The kinetic mechanism of human placental aldose reductase and aldehyde reductase II

Pyridine nucleotide redox potential in coronary smooth muscle couples myocardial blood flow to cardiac metabolism

Adequate oxygen delivery to the heart during stress is essential for sustaining cardiac function. Acute increases in myocardial oxygen demand evoke coronary vasodilation and enhance perfusion via functional upregulation of smooth muscle voltage-gated K⁺ (Kv) channels. Because this response is controlled by Kv1 accessory subunits (i.e., Kvβ), which are NAD(P)(H)-dependent aldo-keto reductases, we tested the hypothesis that oxygen demand modifies arterial [NAD(H)]_i, and that resultant cytosolic pyridine nucleotide redox state influences Kv1 activity. High-resolution imaging mass spectrometry and live-cell imaging reveal cardiac workload-dependent increases in NADH:NAD⁺ in intramyocardial arterial myocytes. Intracellular NAD(P)(H) redox ratios reflecting elevated oxygen demand potentiate native coronary Kv1 activity in a Kvβ2-dependent manner. Ablation of Kvβ2 catalysis suppresses redox-dependent increases in Kv1 activity, vasodilation, and the relationship between cardiac workload and myocardial blood flow. Collectively, this work suggests that the pyridine nucleotide sensitivity and enzymatic activity of Kvβ2 controls coronary vasoreactivity and myocardial blood flow during metabolic stress.

Physiological matching of blood flow to the demand for oxygen by the heart is required for sustained cardiac health, yet the underlying mechanisms are obscure. Here, the authors report a key role for acute modifications to the redox state of intracellular pyridine nucleotides in coronary smooth muscle and their impact on voltage-gated K + channels in metabolic vasodilation

Perspectives of data analysis of enzyme inhibition and activation, Part 3: Equations for calculation of the initial rates of enzymatic reactions

Equations for calculation of the initial rates of activated enzymatic reactions and the inhibited enzymatic reactions, unavailable in experimental enzymology, were obtained. Examples of practically using of these equations are given.

Identification, characterization of levoglucosan kinase, and cloning and expression of levoglucosan kinase cDNA from Aspergillus niger CBX-209 in Escherichia coli

The first enzyme responsible for assimilating levoglucosan in Aspergillus niger CBX-209 was corroborated to be levoglucosan kinase that catalyzes the transfer of a phosphate group from ATP to levoglucosan to yield a glucose 6-phosphate in the presence of magnesium ion and ATP by FAB-mass spectrometric method combined with previous observations from HPLC and enzymological experiments. Levoglucosan kinase was purified to apparent homogeneity by using a combination of seven purification steps. SDS-PAGE revealed a single protein band of 56 KDa. It is a monomeric enzyme and maximal enzyme activity was measured at pH 9.3 and 30 degrees C. This kinase is stable below 20 degrees C at a quite broad pHs ranging from 6 to 10 and levoglucosan could protect the enzyme from thermal inactivation. Exclusive substrate specificity for levoglucosan suggested that not only the structure of the intramolecular glucosidic linkage but also the configuration of the pyranose frame would be specific for recognition by levoglucosan kinase. The K(m) values of this enzyme were 71.2mM for levoglucosan and 0.25 mM for ATP, determined by double reciprocal plottings and ADP inhibited on the enzyme activity competitively with a Ki value of 0.20mM. A cDNA library from A. niger was constructed in Escherichia coli DH5alpha. The library was screened for levoglucosan kinase gene on NCE selective medium and three positive recombinants were selected after a five day culture. Detection of activities of levoglucosan kinase in the cell extracts indicated that levoglucosan kinase gene (lgk) was expressed by the recombinant strain of E. coli DH5alpha.

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.

Human placental aldose reductase: role of Cys-298 in substrate and inhibitor binding

Steady-state kinetic and inhibition properties of human placental aldose reductase carboxymethylated at Cys-298 were investigated. A comparison of the primary deuterium kinetic isotope effect on the reduced and the carboxymethylated enzymes suggests that carboxymethylation did not affect the reaction sequence of substrate binding and release. Values of DV/KD-glyceraldehyde greater than DV suggest that steps in the reaction scheme subsequent to hydride transfer, particularly the release of NADP may be rate limiting. Carboxymethylation of Cys-298 was also found to affect NADPH and aldehyde binding to the enzyme. Carboxymethylation had little effect on the secondary structure of the enzyme, but a comparison of the circular dichroic spectra of the reduced and carboxymethylated enzyme, suggests a weakened interaction between the nicotinamide and 2'-monophosphoadenosine 5'-diphosphoribose of NADPH, and the carboxymethylated enzyme. Interaction between Cys-298 and NADPH appears to determine the rate of isomerization of the E:NADP binary complex and carboxymethylation-induced decrease in kcat may be due to slower isomerization of the E:NADP binary complex. The carboxymethylated enzyme was less sensitive than the reduced enzyme to most aldose reductase inhibitors including sorbinil (d-6-fluoro-spiro[chroman-4,4'-imidazolidine]-2',5'-dione), except tolrestat (N-methyl-N-[(5-trifluromethyl-6-methoxy-1-naphthalenyl)- thiomethyl]glycine) and quercetin. On the basis of these observations it is suggested that Cys-298 may form a part of the 'S'-inhibitor binding site of the enzyme and may be responsible for tight binding of NADPH.

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.

Carboxymethylation-induced activation of bovine lens aldose reductase

Carboxymethylation of bovine lens aldose reductase with 10 mM iodoacetate for 1 h at 25 degrees C led to a more than 4-fold increase in kcat. Carboxymethylation led to a 3- to 5-fold increase in Km NADPH and Km D-glyceraldehyde, whereas Km L-glyceraldehyde increased approx. 30-fold. Activation of the enzyme on carboxymethylation was accompanied by a decrease in the sensitivity of the enzyme to inhibition by 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), sorbinil (Kii increased from 0.4 to 109 microM) and NADP (Kis increased from 0.01 to 0.03 mM), but not tolrestat. Activation of the enzyme was almost completely prevented by NADPH and to a lesser extent by DL-glyceraldehyde. Carboxymethylation of the enzyme did not result in the generation of several partially oxidized enzyme species, indicating the absence of partially carboxymethylated forms. Primary deuterium isotope effects on the reduced enzyme were consistent with a preferred ordered kinetic reaction scheme, in which hydride transfer is not rate limiting. The hydride transfer step does not seem to be significantly affected by carboxymethylation, nor do changes in the substrate binding steps seem to contribute to the observed rate enhancement. Increase in the turnover number of the enzyme on carboxymethylation appears to be due to facilitation of the isomerization of the E:NADP binary complex. The differential effect of carboxymethylation on sorbinil and tolrestat suggests distinct inhibitor sites on the enzyme, an S-site that binds sorbinil and a T-site that binds tolrestat.

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.

Structure-activity correlations in human kidney aldehyde reductase-catalyzed reduction of para-substituted benzaldehyde by 3-acetyl pyridine adenine dinucleotide phosphate

Steady-state kinetic parameters of the human kidney aldehyde reductase-catalyzed reduction of para-substituted benzaldehydes by 3-acetyl pyridine dinucleotide phosphate (3-APADPH) were determined. The kcat of aldehyde reduction by 3-APADPH was 2- to 4-fold lower than by NADPH. The dissociation constant of 3-APADPH from the enzyme-coenzyme complex was higher (77 microM) than that of NADPH (5.3 microM). Primary deuterium kinetic isotope effects on both kcat and kcat/Km for para-substituted benzaldehyde reduction by 3-APADPH (with the exception of para-carboxybenzaldehyde) were equal and on average 2.82 +/- 0.21, suggesting that these reactions follow a rapid equilibrium-ordered reaction scheme in which the hydride transfer step is rate-limiting. Multiple regression analysis of the data suggests that benzaldehyde reduction depends upon electronic substituent effects, characterized by a rho value of 0.5. These data are consistent with a transition state in which the charge on the aldehyde carbonyl increases relative to the charge on this group in the ground state. A positive deviation of para-carboxybenzaldehyde from the linear correlation between other benzaldehydes and the substituent constant sigma + suggests a specific interaction of the carboxyl substituent of the substrate with the enzyme.

Inhibition kinetics of human kidney aldose and aldehyde reductases by aldose reductase inhibitors

Kinetic patterns of inhibition of homogenous human kidney aldose reductase (AR, EC 1.1.1.21) and aldehyde reductase II (AR II, EC 1.1.1.19) by statil, ICI 105552 [1-(3,4-dichlorobenzyl)-3-methyl-1,2-dihydro-2-oxoquinol-4-yl acetic acid], tolrestat, alrestatin, chromone carboxylic acid (CCA), quercetin, phenobarbital and sorbinil were studied. On the basis of the kinetic nature of inhibition, the inhibitors were classified into four distinct categories. For aldose reductase, sorbinil and phenobarbital were noncompetitive (NC; category I) and CCA and alrestatin were uncompetitive (UC; category II) to both the aldehyde substrate and NADPH. Quercetin and ICI 105552 were NC to the aldehyde and UC to NADPH (category III) and tolrestat and statil were UC to the aldehyde and NC to NADPH (category IV). For AR II, sorbinil and alrestatin were category I inhibitors, ICI 105552 and statil belong to category II, phenobarbital, tolrestat and CCA to category III, and quercetin to category IV. To determine the specificity of inhibition, the ratios of the inhibition constants (Kii) for AR and AR II were calculated. A lower ratio indicates greater specificity. With aldehyde as the varied substrate the specificity ratios were: statil less than ICI 105552 less than alrestatin less than tolrestat less than quercetin less than CCA less than sorbinil less than phenobarbital, and with NADPH as the varied substrate, ICI 105552 less than statil less than alrestatin less than tolrestat less than quercetin less than CCA less than sorbinil less than phenobarbital. For AR, double-inhibition plots generated for one inhibitor from each kinetic category versus sorbinil showed that AR inhibitors of categories I-III bind to the same site on the protein molecule as sorbinil. However, tolrestat seemed to bind to a site different from the sorbinil binding site. For AR II, inhibitors from all the four categories appeared to bind to the same inhibitor binding site.

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.