for the conserved serine/threonine residue of the catalytic triad of SDR enzymes.

Steroleosin, a sterol-binding dehydrogenase in seed oil bodies

Besides abundant oleosin, three minor proteins, Sop 1, 2, and 3, are present in sesame (Sesamum indicum) oil bodies. The gene encoding Sop1, named caleosin for its calcium-binding capacity, has recently been cloned. In this study, Sop2 gene was obtained by immunoscreening, and it was subsequently confirmed by amino acid partial sequencing and immunological recognition of its overexpressed protein in Escherichia coli. Immunological cross recognition implies that Sop2 exists in seed oil bodies of diverse species. Along with oleosin and caleosin genes, Sop2 gene was transcribed in maturing seeds where oil bodies are actively assembled. Sequence analysis reveals that Sop2, tentatively named steroleosin, possesses a hydrophobic anchoring segment preceding a soluble domain homologous to sterol-binding dehydrogenases/reductases involved in signal transduction in diverse organisms. Three-dimensional structure of the soluble domain was predicted via homology modeling. The structure forms a seven-stranded parallel beta-sheet with the active site, S-(12X)-Y-(3X)-K, between an NADPH and a sterol-binding subdomain. Sterol-coupling dehydrogenase activity was demonstrated in the overexpressed soluble domain of steroleosin as well as in purified oil bodies. Southern hybridization suggests that one steroleosin gene and certain homologous genes may be present in the sesame genome. Comparably, eight hypothetical steroleosin-like proteins are present in the Arabidopsis genome with a conserved NADPH-binding subdomain, but a divergent sterol-binding subdomain. It is indicated that steroleosin-like proteins may represent a class of dehydrogenases/reductases that are involved in plant signal transduction regulated by various sterols.

Structure-based phylogenetic analysis of short-chain alcohol dehydrogenases and reclassification of the 17beta-hydroxysteroid dehydrogenase family

Short-chain alcohol dehydrogenases (SCAD) constitute a large and diverse family of ancient origin. Several of its members play an important role in human physiology and disease, especially in the metabolism of steroid substrates (e.g., prostaglandins, estrogens, androgens, and corticosteroids). Their involvement in common human disorders such as endocrine-related cancer, osteoporosis, and Alzheimer disease makes them an important candidate for drug targets. Recent phylogenetic analysis of SCAD is incomplete and does not allow any conclusions on very ancient divergences or on a functional characterization of novel proteins within this complex family. We have developed a 3D structure-based approach to establish the deep-branching pattern within the SCAD family. In this approach, pairwise superpositions of X-ray structures were used to calculate similarity scores as an input for a tree-building algorithm. The resulting phylogeny was validated by comparison with the results of sequence-based algorithms and biochemical data. It was possible to use the 3D data as a template for the reliable determination of the phylogenetic position of novel proteins as a first step toward functional predictions. We were able to discern new patterns in the phylogenetic relationships of the SCAD family, including a basal dichotomy of the 17beta-hydroxysteroid dehydrogenases (17beta-HSDs). These data provide an important contribution toward the development of type-specific inhibitors for 17beta-HSDs for the treatment and prevention of disease. Our structure-based phylogenetic approach can also be applied to increase the reliability of evolutionary reconstructions in other large protein families.

Metabolic conversion as a pre-receptor control mechanism for lipophilic hormones

The majority of physiological effects mediated by steroids, retinoids and thyroids is accomplished by binding to members of the nuclear receptor superfamily of ligand activated transcription factors. The complex specific effects of lipid hormones depend not only on receptor expression, distribution and interactions, but also on the availability and metabolic conversion of the hormone itself. The cell-specific metabolic activation of inactive hormone precursors introduces a further level of hormonal regulation, and constitutes an important concept in endocrinology. The metabolic reactions carried out are achieved by dehydrogenases/reductases, hydroxylases and other enzymes, acting on ligands of the steroid/thyroid/retinoic hormone receptor superfamily. The concept implies that these tissue- and cell-specific metabolic conversions contribute to lipid hormone action, thus pointing to novel targets in drug development. All components of this signalling system, the hormone compounds, the receptor proteins, and modifying enzyme families originate from an early metazoan date, emphasizing the essential nature of all elements for development and diversification of vertebrate life.

3alpha-hydroxysteroid dehydrogenase messenger RNA transcription in the immature rat ovary in response to an ovulatory dose of gonadotropin

The ovulatory process in mammals involves a substantial increase in the metabolism of steroids and eicosanoids in response to a surge in LH or to an injection of hCG into experimental animals. This study provides evidence that the ovulatory stimulus causes induction of the gene for 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), an enzyme that belongs to several oxidoreductase superfamilies that affect steroid and eicosanoid metabolism. Immature Wistar rats were primed with 10 IU eCG s.c., and 48 h later the 12-h ovulatory process was initiated by 10 IU hCG s.c. Ovarian RNA was extracted at 0, 2, 4, 8, 12, and 24 h after injecting the animals with hCG. The RNA extracts were used for reverse transcription-polymerase chain reaction (PCR) differential display to detect gene expression in the stimulated ovarian tissue. One of the PCR primer sets differentially amplified a cDNA fragment that is 52.3% homologous with a 3alpha-HSD gene in rat liver. Northern analyses revealed that maximum transcription was at 8 h after the animals had been treated with hCG. The Northerns also indicated that the 3alpha-HSD cDNA probe cross-hybridized with as many as six different bands of mRNA on the blots. In situ hybridization localized 3alpha-HSD mRNA in the granulosa and thecal layers of mature follicles and in newly formed corpora lutea at 24 h after the ovulatory stimulus. In conclusion, gene(s) for 3alpha-HSD are transcribed in ovarian follicles in response to an ovulatory dose of gonadotropin. A possible function of the oxidoreductase enzyme that is translated from the 3alpha-HSD mRNA may be to reduce the toxic aldehyde and ketone components of the steroids and eicosanoids that accumulate in the mammalian ovary at the time of ovulation.

Functional expression, characterization, and purification of the catalytic domain of human 11-beta -hydroxysteroid dehydrogenase type 1

11-beta-hydroxysteroid dehydrogenase type 1 catalyzes the conversion of cortisone to hormonally active cortisol and has been implicated in the pathogenesis of a number of disorders including insulin resistance and obesity. The enzyme is a glycosylated membrane-bound protein that has proved difficult to purify in an active state. Extracted enzyme typically loses the reductase properties seen in intact cells and shows principally dehydrogenase activity. The C-terminal catalytic domain is known to contain a disulfide bond and is located within the lumen of the endoplasmic reticulum, anchored to the membrane by a single N-terminal transmembrane domain. We report here the functional expression of the catalytic domain of the human enzyme, without the transmembrane domain and the extreme N terminus, in Escherichia coli. Moderate levels of soluble active protein were obtained using an N-terminal fusion with thioredoxin and a 6xHis tag. In contrast, the inclusion of a 6xHis tag at the C terminus adversely affected protein solubility and activity. However, the highest levels of active protein were obtained using a construct expressing the untagged catalytic domain. Nonreducing electrophoresis revealed the presence of both monomeric and dimeric disulfide bonded forms; however, mutation of a nonconserved cysteine residue resulted in a recombinant protein with no intermolecular disulfide bonds but full enzymatic activity. Using the optimal combination of plasmid construct and E. coli host strain, the recombinant protein was purified to apparent homogeneity by single step affinity chromatography. The purified protein possessed both dehydrogenase and reductase activities with a K(m) of 1.4 micrometer for cortisol and 9.5 micrometer for cortisone. This study indicates that glycosylation, the N-terminal region including the transmembrane helix, and intermolecular disulfide bonds are not essential for enzyme activity and that expression in bacteria can provide active recombinant protein for future structural and functional studies.