Aldehyde reductase activity in the antennae of Helicoverpa armigera

In the present study, we identified two aldehyde reductase activities in the antennae of Helicoverpa species, NADH and NADPH-dependent activity. We expressed one of these proteins of H. armigera, aldo-keto reductase (AKR), which bears 56% identity to bovine aldose reductase, displays a NADPH-dependent activity and is mainly expressed in the antennae of adults. Whole-mount immunostaining showed that the enzyme is concentrated in the cells at the base of chemosensilla and in the nerves. The enzyme activity of H. armigera AKR is markedly different from those of mammalian enzymes. The best substrates are linear aliphatic aldehydes of 8-10 carbon atoms, but not hydroxyaldehydes. Both pheromone components of H. armigera, which are unsaturated aldehydes of 16 carbons, are very poor substrates. Unlike mammalian AKRs, the H. armigera enzyme is weakly affected by common inhibitors and exhibits a different behaviour from the action of thiols. A model of the enzyme suggests that the four cysteines are in their reduced form, as are the seven cysteines of mammalian enzymes. The occurrence of orthologous proteins in other insect species, that do not use aldehydes as pheromones, excludes the possibility of classifying this enzyme among the pheromone-degrading enzymes, as has been previously described in other insect species.

From a dull enzyme to something else: facts and perspectives regarding aldose reductase

Aldose Reductase (ALR2) is defined as the first enzyme of the "polyol pathway". As such, ALR2 would convert glucose to sorbitol through an NADPH dependent reaction. Considered a promoter of osmotic imbalance under hyperglycemic conditions, the enzyme has been under intense investigation as a critical target to prevent and control diabetic complications through the inhibition of its activity. Further characterization of ALR2 suggests its participation in cell detoxification mechanisms through the reduction of toxic aldehydes. Moreover, intriguing is the apparent involvement of the enzyme in the signalling machinery of inflammatory cell response. Here, the structural and functional assessment of ALR2 as an aldose/aldehyde reducing enzyme, and its involvement in various aspects of cell function from sugar metabolism to redox homeostasis and cell signaling are presented.

Chaperone-like features of bovine serum albumin: a comparison with α-crystallin

The chaperone behaviour of bovine serum albumin was compared with that of alpha-crystallin. The chaperone activity was assessed by measuring: (i) the ability to antagonize protein aggregation induced by heat; (ii) the capability to protect the activity of thermally stressed enzymes and (iii) the effectiveness in assisting the functional recovery of chemically denatured sorbitol dehydrogenase. Despite the lack of structural analogies, both proteins show several functional similarities in preventing inactivation of thermally stressed enzymes and in reactivating chemically denatured sorbitol dehydrogenase. As with alpha-crystallin, the chaperone action of bovine serum albumin appears to be ATP independent. Bovine serum albumin appears significantly less effective than alpha-crystallin only in preventing thermally induced protein aggregation. A possible relationship between chaperone function and structural organization is proposed. Together, our results indicate that bovine serum albumin acts as a molecular chaperone and that, for its particular distribution, can be included in the extracellular chaperone family.

Alpha-crystallin: an ATP-independent complete molecular chaperone toward sorbitol dehydrogenase

alpha-Crystallin, the major component of the vertebrate lens, is known to interact with proteins undergoing denaturation and to protect them from aggregation phenomena. Bovine lens sorbitol dehydrogenase (SDH) was previously shown to be completely protected by alpha-crystallin from thermally induced aggregation and inactivation. Here we report that alpha-crystallin, in the presence of the SDH pyridine cofactor NAD(H), can exert a remarkable chaperone action by favoring the recovery of the enzyme activity from chemically denaturated SDH up to 77%. Indeed, even in the absence of the cofactor, alpha-crystallin present at a ratio with SDH of 20:1 (w:w) allows a recovery of 35% of the enzyme activity. The effect of ATP in enhancing alpha-crystallin-promoted SDH renaturation appears to be both nonspecific and to not involve hydrolysis phenomena, thus confirming that the chaperone action of alpha-crystallin is not dependent on ATP as energy donor.

Thiol/disulfide interconversion in bovine lens aldose reductase induced by intermediates of glutathione turnover

The effectiveness of cysteine and cysteinylglycine to act as protein thiolating agents was investigated using bovine lens aldose reductase (ALR2) as the protein target. Disulfides of both thiol compounds appear to be very effective as ALR2 thiolating agents. Cysteine- and CysGly-modified ALR2 forms (Cys-ALR2 and CysGly-ALR2, respectively) are characterized by the presence of a mixed disulfide bond involving Cys298, as demonstrated by a combined electrospray mass spectrometry and Edman degradation approach. Both Cys-ALR2 and CysGly-ALR2 essentially retain the ability to reduce glyceraldehyde but lose the susceptibility to inhibition by Sorbinil and other ALR2 inhibitors. Cys-ALR2 and CysGly-ALR2 are easily reduced back to the native enzyme form by dithiothreitol and GSH treatment; on the contrary, Cys and 2-mercaptoethanol appear to act as protein trans-thiolating agents, rather than reducing agents. The treatment at 37 degrees C of both Cys-ALR2 and CysGly-ALR2, unlikely what observed for glutathionyl-modified ALR2 (GS-ALR2), promotes the generation of an intramolecular disulfide bond between Cys298 and Cys303 residues. A rationale for the special susceptibility of Cys-ALR2 and CysGly-ALR2, as compared to GS-ALR2, to the thermally induced intramolecular rearrangement is given on the basis of a molecular dynamic and energy minimization approach. A pathway of thiol/disulfide interconversion for bovine lens ALR2 induced, in oxidative conditions, by physiological thiol compounds is proposed.

7-Hydroxy-2-substituted-4-H-1-benzopyran-4-one derivatives as aldose reductase inhibitors: a SAR study

On the basis of the results of molecular modelling studies performed on the aldose reductase (ALR2) inhibitor 7-hydroxy-2-(4'-hydroxybenzyl)-4H-1-benzopyran-4-one (compound A) bound at the active site of the enzyme, we synthesised and tested on bovine and human ALR2 several derivatives modified at position 2 of the benzopyran moiety, in order to confirm the hypothesised binding mode of this compound. The substitution of the methylene bridge with the isosteric sulphur substituent gives an active derivative, while substitution with a polar NH causes a decrease in inhibitory activity; this is in accordance to the previously reported structure in which the methylene linker was found to be adjacent to a hydrophobic aminoacid (Leu300). Among the substituents at 4' position examined, the most favourable for inhibitory activity are those able to act as hydrogen bond donors, supporting the hypothesis of the importance of the interaction with Thr113 for the inhibition of the enzyme.

Modulation of aldose reductase activity through S-thiolation by physiological thiols

The glutathionyl-modified aldose reductase (GS-ALR2) is unique, among different S-thiolated enzyme forms, in that it displays a lower specific activity than the native enzyme (ALR2). Specific interactions of the bound glutathionyl moiety (GS) with the ALR2 active site, were predicted by a low perturbative molecular modelling approach. The outcoming GS allocation, involving interactions with residues relevant for catalysis and substrate allocation, explains the rationale behind the observed differences in the activity between GS-ALR2 and other thiol-modified enzyme forms. The reversible S-glutathionylation of ALR2 observed in cultured intact bovine lens undergoing an oxidative/non oxidative treatment cycle is discussed in terms of the potential of ALR2/GS-ALR2 inter-conversion as a response to oxidative stress conditions.

Aldose reductase does catalyse the reduction of glyceraldehyde through a stoichiometric oxidation of NADPH

In order to define the ability of bovine lens aldose reductase (ALR2) to generate polyols from aldoses, the quantitative determination of glycerol in the presence of glyceraldehyde was performed by gas chromatography after derivatization with trifluoroacetic anhydride. The proposed method appears to be useful in quantifying low amounts of glycerol in the presence of relatively high concentrations of glyceraldehyde and in following glycerol formation in enzyme assay conditions. The generation of one equivalent of glycerol in the presence of ALR2, is paralleled by the oxidation of one equivalent of NADPH. A similar result was obtained when S-glutathionyl-modified ALR2 was used, instead of the native enzyme, as a catalyst of glyceraldehyde reduction. Sorbinil, a classical ALR2 inhibitor, present in the enzyme assay mixture, inhibits to the same extent both NADPH oxidation and glycerol formation. The demonstration of the stoichiometric ratio of 1:1 occurring in the presence of bovine lens ALR2 between the synthesis of glycerol from D, L -glyceraldehyde and the oxidation of NADPH, rules out doubts concerning the ability of the enzyme to catalyse the reduction of aldoses to the corresponding polyalcohols. Possible autooxidation processes of glyceraldehyde, in the enzyme assay conditions, appear to be irrelevant with respect to the enzyme-catalysed reduction of the aldose. This would indicate that the spectrophotometric monitoring of NADPH oxidation at 340 nm, in the presence of ALR2, is a reliable method to assay the enzyme activity.

Complete protection by alpha-crystallin of lens sorbitol dehydrogenase undergoing thermal stress

Sorbitol dehydrogenase (l-iditol:NAD(+) 2-oxidoreductase, E.C. 1.1.1. 14) (SDH) was significantly protected from thermally induced inactivation and aggregation by bovine lens alpha-crystallin. An alpha-crystallin/SDH ratio as low as 1:2 in weight was sufficient to preserve the transparency of the enzyme solution kept for at least 2 h at 55 degrees C. Moreover, an alpha-crystallin/SDH ratio of 5:1 (w/w) was sufficient to preserve the enzyme activity fully at 55 degrees C for at least 40 min. The protection by alpha-crystallin of SDH activity was essentially unaffected by high ionic strength (i.e. 0.5 m NaCl). On the other hand, the transparency of the protein solution was lost at a high salt concentration because of the precipitation of the alpha-crystallin/SDH adduct. Magnesium and calcium ions present at millimolar concentrations antagonized the protective action exerted by alpha-crystallin against the thermally induced inactivation and aggregation of SDH. The lack of protection of alpha-crystallin against the inactivation of SDH induced at 55 degrees C by thiol blocking agents or EDTA together with the additive effect of NADH in stabilizing the enzyme in the presence of alpha-crystallin suggest that functional groups involved in catalysis are freely accessible in SDH while interacting with alpha-crystallin. Two different adducts between alpha-crystallin and SDH were isolated by gel filtration chromatography. One adduct was characterized by a high M(r) of approximately 800,000 and carried exclusively inactive SDH. A second adduct, carrying active SDH, had a size consistent with an interaction of the enzyme with monomers or low M(r) aggregates of alpha-crystallin. Even though it had a reduced efficiency with respect to alpha-crystallin, bovine serum albumin was shown to mimic the chaperone-like activity of alpha-crystallin in protecting SDH from thermal denaturation. These findings suggest that the multimeric structural organization of alpha-crystallin may not be a necessary requirement for the stabilization of the enzyme activity.

Thiol disulfide exchange modulates the activity of aldose reductase in intact bovine lens as a response to oxidative stress

The reversibility of S-thiolation of aldose reductase was shown in intact bovine lens subjected to oxidative stress. The glutathione modified aldose reductase generated in the lens as a consequence of hyperbaric oxygen treatment was recovered in its reduced form following culturing in normobaric air conditions. Nucleus and cortex were differently affected by both oxidative treatment and normobaric air recovery. The extent of S-thiolation of aldose reductase appeared to be higher in the nucleus than in the cortex. Moreover, the nucleus, but not the cortex, was unable to completely recover from the protein S-thiolation process. The ratios of GSH/GSSG and NADPH/NADP(+)as well as the Energy Charge values were determined in the cortex and nucleus both after oxidative stress and recovery. The results are consistent with the existence of a quite well-defined boundary between the two lens regions. Moreover, they are supportive of the hypothesis that thiol/disulfide exchange has the potential to be a regulatory mechanism for certain enzymes which can modulate the flux of NADPH inside the cell.

An atypical form of alphaB-crystallin is present in high concentration in some human cataractous lenses. Identification and characterization of aberrant N- and C-terminal processing

Two unique polypeptides, 22.4 and 16.4 kDa, were prominent in some human cataracts. Both proteins were identified as modified forms of the small heat shock protein, alphaB-crystallin. The concentration of total alphaB-crystallin in most of these cataracts was significantly increased. The 22.4-kDa protein was subsequently designated as alphaB(g). Mass spectrometric analyses of tryptic and Asp-N digests showed alphaB(g) is alphaB-crystallin minus the C-terminal lysine. alphaB(g) constituted 10-90% of the total alphaB-crystallin in these cataracts and was preferentially phosphorylated over the typical form of alphaB-crystallin. Human alphaB(g) and alphaB-crystallin were cloned and expressed in Escherichia coli. The differences in electrophoretic mobility and the large difference in native pI values suggest some structural differences exist. The chaperone-like activity of recombinant human alphaB(g) was comparable to that of recombinant human alphaB-crystallin in preventing the aggregation of lactalbumin induced by dithiothreitol. The mechanism involved in generating alphaB(g) is not known, but a premature termination of the alphaB-crystallin gene was ruled out by sequencing the polymerase chain reaction products of the last exon for the alphaB-crystallin gene from lenses containing alphaB(g). The 16.4-kDa protein was an N-terminally truncated fragment of alphaB(g). The high concentration of alphaB-crystallin in these cataracts is the first observation of this kind in human lenses.

A new approach against sugar cataract through aldose reductase inhibitors

Aldose reductase inhibition is one of the therapeutic strategies that has been proposed to prevent or ameliorate long term diabetic complications including retinopathy and sugar cataract. Rats were fed with a galactose rich diet and the aldose reductase inhibitor Tolrestat was topically delivered by ocular instillation. The levels of lens aldose reductase activity, galactitol and the onset of cataract were evaluated during and after treatment with the inhibitor. Topical application of 1-3% Tolrestat (10 microl) four times daily resulted, after 9 days, in a significant decrease in the enzyme activity. Well after interrupting treatment with the drug, the enzyme activity remained impaired and galactose induced cataract was prevented. Our findings may represent the basis for therapeutic plans to prevent sugar cataract by long term cyclic treatments with aldose reductase inhibitors, with reduction in drug doses and side effects.

1-Benzopyran-4-one antioxidants as aldose reductase inhibitors

Starting from the inhibitory activity of the flavonoid Quercetin, a series of 4H-1-benzopyran-4-one derivatives was synthesized and tested for inhibition of aldose reductase, an enzyme involved in the appearance of diabetic complications. Some of the compounds obtained display inhibitory activity similar to that of Sorbinil but are more selective than Quercetin and Sorbinil with respect to the closely related enzyme, aldehyde reductase, and also possess antioxidant activity. Remarkably, these compounds possess higher pKa values than carboxylic acids, a characteristic which could make the pharmacokinetics of these compounds very interesting. Molecular modeling investigations on the structures of inhibitors bound at the active site of aldose reductase were performed in order to suggest how these new inhibitors might bind to the enzyme and also to interpret structure-activity relationships.

Oxidative modification of aldose reductase induced by copper ion. Factors and conditions affecting the process

Bovine lens aldose reductase (ALR2) is inactivated by copper ion [Cu(II)] through an oxygen-independent oxidative modification process. A stoichiometry of 2 equiv of Cu(II)/enzyme mol is required to induce inactivation. While metal chelators such as EDTA or o-phenantroline prevent but do not reverse the ALR2 inactivation, DTT allows the enzyme activity to be rescued by inducing the recovery of the native enzyme form. The inactive enzyme form is characterized by the presence of 2 equiv of bound copper, at least one of which present as Cu(I), and by the presence of two lesser equivalents, with respect to the native enzyme, of reduced thiol residues. Data are presented which indicate that the Cu-induced protein modification responsible for the inactivation of ALR2 is the generation on the enzyme of an intramolecular disulfide bond. GSH significantly interferes with the Cu-dependent inactivation of ALR2 and induces, through its oxidation to GSSG, the generation of an enzyme form linked to a glutathionyl residue by a disulfide bond.

Thioltransferase activity of bovine lens glutathione S-transferase

A Mu-class glutathione S-transferase purified to electrophoretic homogeneity from bovine lens displayed thioltransferase activity, catalysing the transthiolation reaction between GSH and hydroxyethyldisulphide. The thiol-transfer reaction is composed of two steps, the formation of GSSG occurring through the generation of an intermediate mixed disulphide between GSH and the target disulphide. Unlike glutaredoxin, which is only able to catalyse the second step of the transthiolation process, glutathioneS-transferase catalyses both steps of the reaction. Data are presented showing that bovine lens glutathione S-transferase and rat liver glutaredoxin, which was used as a thioltransferase enzyme model, can operate in synergy to catalyse the GSH-dependent reduction of hydroxyethyldisulphide.

Hirunorms, novel hirudin-like direct thrombin inhibitors

1. Hirunorms are new synthetic peptides designed to interact with thrombin in a similar way to the natural inhibitor hirudin. 2. Hirunorms are specific and efficient in vitro inhibitors of thrombin activity. 3. Hirunorms are potent anticoagulant and antithrombotic agents in in vivo experimental models devoid of hemorrhagic effects at doses that are active in preventing thrombosis.

Site-specific inactivation of aldose reductase by 4-hydroxynonenal

Bovine lens aldose reductase (ALR2), which catalyzes the NADPH-dependent reduction of 4-hydroxy-2-nonenal (HNE), is readily inactivated by its own substrate in a time- and concentration-dependent manner. Both DTT and NADP+ can prevent enzyme inactivation but neither extensive dialysis nor thiol-reducing treatment were able to restore enzyme activity once inactivation had occurred. Unlike the native enzyme, S-glutathionyl-modified ALR2 is unaffected by HNE, and can be easily reverted to the native form under thiol-reducing conditions. Evidence is presented of the involvement of Cys298 in the inactivation process. Zofenoprilat, an antioxidant thiol compound, mimics the effect of GSH. The possibility is raised that enzyme thiolation may function as a protection mechanism against the irreversible modification of ALR2.

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.

Specifically targeted modification of human aldose reductase by physiological disulfides

Aldose reductase is inactivated by physiological disulfides such as GSSG and cystine. To study the mechanism of disulfide-induced enzyme inactivation, we examined the rate and extent of enzyme inactivation using wild-type human aldose reductase and mutants containing cysteine-to-serine substitutions at positions 80 (C80S), 298 (C298S), and 303 (C303S). The wild-type, C80S, and C303S enzymes lost >80% activity following incubation with GSSG, whereas the C298S mutant was not affected. Loss of activity correlated with enzyme thiolation. The binary enzyme-NADP+ complex was less susceptible to enzyme thiolation than the apoenzyme. These results suggest that thiolation of human aldose reductase occurs predominantly at Cys-298. Energy minimization of a hypothetical enzyme complex modified by glutathione at Cys-298 revealed that the glycyl carboxylate of glutathione may participate in a charged interaction with His-110 in a manner strikingly similar to that involving the carboxylate group of the potent aldose reductase inhibitor Zopolrestat. In contrast to what was observed with GSSG and cystine, cystamine inactivated the wild-type enzyme as well as all three cysteine mutants. This suggests that cystamine-induced inactivation of aldose reductase does not involve modification of cysteines exclusively at position 80, 298, or 303.

Kinetics of human thrombin inhibition by two novel peptide inhibitors (Hirunorm IV and Hirunorm V)

A study on the kinetics of human thrombin inhibition by two novel synthetic peptides (Hirunorm IV and Hirunorm V) and a comparison with recombinant hirudin and a commonly used thrombin inhibitor, Hirulog-1, are reported. The dissociation constants for Hirunorm IV and Hirunorm V were determined by varying the concentration of inhibitors at fixed concentrations of the chromogenic substrate Chromozym-TH (N-tosylglycyl-L-prolyl-L-arginine 4-nitroanilide acetate). Both inhibitors behaved as reversible tight-binding inhibitors of amidolytic thrombin activity. The apparent dissociation constants determined showed a linear dependence on the concentration of substrate; this finding, which indicates that the inhibition was competitive, made possible the estimation of the dissociation constants (KI) for Hirunorm IV and Hirunorm V, which were 0.134 +/- 0.014 nM and 0.245 +/- 0.016 nM, respectively. Similar dissociation constants were also obtained for the two inhibitors when thrombin activity was measured with fibrinogen in the clotting assay. When tested for resistance to thrombin proteolytic activity, both inhibitors were inviolate to cleavage by thrombin. The data obtained demonstrate that both Hirunorm IV and Hirunorm V are potent and stable inhibitors of human thrombin activity.

Synthesis, activity, and molecular modeling of a new series of tricyclic pyridazinones as selective aldose reductase inhibitors

Three new series of tricyclic pyridazinones have been synthesized and tested in vitro in order to assess (i) their ability to inhibit aldose reductase enzyme (ALR2) and (ii) their specificity toward the target enzyme with respect to other related oxidoreductases, such as aldehyde reductase, sorbitol dehydrogenase, and glutathione reductase. The inhibitory capability of the most effective compounds (IC50 values ranging from 6.44 to 12.6 microM) appears to be associated with a rather significant specificity for ALR2. Molecular mechanics and molecular dynamic calculations performed on the ALR2-inhibitor complex give indications of specific interaction sites responsible for the binding, thus providing information for the design of new inhibitors with improved affinity for the enzyme.

Alpha-crystallin-like molecular chaperone against the thermal denaturation of lens aldose reductase: the effect of divalent metal ions

A chaperone-like activity of bovine lens alpha-crystallin against the thermal-induced aggregation of bovine lens aldose reductase is reported. While the precipitation of aldose reductase at 55 degrees C is prevented by alpha-crystallin present at a ratio of aldose reductase: alpha-crystallin as low as 1:0.5 (w:w), the heat-induced inactivation of the enzyme occurs regardless of the presence of alpha-crystallin. This would suggest that, irrespective of the functional integrity of the target protein, alpha-crystallin interferes only with aggregation phenomena, having the potential to preserve the lens transparency. Calcium and magnesium ions at mM levels affect the antiaggregation action exerted by alpha-crystallin either interfering on the formation or reducing the stability of the aldose reductase: alpha-crystallin complex.

Occurrence of glutathione-modified aldose reductase in oxidatively stressed bovine lens

The optimization of an affinity chromatography method on Matrex Orange resin allowed the separation of glutathione modified and native aldose reductase in crude extracts of bovine lens. The analysis of hyperbaric oxygen treated lenses revealed the formation in the intact cultured lens of an enzyme form displaying affinity column binding properties, specific activity, sensitivity to inhibition and susceptibility to activation by thiol reducing agents, all comparable to glutathione modified aldose reductase. The extent of the enzyme modification increased with the time of the oxidative treatment and was maximal in the lens nucleus. The relative increase of glutathione modified aldose reductase from cortex to the nucleus is consistent with the increase in these lens regions of the GSSG/GSH ratio.

Thiol dependent oxidation of enzymes: the last chance against oxidative stress

1. A survey of known effects of oxidized thiols on enzyme activity reveals a potential concerted action on metabolic pathways determining an impairment of anabolic reduction processes and an activation of the oxidative arm of the hexose monophosphate shunt. Thus it appears that, following oxidative stress, the increase of disulphides may act in restoring a reduced state in the cell by specifically channelling the metabolic energy flux.

Glutathione dependent modification of bovine lens aldose reductase

Bovine lens aldose reductase (ALR2) is readily modified by glutathione disulphide (GSSG) to an enzyme form (GS-ALR2) exhibiting a reduced catalytic efficiency with all the substrates tested and a reduced susceptibility to inhibition. The modification, which is completely reversed by reduced glutathione (GSH) or dithiothreitol occurs by a pseudo-first-order process with respect to the enzyme and a second order rate constant of 30 +/- 0.1 mol-1 min-1 at 25 degrees C was determined. By measuring the residual activity of ALR2 incubated in different glutathione redox buffers at 25 degrees C, an apparent redox equilibrium constant of 1.4 +/- 0.1 was evaluated. Thus the rate and the maximal extent of ALR2 inactivation are proportional to the redox ratio of the thiol used as modifying agent (i.e. [GSH]/[GSSG]). The stoichiometric reversibility of the enzyme modification might be impaired by a reduced solubility of GS-ALR2 with respect to ALR2 and by an increased susceptibility of the modified enzyme to proteolysis. While the native enzyme form is rather insensitive to proteolytic breakdown. GS-ALR2 is easily degraded by chymotrypsin with the generation of a peptide of 26 kDa with an aminoacid sequence at the aminoterminal side compatible with proteolysis at level of Tyr 7 of aldose reductase. A reduced efficiency in the enzyme-cofactor binding following the GSSG dependent modification of ALR2, appears to be associated to the thiol accessibility of GS-ALR2 measured at different temperatures. GS-ALR2 is characterized by the presence of one glutathione residue, linked through a mixed disulphide bond. This is sustained by: (i) the isoelectric point for the modified enzyme of 4.75, which is 0.1 pH units lower than that observed for the native enzyme, which indicates the contribution of an acidic residue to the pI of GS-ALR2; (ii) the incorporation of radioactivity coming from [3H] labelled GSSG accounting for the presence of one equivalent of glutathione per mole of enzyme. Besides being a general feature of protein reactivity in oxidative conditions, the glutathione-mediated ALR2 modification might be part of a cell strategy to preserve reducing power in conditions of oxidative stress.

Thiol and disulfide determination by free zone capillary electrophoresis

A rapid, sensitive and simple method for the determination of reduced and oxidized glutathione, cysteine, cystine, cysteamine, cystamine and their respective mixed disulfides is described. The compounds were separated and identified in a single step by capillary zone electrophoresis. The method was used to follow the thiol-disulfide interconversion and to measure glutathione levels in lens extracts.

Thiol-dependent metal-catalyzed oxidation of bovine lens aldose reductase. II. Proteolytic susceptibility of the modified enzyme form

Bovine lens aldose reductase (alditol: NADP+ oxidoreductase, EC 1.1.1.21) undergoes a modification induced by 2-mercaptoethanol in the presence of the redox system Fe(II)/Fe(III). The modified form (ARa) exhibits an increased hydrophobicity and tendency to aggregate. Moreover, while the native enzyme form is rather insensitive to proteolytic breakdown, the modified form is susceptible to limited proteolysis by trypsin and chymotrypsin. With both proteases, the degradation correlated with a loss of enzyme activity and results in the appearance of one molecular species of 26 KDa (for chymotrypsin) and two molecular species of 24 and 17 KDa (for trypsin). The decline in solubility and the increase in susceptibility to proteolysis of ARa suggests that the thiol-dependent metal-catalyzed modification is comparable to other oxidative systems that mark proteins for degradation.

Thiol-dependent metal-catalyzed oxidation of bovine lens aldose reductase. I. Studies on the modification process

Bovine lens aldose reductase (alditol: NADP+ oxido-reductase, EC 1.1.1.21) undergoes a thiol-dependent oxidative modification catalyzed by the Fe(II)/Fe(III) redox system. The enzyme is inactivated by various oxygen radical generating systems. However, addition of 2-mercaptoethanol to the oxygen radical generating systems resulted in an initial increase followed by a decrease in the activity of aldose reductase. The net maximal increase in the enzyme activity was observed with 3 mM 2-mercaptoethanol, 0.3 mM FeSO4, and 0.9 mM EDTA, either with or without 1 mM hypoxanthine and 37 mU/ml of xanthine oxidase. The formation of the stable, activated intermediate, ARa, appears to proceed through the reaction between the enzyme and the oxidized form of 2-mercaptoethanol which in the presence of iron, forms a mixed disulfide with a cysteine residue. Reduction of ARa with dithiothreitol released 0.7 mol of 2-mercaptoethanol per mole of enzyme and converted it to a form that resembled the native aldose reductase.

Purine nucleoside phosphorylase from bovine lens: purification and properties

Purine nucleoside phosphorylase (purine nucleoside: orthophosphate ribosyltransferase, EC 2.4.2.1) was purified 38,750-fold to apparent electrophoretic homogeneity from bovine ocular lens. The enzyme appears to be a homotrimer with a molecular weight of 97,000, and displays non-linear kinetics with concave downward curvature in double-reciprocal plots with orthophosphate as variable substrate. The analysis of the kinetic parameters of bovine lens purine nucleoside phosphorylase, determined both for the phosphorolytic activity on nucleosides and for ribosylating activity on purine bases, indicates the occurrence of a rapid equilibrium random Bi-Bi mechanism with formation of abortive complexes. The effect of pH on the enzyme activity and on the sensitivity of the enzyme to photoinactivation, as well as the effect of thiol reagents on the enzyme activity and stability, strongly suggest the involvement of histidine and cysteine residues in the active site. From the measurements of the kinetic parameters at different temperatures, heats of formation of the enzyme-substrate complex for guanosine, guanine, orthophosphate and ribose 1-phosphate were determined. Activation energies of 15,250 and 14,650 cal/mol were obtained for phosphorolysis and synthesis of guanosine, respectively.

Purine salvage as a metabolite and energy saving mechanism in the ocular lens

The ocular lens is an organ which depends mainly on anaerobic processes to obtain the metabolic energy required for the maintenance of its physiological functions. In these circumstances, the purine salvage pathway enzymes, by using preformed purine rings, and allowing the utilization of the activated ribose moiety of nucleosides, might be of relevance as an energy saving device. In this paper we show that the calf lens possesses many enzymes of the purine salvage pathway, with a particularly high specific activity of purine nucleoside phosphorylase (EC 2.4.2.1), and that the isolated lens epithelium can actively convert adenine and adenosine into adenine nucleotides. In addition, as in bacteria and red blood cells, inosine and adenosine in the lens, acting as ribose donors, exert a profound effect on the process of adenine conversion into ATP.

zeta-Crystallin is a major protein in the lens of Camelus dromedarius

Camel (Camelus dromedarius) lenses contain a protein with an apparent subunit Mr 38,000 that constitutes approximately 8-13% of the total protein. The protein has been purified and has a native Mr 140,000 as determined by gel filtration. This is consistent with its being a tetramer. The protein reacts with antibodies raised against both guinea pig zeta-crystallin and peptides corresponding to amino acids 1-10 and 295-308, but not to antibodies raised against amino acids 320-328 of zeta-crystallin. Based on these criteria it is concluded that this protein, which is a major constituent of camel lens, is zeta-crystallin. This may be the first example of a protein (enzyme) being independently utilized as a crystallin in the lens of species from two mammalian orders.

Bovine lens aldose reductase: tight binding of the pyridine coenzyme

Analysis by HPLC of the protein-free supernatant obtained after denaturation of aldose reductase shows that the native form of the enzyme (ARb) contains a tightly bound NADP+, which is absent in the oxidatively modified form (ARa). The absorption, fluorescence, and circular dichroism spectra of ARb and ARa are consistent with the presence of the cofactor only in the native form of aldose reductase. On the other hand, the modified enzyme, in appropriate thiol reducing conditions, can tightly bind NADP+. This indicates a potential reversibility of the modification of aldose reductase, at least in terms of retention of the cofactor.

Liver purine nucleoside phosphorylase in Camelus dromedarius: purification and properties

1. Purine nucleoside phosphorylase (purine nucleoside:orthophosphate ribosyl transferase, EC 2.4.2.1) was purified to electrophoretic homogeneity from the liver of Camelus dromedarius. 2. The enzyme appears to be a dimer with a 44,000 subunit mol. wt and displays non-linear kinetics with concave downward curvature in double reciprocal plots with respect to both inosine and orthophosphate as variable substrates. 3. The effect of thiol compounds on the enzyme activity and of pH on kinetic parameters is reported.

Change in stereospecificity of bovine lens aldose reductase modified by oxidative stress

Bovine lens aldose reductase (alditol:NADP+ oxidoreductase, EC 1.1.1.21) undergoes an oxidative modification, greatly stimulated by high ionic strength, upon incubation in the presence of oxygen radical generating systems (Del Corso, A., Camici, M., and Mura, U. (1987) Biochem. Biophys. Res. Commun. 148, 369-375). The enzyme modification is accompanied by a change in stereospecificity toward the two enantiomers of glyceraldehyde. In particular, the Km for L-glyceraldehyde of the native form increased over 150 times after the enzyme modification, with a decrease in the catalytic efficiency of over 200 times. By contrast, for the D-enantiomer the Km increased only 7 times with respect to the native form, with a concomitant decrease in the catalytic efficiency of only approximately 3 times. This dramatic change in stereospecificity may account for the reported apparent cooperative behavior exhibited also by highly purified electrophoretically homogeneous preparations of aldose reductase.

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.

On the effects of tranexamic acid and its isobenzedrine ester on plasminogen activation and streptokinase induced fibrinolysis

A comparison between the inhibitory capability of Tranexamic acid (AMCA) and its isobenzedrine ester (IB-AMCA) on the streptokinase and urokinase induced plasminogen activation, indicated in vitro a higher potency of the ester derivative. A peculiar activatory rather than inhibitory effect on the plasminogen activation was exerted by AMCA and aminocaproic acid at relatively low concentrations. Attempts to show in vivo the in vitro observed differences between AMCA and IB-AMCA action are reported.

Bovine lens aldose reductase: identification of two enzyme forms

Two structurally different forms of bovine lens aldose reductase have been identified. Freshly prepared lens extracts contain an unactivated "b form" (ARb) which is sensitive to inhibition by Sorbinil. Upon incubation of the extracts with oxygen radical generating systems, ARb is converted to a more active "a form" (ARa), which is not inhibited by Sorbinil. ARa and ARb were purified to electrophoretic homogeneity.

NADPH-dependent reduction of glyceraldehyde: a unusually high activity in the lens of the camel (Camelus dromedarius)

The enzymes of the polyol pathway, namely aldose reductase and sorbitol dehydrogenase, were measured in camel lens extracts. A NADPH-dependent glyceraldehyde and erythrose reductase activity 25 times higher than that of calf lens was observed in camel lens. A preliminary comparison between this enzyme activity present in the camel and aldose reductase of calf lens is reported.

Liver uricase in Camelus dromedarius: purification and properties

1. Uricase (urate: oxygen oxidoreductase, EC 1.7.3.3) was purified 750-fold from the liver of Camelus dromedarius. 2. The enzyme is a tetramer with a Mr of 100,000, displays high specificity for uric acid with a Km of 12 microM and is inhibited by a selected number of purine derivatives carrying oxygen at the C2 position. 3. The effect of pH and the inhibition by thiol compounds and chelating agents on the enzyme activity is reported. 4. Some lines of evidence suggesting the possibility of interaction of camel liver uricase with oligonucleotides are presented.

Alpha-5-phosphoribosyl-1-pyrophosphate-independent salvage of purines in cultured Chinese hamster lung fibroblasts

A variant clone of cultured chinese hamster lung fibroblasts (V79), selected for resistance to 8-azaguanine (V79 azagrst), although lacking hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8), is able to convert hypoxanthine into IMP via purine-nucleoside phosphorylase (EC 2.4.2.1) and nucleoside kinase. In addition to the phosphoribosylation pathway, we also present evidence for the occurrence of a kinase-mediated pathway of recovery of hypoxanthine in the wild-type cells. The lower rate of formation of IMP in the V79 azagrst cells, apparently correlated with the phosphorylation of the nucleoside, suggests possible differences in the catalytic and/or regulatory properties of nucleoside kinase in the two cell lines. This fact might be of particular relevance in evaluating the mechanisms of resistance to purine analogs displayed by several cell types.

Phosphorylase-mediated mobilization of the amino group of adenine in Bacillus cereus

Mobilization of the ribose moiety of purine nucleosides as well as of the amino group of adenine may be realized in Bacillus cereus by the concerted action of three enzymes: adenosine phosphorylase, adenosine deaminase, and purine nucleoside phosphorylase. In this pathway, ribose-1-phosphate and inorganic phosphate act catalytically, being continuously regenerated by purine nucleoside phosphorylase and adenosine phosphorylase, respectively. As a result of such a metabolic pathway, adenine is quantitatively converted into hypoxanthine, thus overcoming the lack of adenase in B. cereus.

Radioenzymatic determination of adenosine

Adenosine has been measured at the nanomolar level by an enzymatic radioactive assay. The nucleoside is converted into [U-14C]ribose-labeled inosine via the following reactions: adenosine + H2O----adenine + ribose (adenosine nucleosidase); adenine + [U-14C]ribose 1-phosphate in equilibrium with T[U-14C]ribose-adenosine + Pi (adenosine phosphorylase); [U-14C]ribose-adenosine + H2O----[U-14C]ribose-inosine + NH3 (adenosine deaminase). The radioactivity of inosine, separated by thin-layer chromatography, is a measure of the adenosine initially present.

In vitro modification of bovine lens aldose reductase activity

Bovine lens aldose reductase can be activated in crude extracts upon incubation at 37 degrees C at relatively high ionic strength. This phenomenon shows a seasonal occurrence, the enzyme being susceptible to activation only in lenses of animals sacrified in summer. Systems generating oxygen activated species induce the enzyme activation, whereas scavengers of "oxygen radicals" preserve the activated state of the enzyme. Glutathione and other thiol compounds appear to prevent the enzyme activation.

A coupled optical assay for adenine phosphoribosyltransferase and its extension for the spectrophotometric and radioenzymatic determination of 5-phosphoribosyl-1-pyrophosphate in mixtures and in tissue extracts

A reliable assay was developed to characterize crude cell homogenates with regard to their adenine phosphoribosyltransferase activities. The 5-phosphoribosyl-1-pyrophosphate (PRPP)-dependent formation of AMP from adenine is followed spectrophotometrically at 265 nm by coupling it with the following two-stage enzymatic conversion: AMP + H2O----adenosine + Pi (5'-nucleotidase); adenosine + H2O----inosine + NH3 (adenosine deaminase). The same principle was applied to develop a spectrophotometric and a radioenzymatic assay for PRPP. The basis of the spectrophotometric assay is the absorbance change at 265 nm associated with the enzymatic conversion of PRPP into inosine, catalyzed by the sequential action of partially purified adenine phosphoribosyltransferase, commercial 5'-nucleotidase, and commercial adenosine deaminase, in the presence of excess adenine. In the radiochemical assay PRPP is quantitatively converted into [14C]inosine via the same combined reaction. Tissue extracts are incubated with excess [14C]adenine. The radioactivity of inosine, separated by a thin-layer chromatographic system, is a measure of PRPP present in tissue extracts. The radioenzymatic assay is at least as sensitive as other methods based on the use of adenine phosphoribosyltransferase. However, it overcomes the reversibility of the reaction and the need to use transferase preparations free of any phosphatase and adenosine deaminase activities.