Singlet oxygen production from the peroxidase-catalyzed oxidation of indole-3-acetic acid

The aerobic oxidation of indole-3-acetic acid catalyzed by horseradish peroxidase produces 1268 nm emission characteristic of singlet oxygen. Lactoperoxidase also oxidizes indole-3-acetic acid to produce singlet oxygen, but in contrast to horseradish peroxidase, this enzyme system requires hydrogen peroxide. In both of these systems, the intensity of the 1268 nm emission is small due to quenching of the singlet oxygen by indole-3-acetic acid and by reaction products derived from indole-3-acetic acid. The biomolecular reaction of peroxyl radicals via a Russell mechanism is a plausible mechanism for the singlet oxygen generation in these systems. Under typical conditions of p2H 4.0, 1 microM horseradish peroxidase, 1 mM indole-3-acetic acid, and 240 microM oxygen, the singlet oxygen yield was 15 +/- 1 microM or 13% of the amount predicted by the Russell mechanism.     

Ethylene enhances reactivity of superoxide with peroxidase to form the oxy-ferrous complex

Ethylene and its analogues acetylene, carbon monoxide, and propylene inhibited the rate of oxidation of indole-3-acetic acid by peroxidase. Annulment of this effect by addition of superoxide dismutase showed that inhibition occurred only in the presence of the superoxide anion radical (O2-.). Kinetic and spectral data established that ethylene and its analogues enhanced markedly the rate of reaction of O2-. with peroxidase. This reaction resulted in the formation of compound III, an oxy-ferrous complex of peroxidase. In the presence of indole-3-acetic acid, the interaction between ethylene, peroxidase, and O2-. activated the reduced peroxidase in equilibrium compound III shuttle. O2-. is a major product of this shuttle, and compound III constitutes the dominant steady-state form of peroxidase. These interactions may help to explain the mechanism of action of ethylene as a plant growth regulator.     

Promotion of peroxidase activity in the cell wall of Nicotiana

Peroxidase catalyzes the oxidation of indole-3-acetic acid. The primary products of this reaction stimulate growth in plants. Therefore, our concept is that an increase in peroxidase activity will increase the effect of indole-3-acetic acid as a growth hormone. Our objective was to study the effect of 2,3,5-triiodobenzoic acid, a growth regulator, on isoperoxidases in the cell wall and cytoplasm of Nicotiana. Isoperoxidases from the cell wall and cytoplasmic fractions were separated by acrylamide gel electrophoresis. We found that 2,3,5-triiodobenzoic acid and indole-3-acetic acid increase peroxidase activity in the cell wall. Since both 2,3,5-triiodobenzoic acid and indole-3-acetic acid increase the activity of the same isoperoxidase, we conclude that 2,3,5-triiodobenzoic acid synergizes rather than antagonizes auxin action, and we suggest that this increase in indole-3-acetic acid oxidase activity sensitizes plant tissues to auxin.     

Effects of Seselin and Coumarin on Growth, Indoleacetic Acid Oxidase, and Peroxidase, with Special Reference to Cucumber (Cucumis sativa L.) Radicles

Seselin, a natural coumarin derivative isolated from citrus roots, inhibited radicle growth in seedlings of cucumber (Cucumis sativa), lettuce (Lactuca sativum), radish (Raphanus sativus), and wheat (Triticum aestivum) grown in the dark. Coumarin similarly inhibited radicle growth of cucumber seedlings. Growth retardation of the cucumber radicles was accompanied by an increased activity of peroxidase and indole-3-acetic acid oxidase. Both compounds antagonized indole-3-acetic acid-induced growth of wheat coleoptiles, whereas coumarin was much less effective than seselin in antagonizing gibberellic acid-induced release of reducing sugars from barley endosperm. It is suggested that seselin plays an important role in the regulation of root growth, and that it is the indole-3-acetic acid oxidase cofactor previously detected in citrus roots.     

Indole-3-methanol as an intermediate in the oxidation of indole-3-acetic acid by peroxidase

During oxidation of indole-3-acetic acid catalyzed by horseradish peroxidase, indole-3-aldehyde and 3-hydroxymethayloxindole cease to be produced a few minutes after initiation of the reaction even though IAA is still being consumed. At the same time an increased accumulation of indole-3-methanol is observed and the ratio of oxygen to indole-3-acetic acid consumed becomes less than unity. Indole-3-niethanol can be a substrate for horseradish peroxidase provided that H₂O₂ is present. In this reaction, indole-3-aldehyde but not 3-hydroxymethyloxindole is formed. H₂O₂ is not merely an activating agent for the enzyme but also a true oxidant because it is consumed stoichiometrically (1 mol of H₂O₂ per mol of indole-3-methanol) and the reaction is independent of the presence of oxygen. Indole-3-methanol is proposed as an intermediate in the process of oxidation of indole-3-acetic acid into indole-3-al-denyde, the second step of which requires peroxide as an oxidant.     

Influence of exogenous hormones on growth and secondary metabolite production in hairy root cultures of Cichorium intybus L. CV. Lucknow local

Summary The effect of exogenously fed hormones on hairy root cultures of Cichorium intybus L. ev. Lucknow Local was studied. It was seen that auxin in the presence of low levels of kinetin induces rapid disorganization in hairy root cultures of C. intybus, ultimately to form suspension cultures, and this process was associated with the decrease in coumarin content in the cells. Of various treatments, it was observed that with an increase in the auxin: cytokinin ratio, the biomass decreased with the increase in disorganization index during the culture period of 28 d. The disorganization index was less when the inoculum size was enhanced to 10-fold. The total endogenous indole-3-acetic acid titers and indole-3-acetic acid oxidase activity also decreased with an increase in disorganization index, and was independent of initial inoculum size, with only a magnitude difference. The total coumarin content strictly correlated with growth in all the treatments. In contrast, exogenously supplied gibberellic acid at the 0.5 mg l⁻¹ level enhanced growth, coumarin content, and branching patterns over the control and other treatments on day 28. The exogenously fed growth regulators had an effect on growth, auxin and coumarin biosyntheses, wherein transformed roots treated with increasing concentration of auxin to cytokinin ratios lost their ability for coumarin biosynthesis. The behavior of hairy roots from an Indian cultivar of chicory upon growth regulator treatment is discussed in terms of growth, coumarin and auxin biosyntheses.     

The reaction of coumarins with horseradish peroxidase

The peroxidase catalyzed oxidation of indole-3-acetate is inhibited by naturally occurring coumarins such as scopoletin. This inhibition is due to the preferential reactivity of the coumarins with the peroxidase compounds I, II, and III. In view of the possible growth regulatory role of coumarins in plants, the mechanism of oxidation of scopoletin by horse-radish peroxidase has been investigated.     

Inactivity of Oxidation Products of Indole-3-acetic Acid on Ethylene Production in Mung Bean Hypocotyls

The suggestion that indole-3-acetic acid (IAA)-stimulated ethylene production is associated with oxidative degradation of IAA and is mediated by 3-methyleneoxindole (MOI) has been tested in mung bean (Phaseolus aureus Roxb.) hypocotyl segments. While IAA actively stimulated ethylene production, MOI and indole-3-aldehyde, the major products of IAA oxidation, were inactive. Tissues treated with a mixture of intermediates of IAA oxidation, obtained from a 1-hour incubation of IAA with peroxidase, failed to stimulate ethylene production. Furthermore, chlorogenic acid and p-coumaric acid, which are known to interfere with the enzymic oxidation of IAA to MOI, had no effect on IAA-stimulated ethylene production. Other oxidation products of IAA, including oxindole-3-acetic acid, indole-3-carboxylic acid, (2-sulfoindole)-3-acetic acid, and dioxindole-3-acetic acid, were all inactive. 1-Naphthaleneacetic acid was as active as IAA in stimulating ethylene production but was decarboxylated at a much lower rate than IAA, suggesting that oxidative decarboxylation of auxins is not linked to ethylene production. These results demonstrate that IAA-stimulated ethylene production in mung bean hypocotyl tissue is not mediated by MOI or other associated oxidative products of IAA.     

Horseradish peroxidase-catalyzed aerobic oxidation and peroxidation of indole-3-acetic acid. I. Optical spectra.

A study of the indole-3-acetate reaction with horse-radish peroxidase, in the absence or presence of hydrogen peroxide, has been performed, employing rapid scan and conventional spectrophotometry. We present here the first clear spectral evidence, obtained on the millisecond time scale, indicating that at pH 5.0 and for high [enzyme/substrate] ratios peroxidase compound III is formed. Most, if not all, of the compound III is formed by oxygenation of the ferrous peroxidase. There is an inhibitory effect of superoxide dismutase and histidine on compound III formation which indicates the involvement of the active oxygen species superoxide and singlet oxygen. It is concluded that the oxidation of indole-3-acetate by horseradish peroxidase at pH 5.0 proceeds through compound III formation to the catalytically inactive forms P-670 and P-630. A reaction path in which the enzyme is directly reduced by indole-3-acetate might be involved as an initiation step. Rapid scan spectral data, which indicate differences in the formation and decay of enzyme intermediate compounds at pH 7.0, in comparison with those observed at pH 5.0, are also presented. At pH 7.0 compound II is a key intermediate in oxidation--peroxidation of substrate. Mechanisms of reactions consistent with the experimental data are proposed and discussed.     

Stimulation of the oxidative decarboxylation of indole-3-acetic acid in citrus tissues by ethylene

Ethylene has been shown to stimulate the degradation of indole-3-acetic acid (IAA) in citrus leaf tissues via the oxidative decarboxylation pathway, resulting in the accumulation of indole-3-carboxylic acid (ICA). Preliminary data indicated that ethylene stimulates only the first step of this pathway, i.e. the decarboxylation of IAA which leads to the formation of indole-3-methanol. The effect of ethylene seems to be a specific one since 2,5-norbornadiene, an ethylene action inhibitor, significantly inhibited the stimulation of IAA decarboxylation by ethylene. It has long been suggested that peroxidase or a specific form of the peroxidase complex (`IAA oxidase') catalyse this step. However, we did not observe a clear effect of ethylene on the peroxidase system. An alternative possibility, that the stimulatory effect of ethylene on IAA catabolism results from increased formation of hydrogen peroxide (H₂O₂), a co-factor for peroxidase activity, was verified by direct measurements of H₂O₂ in the tissues or by assaying the activity of gluthathione reductase, which has been shown to be induced by oxygen species. This possibility is further supported by the observations showing that IAA decarboxylation in control tissues was enhanced to the level detected in ethylene-treated tissues by application of H₂O₂.     

Oxidation of indole-3-acetic acid by peroxidase: involvement of reduced peroxidase and compound III with superoxide as a product

Kinetic and spectral data establish that peroxidase may oxidize indole-3-acetic acid by either of two pathways depending on the enzyme/substrate ratio. When relatively low enzyme/substrate ratios are employed, the oxidation proceeds through a reduced peroxidase in equilibrium compound III shuttle. Conversely, peroxidase operates through the conventionally accepted pathway involving native enzyme and compounds I and II only when high enzyme/substrate ratios are used. Compound III, a specific oxidase, constitutes the dominant steady-state form of peroxidase when the reduced peroxidase in equilibrium compound III shuttle is operational. Activation of this shuttle also produces a flux of superoxide anion radical at the expense of molecular oxygen. Thus, important biological consequences may follow activation of this shuttle under physiological conditions.     

Proposed Model for the Peroxidase-Catalyzed Oxidation of Indole-3-acetic Acid in the Presence of the Inhibitor Ferulic Acid

Linear increments in ferulic acid concentration produce logarithmic increases in the ferulic acid-induced lag periods prior to the peroxidase-catalyzed oxidation of indole-3-acetic acid in a system containing 2,4-dichlorophenol and MnCl₂ in acetate buffer at pH 5.6. Maintaining the ratio of indole-3-acetic acid to ferulic acid constant at 100 while linearly raising the ferulic acid concentration results in linear increases in the lag period. Both indole-3-acetic acid and ferulic acid are substrates of horseradish peroxidase in the presence of H₂O₂, and indole-3-acetic acid competitively inhibits the oxidation of ferulic acid. A model for the enzymatic oxidation of indole-3-acetic acid catalyzed by peroxidase is proposed.     

The Role of Indole-3-Acetic Acid in Peroxidase Oxidation of Ascorbic Acid Catalyzed by Horseradish Peroxidase

We studied stationary kinetics of ascorbic acid oxidation in the presence of indole-3-acetic acid catalyzed by horseradish peroxidase. The catalytic (k_cat and K_m) and inhibition (K_i) constants were determined for pH from 4.5 to 7.0. The auxin proved to competitively inhibit the enzyme when a single ascorbic acid molecule is bound, while a non-competitive inhibition by IAA is observed for peroxidase oxidation of two or more substrate molecules. A mechanism of ascorbic acid oxidation in the presence of indole-3-acetic acid is proposed.     

Oxidation of Indole-3-Acetic Acid to Oxindole-3-Acetic Acid by an Enzyme Preparation from Zea mays

Indole-3-acetic acid is oxidized to oxindole-3-acetic acid by Zea mays tissue extracts. Shoot, root, and endosperm tissues have enzyme activities of 1 to 10 picomoles per hour per milligram protein. The enzyme is heat labile, is soluble, and requires oxygen for activity. Cofactors of mixed function oxygenase, peroxidase, and intermolecular dioxygenase are not stimulatory to enzymic activity. A heat-stable, detergent-extractable component from corn enhances enzyme activity 6- to 10-fold. This is the first demonstration of the in vitro enzymic oxidation of indole-3-acetic acid to oxindole-3-acetic acid in higher plants.     

An electron spin resonance study of free radical intermediates in the oxidation of indole acetic acid by horseradish peroxidase

The oxidation of indole-3-acetic acid by horseradish peroxidase was studied using the spin traps t-nitrosobutane and 5,5-dimethyl-1-pyrroline N-oxide to trap free radical intermediates. The major free radical metabolite of indole acetic acid was unambiguously determined by the use of indole-3-[2,2-2H2]acetic acid to be the skatole carbon-centered free radical. In the presence of oxygen, superoxide was also trapped.     

The inhibition of peroxidase and indole-3-acetic acid oxidase activity by British Anti-Lewisite

British Anti-Lewisite (BAL) binds to horseradish peroxidase in a manner which results in inhibition of both peroxidatic and oxidative functions of the enzyme. BAL competes with hydrogen peroxide for binding on peroxidase, and the inhibition of peroxidatic activity is irreversible. Solutions of purified horseradish peroxidase and individually resolved peroxidase isozymes show a gradual loss of peroxidatic activity with time when incubated with BAL. In these same treatments, however, the inhibition of indole-3-acetic acid (IAA) oxidase activity is immediate. With increasing amounts of enzyme in the incubation mixture, IAA oxidase activity is not completely inhibited and is observed following a lag period in the assay which shortens with longer incubation times. Peroxidase activity during this same time interval shows a lag period which increases with longer incubation times. Lowering the pH removed the lag period for oxidase activity, but did not change the pattern of peroxidase activity. These results suggest that the sites for the oxidation of indole-3-acetic acid and for peroxidatic activity may not be identical in horseradish peroxidase isozymes.     

Scopoletin and Polyphenol-Induced Lag in Peroxidase Catalyzed Oxidation of Indole-3-acetic Acid

The rate of initial indole-3-acetic acid (IAA) oxidation with horseradish peroxidase is modified with scopoletin, scopolin and other phenolic derivatives. In the presence of phenolics there is an initial lag phase in the oxidation. The early lag is dissipated enzymatically after which the rale of IAA oxidation again returns to normal. Chlorogenic and sinapic acids produce the longest lag periods of the compounds reported here, whereas the glucoside, scopolin, produced the least inhibition. Scopoletin is more than 10× as inhibitory as scopolin.     

Glucuronidation of oxygenated benzo(a)pyrene derivatives by UDP-glucuronyl transferase of nuclear envelope

The variation of the spectra and its reactivity towards 2-methylpropanal, indole-3-acetic acid and malonaldehyde of solutions of horseradish peroxidase in dimethyl sulfoxide-water mixtures has been studied. A broad pattern of changes was observed in the CD spectra of peroxidase, especially in the 400 nm region. These variations influenced strongly the excited triplet acetone emission from the 2-methylpropanal system which is generated in the active site of the enzyme protected from external quenching. This means that presumably the active site is more uncovered in the presence of dimethyl sulfoxide than the native form. Energy transfer parameters indicate that in fact there is a conformational effect produced by dimethyl sulfoxide in the horseradish peroxide active site. Dimethyl sulfoxide appears to be an important conformational probe in biochemistry.     

Exogenous auxin affects the oxidative burst in barley roots colonized by Piriformospora indica

Beside a cardinal role in coordination of many developmental processes in the plant, the phytohormone auxin has been recognized as a regulator of plant defense. The molecular mechanisms involved are still largely unknown. Using a sensitive chemiluminescence assay, which measures the oxidation of luminol in the presence of H₂O₂ by horseradish peroxidase (HRP), we report here on the ability of exogenously added indole-3-acetic acid (IAA) to enhance the suppressive effect of the root endophyte Piriformospora indica on the chitin-elicited oxidative burst in barley roots. Thus, the potential of P. indica to produce free IAA during the early colonization phase in barley might provide the symbiont with a means to interfere with the microbe-associated molecular patterns (MAMP)-triggered immunity.     

Horseradish peroxidase: a modern view of a classic enzyme

Horseradish peroxidase is an important heme-containing enzyme that has been studied for more than a century. In recent years new information has become available on the three-dimensional structure of the enzyme and its catalytic intermediates, mechanisms of catalysis and the function of specific amino acid residues. Site-directed mutagenesis and directed evolution techniques are now used routinely to investigate the structure and function of horseradish peroxidase and offer the opportunity to develop engineered enzymes for practical applications in natural product and fine chemicals synthesis, medical diagnostics and bioremediation. A combination of horseradish peroxidase and indole-3-acetic acid or its derivatives is currently being evaluated as an agent for use in targeted cancer therapies. Physiological roles traditionally associated with the enzyme that include indole-3-acetic acid metabolism, cross-linking of biological polymers and lignification are becoming better understood at the molecular level, but the involvement of specific horseradish peroxidase isoenzymes in these processes is not yet clearly defined. Progress in this area should result from the identification of the entire peroxidase gene family of Arabidopsis thaliana, which has now been completed.