Zinc coordination in mammalian sorbitol dehydrogenase. Replacement of putative zinc ligands by site-directed mutagenesis

Substitution of cysteine-153 ligated to the catalytic zinc in yeast alcohol dehydrogenase with aspartic acid and analysis of mechanisms of related medium chain dehydrogenases

The catalytic zincs in complexes of horse liver and yeast alcohol dehydrogenases (ADH) with NAD⁺ and the substrate analogue, 2,2,2-trifluoroethanol, are ligated to two cysteine residues and one histidine residue from the protein and the oxygen from the alcohol. The zinc facilitates deprotonation of the alcohol and is essential for catalysis. In the yeast apoenzyme, the zinc is coordinated to a nearby glutamic acid, which is displaced by the alcohol in the complex with NAD⁺. Some homologous medium chain dehydrogenases have a cysteine replaced by aspartic or glutamic acid residues. How an aspartic acid would affect catalysis was studied by replacing Cys-153 in Saccharomyces cerevisiae ADH1 by using site-directed mutagenesis. The C153D enzyme was about as stable as the wild-type enzyme, if EDTA was not included in the buffers. The substitution increased affinity for NAD⁺ by 3-fold, but did not affect NADH binding. At pH 7.3, the turnover number for ethanol oxidation (V₁/E_t) decreased by 7-fold and catalytic efficiency decreased 18-fold (V₁/E_tK_b), but turnover for acetaldehyde reduction (V₂/E_t) was the same as for wild-type enzyme and catalytic efficiency decreased 8-fold (V₂/E_tK_p). Deuterium isotope effects of 3.0 on V₁/E_t and 3.8 on V₁/E_tK_b for ethanol oxidation suggest that hydride transfer is more rate-limiting for turnover for the C153D enzyme than by wild-type enzyme. The patterns of pH dependence for V₁/E_tK_b for ethanol oxidation were similar for both enzymes in the pH range from 7 to 9. The C153D substitution decreased binding of trifluoroethanol by 5-fold and of pyrazole by 65-fold. Substrate specificities for C153D and wild-type ADHs for primary alcohols have similar patterns. Efficiency for secondary alcohols decreased only about 4-fold, and efficiencies for 1,2-propanediol and acetone were about the same as for wild-type enzyme. The C153D substitution modestly affects catalysis by altering ligand exchange on the zinc or local structure. Structures and mechanisms for acid-base catalysis in related medium chain dehydrogenases with substitutions of the homologous cysteine are reviewed and analyzed.

Catalytic mechanism of Zn2+-dependent polyol dehydrogenases: kinetic comparison of sheep liver sorbitol dehydrogenase with wild-type and Glu154-->Cys forms of yeast xylitol dehydrogenase

Co-ordination of catalytic Zn2+ in sorbitol/xylitol dehydrogenases of the medium-chain dehydrogenase/reductase superfamily involves direct or water-mediated interactions from a glutamic acid residue, which substitutes a homologous cysteine ligand in alcohol dehydrogenases of the yeast and liver type. Glu154 of xylitol dehydrogenase from the yeast Galactocandida mastotermitis (termed GmXDH) was mutated to a cysteine residue (E154C) to revert this replacement. In spite of their variable Zn2+ content (0.10-0.40 atom/subunit), purified preparations of E154C exhibited a constant catalytic Zn2+ centre activity (kcat) of 1.19+/-0.03 s(-1) and did not require exogenous Zn2+ for activity or stability. E154C retained 0.019+/-0.003% and 0.74+/-0.03% of wild-type catalytic efficiency (kcat/K(sorbitol)=7800+/-700 M(-1) x s(-1)) and kcat (=161+/-4 s(-1)) for NAD+-dependent oxidation of sorbitol at 25 degrees C respectively. The pH profile of kcat/K(sorbitol) for E154C decreased below an apparent pK of 9.1+/-0.3, reflecting a shift in pK by about +1.7-1.9 pH units compared with the corresponding pH profiles for GmXDH and sheep liver sorbitol dehydrogenase (termed slSDH). The difference in pK for profiles determined in 1H2O and 2H2O solvent was similar and unusually small for all three enzymes (approximately +0.2 log units), suggesting that the observed pK in the binary enzyme-NAD+ complexes could be due to Zn2+-bound water. Under conditions eliminating their different pH-dependences, wild-type and mutant GmXDH displayed similar primary and solvent deuterium kinetic isotope effects of 1.7+/-0.2 (E154C, 1.7+/-0.1) and 1.9+/-0.3 (E154C, 2.4+/-0.2) on kcat/K(sorbitol) respectively. Transient kinetic studies of NAD+ reduction and proton release during sorbitol oxidation by slSDH at pH 8.2 show that two protons are lost with a rate constant of 687+/-12 s(-1) in the pre-steady state, which features a turnover of 0.9+/-0.1 enzyme equivalents as NADH was produced with a rate constant of 409+/-3 s(-1). The results support an auxiliary participation of Glu154 in catalysis, and possible mechanisms of proton transfer in sorbitol/xylitol dehydrogenases are discussed.

Sorbitol dehydrogenase: a novel target for adjunctive protection of ischemic myocardium

Sorbitol dehydrogenase (SDH) is a polyol pathway enzyme that catalyzes conversion of sorbitol to fructose. Recent studies have demonstrated that activation of aldose reductase, the first enzyme of the polyol pathway, is a key response to ischemia and that inhibition of aldose reductase reduces myocardial ischemic injury. In our efforts to understand the role of pathway in affecting metabolism under normoxic and ischemic conditions, as well as in ischemic injury in myocardium, we investigated the importance of SDH by use of a specific inhibitor (SDI), CP-470,711. SDH inhibition increased glucose oxidation, whereas palmitate oxidation remained unaffected. Global ischemia increased myocardial SDH activity by approximately 1.5 fold. The tissue lactate/pyruvate ratio, a measure of cytosolic NADH/NAD+, was reduced by SDH inhibition under both normoxic and ischemic conditions. ATP was higher in SDI hearts during ischemia and reperfusion. Creatine kinase release during reperfusion, a marker of myocardial ischemic injury, was markedly attenuated in SDH-inhibited hearts. These data indicate that myocardial SDH activation is a component of ischemic response and that interventions that inhibit SDH protect ischemic myocardium. Furthermore, these data identify SDH as a novel target for adjunctive cardioprotective interventions.

X-ray crystallographic and kinetic studies of human sorbitol dehydrogenase

Sorbitol dehydrogenase (hSDH) and aldose reductase form the polyol pathway that interconverts glucose and fructose. Redox changes from overproduction of the coenzyme NADH by SDH may play a role in diabetes-induced dysfunction in sensitive tissues, making SDH a therapeutic target for diabetic complications. We have purified and determined the crystal structures of human SDH alone, SDH with NAD(+), and SDH with NADH and an inhibitor that is competitive with fructose. hSDH is a tetramer of identical, catalytically active subunits. In the apo and NAD(+) complex, the catalytic zinc is coordinated by His69, Cys44, Glu70, and a water molecule. The inhibitor coordinates the zinc through an oxygen and a nitrogen atom with the concomitant dissociation of Glu70. The inhibitor forms hydrophobic interactions to NADH and likely sterically occludes substrate binding. The structure of the inhibitor complex provides a framework for developing more potent inhibitors of hSDH.

Characterization of recombinant xylitol dehydrogenase from Galactocandida mastotermitis expressed in Escherichia coli

The plasmid-encoded gene of xylitol dehydrogenase from the yeast Galactocandida mastotermitis was expressed in Escherichia coli at 25 degrees C. Recombinant enzyme was isolated in 70% yield using two steps of biomimetic affinity chromatography with the dye ligand Procion Red HE3B immobilized onto Sepharose 4B-CL. Similar to natural enzyme, recombinant xylitol dehydrogenase is a functional homotetramer with a stoichiometric content of catalytic zinc in each 37-kDa subunit. Though lacking bound Mg(2+) found in xylitol dehydrogenase isolated from yeast cell extracts, the recombinant enzyme is as active and stable as the native enzyme. Stereospecificity of enzymic hydrogen transfer from NADH has been determined by 1H-NMR and is 4-pro-R. A detailed steady-state kinetic analysis has been carried out for the enzymic reaction, polyol+NAD(+)<-->ketose+NADH+H(+), at pH 7.5 and 25 degrees C using xylitol and D-xylulose, the physiological polyol-ketose pair, as well as D-sorbitol and D-fructose. Primary deuterium kinetic isotope effects on steady-state kinetic parameters for oxidation of D-sorbitol and reduction of D-fructose have been measured at pH 7.5. Combined results of initial-rate analysis and isotope effect studies suggest that the enzyme utilizes a preferentially ordered kinetic mechanism in which NAD(+) binds before D-sorbitol and D-fructose is released before NADH. Dissociation of NADH appears to be the main rate-limiting step for D-sorbitol oxidation under substrate-saturated reaction conditions.

Polyol pathway and diabetic peripheral neuropathy

This chapter critically examines the concept of the polyol pathway and how it relates to the pathogenesis of diabetic peripheral neuropathy. The two enzymes of the polyol pathway, aldose reductase and sorbitol dehydrogenase, are reviewed. The structure, biochemistry, physiological role, tissue distribution, and localization in peripheral nerve of each enzyme are summarized, along with current informaiton about the location and structure of their genes, their alleles, and the possible links of each enzyme and its alleles to diabetic neuropathy. Inhibitors of pathway enzyme and results obtained to date with pathway inhibitors in experimental models and human neuropathy trials are updated and discussed. Experimental and clinical data are analyzed in the context of a newly developed metabolic odel of the in vivo relationship between nerve sorbitol concentration and metabolic flux through aldose reuctase. Overall, the data will be interpreted as supporting the hypothesis that metabolic flux through the polyol pathway, rather than nerve concentration of sorbitol, is the predominant polyol pathway-linked pathogeneic factor in diabetic preipheral nerve. Finally, key questions and future directions for bsic and clinical research in this area are considered. It is concluded that robust inhibition of metabolic flux through the polyol pathway in peripheral nerve will likely result in substantial clinical benefit in treating and preventing the currently intractable condition of diabetic peripheral neuropathy. To accomplish this, it is imperative to develop and test a new generation of "super-potent" polyol pathway inhibitors.

Crystal structure of the NADP(H)-dependent ketose reductase from Bemisia argentifolii at 2.3 A resolution

Polyhydric alcohols are widely found in nature and can be accumulated to high concentrations as a protection against a variety of environmental stresses. It is only recently, however, that these molecules have been shown to be active in protection against heat stress, specifically in the use of sorbitol by the silverleaf whitefly, Bemisia argentifolii. We have determined the structure of the enzyme responsible for production of sorbitol in Bemisia argentifolii, NADP(H)-dependent ketose reductase (BaKR), to 2.3 A resolution. The structure was solved by multiwavelength anomalous diffraction (MAD) using the anomalous scattering from two zinc atoms bound in the structure, and was refined to an R factor of 21.9 % (R(free)=25.1 %). BaKR belongs to the medium-chain dehydrogenase family and its structure is the first for the sorbitol dehydrogenase branch of this family. The enzyme is tetrameric, with the monomer having a very similar fold to the alcohol dehydrogenases (ADHs). Although the structure determined is for the apo form, a phosphate ion in the active site marks the likely position for the adenyl phosphate of NADP(H). The catalytic zinc ion is tetrahedrally coordinated to Cys41, His66, Glu67 and a water molecule, in a modification of the zinc site usually found in ADHs. This modified zinc site seems likely to be a conserved feature of the sorbitol dehydrogenase sub-family. Comparisons with other members of the ADH family have also enabled us to model a ternary complex of the enzyme, and suggest how structural differences may influence coenzyme binding and substrate specificity in the reduction of fructose to sorbitol.

Crystal structure of sorbitol dehydrogenase

Sorbitol dehydrogenase (SDH) is a distant relative to the alcohol dehydrogenases (ADHs) with sequence identities around 20%. SDH is a tetramer with one zinc ion per subunit. We have crystallized rat SDH and determined the structure by molecular replacement using a tetrameric bacterial ADH as search object. The conformation of the bound coenzyme is extended and similar to NADH bound to mammalian ADH but the interactions with the NMN-part have several differences with those of ADH. The active site zinc coordination in SDH is significantly different than in mammalian ADH but similar to the one found in the bacterial tetrameric NADP(H)-dependent ADH of Clostridiim beijerinckii. The substrate cleft is significantly more polar than for mammalian ADH and a number of residues are ideally located to position the sorbitol molecule in the active site. The SDH molecule can be considered to be a dimer of dimers, with subunits A-B and C-D, where the dimer interactions are similar to those in mammalian ADH. The tetramers are composed of two of these dimers, which interact with their surfaces opposite the active site clefts, which are accessible on the opposite side. In contrast to the dimer interactions, the tetramer-forming interactions are small with only few hydrogen bonds between side-chains.

Activities of human alcohol dehydrogenases in the metabolic pathways of ethanol and serotonin

Alcohols and aldehydes in the metabolic pathways of ethanol and serotonin are substrates for alcohol dehydrogenases (ADH) of class I and II. In addition to the reversible alcohol oxidation/aldehyde reduction, these enzymes catalyse aldehyde oxidation. Class-I gammagamma ADH catalyses the dismutation of both acetaldehyde and 5-hydroxyindole-3-acetaldehyde (5-HIAL) into their corresponding alcohols and carboxylic acids. The turnover of acetaldehyde dismutation is high (kcat = 180 min-1) but saturation is reached first at high concentrations (Km = 30 mm) while dismutation of 5-HIAL is saturated at lower concentrations and is thereby more efficient (Km = 150 microm; kcat = 40 min-1). In a system where NAD+ is regenerated, the oxidation of 5-hydroxytryptophol to 5-hydroxyindole-3-acetic acid proceeds with concentration levels of the intermediary 5-HIAL expected for a two-step oxidation. Butanal and 5-HIAL oxidation is also observed for class-I ADH in the presence of NADH. The class-II enzyme is less efficient in aldehyde oxidation, and the ethanol-oxidation activity of this enzyme is competitively inhibited by acetate (Ki = 12 mm) and 5-hydroxyindole-3-acetic acid (Ki = 2 mm). Reduction of 5-HIAL is efficiently catalysed by class-I gammagamma ADH (kcat = 400 min-1; Km = 33 microm) in the presence of NADH. This indicates that the increased 5-hydroxytryptophol/5-hydroxyindole-3-acetic acid ratio observed after ethanol intake may be due to the increased NADH/NAD+ ratio on the class-I ADH.

Mutagenesis of histidinol dehydrogenase reveals roles for conserved histidine residues

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

Molecular basis for thermoprotection in Bemisia: structural differences between whitefly ketose reductase and other medium-chain dehydrogenases/reductases

The silverleaf whitefly (Bemisia argentifolii, Bellows and Perring) accumulates sorbitol as a thermoprotectant in response to elevated temperature. Sorbitol synthesis in this insect is catalyzed by an unconventional ketose reductase (KR) that uses NADPH to reduce fructose. A cDNA encoding the NADPH-KR from adult B. argentifolii was cloned and sequenced to determine the primary structure of this enzyme. The cDNA encoded a protein of 352 amino acids with a calculated molecular mass of 38.2 kDa. The deduced amino acid sequence of the cDNA shared 60% identity with sheep NAD(+)-dependent sorbitol dehydrogenase (SDH). Residues in SDH involved in substrate binding were conserved in the whitefly NADPH-KR. An important structural difference between the whitefly NADPH-KR and NAD(+)-SDHs occurred in the nucleotide-binding site. The Asp residue that coordinates the adenosyl ribose hydroxyls in NAD(+)-dependent dehydrogenases (including NAD(+)-SDH), was replaced by an Ala in the whitefly NADPH-KR. The whitefly NADPH-KR also contained two neutral to Arg substitutions within four residues of the Asp to Ala substitution. Molecular modeling indicated that addition of the Arg residues and loss of the Asp decreased the electric potential of the adenosine ribose-binding pocket, creating an environment favorable for NADPH-binding. Because of the ability to use NADPH, the whitefly NADPH-KR synthesizes sorbitol under physiological conditions, unlike NAD(+)-SDHs, which function in sorbitol catabolism.

Sorbitol dehydrogenase of Drosophila. Gene, protein, and expression data show a two-gene system

The Drosophila melanogaster sorbitol dehydrogenase (SDH) is characterized as a two-enzyme system of the medium chain dehydrogenase/reductase family (MDR). The SDH-1 enzyme has an enzymology with Km and kcat values an order of magnitude higher than those for the human enzyme but with a similar kcat/Km ratio. It is a tetramer with identical subunits of approximately 38 kDa. At the genomic level, two genes, Sdh-1 and Sdh-2, have a single transcriptional start site and no functional TATA box. Expression is greater in larvae and adults than in pupae, where it is very low. At all three stages, Sdh-1 constitutes the major transcript. Sdh-1 and Sdh-2 genes were located at positions 84E-F and 86D in polytene chromosomes. The deduced amino acid sequences of the two genes show 90% residue identity. Evaluation of the sequence and modeling of the structure toward that of class I alcohol dehydrogenase (ADH) show altered loop and gap arrangements as in mammalian SDH and establishes that SDH, despite gene multiplicity and larger variability than the "constant" ADH of class III, is an enzyme conserved over wide ranges.

The use of fluoro- and deoxy-substrate analogs to examine binding specificity and catalysis in the enzymes of the sorbitol pathway

The carbohydrate specificity of the two enzymes that catalyze the metabolic interconversions in the sorbitol pathway, aldose reductase and sorbitol dehydrogenase, has been examined through the use of fluoro- and deoxy-substrate analogs. Hydrogen bonding has been shown to be the primary mode of interaction by which these enzymes specifically recognize and bind their respective polyol substrates. Aldose reductase has broad substrate specificity, and all of the fluoro- and deoxysugars that were examined are substrates for this enzyme. Unexpectedly, both 3-fluoro- and 4-fluoro-D-glucose were found to be better substrates, with significantly lower K(m) and higher Kcat/K(m) values than those of D-glucose. A more discriminating pattern of substrate specificity is observed for sorbitol dehydrogenase. Neither the 2-fluoro nor the 2-deoxy analogs of D-glucitol were found to be substrates or inhibitors, suggesting that the 2-hydroxyl group of sorbitol is a hydrogen bond donor. The 4-fluoro and 4-deoxy analogs are poorer substrates than sorbitol, also implying a binding role for this hydroxyl group. In contrast, both 6-fluoro- and 6-deoxy-D-glucitol are very good substrates for sorbitol dehydrogenase, indicating that the primary hydroxyl group at this position is not involved in substrate recognition by this enzyme.

Structural and functional properties of a yeast xylitol dehydrogenase, a Zn2+-containing metalloenzyme similar to medium-chain sorbitol dehydrogenases

The NAD+-dependent xylitol dehydrogenase from the xylose-assimilating yeast Galactocandida mastotermitis has been purified in high yield (80%) and characterized. Xylitol dehydrogenase is a heteronuclear multimetal protein that forms homotetramers and contains 1 mol of Zn2+ ions and 6 mol of Mg2+ ions per mol of 37.4 kDa protomer. Treatment with chelating agents such as EDTA results in the removal of the Zn2+ ions with a concomitant loss of enzyme activity. The Mg2+ ions are not essential for activity and are removed by chelation or extensive dialysis without affecting the stability of the enzyme. Results of initial velocity studies at steady state for d-sorbitol oxidation and d-fructose reduction together with the characteristic patterns of product inhibition point to a compulsorily ordered Theorell-Chance mechanism of xylitol dehydrogenase in which coenzyme binds first and leaves last. At pH 7.5, the binding of NADH (Ki approximately 10 microM) is approx. 80-fold tighter than that of NAD+. Polyhydroxyalcohols require at least five carbon atoms to be good substrates of xylitol dehydrogenase, and the C-2 (S), C-3 (R) and C-4 (R) configuration is preferred. Therefore xylitol dehydrogenase shares structural and functional properties with medium-chain sorbitol dehydrogenases.

Reversible inhibition of sheep liver sorbitol dehydrogenase by the antidiabetogenic drug 2-hydroxymethyl-4-(4-N,N-dimethylaminosulfonyl-1-piperazino) pyrimidine

The mechanism of the inhibition of sheep liver sorbitol dehydrogenase by the novel antidiabetogenic drug 2-hydroxymethyl-4-(4-N,N-dimethylaminosulfonyl-1-piperazino) pyrimidine has been investigated by steady-state kinetics over the range pH 5-10. The pyrimidine derivative exhibits mixed inhibition with respect to sorbitol, fructose and coenzyme, due to the formation of enzyme-inhibitor and enzyme-NAD(H)-inhibitor complexes. The formation of each of the binary and ternary complexes is inhibited by protonation and deprotonation of groups which, in the enzyme-inhibitor complex, have pK values of 6.6 and 8.0, respectively.

Sorbitol dehydrogenase from bovine lens: purification and properties

Bovine lens sorbitol dehydrogenase (L-iditol:NAD+ 2-oxidoreductase, EC 1.1.1.14) (SDH) was purified to electrophoretic homogeneity (51 U/mg of protein) and characterized for both kinetic and some structural properties. The enzyme proves to be a homotetramer of 156 kDa containing one equivalent of zinc ion per subunit. Metal chelators such as EDTA and 1,10-phenanthroline determine a loss of enzyme activity which can be specifically recovered by addition of either zinc or manganese ions. Inactivation induced not only by metal chelators but also by thiol reagents is effectively prevented by the pyridine cofactor. Bovine lens SDH is active on polyalcohols and keto-sugars with more than three carbon atoms, and also requires special steric constraints for substrate recognition. Of the polyols, xylitol is the most effective substrate (kcat/KM of 8.1 s-1 mM-1), followed by sorbitol (kcat/KM of 1.59 s-1 mM-1); fructose, the most effective carbonyl substrate, displays a kcat/KM of only 0.9 s-1 mM-1. Analysis at the steady state of initial velocities as a function of the concentration of different substrates and cofactors and studies of product inhibition indicate for both fructose reduction and sorbitol oxidation a Theorell and Chance-type kinetic mechanism of action.

Thermoanaerobacter brockii alcohol dehydrogenase: characterization of the active site metal and its ligand amino acids

The active-site metal ion and the associated ligand amino acids in the NADP-linked, tetrameric enzyme Thermoanaerobacter brockii alcohol dehydrogenase (TBADH) were characterized by atomic absorption spectroscopy analysis and site-directed mutagenesis. Our preliminary results indicating the presence of a catalytic zinc and the absence of a structural metal ion in TBADH (Peretz & Burstein. 1989. Biochemistry 28:6549-6555) were verified. To determine the role of the putative active-site zinc, we investigated whether exchanging the zinc for other metal ions would affect the structural and/or the enzymatic properties of the enzyme. Substituting various metal ions for zinc either enhanced or diminished enzymatic activity, as follows: Mn2+ (240%); Co2+ (130%); Cd2+ (20%); Cu2+ or V3+ (< 5%). Site-directed mutagenesis to replace any one of the three putative zinc ligands of TBADH, Cys 37, His 59, or Asp 150, with the non-chelating residue, alanine, abolished not only the metal-binding capacity of the enzyme but also its catalytic activity, without affecting the overall secondary structure of the enzyme. Replacing the three putative catalytic zinc ligands of TBADH with the respective chelating residues serine, glutamine, or cysteine damaged the zinc-binding capacity of the mutated enzyme and resulted in a loss of catalytic activity that was partially restored by adding excess zinc to the reaction. The results imply that the zinc atom in TBADH is catalytic rather than structural and verify the involvement of Cys 37, His 59, and Asp 150 of TBADH in zinc coordination.

Structure of the Bombyx sorbitol dehydrogenase gene: a possible alternative use of the promoter

In an initial effort to understand the molecular mechanism of how low temperature induces sorbitol dehydrogenase gene expression in diapause eggs of the silkworm, the sorbitol dehydrogenase gene was isolated from a Bombyx genomic library using a cDNA encoding the Bombyx homologue of mammalian sorbitol dehydrogenase as a probe. The gene extended for about 10 kb, consisting of eight exons and seven introns. Four TATA motifs were found in the 5' upstream region of the gene, without CCAAT. AATTAA, instead of AATAAA, was localized in the upstream region of the polyadenylation site. Although a single copy of this gene was present per haploid genome, 1.2 kb and 1.1 kb transcripts were found from yolk cells in diapause eggs and from larval fat-body cells, respectively. The two major transcription initiation sites corresponding to both transcripts were localized at 355 and 226 base pairs upstream from the transition start site, indicating an alternative use of promoter. The 5'-upstream region of the gene contained a consensus sequence, TGA(A/T)AA(A/G/T), that has been found in insect genes expressed mainly in larval and pupal fat bodies. It also contained three kinds of sequences similar to cis-elements recognized by members of the steroid receptor superfamily, such as chicken ovalbumin upstream promoter transcription factor (COUP-TF)/Drosophila Seven up (SVP), Drosophila hormone receptor 39 (DHR39) and Bombyx fushi tarazu transcriptional factor 1 (BmFTZ-F1).

Stereo-selective affinity labelling of sheep liver sorbitol dehydrogenase by chloro-substituted analogues of 2-bromo-3-(5-imidazolyl)propionic acid

The role of configuration for the affinity labelling of sheep liver sorbitol dehydrogenase by chloro-substituted analogues of 2-bromo-3-(5-imidazolyl)propionate (BrImPpOH) has been studied. A saturation kinetics mechanism applies which includes formation of a reversible complex with the enzyme prior to alkylation of Cys-43. The pseudo first-order inactivation rate-constant, k2, and the dissociation constant for the reversible enzyme-affinity label complex. KEI, were determined at pH 7.4 and 23.5 degrees C. The stereo isomers of each affinity label exhibit different kinetic characteristics but, unlike with horse liver alcohol dehydrogenase, the discrimination between them is not absolute. For the different affinity labels, k2 varies with 2-chloro-3-(5-imidazolyl)methylpropionate (Me-ClImPpOH) > 2-chloro-3-(5-imidazolyl)propionate (ClImPpOH) > 2-chloro-3-(5-imidazolyl)propanol (ClImPOH), consistent with their order of inherent reactivity, and the specificity constant k2/KEI varies with (S)-Me-ClImPpOH > (S)-ClImPpOH > (S)-ClImPpOH > (R)-Me-ClImPpOH > (R)-ClImPpOH. Models of the affinity labels were built into the active site of the predicted subunit structure of the enzyme by using a computer-controlled display system. In each binary complex, the imidazole moiety of the affinity label was liganded to the catalytic zinc atom, and the angle Scys-C alpha-Cl was linear, in accordance with an SN2 mechanism. Both enantiomers of each label could form plausible complexes with the enzyme model, in agreement with the kinetic data. The enantiomeric selectivity, rather than absolute specificity, of the reaction appears due to the anion-binding site in sorbitol dehydrogenase being less developed than in horse liver alcohol dehydrogenase.

Effect of pH on sheep liver sorbitol dehydrogenase steady-state kinetics

The variation with pH of the kinetic parameters for sorbitol oxidation and fructose reduction by sheep liver sorbitol dehydrogenase has been studied over the pH 5-10 range. The reaction is compulsory ordered in both directions with the coenzyme as the leading substrate, and the rate-determining step in either direction is the enzyme-coenzyme product dissociation. Throughout the pH range, the lack of a primary kinetic isotope effect on Vm with (2H8) sorbitol confirms that the ternary complexes are not of rate-determining significance under maximum velocity conditions. The association rate constants for NAD and NADH increase and decrease, respectively, towards high pH. NAD binding to the enzyme is dependent upon pK values of 9.2 and 9.6. Whereas the dissociation rate constant for NAD release from the enzyme shows no pronounced variation with pH, NADH release is dependent upon pK values of 7.2 and 7.7. The kinetic constants that characterize the dependence on substrate concentration of the steady-state rate of catalysis vary with pH in accordance with a single pK of 7.1 for sorbitol oxidation and of 7.7 for fructose reduction. These pK values reflect the ionization properties of a catalytically essential group, which is tentatively considered to be either the H2O/OH- ligand binding to the catalytic zinc atom or a histidine residue. Catalysis by sorbitol dehydrogenase, due to the absence of a second ionization contribution, appears not to involve any obligatory step of proton transfer to solution at the ternary complex level. A mechanism for sorbitol dehydrogenase catalysis is proposed.

The phylogenetically conserved histidines of Escherichia coli porphobilinogen synthase are not required for catalysis

Porphobilinogen synthase (PBGS) is a metalloenzyme that catalyzes the first common step of tetrapyrrole biosynthesis, the asymmetric condensation of two molecules of 5-aminolevulinic acid (ALA) to form porphobilinogen. Chemical modification data implicate histidine as a catalytic residue of PBGS from both plants and mammals. Histidine may participate in the abstraction of two non-ionizable protons from each substrate molecule at the active site. Only one histidine is species-invariant among 17 known sequences of PBGS which have high overall sequence similarity. In Escherichia coli PBGS, this histidine is His128. We performed site-directed mutagenesis on His128, replacing it with alanine. The mutant protein H128A is catalytically active. His128 is part of a histidine- and cysteine-rich region of the sequence that is implicated in metal binding. The apparent Kd for Zn(II) binding to H128A is about an order of magnitude higher than for the wild type protein. E. coli PBGS also contains His126 which is conserved through the mammalian, fungal, and some bacterial PBGS. We mutated His126 to alanine, and both His126 and His128 simultaneously to alanine. All mutant proteins are catalytically competent; the Vmax values for H128A (44 units/mg), H126A (75 units/mg), and H126/128A (61 units/mg) were similar to wild type PBGS (50 units/mg) in the presence of saturating concentrations of metal ions. The apparent Kd for Zn(II) of H126A and H126/128A is not appreciably different from wild type. The activity of wild type and mutant proteins are all stimulated by an allosteric Mg(II); the mutant proteins all have a reduced affinity for Mg(II). We observe a pKa of approximately 7.5 in the wild type PBGS kcat/Km pH profile as well as in those of H128A and H126/128A, suggesting that this pKa is not the result of protonation/deprotonation of one of these histidines. H128A and H126/128A have a significantly increased Km value for the substrate ALA. This is consistent with a role for one or both of these histidines as a ligand to the required Zn(II) of E. coli PBGS, which is known to participate in substrate binding. Past chemical modification may have inactivated the PBGS by blocking Zn(II) and ALA binding. In addition, the decreased Km for E. coli PBGS at basic pH allows for the quantitation of active sites at four per octamer.

Polyol metabolism of Rhodobacter sphaeroides: biochemical characterization of a short-chain sorbitol dehydrogenase

A sorbitol dehydrogenase (SDH; L-iditol:NAD+ 2-oxidoreductase; EC 1.1.1.14) was isolated from the phototrophic bacterium Rhodobacter sphaeroides strain M22, a transposon mutant of R. sphaeroides Si4 with the transposon inserted in the mannitol dehydrogenase (MDH) gene. SDH was purified 470-fold to apparent homogeneity by ammonium sulfate precipitation, chromatography on Phenyl-Sepharose, Q-Sepharose and Matrex Gel Red-A, and by gel filtration on Superdex 200. The relative molecular mass (M(r)) of the native SDH was 61000 as calculated from its Stokes' radius (rs = 3.5 nm) and sedimentation coefficient (S20,w = 4.23S). SDS-PAGE resulted in one single band representing a polypeptide with a M(r) of 29,000, indicating that the native protein is a dimer. The isoelectric point of SDH was determined to be pH 4.8. The enzyme was specific for NAD+ and catalysed the oxidation of D-glucitol (sorbitol) to D-fructose, galactitol to D-tagatose and of L-iditol. The apparent Km values were NAD+, 0.06 mM; D-glucitol, 6.2 mM; galactitol, 1.5 mM; NADH, 0.13 mM; D-fructose, 160 mM; and D-tagatose, 13 mM. The pH-optimum of substrate oxidation was 11.0 and that of substrate reduction 6.0-7.2. It was demonstrated that SDH is expressed in the wild-type strain R. sphaeroides Si4 together with MDH during growth on D-glucitol. Forty-four amino acids of the SDH N terminus were sequenced. This sequence exhibited 45-55% identity to the N-terminal sequence of 10 enzymes belonging to the short-chain alcohol dehydrogenase family.

Cloning, sequencing, and determination of the sites of expression of mouse sorbitol dehydrogenase cDNA

Sorbitol dehydrogenase is one of the enzymes in the polyol pathway, which is thought to be implicated in the pathogenesis of diabetic complications. The cDNA encoding mouse sorbitol dehydrogenase was cloned from a liver library and its sequence was determined. The open reading frame encodes a product of 356 amino acids that shares high similarity with the human and rat liver sorbitol dehydrogenases (83% and 93% identity, respectively). The 3'-untranslated region contains a truncated L1Md repeat element inserted in reverse relative to the sorbitol dehydrogenase cDNA. Northern-blot hybridization showed that the testis has the highest level of expression, followed by kidney, liver, and lung. Low levels of expression were also observed in lens, brain, and skeletal muscle. In situ hybridization revealed that in the kidney, the highest concentration of sorbitol dehydrogenase mRNA is observed in the cortex, but is absent from the inner medulla. The parenchymal cells of the liver showed strong expression while the cells of the hepatic vasculature did not hybridize. The sorbitol dehydrogenase expression in the seminiferous tubules was mostly associated with the mature cells of the developing germ cells, confirming the usefulness of sorbitol dehydrogenase as an enzyme indicator for sexual maturation. The seminal vesicle, where most of the seminal fructose is produced, also showed a high level of expression in the epithelial cells. The mouse sorbitol dehydrogenase cDNA will be useful in the studies of the involvement of the polyol pathway in diabetic complications.

Structural organization of the human sorbitol dehydrogenase gene (SORD)

The primary structure of human sorbitol dehydrogenase (SORD) was determined by cDNA and genomic cloning. The nucleotide sequence of the mRNA covers 2471 bp including an open reading frame that yields a protein of 356 amino acid residues. The gene structure of SORD spans approximately 30 kb divided into 9 exons and 8 introns. The gene was localized to chromosome 15q21.1 by in situ hybridization. Two transcription initiation sites were detected. Three Sp1 sites and a repetitive sequence (CAAA)5 were observed in the 5' noncoding region; no classical TATAA or CCAAT elements were found. The related alcohol dehydrogenases and zeta-crystallin have the same gene organization split by 8 introns, but no splice points coincide between SORD and these gene types. The deduced amino acid sequence of the SORD structure differs at a few positions from the directly determined protein sequence, suggesting allelic forms of the enzyme. High levels of SORD transcripts were observed in lens and kidney, as judged from Northern blot analysis.

Cloning and high-level expression of the glutathione-independent formaldehyde dehydrogenase gene from Pseudomonas putida

A DNA fragment of 485 bp was specifically amplified by PCR with primers based on the N-terminal sequence of the purified formaldehyde dehydrogenase (EC 1.2.1.46) from Pseudomonas putida and on that of a cyanogen bromide-derived peptide. With this product as a probe, a gene coding for formaldehyde dehydrogenase (fdhA) in P. putida chromosomal DNA was cloned in Escherichia coli DH5 alpha. Sequencing analysis revealed that the fdhA gene contained 1,197-bp open reading frame, encoding a protein composed of 399 amino acid residues whose calculated molecular weight was 42,082. The transformant of E. coli DH5 alpha harboring the hybrid plasmid, pFDHK3DN71, showed about 50-fold-higher formaldehyde dehydrogenase activity than P. putida. The predicted amino acid sequence contained several features characteristic of the zinc-containing medium-chain alcohol dehydrogenase (ADH) family. Most of the glycine residues strictly conserved within the family, including a Gly-Xaa-Gly-Xaa-Xaa-Gly pattern in the coenzyme binding domain, were well conserved in this enzyme. Regions around both the catalytic and the structural zinc atoms were also conserved. Analyses of structural and enzymatic characteristics indicated that P. putida FDH belongs to the medium-chain ADH family, with mixed properties of mammalian class I and III ADHs.

Inhibition and activation studies on sheep liver sorbitol dehydrogenase

Reversible inhibition and activation, as well as protection against affinity labelling with DL-2-bromo-3-(5-imidazolyl)propionic acid, of sheep liver sorbitol dehydrogenase have been studied. The results presented are discussed in terms of enzyme active-site properties and may have potential applications for drug design. Kinetics with mainly sorbitol competitive inhibitors reveals that aliphatic thiols are generally the most potent inhibitors of enzyme activity. Inhibition and inactivation by heterocyclics parallel that seen previously with sorbitol dehydrogenase from other sources as well as with alcohol dehydrogenase from yeast. However, there are significant differences in relation to the structurally similar horse liver alcohol dehydrogenase, as the catalytic zinc of sorbitol dehydrogenase is more easily removed by chelating molecules. Several aldose reductase inhibitors are shown to also inhibit sorbitol dehydrogenase, but at concentrations unlikely to be reached clinically. Enzyme activation has been observed with various compounds, in particular halo-alcohols and detergents. Several inhibitors provide competitive protection against enzyme inactivation by DL-2-bromo-3-(5-imidazolyl)propionic acid. This enables the dissociation constants for binary enzyme-inhibitor complexes to be determined. NADH protects noncompetitively against inactivation. The presence of some binary and ternary enzyme-NADH complexes is indicated from fluorescence emission spectra, as a shift in the fluorescence maximum and intensity is observed due to their formation.