Purification and characterization of a microbial, NADP-dependent bile acid 7 alpha-hydroxysteroid dehydrogenase

Reconstruction of Acinetobacter johnsonii ICE_NC genome using hybrid de novo genome assemblies and identification of the 12α-hydroxysteroid dehydrogenase gene

AIMS

The role of a Acinetobacter johnsonii strain, isolated from a soil sample, in the biotransformation of bile acids (BAs) was already described but the enzymes responsible for these transformations were only partially purified and molecularly characterized.

METHODS AND RESULTS

This study describes the use of hybrid de novo assemblies, that combine long-read Oxford Nanopore and short-read Illumina sequencing strategies, to reconstruct the entire genome of A. johnsonii ICE_NC strain and to identify the coding region for a 12α-hydroxysteroid dehydrogenase (12α-HSDH), involved in BAs metabolism. The de novo assembly of the A. johnsonii ICE_NC genome was generated using Canu and Unicycler, both strategies yielded a circular chromosome of about 3.6 Mb and one 117 kb long plasmid. Gene annotation was performed on the final assemblies and the gene for 12α-HSDH was detected on the plasmid.

CONCLUSIONS

Our findings illustrate the added value of long read sequencing in addressing the challenges of whole genome characterization and plasmid reconstruction in bacteria. These approaches also allowed the identification of the A. johnsonii ICE_NC gene for the 12α-HSDH enzyme, whose activity was confirmed at the biochemical level.

SIGNIFICANCE AND IMPACT OR THE STUDY

At present, this is the first report on the characterization of a 12α-HSDH gene in an A. johnsonii strain able to biotransform cholic acid into ursodeoxycholic acid, a promising therapeutic agent for several diseases.

Metabolism of hydrogen gases and bile acids in the gut microbiome

The human gut microbiome refers to a highly diverse microbial ecosystem, which has a symbiotic relationship with the host. Molecular hydrogen (H₂ ) and carbon dioxide (CO₂ ) are generated by fermentative metabolism in anaerobic ecosystems. H₂ generation and oxidation coupled to CO₂ reduction to methane or acetate help maintain the structure of the gut microbiome. Bile acids are synthesized by hepatocytes from cholesterol in the liver and are important regulators of host metabolism. In this Review, we discuss how gut bacteria metabolize hydrogen gases and bile acids in the intestinal tract and the consequences on host physiology. Finally, we focus on bile acid metabolism by the Actinobacterium Eggerthella lenta. Eggerthella lenta appears to couple hydroxyl group oxidations to reductive acetogenesis under a CO₂ or N₂ atmosphere, but not under H₂ . Hence, at low H₂ levels, E. lenta is proposed to use NADH from bile acid hydroxyl group oxidations to reduce CO₂ to acetate.

Cloning, expression, and biochemical characterization of a novel NADP+-dependent 7α-hydroxysteroid dehydrogenase from Clostridium difficile and its application for the oxidation of bile acids

A gene encoding a novel 7α-specific NADP⁺-dependent hydroxysteroid dehydrogenase from Clostridium difficile was cloned and heterologously expressed in Escherichia coli. The enzyme was purified using an N-terminal hexa-his-tag and biochemically characterized. The optimum temperature is at 60°C, but the enzyme is inactivated at this temperature with a half-life time of 5min. Contrary to other known 7α-HSDHs, for example from Clostridium sardiniense or E. coli, the enzyme from C. difficile does not display a substrate inhibition. In order to demonstrate the applicability of this enzyme, a small-scale biotransformation of the bile acid chenodeoxycholic acid (CDCA) into 7-ketolithocholic acid (7-KLCA) was carried out with simultaneous regeneration of NADP⁺ using an NADPH oxidase that resulted in a complete conversion (<99%). Furthermore, by a structure-based site-directed mutagenesis, cofactor specificity of the 7α-HSDH from Clostridium difficile was altered to accept NAD(H). This mutant was biochemically characterized and compared to the wild-type.

Development of analytical method for simultaneous determination of five rodent unique bile acids in rat plasma using ultra-performance liquid chromatography coupled with time-of-flight mass spectrometry

Bile acids (BAs) are crucial for the diagnosis, follow-up, and prognostics of liver injuries and other BA metabolism related diseases. In particular, rodent unique BAs, α-muricholic acid (α-MCA), β-MCA, ω-MCA, tauro-α-MCA (α-TMCA), and β-TMCA, are valuable biomarkers for preclinical drug development. To the best of our knowledge, however, a simple, selective, sensitive, and robust analytical method for ω-MCA and taurine-conjugated MCAs has never been reported. We have developed a simple, selective, and sensitive analytical method for measurement of 16 BAs including the five rodent unique BAs in rat plasma using an ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-TOF-MS) method. Activated charcoal was utilized to prepare BA-free plasma, which served as the surrogate matrix for the preparation of calibration standards and quality control (QC) samples. Results of matrix effects evaluation suggested that the BA-free plasma could be adequate as a surrogate matrix for BAs determination. Three stable isotope labelled internal standards were separated by reverse phase UPLC using gradient elution and were detected by TOF-MS in negative ion mode. The calibration curve was linear for all BAs over a range of 10-25ng/mL to 1000-10,000ng/mL, with overall imprecision below 15% and 20% at lower limit of quantification (LLOQ), respectively. This analytical method was used to determine BA concentrations in more than 300 plasma samples from rats with liver injuries induced using α-naphthylisocyanate, carbon tetrachloride, or flutamide. The alteration of BA concentrations was most evident for necrosis, and cholestasis hepatotoxins, with more subtle effects by steatosis and idiosyncratic hepatotoxins. In conclusion, we have developed a simple, selective, and sensitive analytical method to measure plasma 16 BAs including 5 rodent unique BAs, α-MCA, β-MCA, ω-MCA, α-TMCA, and β-TMCA. Our data suggested that α-TMCA and β-TMCA could be useful for identification or prediction of liver injuries, a currently unmet need in preclinical toxicity. Our method using TOF-MS is useful to determine BAs in rat plasma and of use in structural analyses of metabolites in early stage of drug development.

Metabolic profiling of bile acids in human and mouse blood by LC-MS/MS in combination with phospholipid-depletion solid-phase extraction

To obtain a more comprehensive profile of bile acids (BAs) in blood, we developed an ultrahigh performance liquid chromatography/multiple-reaction monitoring-mass spectrometry (UPLC-MRM-MS) method for the separation and detection of 50 known BAs. This method utilizes phospholipid-depletion solid-phase extraction as a new high-efficiency sample preparation procedure for BA assay. UPLC/scheduled MRM-MS with negative ion electrospray ionization enabled targeted quantitation of 43 and 44 BAs, respectively, in serum samples from seven individuals with and without fasting, as well as in plasma samples from six cholestatic gene knockout mice and six age- and gender-matched wild-type (FVB/NJ) animals. Many minor BAs were identified and quantitated in the blood for the first time. Method validation indicated good quantitation precision with intraday and interday relative standard deviations of ≤9.3% and ≤10.8%, respectively. Using a pooled human serum sample and a pooled mouse plasma sample as the two representative test samples, the quantitation accuracy was measured to be 80% to 120% for most of the BAs, using two standard-substance spiking approaches. To profile other potential BAs not included in the 50 known targets from the knockout versus wild-type mouse plasma, class-specific precursor/fragment ion transitions were used to perform UPLC-MRM-MS for untargeted detection of the structural isomers of glycine- and taurine-conjugated BAs and unconjugated tetra-hydroxy BAs. As a result, as many as 36 such compounds were detected. In summary, this UPLC-MRM-MS method has enabled the quantitation of the largest number of BAs in the blood thus far, and the results presented have revealed an unexpectedly complex BA profile in mouse plasma.

Enzymatic routes for the synthesis of ursodeoxycholic acid

Ursodeoxycholic acid, a secondary bile acid, is used as a drug for the treatment of various liver diseases, the optimal dose comprises the range of 8-10mg/kg/day. For industrial syntheses, the structural complexity of this bile acid requires the use of an appropriate starting material as well as the application of regio- and enantio-selective enzymes for its derivatization. Most strategies for the synthesis start from cholic acid or chenodeoxycholic acid. The latter requires the conversion of the hydroxyl group at C-7 from α- into β-position in order to obtain ursodeoxycholic acid. Cholic acid on the other hand does not only require the same epimerization reaction at C-7 but the removal of the hydroxyl group at C-12 as well. There are several bacterial regio- and enantio-selective hydroxysteroid dehydrogenases (HSDHs) to carry out the desired reactions, for example 7α-HSDHs from strains of Clostridium, Bacteroides or Xanthomonas, 7β-HSDHs from Clostridium, Collinsella, or Ruminococcus, or 12α-HSDH from Clostridium or from Eggerthella. However, all these bioconversion reactions need additional steps for the regeneration of the coenzymes. Selected multi-step reaction systems for the synthesis of ursodeoxycholic acid are presented in this review.

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

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

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.

Xanthomonas maltophilia CBS 897.97 as a source of new 7beta- and 7alpha-hydroxysteroid dehydrogenases and cholylglycine hydrolase: improved biotransformations of bile acids

The paper reports the partial purification and characterization of the 7beta- and 7alpha-hydroxysteroid dehydrogenases (HSDH) and cholylglycine hydrolase (CGH), isolated from Xanthomonas maltophilia CBS 897.97. The activity of 7beta-HSDH and 7alpha-HSDH in the reduction of the 7-keto bile acids is determined. The affinity of 7beta-HSDH for bile acids is confirmed by the reduction, on analytical scale, to the corresponding 7beta-OH derivatives. A crude mixture of 7alpha- and 7beta-HSDH, in soluble or immobilized form, is employed in the synthesis, on preparative scale, of ursocholic and ursodeoxycholic acids starting from the corresponding 7alpha-derivatives. On the other hand, a partially purified 7beta-HSDH in a double enzyme system, where the couple formate/formate dehydrogenase allows the cofactor recycle, affords 6alpha-fluoro-3alpha, 7beta-dihydroxy-5beta-cholan-24-oic acid (6-FUDCA) by reduction of the corresponding 7-keto derivative. This compound is not obtainable by microbiological route. The efficient and mild hydrolysis of glycinates and taurinates of bile acids with CGH is also reported. Very promising results are also obtained with bile acid containing raw materials.

Bile salt biotransformations by human intestinal bacteria

Secondary bile acids, produced solely by intestinal bacteria, can accumulate to high levels in the enterohepatic circulation of some individuals and may contribute to the pathogenesis of colon cancer, gallstones, and other gastrointestinal (GI) diseases. Bile salt hydrolysis and hydroxy group dehydrogenation reactions are carried out by a broad spectrum of intestinal anaerobic bacteria, whereas bile acid 7-dehydroxylation appears restricted to a limited number of intestinal anaerobes representing a small fraction of the total colonic flora. Microbial enzymes modifying bile salts differ between species with respect to pH optima, enzyme kinetics, substrate specificity, cellular location, and possibly physiological function. Crystallization, site-directed mutagenesis, and comparisons of protein secondary structure have provided insight into the mechanisms of several bile acid-biotransforming enzymatic reactions. Molecular cloning of genes encoding bile salt-modifying enzymes has facilitated the understanding of the genetic organization of these pathways and is a means of developing probes for the detection of bile salt-modifying bacteria. The potential exists for altering the bile acid pool by targeting key enzymes in the 7alpha/beta-dehydroxylation pathway through the development of pharmaceuticals or sequestering bile acids biologically in probiotic bacteria, which may result in their effective removal from the host after excretion.

Biotransformations on steroid nucleus of bile acids

The bile acids in mammals are all derivatives of 5 beta-cholan-26-oic acid. They represent the major quantitative pathway by which cholesterol is metabolized in the body. This article covers the microbial and enzymatic transformations of free, saturated bile acids, that kept unaltered the C-24 cyclopentane-perhydrophenantrene nucleus. The bile acids that have been considered include the primary cholic and chenodeoxycholic acids, the secondary deoxycholic and lithocholic acids as well as the relevant dehydrocholic, ursocholic and ursodeoxycholic acids. Among the bile acid biotransformations, attention is paid to reactions that lead to pharmaceutically significant compounds. This is the case of 7 alpha-hydroxy epimerization of chenodeoxycholic acid to ursodeoxycholic acid, currently used for cholesterol galistone dissolution therapy and in the treatment of cholestatic liver diseases. Emphasis has placed on reporting reactions that may be of general interest and on the practical aspects of work in the field of biotransformations.

Crystal structures of the binary and ternary complexes of 7 alpha-hydroxysteroid dehydrogenase from Escherichia coli

7 alpha-Hydroxysteroid dehydrogenase (7 alpha-HSDH;1 EC 1.1.1.159) is an NAD+-dependent oxidoreductase belonging to the short-chain dehydrogenase/reductase (SDR) 1 family. It catalyzes the dehydrogenation of a hydroxyl group at position 7 of the steroid skeleton of bile acids. The crystal structure of the binary (complexed with NAD+) complex of 7 alpha-HSDH has been solved at 2.3 A resolution by the multiple isomorphous replacement method. The structure of the ternary complex [the enzyme complexed with NADH, 7-oxoglycochenodeoxycholic acid (as a reaction product), and possibly partially glycochenodeoxycholic acid (as a substrate)] has been determined by a difference Fourier method at 1.8 A resolution. The enzyme 7 alpha-HSDH is an alpha/beta doubly wound protein having a Rossmann-fold domain for NAD (H) binding. Upon substrate binding, large conformation changes occur at the substrate binding loop (between the beta F strand and alpha G helix) and the C-terminal segment (residues 250-255). The variable amino acid sequences of the substrate-binding loop appear to be responsible for the wide variety of substrate specificities observed among the enzymes of the SDR family. The crystal structure of the ternary complex of 7 alpha-HSDH, which is the only structure available as the ternary complex among the enzymes of the SDR family, indicates that the highly conserved Tyr159 and Ser146 residues most probably directly interact with the hydroxyl group of the substrates although this observation cannot be definite due to an insufficiently characterized nature of the ternary complex. The strictly conserved Lys163 is hydrogen-bonded to both the 2'- and 3'-hydroxyl groups of the nicotinamide ribose of NAD(H). We propose a new catalytic mechanism possibly common to all the enzymes belonging to the SDR family in which a tyrosine residue (Tyr159) acts as a catalytic base and a serine residue (Ser146) plays a subsidiary role of stabilizing substrate binding.

Purification and cDNA cloning of the alcohol dehydrogenase of the flesh fly Sarcophaga peregrina. A structural relationship between alcohol dehydrogenase and a 25-kDa protein

We purified to homogeneity two proteins with molecular masses of 25 kDa from the fat body of the Sarcophaga larva. One was alcohol dehydrogenase (ADH) and the other was a 25-kDa protein of which the genomic DNA had been cloned. We isolated the cDNA for ADH and determined its amino acid sequence. Amino acid sequence identity between ADH and the 25-kDa protein was 40%, indicating that they are structurally related proteins. The amount of ADH in Sarcophaga was almost constant through the larval stage to the adult stage, but the 25-kDa protein was detected only within a restricted period between the final larval instar and the early pupal stage.

Suppression of a signaling defect during Myxococcus xanthus development

The csgA gene encodes an extracellular protein that is essential for cell-cell communication (C-signaling) during fruiting body development of Myxococcus xanthus. Two transposon insertions in the socABC operon, soc-560 and socC559, restore development to csgA null mutants. Mixing soc-560 csgA cells or socC559 csgA cells with csgA cells at a ratio of 1:1 stimulated the development of csgA cells, suggesting that soc mutations allow cells to produce the C-signal or a similar molecule via a csgA-independent mechanism. The socABC operon contains the following three genes: socA, a member of the short-chain alcohol dehydrogenase gene family; socB, a gene encoding a putative membrane anchoring protein; and socC, a negative autoregulator of socABC operon expression. Both suppressor mutations inactivate socC, leading to a 30- to 100-fold increase in socA transcription; socA expression in suppressor strains is at least 100-fold higher than csgA expression during all stages of development. The amino acid sequence of SocA has 28% identity and 51% similarity with that of CsgA. We suggest that CsgA suppression is due to overproduction of SocA, which can substitute for CsgA. These results raise the possibility that a cell surface dehydrogenase plays a role in C-signaling.

Expression of the bile acid-inducible NADH:flavin oxidoreductase gene of Eubacterium sp. VPI 12708 in Escherichia coli

The intestinal microorganism Eubacterium sp. VPI 12708 synthesizes a bile acid-inducible NADH:flavin oxidoreductase (NADH:FOR) which presumably functions in the 7 alpha-dehydroxylation of cholic acid to deoxycholic acid. The baiH gene encoding NADH:FOR was subcloned into an IPTG-inducible expression vector, pBaiH2.2. Escherichia coli DH5 alpha cells transformed with pBaiH2.2 expressed 10-fold higher levels of NADH:FOR upon induction with IPTG than did Eubacterium sp. VPI 12708 cells induced with cholic acid. The NADH:FOR produced by E. coli DH5 alpha(pBaiH2.2) was purified to > 95% electrophoretic homogeneity in three steps. The purified NADH:FOR was similar to that of Eubacterium sp. VPI 12708 in subunit and native M(r) (ca. 72,000 and 210,000, respectively), pH optimum, sensitivity to inhibitors, and electron acceptor specificity. It contained 1 mol of FAD, up to 2 mol of iron, and 1 mol of copper per mol of subunit. The enzyme reduced synthetic quinones, dyes, flavins, and O2 with NADH as the electron donor, but did not reduce disulfide compounds, various unsaturated bile acids, cytochrome c, physiological quinones, or cell fractions from Eubacterium sp. VPI 12708. Addition of purified NADH:FOR to Eubacterium sp. VPI 12708 cell extracts altered the balance of oxidized and reduced bile acid intermediates produced during cholic acid 7 alpha-dehydroxylation, suggesting that the enzyme may regulate the cellular ratio of NAD to NADH.

Expression in Escherichia coli and characterization of a bile acid-inducible 3 alpha-hydroxysteroid dehydrogenase from Eubacterium sp. strain VPI 12708

We have previously cloned and sequenced three members of a bile acid-inducible gene family from Eubacterium sp. strain VPI 12708 that encode 27,000-M(r) polypeptides. Two copies of these genes (baiA1 and baiA3) are identical, while the third copy (baiA2) encodes a polypeptide sharing 92% amino acid identity with the baiA1 and baiA3 gene products. We have overexpressed the baiA1 gene in Escherichia coli and analyzed the expressed activity. Thin-layer chromatography of 14C-labeled bile acid products from reactions using cell-free extracts revealed a 3 alpha-hydroxysteroid dehydrogenase activity for the BaiA1 protein. The BaiA1 protein could utilize both NAD+ and NADP+, and the preferred steroid substrate was the cholyl-coenzyme A conjugate rather than free cholic acid. These results show that the BaiA proteins are novel 3 alpha-hydroxysteroid dehydrogenases.

Characterization and regulation of the NADP-linked 7 alpha-hydroxysteroid dehydrogenase gene from Clostridium sordellii

A bile acid-inducible NADP-linked 7 alpha-hydroxysteroid dehydrogenase (7 alpha-HSDH) from Clostridium sordellii ATCC 9714 was purified 310-fold by ion-exchange, gel filtration, and dye-ligand affinity chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the purified enzyme showed one predominant peptide band (30,000 Da). The N-terminal sequence was determined, and the corresponding oligonucleotides were synthesized and used to screen EcoRI and HindIII genomic digests of C. sordellii. Two separate fragments (4,500 bp, EcoRI; 3,200 bp, HindIII) were subsequently cloned by ligation to pUC19 and transformation into Escherichia coli DH5 alpha-MCR. The EcoRI fragment was shown to contain a truncated 7 alpha-HSDH gene, while the HindIII fragment contained the entire coding region. E. coli clones containing the HindIII insert expressed high levels of an NADP-linked 7 alpha-HSDH. Nucleotide sequence analyses suggest that the 7 alpha-HSDH is encoded by a monocistronic transcriptional unit, with DNA sequence elements resembling rho-independent terminators located in both the upstream and downstream flanking regions. The transcriptional start site was located by primer extension analysis. Northern (RNA) blot analysis indicated that induction is mediated at the transcriptional level in response to the presence of bile acid in the growth medium. In addition, growth-phase-dependent expression is observed in uninduced cultures. Analysis of the predicted protein sequence indicates that the enzyme can be classified in the short-chain dehydrogenase group.

Characterization of the baiH gene encoding a bile acid-inducible NADH:flavin oxidoreductase from Eubacterium sp. strain VPI 12708

A cholate-inducible, NADH-dependent flavin oxidoreductase from the intestinal bacterium Eubacterium sp. strain VPI 12708 was purified 372-fold to apparent electrophoretic homogeneity. The subunit and native molecular weights were estimated to be 72,000 and 210,000, respectively, suggesting a homotrimeric organization. Three peaks of NADH:flavin oxidoreductase activity (forms I, II, and III) eluted from a DEAE-high-performance liquid chromatography column. Absorption spectra revealed that purified form III, but not form I, contained bound flavin, which dissociated during purification to generate form I. Enzyme activity was inhibited by sulfhydryl-reactive compounds, acriflavine, o-phenanthroline, and EDTA. Activity assays and Western blot (immunoblot) analysis confirmed that expression of the enzyme was cholate inducible. The first 25 N-terminal amino acid residues of purified NADH:flavin oxidoreductase were determined, and a corresponding oligonucleotide probe was synthesized for use in cloning of the associated gene, baiH. Restriction mapping, sequence data, and RNA blot analysis suggested that the baiH gene was located on a previously described, cholate-inducible operon > or = 10 kb long. The baiH gene encoded a 72,006-Da polypeptide containing 661 amino acids. The deduced amino acid sequence of the baiH gene was homologous to that of NADH oxidase from Thermoanaerobium brockii, trimethylamine dehydrogenase from methylotrophic bacterium W3A1, Old Yellow Enzyme from Saccharomyces carlsbergensis, and the product of the baiC gene of Eubacterium sp. strain VPI 12708, located upstream from the baiH gene in the cholate-inducible operon. Alignment of these five sequences revealed potential ligands for an iron-sulfur cluster, a putative flavin adenine dinucleotide-binding domain, and two other well-conserved domains of unknown function.

Cloning, sequencing, and expression of the gene coding for bile acid 7 alpha-hydroxysteroid dehydrogenase from Eubacterium sp. strain VPI 12708

Southern blot analysis indicated that the gene encoding the constitutive, NADP-linked bile acid 7 alpha-hydroxysteroid dehydrogenase of Eubacterium sp. strain VPI 12708 was located on a 6.5-kb EcoRI fragment of the chromosomal DNA. This fragment was cloned into bacteriophage lambda gt11, and a 2.9-kb piece of this insert was subcloned into pUC19, yielding the recombinant plasmid pBH51. DNA sequence analysis of the 7 alpha-hydroxysteroid dehydrogenase gene in pBH51 revealed a 798-bp open reading frame, coding for a protein with a calculated molecular weight of 28,500. A putative promoter sequence and ribosome binding site were identified. The 7 alpha-hydroxysteroid dehydrogenase mRNA transcript in Eubacterium sp. strain VPI 12708 was about 0.94 kb in length, suggesting that it is monocistronic. An Escherichia coli DH5 alpha transformant harboring pBH51 had approximately 30-fold greater levels of 7 alpha-hydroxysteroid dehydrogenase mRNA, immunoreactive protein, and specific activity than Eubacterium sp. strain VPI 12708. The 7 alpha-hydroxysteroid dehydrogenase purified from the pBH51 transformant was similar in subunit molecular weight, specific activity, and kinetic properties to that from Eubacterium sp. strain VPI 12708, and it reached with antiserum raised against the authentic enzyme on Western immunoblots. Alignment of the amino acid sequence of the 7 alpha-hydroxysteroid dehydrogenase with those of 10 other pyridine nucleotide-linked alcohol/polyol dehydrogenases revealed six conserved amino acid residues in the N-terminal regions thought to function in coenzyme binding.

Cloning and sequencing of the 7 alpha-hydroxysteroid dehydrogenase gene from Escherichia coli HB101 and characterization of the expressed enzyme

The 7 alpha-hydroxysteroid dehydrogenase (EC 1.1.1.159) gene from Escherichia coli HB101 was cloned and expressed in E. coli DH1. The hybrid plasmid pSD1, with a 2.8-kbp insert of chromosomal DNA at the BamHI site of pBR322, was subcloned into pUC19 to construct plasmid pSD3. The entire nucleotide sequence of an inserted PstI-BamHI fragment of plasmid pSD3 was determined by the dideoxy chain-termination method. Within this sequence, the mature enzyme protein-encoding sequence was found to start at a GTG initiation codon and to comprise 765 bp, as judged by comparison with the protein sequence. The deduced amino acid sequence of the enzyme indicated that the molecular weight is 26,778. The transformant of E. coli DH1 harboring pSD3 with a 1.8-kbp fragment showed about 200-fold-higher enzyme activity than the host. The enzyme was purified by a single chromatography step on DEAE-Toyopearl and obtained as crystals, with an activity yield of 39%. The purified enzyme was homogeneous, as judged by sodium dodecyl sulfate gel electrophoresis. The enzyme was most active at pH 8.5 and stable between pH 8 and 9. The enzyme was NAD+ dependent and had a pI of 4.3. The molecular mass was estimated to be 120 kDa by the gel filtration method and 28 kDa by electrophoresis, indicating that the enzyme exists in a tetrameric form.