Inactivation of cytochrome-c with glucose oxidase

A novel reaction of cytochrome-c from the horse heart with the enzyme glucose oxidase from Aspergillus niger (EC 1.1.3.4), in acidic media is described. Glucose oxidase is able to induce a rapid, profound and irreversible physico-chemical change in cytochrome-c, under anaerobic conditions and in the presence of glucose. The initial rate of reaction is almost independent of the concentration of enzyme and glucose. The striking feature of this reaction is the fact that the reaction proceeds efficiently even below a concentration of 10 nM enzyme.     

Inactivation of plasminogen activator inhibitor by oxidants

The rapidly acting plasminogen activator inhibitor (PAI) purified from cultured bovine aortic endothelial cells (BAEs) was inactivated during iodination with chloramine T and other oxidizing iodination systems. Inactivation was observed in the absence of iodine, suggesting that the loss of activity resulted from the oxidizing conditions employed. In an attempt to further study the nature of this inactivation, the PAI was treated with chloramine T under conditions that specifically oxidize methionine and cysteine residues. Both PAI inhibitory activity and the ability of the PAI to form complexes with tissue-type PA were decreased in a dose-dependent manner by such treatment. The PAI was more sensitive to oxidative inactivation than urokinase, elastase, and alpha 1-protease inhibitor. Incubation of the chloramine T inactivated PAI with methionine sulfoxide peptide reductase in the presence of dithiothreitol (DTT) restored more than 90% of the PAI activity. The reductase is a DTT-dependent enzyme that specifically converts methionine sulfoxide to methionine. Little activity was restored by either the reductase or DTT alone. These results indicate that the oxidation of at least one critical methionine residue is responsible for the loss of PAI activity upon iodination. In this respect, the BAE PAI resembles alpha 1-protease inhibitor, a well-characterized elastase inhibitor that also is inactivated by oxidants. Both inhibitors are members of the serine protease inhibitor superfamily (Serpins), and both have a methionine residue in their reactive center.     

Inactivation of bovine plasma amine oxidase by haloallylamines

Various 2- and 3-haloallylamines were synthesized and evaluated as inhibitors of the quinone-dependent bovine plasma amine oxidase (BPAO). 3-Haloallylamines, which were previously found to be good inhibitors of the flavin-dependent mitochondrial monoamine oxidase (MAO), exhibited a time-dependent inactivation of BPAO, with the 2-phenyl analogs being more potent than the 2-methyl analogs. No plateau of enzyme activity loss was observed, suggestive of a lack of competitive partitioning to normal turnover. The (E)- and (Z)-2-phenyl-3-fluoro analogs were the most potent (low microM IC(50)s), with the corresponding 3-bromo and 3-chloro analogs being >10-fold less potent. In each case, the Z-isomers were more potent than the E-isomers, the reverse of the configurational inhibitory preference observed with MAO. In contrast to the 2-phenyl analogs, 3-phenyl-2(or 3)-chloroallylamines displayed a partitioning behavior, consistent with these being both substrates and inactivators of BPAO.     

Inactivation of mitochondrial monoamine oxidase B by methylthio-substituted benzylamines

Mitochondrial monoamine oxidase was inactivated by o-mercaptobenzylamine (1) and o- (2) and p-methylthiobenzylamine (5). Experiments were carried out to provide evidence for possible mechanisms of inactivation. The corresponding o- (3) and p-hydroxybenzylamine (4) are not inactivators. Four radiolabeled analogues of 2 and 5, having radioactivity at either the methyl or benzyl groups, were synthesized, and all were shown to incorporate multiple equivalents of radioactivity into the enzyme. Inactivation in the presence of an electrophile scavenger decreased the number of molecules incorporated, but still multiple molecules became incorporated; catalase did not further reduce the number of inactivator molecules bound. Two inactivation mechanisms are proposed, one involving a nucleophilic aromatic substitution (SNAr) mechanism and the other a dealkylation mechanism. Evidence for both mechanisms is that inactivation leads to reduction of the flavin (oxidation of the inactivator), but upon denaturation the flavin is reoxidized, indicating that attachment is not at the flavin. A cysteine titration indicates the loss of four cysteines after inactivation and denaturation. Support for the SNAr mechanism was obtained by showing that o- and p-chlorobenzylamine also inactivate MAO. Chemical model studies were carried out that also support both SNAr and dealkylation mechanisms.     

The dissociation of glucose oxidase by sodium n-dodecyl sulphate.

1. The enzymic activity of glucose oxidase was determined as a function of pH and sodium n-dodecyl sulphate (SDS) concentration. 2. Glucose oxidase is not deactivated by SDS at pH 6 even after prolonged incubation, but is deactivated at pH 4.3 and 3.65. 3. Sedimentation-rate analysis showed that glucose oxidase dissociates into its two subunits at pH 5 and below, and sedimentation-equilibrium experiments in the presence of SDS gave a subunit molecular weight of 73,500. 4. SDS binds to glucose oxidase in acid solutions; specific binding occurs ap pH 3.65, but at pH 6 only co-operative binding was observed. 5. Glucose oxidases in which some of the carboxy groups were blocked with glycine methyl ester were deactivated by SDS at pH 6.0; the rate of deactivation increased with the extent of esterification. 6. Deactivation of esterified glucose oxidases correlated with thermal analysis of the initial SDS interaction, the exothermicity of the interaction increasing with the extent of esterification. 7. The results show that carboxy groups confer resistance to deactivation by SDS on glucose oxidase by screening cationic residues and inhibiting specific interactions that facilitate dissociation into subunits.     

Inactivation and modification of lactate oxidase with fluorodinitrobenzene.

1. Dinitrophenylation of 2 +/- 0.2mol of residues/mol of enzyme-bound FMN resulted in the complete inactivation of the flavoenzyme L-lactate oxidase. 2. Hydrolysates of the inactivated enzyme contained 1mol each of Nim-Dnp-histidine (abbreviation: Dnp-,2,4-dinitrophenyl-; Nim indicates that either of the N atoms in the imidazole ring is substituted) and epsilon-Dnp-lysine/mol of FMN. 3. Competitive inhibitors decreased the extent of inactivation to a 10% loss of activity, and dinitrophenylation was decreased from 2 to approx. 0.5mol/mol of FMN. Only Nim-Dnp-histidine was detected in the hydrolysates. 4. Although the dinitrophenylated enzyme did not possess enzyme activitiy, L-lactate reduced approx. 50% of the enzyme-bound flavin slowly (0.6min-1), and approx. 50% of the flavin in the modified enzyme-bound flavin slowly (0.6min-1), and approx. 50% of the flavin in the modified enzyme formed a complex with bisulphite. 6. The modified enzyme (2mol of Dnp/mol of FMN) was unable to bind substrate analogues and competitive inhibitors.     

Inactivation of xanthine oxidase by hydrogen peroxide involves site-directed hydroxyl radical formation.

The mechanism of xanthine oxidase (XO) inactivation by hydrogen peroxide (H2O2) and its biologic significance are unclear. We found that addition of increasing concentrations of H2O2 progressively decreased xanthine oxidase activity in the presence but not the absence of xanthine in vitro. Inactivation of XO by H2O2 was also enhanced by anaerobic reduction of XO by xanthine. Inactivation of XO by H2O2 was accompanied by production of hydroxyl radical (.OH), measured as formation of formaldehyde from dimethylsulfoxide (DMSO). In contrast, addition of H2O2 to deflavo XO did not produce .OH. Inactivation of XO by H2O2 was decreased by simultaneous addition of the .OH scavenger, DMSO. However, inactivation of XO by H2O2 and formation of .OH were not decreased following addition of the metal chelator. DETAPAC, and/or the O2 scavenger, superoxide dismutase. The results suggest that inactivation of XO by H2O2 occurs by production of .OH following direct reduction of H2O2 by XO at the flavin site.     

Inactivation of serine proteinase inhibitors (serpins) in human plasma by reactive oxidants

Activated polymorphonuclear neutrophils (PMN) and macrophages generate oxidizing agents similar to or identical with N-chloroamines. Mimicking this oxidation in normal human plasma by usage of chloramine T (CT), we observed an oxidant concentration-dependent inactivating effect on plasma alpha 2-plasmin inhibitor (alpha 2-PI), antithrombin III (AT III), and alpha 1-proteinase inhibitor (alpha 1-PI). 20-50 mumol CT/ml plasma are necessary for almost complete inactivation of alpha 2-PI and AT III-activity, i.e. about 2-5 times the dose necessary for inactivation of alpha 1-PI which has already been classified as "oxidant sensitive". The inactivation of alpha 1-PI, alpha 2-PI and AT III in plasma by oxidants is the result of a specific oxidative damage since C1-inhibitor, serine proteinases and complexes of plasmin and alpha 2-PI were chloramine resistant under the conditions used. According to our results, the amount of chloramines released by 1 x 10(6) activated PMN, namely ca. 10 nmol (see Weiss et al. Science 222 625-628, 1983) would be sufficient to destroy alpha 1-PI and alpha 2-PI activity of 1.5 and 0.4 microliter of human plasma, respectively. Consequently, activated leukocytes may be able to create a microenvironment in which elastase as well as plasmin and thrombin can display their proteolytic activity unchecked by their regulator proteins. Oxidation may provide a general basis for altering enzyme/inhibitor balances.     

Thermal stabilization of glucose oxidase and glucoamylase by physical entrapment.

Physical entrapment was used as an approach to achieve thermal stabilization of enzymes. The t 1/2 values for the thermoinactivation of glucose oxidase and glucoamylase were increased several-fold by their entrapment in polyacrylamide gels. In polyacrylate gels the individual enzymes behaved differently, probably owing to microenvironmental effects arising by the polyelectrolyte nature of the carrier.     

Inactivation of glutathione S-transferases by nitric oxide-derived oxidants: exploring a role for tyrosine nitration

Reactive intermediates derived from nitric oxide ((*)NO) are thought to play a contributing role in disease states associated with inflammation and infection. We show here that glutathione S-transferases (GSTs), principal enzymes responsible for detoxification of endogenous and exogenous electrophiles, are susceptible to inactivation by reactive nitrogen species (RNS). Treatment of isolated GSTs or rat liver homogenates with either peroxynitrite, the myeloperoxidase/hydrogen peroxide/nitrite system, or tetranitromethane, resulted in loss of GST activity with a concomitant increase in the formation of protein-associated 3-nitrotyrosine (NO(2)Tyr). This inactivation was only partially (<25%) reversible by dithiothreitol, and exposure of GSTs to hydrogen peroxide or S-nitrosoglutathione was only partially inhibitory (<25%) and did not result in protein nitration. Thus, irreversible modifications such as tyrosine nitration may have contributed to GST inactivation by RNS. Since all GSTs contain a critical, highly conserved, active-site tyrosine residue, we postulated that this Tyr residue might present a primary target for nitration by RNS, thus leading to enzyme inactivation. To directly investigate this possibility, we analyzed purified mouse liver GST-mu, following nitration by several RNS, by trypsin digestion, HPLC separation, and matrix-assisted laser desorption/ionization-time of flight analysis, to determine the degree of tyrosine nitration of individual Tyr residues. Indeed, nitration was found to occur preferentially on several tyrosine residues located in and around the GST active site. However, RNS concentrations that resulted in near complete GST inactivation only caused up to 25% nitration of even preferentially targeted tyrosine residues. Hence, nitration of active-site tyrosine residues may contribute to GST inactivation by RNS, but is unlikely to fully account for enzyme inactivation. Overall, our studies illustrate a potential mechanism by which RNS may promote (oxidative) injury by environmental pollutants in association with inflammation.     

Inactivation by glucose of phosphoenolpyruvate carboxykinase from Saccharomyces cerevisiae

Phosphoenolpyruvate carboxykinase showed high activity in Saccharomyces cerevisiae grown on gluconeogenic carbon sources. Addition of glucose to such cultures caused a rapid loss of the phosphoenolpyruvate carboxykinase activity. Fructose or mannose had the same effect as glucose, while 2-deoxyglucose or galactose were without effect. The inactivation was an irreversible process, since the regain of the activity was dependent of de novo protein synthesis. Cycloheximide did not prevent inactivation. All strains of the genus Saccharomyces tested showed inactivation of their phosphoenolpyruvate carboxykinase upon addition of glucose; this behaviour was not restricted to this genus.Non-Standard Abbreviations FbPase fructose bisphosphatase [EC 3.1.3.11 fructose-1,6-bisphosphate hydrolase] - PEPCK phosphoenolpyruvate carboxykinase [EC 4.1.49 ATP: oxalacetate carboxylase (transphosphorylating)] - YPE yeast-peptone-ethanol A preliminary account of these results was presented at the Fourth International Symposium on Yeasts, Vienna, Austria, July 1974     

Transport of gluconate in Rhodotorula glutinis. Inactivation by glucose of the uptake system.

Transport of gluconate has been studied in a wild-type strain of Rhodotorula glutinis and in a mutant derived from it which has acquired the ability to grow on gluconate as the only carbon and energy source. The transport is energy dependent. It shows the same K_m for gluconate (0.1 mm) between pH 4.7 and 7, which suggests that the negatively charged gluconate is the true substrate for the transport system. The rate of gluconate uptake is much lower in the wild type than in the mutant. The mutant grown on gluconate transports gluconate much faster than if grown on other carbon sources. Glucose rapidly and irreversibly inactivates the transport system. This inactivation can also be effected by δ-gluconolactone and to a lesser extent by acetate; it is not prevented by gluconate and occurs also in the presence of cycloheximide.     

Inactivation of active glucose transport in Candida wickerhamii is triggered by exocellular glucose

Abstract Under conditions of derepression the yeast Candida wickerhamii formed a high-affinity glucose proton symport. Glucose and glucose analogues induced inactivation of the glucose proton symport and its interconversion into a low-affinity facilitated diffusion system. The specific inactivation rate increased with the concentration of the inactivating sugar and did not obey saturation kinetics. This dependence was still pronounced at sugar concentrations far above saturation of the glucose transport systems. This suggested that the inactivation and interconversion mechanism was triggered by interaction of the inactivating sugar with receptor sites located on the cell surface.