Inactivation of carbonyl reductase from human brain by phenylglyoxal and 2,3-butanedione: a comparison with aldehyde reductase and aldose reductase

Reactivity of (Vicinal) Carbonyl Compounds with Urea

Urea removal from dialysate is the major obstacle in realization of a miniature dialysis device, based on continuous dialysate regeneration in a closed loop, used for the treatment of patients suffering from end-stage kidney disease. For the development of a polymeric urea sorbent, capable of removing urea from dialysate with high binding capacities and fast reaction kinetics, a systematic kinetic study was performed on the reactivity of urea with a library of low-molecular-weight carbonyl compounds in phosphate-buffered saline (pH 7.4) at 323 K. It was found that dialdehydes do not react with urea under these conditions but need to be activated under acidic conditions and require aldehyde groups in close proximity to each other to allow a reaction with urea. Among the 31 (hydrated) carbonyl compounds tested, triformylmethane, ninhydrin, and phenylglyoxaldehyde were the most reactive ones with urea. This is attributed to the low dehydration energies of these compounds, as calculated by Gibbs free energy differences between the hydrated and dehydrated carbonyl compounds, which are favorable for the reaction with urea. Therefore, future urea sorbents should contain such functional groups at the highest possible density.

Aldehyde sources, metabolism, molecular toxicity mechanisms, and possible effects on human health

Aldehydes are organic compounds that are widespread in nature. They can be formed endogenously by lipid peroxidation (LPO), carbohydrate or metabolism ascorbate autoxidation, amine oxidases, cytochrome P-450s, or myeloperoxidase-catalyzed metabolic activation. This review compares the reactivity of many aldehydes towards biomolecules particularly macromolecules. Furthermore, it includes not only aldehydes of environmental or occupational concerns but also dietary aldehydes and aldehydes formed endogenously by intermediary metabolism. Drugs that are aldehydes or form reactive aldehyde metabolites that cause side-effect toxicity are also included. The effects of these aldehydes on biological function, their contribution to human diseases, and the role of nucleic acid and protein carbonylation/oxidation in mutagenicity and cytotoxicity mechanisms, respectively, as well as carbonyl signal transduction and gene expression, are reviewed. Aldehyde metabolic activation and detoxication by metabolizing enzymes are also reviewed, as well as the toxicological and anticancer therapeutic effects of metabolizing enzyme inhibitors. The human health risks from clinical and animal research studies are reviewed, including aldehydes as haptens in allergenic hypersensitivity diseases, respiratory allergies, and idiosyncratic drug toxicity; the potential carcinogenic risks of the carbonyl body burden; and the toxic effects of aldehydes in liver disease, embryo toxicity/teratogenicity, diabetes/hypertension, sclerosing peritonitis, cerebral ischemia/neurodegenerative diseases, and other aging-associated diseases.

Coenzyme specificity of human monomeric carbonyl reductase: contribution of Lys-15, Ala-37 and Arg-38

Short-chain dehydrogenases/reductases catalyze the oxidoreduction of alcohol and carbonyl compounds using either NAD or NADPH as coenzyme. Structural analysis suggests that specificity for NADPH is conferred by two highly conserved basic residues in the N-terminal part of the peptide chain, whereas specificity for NAD correlates with the presence of an Asp adjacent to the position of the distal basic residue in NADP-dependent enzymes. We carried out site-directed mutagenesis of the two basic residues: Lys-15 and Arg-38, as well as of Ala-37 of human monomeric carbonyl reductase in order to investigate their contribution to coenzyme binding and specificity. Substitution of Lys-15 or Arg-38 by Gln and, even more pronounced Asp decreased the catalytic efficiency (k(cat)/K(m,NADPH)) by more than three orders of magnitude. Similarly, substitution of Asp for Ala-37 decreased k(cat)/K(m,NADPH) 1000-fold but had little effect on k(cat)/K(m,NADH). The results demonstrate the importance of basic residues at positions 15 and 38 and the absence of an acidic residue at position 37 for NADPH binding and catalysis.

Perspectives for biophysicochemical modifications of enzymes

This article reviews the strategies and successes of modifying enzymes by means of biophysicochemical transformations. By judicious choice of methods, it has been possible to modify enzymes through physical interactions, chemical reactions and/or mutagenesis to alter a very wide range of properties ranging from stability and solubility on the one hand to catalytic activity and selectivity on the other.

Kinetic characteristics of ZENECA ZD5522, a potent inhibitor of human and bovine lens aldose reductase

Aldose reductase (aldehyde reductase 2) catalyses the conversion of glucose to sorbitol, and methylglyoxal to acetol. Treatment with aldose reductase inhibitors (ARIs) is a potential approach to decrease the development of diabetic complications. The sulphonylnitromethanes are a recently discovered class of aldose reductase inhibitors, first exemplified by ICI215918. We now describe enzyme kinetic characterization of a second sulphonylnitromethane, 3',5'-dimethyl-4'-nitromethylsulphonyl-2-(2-tolyl)acetanilide (ZD5522), which is at least 10-fold more potent against bovine lens aldose reductase in vitro and which also has a greater efficacy for reduction of rat nerve sorbitol levels in vivo (ED95 = 2.8 mg kg-1 for ZD5522 and 20 mg kg-1 for ICI 215918). ZD5522 follows pure noncompetitive kinetics against bovine lens aldose reductase when either glucose or methylglyoxal is varied (K(is) = K(ii) = 7.2 and 4.3 nM, respectively). This contrasts with ICI 215918 which is an uncompetitive inhibitor (K(ii) = 100 nM) of bovine lens aldose reductase when glucose is varied. Against human recombinant aldose reductase, ZD5522 displays mixed noncompetitive kinetics with respect to both substrates (K(is) = 41 nM, K(ii) = 8 nM with glucose and K(is) = 52 nM, K(ii) = 3.8 nM with methylglyoxal). This is the first report of the effects of a sulphonylnitromethane on either human aldose reductase or utilization of methylglyoxal. These results are discussed with reference to a Di Iso Ordered Bi Bi mechanism for aldose reductase, where the inhibitors compete with binding of both the aldehyde substrate and alcohol product. This model may explain why aldose reductase inhibitors follow noncompetitive or uncompetitive kinetics with respect to aldehyde substrates, and X-ray crystallography paradoxically locates an ARI within the substrate binding site. Aldehyde reductase (aldehyde reductase 1) is closely related to aldose reductase. Inhibition of bovine kidney aldehyde reductase by ZD5522 follows uncompetitive kinetics with respect to glucuronate (K(ii) = 39 nM), indicating a selectivity greater than 5-fold for bovine aldose reductase relative to aldehyde reductase.

Chemical modification of arginine and lysine residues in coenzyme-binding domain of carbonyl reductase from rabbit kidney: indomethacin affords a significant protection against inactivation of the enzyme by phenylglyoxal

Carbonyl reductase from rabbit kidney was inactivated by phenylglyoxal (PGO) and 2,4,6-trinitrobenzenesulfonate sodium (TNBS). NADP+ protected the enzyme from the inactivations by PGO and TNBS, suggesting that essential arginine and lysine residues are located in coenzyme-binding domain of the enzyme. Judging from the effects of PGO-treated enzymes in the presence and in the absence of NADP+ on the fluorescence intensity of NADPH, one essential arginine residue in coenzyme-binding domain was found to have a role in the binding of NADPH to the enzyme. Indomethacin afforded a significant protection against inactivation of the enzyme by PGO, whereas it could not protect the enzyme from the inactivation by TNBS. It is reasonable to postulate that indomethacin interacts at least in part with or near one essential arginine residue in coenzyme-binding domain of carbonyl reductase from rabbit kidney.

Autocatalytic modification of human carbonyl reductase by 2-oxocarboxylic acids

Carbonyl reductase occurs in multiple molecular forms. Sequence analysis has yielded a carboxyethyllysine residue in one of the enzyme forms, suggesting that pyruvate has been incorporated in a posttranslational enzymatic reaction [Krook, M., Ghosh, D., Strömberg R., Carlquist, M. and Jörnvall, H. (1993) Proc. Natl. Acad. Sci. USA 90, 502-506]. Using highly purified carbonyl reductase from human brain we show that pyruvate and other 2-oxocarboxylic acids are bound to the enzyme in an autocatalytic reaction. The resulting enzyme forms were indistinguishable from the native enzyme forms by electrophoresis and isoelectric focusing.

Inhibition of aldose reductase by (2,6-dimethylphenylsulphonyl)nitromethane: possible implications for the nature of an inhibitor binding site and a cause of biphasic kinetics

Aldose reductase (aldehyde reductase 2, ALR2) is often isolated as a mixture of two forms which are sensitive (ALR2S), or insensitive (ALR2I), to inhibitors. We show that ICI 215918 ((2-6-dimethylphenylsulphonyl)-nitromethane) follows either noncompetitive, or uncompetitive kinetics with respect to aldehyde for ALR2S, or the closely related enzyme, aldehyde reductase (aldehyde reductase 1, ALR1). Similar behaviour is exhibited by two other structural types of aldose reductase inhibitor (ARI), spirohydantoins and acetic acids, when either aldehyde, or NADPH is varied. For ALR2S, we have demonstrated kinetic competition between a sulphonylnitromethane, an acetic acid and a spirohydantoin. Thus, different ARIs probably have overlapping binding sites. Published studies imply that ALR2 follows an ordered mechanism where coenzyme binds first and induces a reversible conformation change (E.NADPH-->E*.NADPH). Reduction of aldehyde appears rate-limited by the step E*.NADP+-->E.NADP+. Spontaneous activation converts ALR2S into ALR2I and increases kcat. This must be associated with acceleration of the rate-determining step. We now propose the following hypothesis to explain characteristics of ARIs. (1) Inhibitors preferentially bind to the E* conformation. (2) The ARI binding site contains residues in common with that for aldehyde substrates. When aldehyde is varied, uncompetitive inhibition arises from association at the site for alcohol product in the E*.NADP+ complex which has little affinity for the substrate. Any competitive inhibition arises from use of the aldehyde site in the E*.NADPH complex. (3) Acceleration of the E*.NADP+-->E.NADP+ step upon activation of ALR2 reduces steady state levels of E* and so decreases sensitivity to ARIs.

Chemical modification of cysteinyl, lysyl and histidyl residues of mouse liver 17 beta-hydroxysteroid dehydrogenase

Monomeric 17 beta-hydroxysteroid dehydrogenase from mouse liver was rapidly inactivated by 5,5'-dithiobis(2-nitrobenzoic acid) and 2,4,6-trinitrobenzene-1-sulfonate, and the absorption spectra of the inactivated enzymes indicated that cysteine and lysine residues were modified. The kinetics of inactivation and spectrophotometric quantification of the modified residues suggested that complete inactivation was caused by modification of two cysteine residues or one lysine residue per active site. The inactivation by the two reagents was protected by NADP+ and some coenzyme analogs, but not by a steroid substrate, testosterone. Moreover, chemical modification by diethyl pyrocarbonate also produced inactivation of the enzyme, and showed a difference spectrum with a peak at 242 nm characteristic of N-carbethoxyhistidine residues, which decreased with the addition of hydroxylamine. The inactivation by this reagent, following pseudo-first-order kinetics, was protected partially by either NADP+ or testosterone and completely in the presence of both the coenzyme and substrate. The results suggest the presence of essential cysteine and lysine residues at or near the coenzyme-binding site and that of essential histidine residue(s) in the catalytic region of the active site of mouse liver 17 beta-hydroxysteroid dehydrogenase.

Human liver aldehyde reductase: pH dependence of steady-state kinetic parameters

The pH dependence of steady-state parameters for aldehyde reduction and alcohol oxidation were determined in the human liver aldehyde reductase reaction. The maximum velocity of aldehyde reduction with NADPH or 3-acetyl pyridine adenine dinucleotide phosphate (3-APADPH) was pH independent at low pH but decreased at high pH with a pK of 8.9-9.6. The V/K for both nucleotides decreased below a pK of 5.7-6.2, as did the pKi of competitive inhibitors NADP and ATP-ribose, suggesting that the 2'-phosphate of the nucleotide has to be deprotonated for binding to the enzyme. The pK of the 2'-phosphate of NADPH appears to be perturbed in the ternary complexes to 5.2-5.4. The V/K for NADPH, the V/K for 3-APADPH, and the pKi of ATP-ribose also decreased above a pK of 9-10, suggesting interaction of the 2'-phosphate of the nucleotide with a protonated base, perhaps lysine. Since protonation of a residue with a pK of 8 (evident in V/K for DL-glyceraldehyde and V/K for L-gulonate versus pH profiles) appears to be essential for aldehyde reduction, and deprotonation for alcohol oxidation, this residue appears to act as a general acid-base catalyst. An additional anion binding site with a pK of 9.94 facilitates the binding of carboxylic substrates such as D-glucuronate. With NADPH as the coenzyme the primary deuterium isotope effects on V and V/K for NADPH were close to unity and pH independent, suggesting that the hydride transfer step is not rate determining over the experimental pH range. With 3-APADPH as the coenzyme, the maximum velocity, relative to NADPH was three- to four-fold lower. Isotope effects on V, V/K for 3-APADPH, and V/K for D-glucuronate were pH independent and equal to 2.2-2.8, indicating that the chemical step of the reaction is relatively insensitive to pH. These data suggest that substrates bind to both the protonated and the deprotonated forms of the enzyme, though only the protonated enzyme catalyzes aldehyde reduction and the deprotonated enzyme catalyzes alcohol oxidation. On the basis of these results a scheme for the chemical mechanism of aldehyde reductase is postulated.

Functional cysteinyl residues in human placental aldose reductase

Incubation of human placental aldose reductase (EC 1.1.1.21) with the sulfhydryl oxidizing reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM) results in a biexponential loss of catalytic activity. Inactivation by DTNB or NEM is prevented by saturating concentrations of NADPH. ATP-ribose offers partial protection against inactivation by DTNB, whereas NADP, nicotinamide mononucleotide (NMN), and the substrates glyceraldehyde and glucose offer little or no protection. The inactivation by DTNB was reversed by dithiothreitol and partially by 2-mercaptoethanol but not by KCN. When the release of 2-nitro-5-mercaptobenzoic acid was measured, 3 mol of sulfhydryl residues was found to be modified per mole of the enzyme by DTNB. Correlation of the fractional activity remaining with the extent of modification by the statistical method of C.-L. Tsou (1962, Sci. Sin. 11, 1535-1558) indicates that of the three reactive residues, one reacts at a faster rate than the other two, and that two residues are essential for the catalytic activity of the enzyme. Labeling of the total sulfhydryl by [14C]NEM and quantification of DTNB-reactive residues in the enzyme denatured by 6 M urea indicates that a total of seven sulfhydryl residues are present in the protein. The modification of the enzyme did not affect Km glyceraldehyde, but the modified enzyme had a lower Km NADPH. Kinetic analysis of the data suggests that a biexponential nature of inactivation could be due to the formation of a dissociable E:DTNB complex and the presence of a partially active enzyme species.

Lens aldo-keto reductase of Camelus dromedarius: purification and properties

Aldo-keto reductase has been purified 13,000-fold from the lens of the camel (Camelus dromedarius) to a specific activity of 85 U/mg protein. The enzyme is a monomeric protein, exhibiting a Mr = 40,000 upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. Camel lens aldo-keto reductase shows a broad substrate specificity, which is strictly dependent on NADPH, and is insensitive to inhibition by Sorbinil and valproate. Aldoses with a carbon chain with more than four residues, as well as glucuronate, are not reduced by the enzyme. On the basis of substrate specificity and sensitivity to inhibition, camel lens aldo-keto reductase appears to be distinct from the so far described aldose, aldehyde and carbonyl reductases.