The kinetic mechanism of sheep liver sorbitol dehydrogenase

Paeonol attenuates retinopathy in streptozotocin-induced diabetes in rats by regulating the oxidative stress and polyol pathway

The current research work was planned to study the effects of paeonol in the management of diabetic retinopathy. Diabetes was induced in male Sprague Dawley rats using Streptozotocin (55 mg/kg, i.p.). After 4 weeks, the diabetic animals were treated with paeonol at a dose of 50, 100, and 200 mg/kg body weight daily for the next 4 weeks. At the end of treatment, retinal physiology was studied by recording an electroretinogram (ERG); biochemical parameters and oxidative stress were estimated. The histopathology of the retina was also carried out at the end of the study. The ERG of paeonol-treated animals showed a significant improvement in a-wave amplitude, b-wave amplitude, a-wave latency, and b-wave latency (p < 0.001) at 15 cd s/m² when compared with the diabetic control animals. The paeonol treatment (200 mg/kg) in diabetic animals showed a significant decrease in the plasma glucose level (p < 0.001) when compared to the animals in diabetic control group. Paeonol also significantly decreased the lactate dehydrogenase, aldose reductase, and sorbitol dehydrogenase levels when compared with the diabetic control animals. The oxidative stress in the eye was significantly reduced after the paeonol treatment in the diabetic rats. The histopathology showed a significant reduction (p < 0.05) in the retinal thickness after the paeonol treatment. The results of the study indicate that paeonol can be considered an effective management option for diabetic retinopathy.

Cofactor Specificity Switch on Peach Glucitol Dehydrogenase

Most oxidoreductases that use NAD⁺ or NADP⁺ to transfer electrons in redox reactions display a strong preference for the cofactor. The catalytic efficiency of peach glucitol dehydrogenase (GolDHase) for NAD⁺ is 1800-fold higher than that for NADP⁺. Herein, we combined structural and kinetic data to reverse the cofactor specificity of this enzyme. Using site-saturation mutagenesis, we obtained the D216A mutant, which uses both NAD⁺ and NADP⁺, although with different catalytic efficiencies (1000 ± 200 and 170 ± 30 M^-1 s^-1, respectively). This mutant was used as a template to introduce further mutations by site-directed mutagenesis, using information from the fruit fly NADP-dependent GolDHase. The D216A/V217R/D218S triple mutant displayed a 2-fold higher catalytic efficiency with NADP⁺ than with NAD⁺. Overall, our results indicate that the triple mutant has the potential to be used for metabolic and cellular engineering and for cofactor recycling in industrial processes.

New insights into the evolutionary history of plant sorbitol dehydrogenase

BACKGROUND

Sorbitol dehydrogenase (SDH, EC 1.1.1.14) is the key enzyme involved in sorbitol metabolism in higher plants. SDH genes in some Rosaceae species could be divided into two groups. L-idonate-5-dehydrogenase (LIDH, EC 1.1.1.264) is involved in tartaric acid (TA) synthesis in Vitis vinifera and is highly homologous to plant SDHs. Despite efforts to understand the biological functions of plant SDH, the evolutionary history of plant SDH genes and their phylogenetic relationship with the V. vinifera LIDH gene have not been characterized.

RESULTS

A total of 92 SDH genes were identified from 42 angiosperm species. SDH genes have been highly duplicated within the Rosaceae family while monocot, Brassicaceae and most Asterid species exhibit singleton SDH genes. Core Eudicot SDHs have diverged into two phylogenetic lineages, now classified as SDH Class I and SDH Class II. V. vinifera LIDH was identified as a Class II SDH. Tandem duplication played a dominant role in the expansion of plant SDH family and Class II SDH genes were positioned in tandem with Class I SDH genes in several plant genomes. Protein modelling analyses of V. vinifera SDHs revealed 19 putative active site residues, three of which exhibited amino acid substitutions between Class I and Class II SDHs and were influenced by positive natural selection in the SDH Class II lineage. Gene expression analyses also demonstrated a clear transcriptional divergence between Class I and Class II SDH genes in V. vinifera and Citrus sinensis (orange).

CONCLUSIONS

Phylogenetic, natural selection and synteny analyses provided strong support for the emergence of SDH Class II by positive natural selection after tandem duplication in the common ancestor of core Eudicot plants. The substitutions of three putative active site residues might be responsible for the unique enzyme activity of V. vinifera LIDH, which belongs to SDH Class II and represents a novel function of SDH in V. vinifera that may be true also of other Class II SDHs. Gene expression analyses also supported the divergence of SDH Class II at the expression level. This study will facilitate future research into understanding the biological functions of plant SDHs.

Inhibition of sorbitol dehydrogenase by nucleosides and nucleotides

Sorbitol dehydrogenase inhibitors have been found to prevent, or alleviate, various secondary complications of diabetes mellitus. In the present study, the effects of nucleosides and nucleotides on the rate of sorbitol oxidation catalyzed by the sheep liver enzyme were studied by steady-state kinetics at pH 7.4. Various such compounds, including ATP and the 2'-deoxy-analogues of ATP, ADP and AMP, reversibly inhibit enzyme activity by formation of enzyme-coenzyme-inhibitor ternary complexes. In each case, no deviations from linearity were seen in the double-reciprocal plots using sorbitol or NAD(+) as the varied substrate and there was a linear relationship between inhibitor concentration and the observed inhibitory effects. Sorbitol was docked into a model of the sheep SDH-NAD(+) complex based upon the structure of the human SDH-NAD(+) holoenzyme. The resulting structure of the ternary complex of sheep SDH, NAD(+) and sorbitol (PMDB ID code PM 0078068) shows that the reactive C-2 hydroxyl group of sorbitol is oriented toward the 4'-position of the nicotinamide moiety of the coenzyme, and that the adjacent primary hydroxyl group of sorbitol interacts with the catalytic zinc. The results indicate that the ribose moiety of the inhibitor structures is an important determinant for the observed effects. Specifically, the 2'-position of the ribose ring exerts an effect with respect to inhibitor potency.

Effects of some anti-neoplastic drugs on sheep liver sorbitol dehydrogenase

Stress is an important factor for many diseases in living metabolisms. The mini pathway named as polyol is a critical junction for stress factors. This pathway has two enzymes: aldose reductase (AR) and sorbitol dehydrogenase (SDH). It is linked with some diseases such as diabetes mellitus and some cancer types. In particular, SDH is very sensitive and unstable in in vitro conditions. In this study, SDH was purified by using simple and rapid chromatographic methods such as DEAE-Sephadex and CM-Sephadex C-50 columns. Subunit and active form molecular weights were found as 39.8 kDa and 150 kDa, respectively. The in vitro effects of some antineoplastic drugs were investigated. IC(50) values were 0.025, 0.081, 0.291, 1.62, 4.86, 6.54 mM for dacarbazine, methotrexate, epirubicin hydrochloride, calcium folinate, gemcitabine hydrochloride, oxaliplatin, respectively. From these results, dacarbazine was lowest IC(50) value and it is the strongest inhibitor for liver SDH enzyme activity compared to the other drugs.

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.

Identification and characterization of the Tuber borchii D-mannitol dehydrogenase which defines a new subfamily within the polyol-specific medium chain dehydrogenases

A novel NADP(+)-dependent D-mannitol dehydrogenase and the corresponding gene from the plant symbiotic ascomycete fungus Tuber borchii was identified and characterized. The enzyme, called TbMDH, is a homotetramer with two zinc atoms per subunit. It catalyzed both D-fructose reduction and D-mannitol oxidation, although it showed the highest substrate specificity and catalytic efficiency for D-fructose. Co-factor specificity was restricted to NADP(H) and the reaction proceeded via a sequential ordered Bi Bi mechanism. The carbon responsive transcriptional pattern showed that Tbmdh is up-regulated when mycelia are transferred to a culture medium containing D-mannitol or D-fructose. The phylogenetic analysis showed TbMDH to be the first example of a fungal D-mannitol-2-dehydrogenase belonging to the medium-chain dehydrogenase/reductases (MDRs). The enzyme identified a new group of proteins, most of them annotated in databases as hypothetical zinc-dependent dehydrogenases, forming a distinct subfamily among the polyol dehydrogenase family.

Steady-state kinetic properties of sorbitol dehydrogenase from chicken liver

The steady-state kinetic properties of partially purified chicken liver sorbitol dehydrogenase (SDH) were determined spectrophotometrically at 25 degrees C, in 50 mM 3-(N-morpholino)propanesulfonic acid (MOPS) buffer, pH 8.0. In the sorbitol-to-fructose direction, analysis was based on initial rate data obtained at [NAD(+)](o)=0.1-0.4 mM and [sorbitol](o)=1.25-10 mM. The reverse process was analyzed by recording progress curves for NADH consumption, starting with [NADH](o)=0.2 mM and [fructose](o)=66.7-267 mM. The kinetics conformed to an ordered sequential model, with the cofactors adding first. The steady-state parameters in the forward direction, K(NAD(+)), K(iNAD(+)) and K(sorbitol), were found to be 210+/-62 muM, 220+/-69 microM and 3.2+/-0.54 mM, respectively. The corresponding parameters in the reverse direction were K(NADH)=240+/-58 microM, K(iNADH)=10+/-2.8 microM and K(fructose)=1000+/-140 mM. The results indicated a close parallelism with human SDH, yet up to 40-fold differences were observed when compared to related reports on other mammalian species. The structural and adaptive bases of the variation in substrate and cofactor affinities need to be accounted for.

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.

Kinetic study of the catalytic mechanism of mannitol dehydrogenase from Pseudomonas fluorescens

To characterize catalysis by NAD-dependent long-chain mannitol 2-dehydrogenases (MDHs), the recombinant wild-type MDH from Pseudomonas fluorescens was overexpressed in Escherichia coli and purified. The enzyme is a functional monomer of 54 kDa, which does not contain Zn(2+) and has B-type stereospecificity with respect to hydride transfer from NADH. Analysis of initial velocity patterns together with product and substrate inhibition patterns and comparison of primary deuterium isotope effects on the apparent kinetic parameters, (D)k(cat), (D)(k(cat)/K(NADH)), and (D)(k(cat)/K(fructose)), show that MDH has an ordered kinetic mechanism at pH 8.2 in which NADH adds before D-fructose, and D-mannitol and NAD are released in that order. Isomerization of E-NAD to a form which interacts with D-mannitol nonproductively or dissociation of NAD from the binary complex after isomerization is the slowest step (>/=110 s(-)(1)) in D-fructose reduction at pH 8.2. Release of NADH from E-NADH (32 s(-)(1)) is the major rate-limiting step in mannitol oxidation at this pH. At the pH optimum for D-fructose reduction (pH 7.0), the rate of hydride transfer contributes significantly to rate limitation of the catalytic cascade and the overall reaction. (D)(k(cat)/K(fructose)) decreases from 2.57 at pH 7.0 to a value of </=1 above pH 9.6, corresponding to the pK of 9.34 observed in the pH profile of k(cat)/K(fructose). Therefore, hydride transfer is not pH-dependent, and D-fructose is not sticky at pH 7.0. A comparison of the kinetic data of MDH and mammalian sorbitol dehydrogenase, presumably involved in detoxification metabolism, is used to point out a physiological function of MDH in the oxidation of D-mannitol with high specificity and fluxional efficiency under prevailing reaction conditions in vivo.

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.

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.

Substrate specificity of sheep liver sorbitol dehydrogenase

The substrate specificity of sheep liver sorbitol dehydrogenase has been studied by steady-state kinetics over the range pH 7-10. Sorbitol dehydrogenase stereo-selectively catalyses the reversible NAD-linked oxidation of various polyols and other secondary alcohols into their corresponding ketones. The kinetic constants are given for various novel polyol substrates, including L-glucitol, L-mannitol, L-altritol, D-altritol, D-iditol and eight heptitols, as well as for many aliphatic and aromatic alcohols. The maximum velocities (kcat) and the substrate specificity-constants (kcat/Km) are positively correlated with increasing pH. The enzyme-catalysed reactions occur by a compulsory ordered kinetic mechanism with the coenzyme as the first, or leading, substrate. With many substrates, the rate-limiting step for the overall reaction is the enzyme-NADH product dissociation. However, with several substrates there is a transition to a mechanism with partial rate-limitation at the ternary complex level, especially at low pH. The kinetic data enable the elucidation of new empirical rules for the substrate specificity of sorbitol dehydrogenase. The specificity-constants for polyol oxidation vary as a function of substrate configuration with D-xylo> D-ribo > L-xylo > D-lyxo approximately L-arabino > D-arabino > L-lyxo. Catalytic activity with a polyol or an aromatic substrate and various 1-deoxy derivatives thereof varies with -CH2OH > -CH2NH2 > -CH2OCH3 approximately -CH3. The presence of a hydroxyl group at each of the remaining chiral centres of a polyol, apart from the reactive C2, is also nonessential for productive ternary complex formation and catalysis. A predominantly nonpolar enzymic epitope appears to constitute an important structural determinant for the substrate specificity of sorbitol dehydrogenase. The existence of two distinct substrate binding regions in the enzyme active site, along with that of the catalytic zinc, is suggested to account for the lack of stereospecificity at C2 in some polyols.

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.

Investigation on the mechanism by which fructose, hexitols and other compounds regulate the translocation of glucokinase in rat hepatocytes

In isolated hepatocytes in suspension, the effect of sorbitol but not that of fructose to increase the concentration of fructose 1-phosphate and to stimulate glucokinase was abolished by 2-hydroxymethyl-4-(4-N,N-dimethylamino-1-piperazino)-pyrimidine (SDI 158), an inhibitor of sorbitol dehydrogenase. In hepatocytes in primary culture, fructose was metabolized at approximately one-quarter of the rate of sorbitol, and was therefore much less potent than the polyol in increasing the concentration of fructose 1-phosphate and the translocation of glucokinase. In cultures, sorbitol, commercial mannitol, fructose, D-glyceraldehyde or high concentrations of glucose caused fructose 1-phosphate formation and glucokinase translocation in parallel. Commercial mannitol was contaminated by approx. 1% sorbitol, which accounted for its effects. The effects of sorbitol, fructose and elevated concentrations of glucose were partly inhibited by ethanol, glycerol and glucosamine. Mannoheptulose increased translocation without affecting fructose 1-phosphate concentration. Kinetic studies performed with recombinant human beta-cell glucokinase indicated that this sugar, in contrast with N-acetylglucosamine, binds to glucokinase competitively with the regulatory protein. All these observations indicate that translocation is promoted by agents that favour the dissociation of the glucokinase-regulatory-protein complex either by binding to the regulatory protein (fructose I-phosphate) or to glucokinase (glucose, mannoheptulose). They support the hypothesis that the regulatory protein of glucokinase acts as an anchor for this enzyme that slows down its release from digitonin-permeabilized cells.

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).

Reversible inhibition of sheep liver sorbitol dehydrogenase by thiol compounds

Reversible inhibition of sheep liver sorbitol dehydrogenase by various thiol compounds has been studied. Most species inhibit the enzyme-catalyzed reaction competitively with respect to sorbitol, due to the formation of ternary enzyme-NAD-thiol complexes. The primary interaction of thiol inhibitors with the enzyme active site involves the catalytic zinc atom, and a bidentate mode of binding to the active site metal is indicated for some bifunctional thiols in their ternary complexes. Enzyme-bound thiolate facilitates NAD binding to the enzyme and vice versa, mainly due to mutual electrostatic stabilization. The aromatic thiols 1-thio-1-phenylmethane and 1-thio-2-phenylethane are especially potent inhibitors with an inhibition constant of 0.30 microM at pH 9.9. The inhibitory effect of aliphatic thiols, which is positively correlated with alkyl chain length, parallels that observed previously with the related enzyme horse liver alcohol dehydrogenase and indicates that interaction with an enzymic hydrophobic site is important for inhibitor binding. Several reversible inhibitors afford competitive protection against affinity labelling of the enzyme by 2-bromo-3-(5-imidazolyl) propionic acid due to the formation of binary enzyme-thiol complexes. The present study establishes thionucleosides as a novel class of potent sorbitol dehydrogenase inhibitors. The thionucleosides 6-thioguanosine and 6-thioinosine gave mixed inhibition with respect to sorbitol, due to the formation of enzyme-NAD-inhibitor and enzyme-NADH-inhibitor complexes. In order to enable a correlation of the substrate and inhibitor specificities of the enzyme, the kinetic constants for several sorbitol dehydrogenase substrates were determined. L-threitol and DL-1-phenyl-1,2-ethanediol are good substrates with, at high pH, kinetic constants similar to those of sorbitol. The potent inhibition by dithiothreitol and the aromatic thiols thus parallels the substrate specificity of the enzyme. The sorbitol competitive inhibitor 1-thiosorbitol is also a substrate with, at pH 7.4, a maximum velocity of 0.17 s-1 and a Michaelis constant of 8.6 mM. Dithiothreitol forms a tight ternary complex with the enzyme-NAD complex with a molar absorbance of 16.4 x 10(3) M-1 . cm-1 at 311 nm. A spectrophotometric titration of the enzyme with NAD in the presence of dithiothreitol is described, which enables an accurate determination of the concentration of sorbitol dehydrogenase active sites and confirms the activity assay of the enzyme.

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.

Reversible and irreversible inhibition of sheep liver sorbitol dehydrogenase with Cibacron Blue 3GA and Eriochrome Black T

Due to the central role of sorbitol dehydrogenase in diabetic cataract, it is important to examine this enzyme's interaction with different inhibitory compounds such as dyes. The aim of the study was to investigate the binding of Cibacron Blue and Eriochrome Black T to the active site in sorbitol dehydrogenase. These dyes' effect on the enzyme was studied by steady state and affinity labelling kinetics. Both dyes were coenzyme competitive inhibitors with KEI values around 0.5 microM. Essentially the same KEI values were obtained using the dyes as protecting ligands against the affinity label D,L-alpha-Bromo-beta-(5-imidazolyl)-propionic acid. Both dyes were also able to inhibit the enzyme irreversibly through an affinity labelling mechanism, with KEI' values for Cibacron Blue and Eriochrome Black T of 2.2 and 3.1 mM, respectively. Dithiothreitol and NADH were competitive protecting ligands against both dyes. The rate of inactivation was fastest for Cibacron Blue at acid pH values, while the opposite was the case with EBT. Both Cibacron Blue and Eriochrome Black T bind to sorbitol dehydrogenase in two different ways. In both cases the complex formed prior to irreversible inhibition is the weakest. The tighter reversible complexes are suggested to share a common epitope in the coenzyme binding region. Both irreversible complexes involve binding close to the zinc ion at the active site and the sugar binding site. Due to different pH dependences it can be concluded that the affinity labelling mechanism is different for the two dyes and in neither case is the inactivation due to removal of the active site zinc ion.

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.

Drosophila melanogaster alcohol dehydrogenase: product-inhibition studies

The Drosophila melanogaster alleloenzymes AdhS and AdhF have been studied with respect to product inhibition by using the two substrate couples propan-2-ol/acetone and ethanol/acetaldehyde together with the coenzyme couple NAD+/NADH. With both substrate couples the reaction was consistent with an ordered Bi Bi mechanism. The substrates added to the enzyme in a compulsory order, with coenzyme as the leading substrate, to give two interconverting ternary complexes. The second ternary complex broke down with release of products in an obligatory order, with the aldehyde/ketone leaving first. Both the acetaldehyde and acetone products formed binary complexes with the enzyme that affected NAD+ binding. However, only an enzyme-acetone complex seemed to affect NADH binding and hence the reverse reaction. The inhibitory pattern with acetaldehyde as product was also affected by the formation of a ternary enzyme-NAD(+)-acetaldehyde complex, which broke down to acetic acid and NADH. The product-inhibition pattern shown in the present work is different from that published for Drosophila Adh previously and this discrepancy can not be explained by the use of different variants of Drosophila Adh.

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.

Methylglyoxal and the polyol pathway. Three-carbon compounds are substrates for sheep liver sorbitol dehydrogenase

Methylglyoxal, 1,2-propanediol and glycerol are shown to be substrates for sheep liver sorbitol dehydrogenase. With 1,2-propanediol the enzyme-catalyzed reaction occurs specifically with the R(-)-enantiomer. The maximum velocities and the specificity constants obtained for the three-carbon substrates are considerably lower than those reported previously for sorbitol, and suggest that rate-determination is imposed by catalytic steps other than the enzyme-coenzyme product dissociation. The present findings are discussed in terms of substrate specificity and stereospecificity, and may indicate novel aspects of sorbitol dehydrogenase function in relation to glucose metabolism and diabetic pathogenesis.

A cold-inducible Bombyx gene encoding a protein similar to mammalian sorbitol dehydrogenase. Yolk nuclei-dependent gene expression in diapause eggs

To facilitate the study of the induction of sorbitol dehydrogenase by acclimation to 5 degrees C in diapause eggs of the silkworm, Bombyx mori, two cDNA libraries from eggs and larval fat bodies were screened with anti-(sorbitol dehydrogenase) serum, and a positive cDNA was cloned from the fat-body cDNA library. 1039 nucleotides determined from the cDNA corresponded to a protein-coding region consisting of 346 amino acids. The missing regions (containing two amino acids at the 5' end and a stop codon at the 3' end) were supplemented with the genome sequence. The deduced amino-acid sequence had 45-47% identity with mammalian sorbitol dehydrogenases. The results led us to conclude that the cDNA for a Bombyx homolog of mammalian sorbitol dehydrogenase was isolated, which was designated as BmSDH. Analyses of Northern hybridization and reverse transcription/polymerase chain reaction showed that the transcript of BmSDH occurred after chilling for 40-50 days when the diapause eggs were exposed to 5 degrees C from two days after oviposition to break the diapause. The changing pattern in the amount of BmSDH transcript was well correlated with those in the activity of sorbitol dehydrogenase and the amount of the enzyme protein in diapause eggs. Further, the transcript of BmSDH was localized in yolk cells. The results indicate that the yolk nuclei-dependent gene expression of BmSDH is induced by acclimation to 5 degrees C in diapause eggs.

Affinity labelling of sorbitol dehydrogenase from sheep liver with alpha-bromo-beta-(5-imidazolyl)propionic acid

The metal-directed alkylating agent DL-alpha-bromo-beta-(5- imidazolyl)propionic acid (BrImPpOH) is shown to be an affinity-labelling reagent for sheep liver sorbitol dehydrogenase (SDH). As previously found for horse liver alcohol dehydrogenase (ADH), it modifies a cysteine ligand to the active-site zinc. In this case it is selectively incorporated (over 90%) at Cys43 in each of the four polypeptide chains/protomers of sheep liver SDH. Incorporated reagent and residual activity correlated. The first order inactivation constant, K2, and KEI, the dissociation constant for SDH and BrImPpOH, have been determined at different pH. The reactivity of BrImPpOH for SDH is higher than that for horse liver and yeast ADH. The protection of SDH against BrImPpOH inactivation by buffers and other molecules shows some similarities to that with horse liver ADH. However, sheep liver SDH bound BrImPpOH, imidazole and phosphate ions much weaker than liver ADH. The pKa values from the plot of log (k2/KEI) against pH are approximately 7.0 and 8.8-8.9. The former pKa value probably represents ionization of an imidazole group and the latter the zinc/water ionization in SDH. These pKa values are similar to those found for horse liver ADH. They are apparently not noticeably influenced by a second cysteine ligand in liver ADH being replaced by a proposed glutamic acid residue as a ligand to the catalytic zinc in SDH. The plot of logk2 against pH shows pKa values around 7.0 and 9.2 for the SDH-BrImPpOH-complex. The pKa of 7.0 is the same as for log(k2/KEI), and indicates no significant perturbation due to the binding of BrImPpOH to SDH. The pKa around 9.2 indicates perturbation of the zinc/water ionization or the ionization of Cys43.