Novel NADPH-binding domain revealed by the crystal structure of aldose reductase

Three-dimensional model of the alpha-subunit of bacterial luciferase

The predicted secondary structure of both subunits of bacterial luciferase is in accordance with a regular 8-fold alpha/beta-barrel structure. The 3D profile confirmed that luciferase subunits are compatible with the alpha/beta-barrel despite the absence of sequence similarity with any alpha/beta-barrel protein. The three-dimensional structure of 260 residues of the alpha-chain of luciferase was modeled from coordinates of glycolate oxidase and then energy minimized. The model obtained satisfies the criteria for the structure of a globular protein and is in accordance with known experimental data. From the model it is possible to predict active site residues involved in binding and catalysis. These predictions, and thus also the model, can be tested by protein engineering experiments.

Mechanism of human aldehyde reductase: characterization of the active site pocket

Human aldehyde reductase is a NADPH-dependent aldo-keto reductase that is closely related (65% identity) to aldose reductase, an enzyme involved in the pathogenesis of some diabetic and galactosemic complications. In aldose reductase, the active site residue Tyr48 is the proton donor in a hydrogen-bonding network involving residues Asp43/Lys77, while His110 directs the orientation of substrates in the active site pocket. Mutation of the homologous Tyr49 to phenylalamine or histidine (Y49F or Y49H) and of Lys79 to methionine (K79M) in aldehyde reductase yields inactive enzymes, indicating similar roles for these residues in the catalytic mechanism of aldehyde reductase. A H112Q mutant aldehyde reductase exhibited a substantial decrease in catalytic efficiency (kcat/Km) for hydrophilic (average 150-fold) and aromatic substrates (average 4200-fold) and 50-fold higher IC50 values for a variety of inhibitors than that of the wild-type enzyme. The data suggest that His112 plays a major role in determining the substrate specificity of aldehyde reductase, similar to that shown earlier for the homologous His110 in aldose reductase [Bohren, K. M., et. al. (1994) Biochemistry 33, 2021-2032]. Mutation of Ile298 or Val299 affected the kinetic parameters to a much lesser degree. Unlike native aldose reductase, which contains a thiol-sensitive Cys298, neither the I298C or V299C mutant exhibited any thiol sensitivity, suggesting a geometry of the active site pocket different from that in aldose reductase. Also different from aldose reductase, the detection of a significant primary deuterium isotope effect on kcat (1.48 +/- 0.02) shows that nucleotide exchange is only partially rate-limiting. Primary substrate and solvent deuterium isotope effects on the H112Q mutant suggest that hydride and proton transfers occur in two discrete steps with hydride transfer taking place first. Dissociation constants and spectroscopic and fluorimetric properties of nucleotide complexes with various mutants suggest that, in addition to Tyr49 and His112, Lys79 plays a hitherto unappreciated role in nucleotide binding. The mode of inhibition of aldehyde reductase by aldose reductase inhibitors (ARIs) is generally similar to that of aldose reductase and involves binding to the E:NADP+ complex, as shown by kinetic and direct inhibitor-binding experiments. The order of ARI potency was AL1576 (Ki = 60 nM) > tolrestat > ponalrestat > sorbinil > FK366 > zopolrestat > alrestatin (Ki = 148 microM). Our data on aldehyde reductase suggest that the active site pocket significantly differs from that of aldose reductase, possibly due to the participation of the C-terminal loop in its formation.

All in the family

Identification of the residues involved in the reaction catalysed by aldehyde reductase should aid in the development of drugs for the treatment of diabetic complications.

The role of cysteine in the alteration of bovine liver dihydrodiol dehydrogenase 3 activity

Bovine liver NADP(+)-dependent dihydrodiol dehydrogenase (DD3) is extremely sensitive to SH reagents such as N-ethylmaleimide (NEM) and 5,5'-dithiobis(2-nitrobenzoic acid). NEM produced time- and concentration-dependent inactivation of DD3 in a pseudo-first-order reaction manner. This inactivation was prevented by NADP+, 3-acetylpyridine-adenine dinucleotide phosphate, 2',5'-ADP and 2'-AMP but not by substrates, NAD+, nicotinamide mononucleotide or 5'-ADP.DD3 was absorbed by an affinity column of thiopropyl-Sepharose 6B, but enzyme incubated with both NEM and NADP+ was not. Moreover, one [14C]NEM molecule was incorporated into a cysteine of DD3 in the presence, and two cysteines of DD3 in the absence, of NADP+. These results suggested that two cysteine residues were modified per enzyme molecule by NEM, one was protected by NADP+ and the other had no significant function for the enzyme activity. Two radiolabelled peptides (P1 and P2) produced by the digestion with lysyl endopeptidase of [14C]NEM-modified DD3 could be separated by reverse-phase HPLC. P1, which was radiolabelled by [14C]NEM only in the absence of NADP+, showed the following sequence; H2N-Tyr-Lys-Pro-Val-Xaa-Asn-Gln-Val-Glu- NEM.Cys-His-Pro-Tyr-Phe-Asn-Gln-Ser-Lys-COOH (Xaa indicates a possible cysteine residue). This sequence was very similar to that of rat liver 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase (3 alpha-HSD/DD) (residues 184 to 201) and was also highly conserved in the aldo-keto reductase superfamily. The sequence of P2, which had radioactivity in both the absence and presence of NADP+, also contained an NEM-modified cysteine and was similar in sequence to the regions located in loop A of rat 3 alpha-HSD/DD. The present study suggests that P1, which may have a cysteine residue corresponding to Cys-193 of rat 3 alpha-HSD/DD, functions in the alteration of DD3 activity depending on the modulation of NADP(+)-binding ability through a thiol/disulphide exchange reaction similar to that of rat 3 alpha-HSD/DD shown in our previous results; while P2, which may have a cysteine residue corresponding to Cys-145 of rat 3 alpha-HSD/DD, may be located near the surface of the enzyme molecule.

Structure of porcine aldehyde reductase holoenzyme

Aldehyde reductase, a member of the aldo-keto reductase superfamily, catalyzes the NADPH-dependent reduction of a variety of aldehydes to their corresponding alcohols. The structure of porcine aldehyde reductase-NADPH binary complex has been determined by x-ray diffraction methods and refined to a crystallographic R-factor of 0.20 at 2.4 A resolution. The tertiary structure of aldehyde reductase is similar to that of aldose reductase and consists of an alpha/beta-barrel with the active site located at the carboxy terminus of the strands of the barrel. Unlike aldose reductase, the N epsilon 2 of the imidazole ring of His 113 in aldehyde reductase interacts, through a hydrogen bond, with the amide group of the nicotinamide ring of NADPH.

A potassium channel beta subunit related to the aldo-keto reductase superfamily is encoded by the Drosophila hyperkinetic locus

Genetic and physiological studies of the Drosophila Hyperkinetic (Hk) mutant revealed defects in the function or regulation of K+ channels encoded by the Shaker (Sh) locus. The Hk polypeptide, determined from analysis of cDNA clones, is a homologue of mammalian K+ channel beta subunits (Kv beta). Coexpression of Hk with Sh in Xenopus oocytes increases current amplitudes and changes the voltage dependence and kinetics of activation and inactivation, consistent with predicted functions of Hk in vivo. Sequence alignments show that Hk, together with mammalian Kv beta, represents an additional branch of the aldo-keto reductase superfamily. These results are relevant to understanding the function and evolutionary origin of Kv beta.

Substrate specificity of human aldose reductase: identification of 4-hydroxynonenal as an endogenous substrate

Aldose reductase, which catalyzes the reduction of glucose to sorbitol as part of the polyol pathway, has been implicated in the development of diabetic complications and is a prime target for drug development. However, aldose reductase exhibits broad specificity for both hydrophilic and hydrophobic aldehydes, which suggests that aldose reductase may also be a detoxification enzyme. Several series of structurally related aldehydes were compared as substrates in order to deduce the structural features that result in low Michaelis constants. Aldehydes that contain an aromatic ring are generally excellent substrates, consistent with crystallographic data which suggest that aldose reductase possesses a large hydrophobic substrate binding site. However, there is little discrimination among different aromatic aldehydes. In addition, small hydrophilic aldehydes exhibit low Km values if the alpha-carbon is oxidized. Analysis of the binding of NADPH by fluorescence quenching techniques indicates that aldose reductase exhibits higher affinity for NADPH than NADP, suggesting that this enzyme is normally primed for reductive metabolism. Thus aldose reductase appears to have evolved to catalyze the reduction of a very broad range of aldehydes. Structural features of substrates that bind to aldose reductase with low Km values were used to identify potential endogenous substrates. 4-Hydroxynonenal, a reactive alpha-beta unsaturated aldehyde produced during oxidative stress, is an excellent substrate (Km = 22 microM, kcat/Km = 4.6 x 10(6) M-1 min-1). Reductive metabolism of endogenous aldehydes in addition to glucose, catalyzed by aldose reductase, may play an important role in the development of diabetic complications.

Catalysis of reduction of carbohydrate 2-oxoaldehydes (osones) by mammalian aldose reductase and aldehyde reductase

In mammalian tissues, carbohydrate 2-oxoaldehydes, or 'osones', formed by cleavage of carbohydrate residues from glycated proteins, cause damage to cells and tissues by cross-linking of proteins. In the substrate specificity study reported here, we show that several osones are relatively good substrates for the reduced, unactivated form of aldose reductase (EC 1.1.1.21) from human and pig muscle, and aldehyde reductase (EC 1.1.1.2) from pig kidney, enzymes that have been well characterised both structurally and mechanistically. Since these enzymes are relatively ubiquitous, they may serve to protect a large number of tissues from damage, by catalysing the reduction of locally-produced osones. Reduction of all substrates by aldehyde reductase obeyed Michaelis-Menten kinetics. In contrast, a Hill constant of about 0.5 was obtained for aldose reductase-catalysed reduction of each of the carbohydrate 2-oxoaldehydes, and for several other substrates that were examined. Although this deviation from Michaelis-Menten kinetics has been ascribed to the presence of two forms of the enzyme, activated and unactivated, our results suggest that it is a characteristic of the unactivated form.

Role of lysine residues in the nucleotides binding to bovine liver high-Km aldehyde reductase

The inactivation of bovine liver high-Km aldehyde reductase (ALR) by heat (47 degrees C), 0.3 mM 2,4,6-trinitrobenzene sulfonate (TNBS) and 0.03 mM pyridoxal 5'-phosphate (PAL-P) followed pseudo-first-order kinetic as a function of incubation-time and concentration of TNBS. Nucleotides which have a 2'-phosphate group, especially beta-NADPH and beta-NADP+, showed effective protection on ALR-inactivation. However, typical substrates for ALR such as D,L-glyceraldehyde, D-erythrose, D-glucuronate and p-carboxybenzaldehyde could not protect the enzyme from inactivation. Completely inactivated enzyme was estimated to have 2.07 TNBS-modified lysine residues/mol enzyme from the determination of free amino group using fluorescamine (Ex = 390 nm, Em = 475 nm). Enzyme protected by beta-NADP+ (96.5% remaining activity) did not lose a significant number of lysine residues. Kd-values for beta-NADPH and beta-NADP+ were estimated to be 0.48 microM and 4.7 microM, respectively and TNBS-treated enzyme lost its ability to bind to these nucleotides.

Residues affecting the catalysis and inhibition of rat lens aldose reductase

Aldose reductase (AR), the first enzyme of the polyol pathway, has been implicated in diabetic complications. Results of recent clinical studies have shown that compounds that inhibit aldose reductase (ARIs) and block the flux of glucose through the polyol pathway have provided benefit to diabetic neuropathic patients. Since many ARIs show broad substrate specificity, emphasis on the structure-function properties of the AR enzyme will help in the refinement and design of future inhibitors. To this end, catalysis and inhibition of rat lens aldose reductase was examined following site-directed mutagenesis. Replacement of tyrosine 48 with phenylalanine (Y48F) resulted in an enzyme form with less than 0.25% activity with DL-glyceraldehyde and no detectable activity with p-nitrobenzaldehyde or xylose, although circular dichroism spectra and NADPH binding affinity were similar to wild-type AR. Mutation of histidine 110 to glutamine (H110Q) also resulted in a less active protein with an approximate 3-fold decrease in kcat for the reduction of DL-glyceraldehyde; slight or no activity was measured with other substrates and an increase of 195-fold over wild type was observed in the Km for glyceraldehyde. H110Q was less sensitive to inhibition by aldose reductase inhibitors. The most dramatic change was seen with imeristat, which showed an 1800-fold increase in IC50. Mutation of cysteine 298 to serine (C298S) affected enzyme function by increasing kcat 2- to 4-fold and increasing Km 15- to 48-fold, with DL-glyceraldehyde, p-nitrobenzaldehyde or xylose as substrates. As a result kcat/Km, catalytic efficiency, dropped to approx. 10% of control. Inhibition of C298S was not noticeably different from wild type. Substitution of histidine 187 or 200 with glutamine (H187Q, H200Q) had little effect on AR catalysis or inhibition. Based on structural and mutagenesis studies of human AR and the conservation of amino acids between human and rat, these data would indicate that Y48, H110, and C298 are important residues in the active site of rat AR and that Y48 is most likely the proton donor during substrate reduction by rat lens aldose reductase. In addition, these studies indicate that mutagenesis of H110 also affects aldose reductase inhibition.

Molecular cloning of two human liver 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase isoenzymes that are identical with chlordecone reductase and bile-acid binder

Human liver contains two dihydrodiol dehydrogenases, DD2 and DD4, associated with 3 alpha-hydroxysteroid dehydrogenase activity. We have raised polyclonal antibodies that cross-reacted with the two enzymes and isolated two 1.2 kb cDNA clones (C9 and C11) for the two enzymes from a human liver cDNA library using the antibodies. The clones of C9 and C11 contained coding sequences corresponding to 306 and 321 amino acid residues respectively, but lacked 5'-coding regions around the initiation codon. Sequence analyses of several peptides obtained by enzymic and chemical cleavages of the two purified enzymes verified that the C9 and C11 clones encoded DD2 and DD4 respectively, and further indicated that the sequence of DD2 had at least additional 16 residues upward from the N-terminal sequence deduced from the cDNA. There was 82% amino acid sequence identity between the two enzymes, indicating that the enzymes are genetic isoenzymes. A computer-based comparison of the cDNAs of the isoenzymes with the DNA sequence database revealed that the nucleotide and amino acid sequences of DD2 and DD4 are virtually identical with those of human bile-acid binder and human chlordecone reductase cDNAs respectively.

Sequence of pig lens aldose reductase and electrospray mass spectrometry of non-covalent and covalent complexes

The complete sequence of pig lens aldose reductase (EC 1.1.1.21), a member of the nicotinamide coenzyme-dependent aldo-keto reductase super family, was determined by the combined use of data obtained from Edman degradation, fast-atom-bombardment mass spectrometry and electrospray mass spectrometry. The N-terminal residue of human and pig aldose reductase was shown to be acetylated. The assignment of a disulfide bridge (Cys298-Cys303) was obtained by mass spectrometry. Electrospray mass spectrometry has been used for molecular mass measurement of human muscle (35758 +/- 7 Da) and pig lens (35778 +/- 3Da) aldose reductase; using mild ionization conditions, it has also been used to study the reversible interaction involved in a non-covalent complex with NADP+ (36527 +/- 4Da). An alkylating analog of NADP+ (3-chloroacetylpyridine-adenine dinucleotide phosphate) was used as an irreversible inhibitor to investigate the NADP binding site and the mass of the covalent complex was measured (36521 +/- 3 Da).

An ethoxyquin-inducible aldehyde reductase from rat liver that metabolizes aflatoxin B1 defines a subfamily of aldo-keto reductases

Protection of liver against the toxic and carcinogenic effects of aflatoxin B1 (AFB1) can be achieved through the induction of detoxification enzymes by chemoprotectors such as the phenolic antioxidant ethoxyquin. We have cloned and sequenced a cDNA encoding an aldehyde reductase (AFB1-AR), which is expressed in rat liver in response to dietary ethoxyquin. Expression of the cDNA in Escherichia coli and purification of the recombinant enzyme reveals that the protein exhibits aldehyde reductase activity and is capable of converting the protein-binding dialdehyde form of AFB1-dihydrodiol to the nonbinding dialcohol metabolite. We show that the mRNA encoding this enzyme is markedly elevated in the liver of rats fed an ethoxyquin-containing diet, correlating with acquisition of resistance to AFB1. AFB1-AR represents the only carcinogen-metabolizing aldehyde reductase identified to date that is induced by a chemoprotector. Alignment of the amino acid sequence of AFB1-AR with other known and putative aldehyde reductases shows that it defines a subfamily within the aldo-keto reductase superfamily.

Refined 1.8 A structure of human aldose reductase complexed with the potent inhibitor zopolrestat

As the action of aldose reductase (EC 1.1.1.21) is believed to be linked to the pathogenesis of diabetic complications affecting the nervous, renal, and visual systems, the development of therapeutic agents has attracted intense effort. We report the refined 1.8 A x-ray structure of the human holoenzyme complexed with zopolrestat, one of the most potent noncompetitive inhibitors. The zopolrestat fits snugly in the hydrophobic active site pocket and induces a hinge-flap motion of two peptide segments that closes the pocket. Excellent complementarity and affinity are achieved on inhibitor binding by the formation of 110 contacts (< or = 4 A) with 15 residues (10 hydrophobic), 13 with the NADPH coenzyme and 9 with four water molecules. The structure is key to understanding the mode of action of this class of inhibitors and for rational design of better therapeutics.

Aldehyde dehydrogenases: widespread structural and functional diversity within a shared framework

Sequences of 16 NAD and/or NADP-linked aldehyde oxidoreductases are aligned, including representative examples of all aldehyde dehydrogenase forms with wide substrate preferences as well as additional types with distinct specificities for certain metabolic aldehyde intermediates, particularly semialdehydes, yielding pairwise identities from 15 to 83%. Eleven of 23 invariant residues are glycine and three are proline, indicating evolutionary restraint against alteration of peptide chain-bending points. Additionally, another 66 positions show high conservation of residue type, mostly hydrophobic residues. Ten of these occur in predicted beta-strands, suggesting important interior-packing interactions. A single invariant cysteine residue is found, further supporting its catalytic role. A previously identified essential glutamic acid residue is conserved in all but methyl malonyl semialdehyde dehydrogenase, which may relate to formation by that enzyme of a CoA ester as a product rather than a free carboxylate species. Earlier, similarity to a GXGXXG segment expected in the NAD-binding site was noted from alignments with fewer sequences. The same region continues to be indicated, although now only the first glycine residue is strictly conserved and the second (usually threonine) is not present at all, suggesting greater variance in coenzyme-binding interactions.

Evolution of parallel beta/alpha-barrel enzyme family lightened by structural data on starch-processing enzymes

The parallel beta/alpha-barrel domain consisting of eight parallel beta-sheets surrounded by eight alpha-helices has been currently identified in crystal structures of more than 20 enzymes. This type of protein folding motif makes it possible to catalyze various biochemical reactions on a variety of substrates (i.e., it seems to be robust enough so that different enzymatic functionalities could be designed on it). In spite of many efforts aimed at elucidation of evolutionary history of the present-day beta/alpha-barrels, a challenging question remains unanswered: How has the parallel beta/alpha-barrel fold arisen? Although the complete sequence comparison of all beta/alpha-barrel amino acid sequences is not yet available, several sequence similarities have been revealed by using the highly conserved regions of alpha-amylase as structural templates. Since many starch-processing enzymes adopt the parallel beta/alpha-barrel structure these enzymes might be useful in the search for evolutionary relationships of the whole parallel eight-folded beta/alpha-barrel enzyme family.

Structure of isocitrate dehydrogenase with isocitrate, nicotinamide adenine dinucleotide phosphate, and calcium at 2.5-A resolution: a pseudo-Michaelis ternary complex

The structure of isocitrate dehydrogenase (IDH) with a bound complex of isocitrate, NADP+, and Ca2+ was solved at 2.5-A resolution and compared by difference mapping against previously determined enzymatic complexes. Calcium replaces magnesium in the binding of metal-substrate chelate complex, resulting in a substantially reduced turnover rate. The structure shows the following: (i) A complete, structurally ordered ternary complex (enzyme, isocitrate, NADP+, and Ca2+) is observed in the active site, with the nicotinamide ring of NADP+ exhibiting a specific salt bridge with isocitrate. The binding of the cofactor nicotinamide ring is dependent on this interaction. (ii) Isocitrate is bound by the enzyme with the same interactions as those found for the magnesium/substrate binary complex, but the entire molecule is shifted in the active site by approximately 1 A in order to accommodate the larger metal species and to interact with the nicotinamide ring. The distances from isocitrate to the bound calcium are substantially longer than those previously found with magnesium. (iii) NADP in the Escherichia coli IDH has a novel binding site and conformation as compared to previously solved dehydrogenases. (iv) The orientation and interactions of the nicotinamide ring with the substrate are consistent with the stereospecificity of the enzyme-catalyzed reaction.

Identification of the reactive cysteine residue in human placenta aldose reductase

Modification of human placental aldose reductase by iodoacetate (IAA) led to a mol/mol binding of IAA, a 40% decrease in the kcat, a 3-5-fold increase in the Km,NADPH and Km,glyceraldehyde and a 600-fold increase in the Ki,sorbinil; determined at pH 6.0. NADPH and 2'-monophosphoadenosine 5'-diphosphoribose but neither glyceraldehyde nor sorbinil, prevented carboxymethylation-induced changes. Cleavage of [14C]IAA-modified enzyme by trypsin resulted in two radiolabeled peptides: Val-297 to Lys-307 and Val-297 to Phe-315. In both these peptides Cys-298 was the only radiolabeled residue. It is suggested that Cys-298 regulates the kinetic and inhibition properties of the enzyme, but does not participate in catalysis.

Nucleotide sequence and over-expression of morphine dehydrogenase, a plasmid-encoded gene from Pseudomonas putida M10

Pseudomonas putida M10 was originally isolated from factory waste liquors by selection for growth on morphine. The NADP(+)-dependent morphine dehydrogenase that initiates morphine catabolism is encoded by a large plasmid of 165 kb. Treatment of P. putida M10 with ethidium bromide led to the isolation of a putative plasmid-free strain that was incapable of growth on morphine. The structural gene for morphine dehydrogenase, morA, has been located on the plasmid by oligonucleotide hybridization, by coupled transcription-translation of cloned restriction fragments and by nucleotide sequence analysis and is contained within a 1.7 kb SphI fragment that has been cloned into Escherichia coli. The cloned dehydrogenase enzyme is expressed at high levels in E. coli resulting in a 65-fold increase in morphine dehydrogenase activity in cell-free extracts compared with P. putida M10. Morphine dehydrogenase was rapidly purified to homogeneity, as judged by SDS/PAGE, by a one-step affinity chromatography procedure on Mimetic Orange 3 A6XL. The properties of the purified enzyme were identical with those previously reported for P. putida M10 morphine dehydrogenase. The morA gene was sequenced and the deduced amino acid sequence confirmed by N-terminal amino acid sequencing of the over-expressed protein. The predicted amino acid sequence of morA, deduced from the nucleotide sequence, indicated that morphine dehydrogenase did not belong to the non-metal-requiring short-chain class of dehydrogenases, but was more closely related to the aldo-ketoreductases.

Chemical modification of an arginine residue in aldose reductase is enhanced by coenzyme binding: further evidence for conformational change during the reaction mechanism

Chemical modification of pig muscle aldose reductase (ALR2) with the arginine specific reagent phenylglyoxal resulted in the inactivation of the enzyme. This inactivation exhibited pseudo-first order kinetics typical of active-site directed chemical modification. Inactivation of ALR2 by [7-C14] phenylglyoxal in the absence of NADPH or NADP+ followed by tryptic digestion resulted in the isolation by HPLC of one major and one minor radioactive peptide. Protein sequencing revealed that the major peptide contained a modified arg268, a residue located in the coenzyme binding site. The minor radioactive peptide and the single radioactive peptide isolated from ALR2 inactivated in the presence of NADP+ contained chemically modified arg293. The arginine residue modified at the active site is positioned to bind the 2'-OH phosphate group of the ribose sugar of the adenine moiety of NADP+. Arg293 is present on the C-terminal loop of ALR2. The enhancement of Arg293 modification by phenylglyoxal in the presence of NADP+ indicates that this C-terminal loop may be involved in the slow conformational change that occurs during the reaction sequence upon coenzyme binding.

Carbonyl-metabolizing enzymes and their relatives recruited as structural proteins in the eye lens

The refractive properties of the eye lens are determined by abundant soluble structural proteins known as crystallins. While some crystallins are common to most vertebrates, others are abundant only in groups of related species. These taxon-specific crystallins all turn out to be enzymes, apparently recruited by modification of gene expression without prior gene duplication. They include eta-crystallin, accounting for up to 25% of protein in elephant shrew lenses and apparently identical to cytoplasmic aldehyde dehydrogenase; rho-crystallin from frog lenses, a member of the same superfamily as aldose and aldehyde reductases; and zeta-crystallin, found in guinea pig and camel lenses, which is structurally related to alcohol dehydrogenase (ADH). Unlike ADH, zeta-crystallin requires NADPH rather than NAD+/NADH as cofactor. Molecular modelling of zeta-crystallin shows that amino-acid changes around the co-factor binding site are responsible for this change in affinity. Purified guinea pig lens zeta-crystallin has a substrate preference for orthoquinones which are reduced by a single electron transfer mechanism. cDNA sequencing of zeta-crystallin suggests that the expression in lens as a crystallin depends on a different gene promoter from that used predominantly in liver. The putative guinea pig zeta-crystallin lens promoter has now been assayed for function in transfection studies. Elements with positive and negative effects on transcription, at least one of which has tissue preferred function, have been defined. When introduced into transgenic mice this promoter exhibits tissue-specific expression in the lens. This is the first identification of a lens-specific, alternative promoter in an enzyme crystallin gene.

Does sorbinil bind to the substrate binding site of aldose reductase?

With benzyl alcohol as the varied substrate, sorbinil was found to be a competitive inhibitor of aldose reductase, an enzyme implicated in the etiology of secondary diabetic complications. The K(is sorbinil) and the Vmax/Km (V/K) benzyl alcohol decreased at low pH with a pK of 7.5 and 7.7, respectively. These observations suggest that both sorbinil and benzyl alcohol bind to the same site on the enzyme. Active site inhibition by sorbinil is consistent with non-competitive inhibition patterns of sorbinil with nucleotide coenzyme or aldehyde as the varied substrate in the direction of aldehyde reduction.

Evaluation of the sequence template method for protein structure prediction. Discrimination of the (beta/alpha)8-barrel fold

A multiple alignment of five (beta/alpha)8-barrel enzymes has been derived from their structure. The eight beta-strands and eight alpha-helices of the (beta/alpha)8-barrel are correctly aligned and the equivalenced residues in these regions fulfil similar structural roles. Each beta-strand has a central core of usually four residues, two residues contribute side-chains to the barrel core and the other two residues are involved in beta-strand/alpha-helix contacts. However, the fold imposes no constraints on the volumes of the residues at either a local or global level: the volume of the beta-barrel core varies between 1088 A3 in glycolate oxidase and 1571 A3 in taka-amylase. Sequence motifs derived from the multiple alignment were scanned against a database of 124 protein sequences, including 17 (beta/alpha)8-barrel enzymes. The results were evaluated in terms of the discrimination of (beta/alpha)8-barrel sequences and the quality of the alignments obtained. One motif was able to identify the top 12% of high scoring sequences as forming (beta/alpha)8-barrels with 50% accuracy and the bottom 50% of sequences as not being (beta/alpha)8-barrel proteins with 100% accuracy. However, in most instances the alignments were poor. The reasons for this are discussed with reference to the (beta/alpha)8-barrel proteins and the sequence motif method in general.

Purification and electrospray mass spectrometry of aldose reductase from pig lens

Aldose reductase (alditol: NADP+ 1-oxidoreductase, EC 1.1.1.21) has been purified from pig lens to homogeneity by a rapid and efficient three-step procedure involving poly(ethylene glycol) fractionation, ion-exchange chromatography and chromatofocusing. The homogeneity of the purified enzyme was examined by polyacrylamide gel electrophoresis under native and denaturing conditions, by isoelectric focusing and by high-performance liquid chromatography on a size-exclusion column. The highly purified enzyme is a monomeric protein with a molecular mass of 35,775 +/- 3 Da as determined by electrospray mass spectrometry (ESMS). This purification procedure is particularly suited for the preparation of triclinic single crystals of pig lens aldose reductase, which are currently used in X-ray studies of this enzyme.