Enzymatic oxidation of phenothiazines by lipoxygenase/H2O2 system

The Effects of Natural Flavonoids on Lipoxygenase-Mediated Oxidation of Compounds with a Benzene Ring Structure—A New Possible Mechanism of Flavonoid Anti-Chemical Carcinogenesis and Other Toxicities

Numerous studies have strongly suggested that flavonoids exhibit antimutagenic, anticarcinogenic, antiallergic, and anti-inflammatory properties, but the mechanism is still far from clear. In this study, the effect of natural flavonoid compounds, such as green tea polyphenol, epigallocatechin gallate, quercetin, and rutin on lipoxygenase-mediated co-oxidation of guaiacol, benzidine, paraphenylenediamine, and dimethoxybenzidine was investigated. Green tea polyphenol, epigallocatechin gallate, quercetin, and rutin can reduce the co-oxidation reaction speed of tested compounds mediated by soybean lipoxygenase and the production of oxidative products and free radical intermediates. Their median inhibition concentrations on guaiacol oxidation mediated by soybean lipoxygenase were 8.22 mg·L−1, 17.8 μmol·L−1, 41.5 μmol·L−1, and 46.3 μmol·L−1, respectively. These were all significantly lower than glutathione, dithiothreitol, butylated hydroxyanisole and gossypol. The data collected in this study suggest that flavonoids may have an anticarcinogenicity and antitoxicity effect through inhibition of oxidative activation.

Lipoxygenase-Mediated N-Demethylation of Imipramine and Related Tricyclic Antidepressants in the Presence of Hydrogen Peroxide

In this study, we examined the ability of soybean lipoxygenase to mediate the N-demethylation of imipramine and related drugs in the presence of hydrogen peroxide. Formaldehyde generation resulting from the N-demethylation of imipramine, a prototype drug, was found to depend on incubation time, and the concentration of the enzyme, imipramine, and hydrogen peroxide. Under optimal assay conditions, Vmax values of 14 to 18 nmol formaldehyde/min/nmol enzyme or 133 to 164 nmol formaldehyde/min/mg protein were observed. An inhibition of formaldehyde and desipramine formation by nordihydroguaiaretic acid confirmed the lipoxygenase involvement. The blockade of the reaction by glutathione, dithiothreitol, butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT) indicated the generation of a free radical intermediate from imipramine. Desipramine, trimipramine, clomipramine, and diltiazem, but not amitriptyline and doxepin, were also oxidized, albeit at a lower rate. Collectively, the evidence gathered in this study suggests, for the first time, that tricyclic antidepressant drugs may undergo lipoxygenase-catalyzed N-demethylation.

Anomalous oxidation of MDL 73,404 by horseradish peroxidase

3,4-Dihydro-6-hydroxy-N,N,N-2,5,7,8-heptamethyl-2H-1-benzopyran-2-ethanaminium-4-methylbenzene sulfonate (MDL 73,404) is a cardioselective water-soluble quaternary ammonium analogue of Vitamin E which is synthesized to augment the antioxidant defence in situations of free radical injury such as myocardial infarction/reperfusion. Its oxidation by any peroxidative enzyme has not been studied kinetically. This paper describes its enzymatic oxidation by horseradish peroxidase (HRP). The activity was followed spectrophotometrically at 255nm, and the experimental results were simulated using the program "KINETIC 3.1" for Windows 3.x. The MDL 73,404 was oxidized by horseradish peroxidase in the presence of H2O2 to its corresponding MDL 73,404 quinone. During this oxidation, the horseradish peroxidase showed an unexpectedly slow kinetic response with time, which contrast with the linear product accumulation curve measured with 2,2'-azino-bis-(3-estilbenzotiazol-6-sulfonic acid) (ABTS). This response was dependent on the respective concentrations of enzyme, MDL 73,404 and H2O2. However, when the enzyme was incubated with H2O2, the slow kinetic response disappeared and a lag period was observed. Furthermore, when p-coumaric acid (PCA) was added, the activity increased and the slow kinetic response became a straight line. In order to explain this anomalous behaviour, a kinetic model has been proposed and its differential equations simulated. From the correlation between experimental and simulated results it is concluded that MDL 73,404 can act as a slow response substrate for peroxidase, probably due to the presence of a quaternary ammonium side chain that confers on it a slow capacity to convert compound III into ferriperoxidase.

Purification of a novel lipoxygenase from eggplant (Solanum melongena) fruit chloroplasts

A novel membrane lipoxygenase (LOX; EC 1.13.11.12) from eggplant (Solanum melongena L. cv. Belleza negra) fruit chloroplasts has been purified 20-fold to a specific activity of 207 enzymatic units per mg of protein with a yield of 72%. The purification was carried out by sonicating the chloroplastic membranes in the presence of Triton X-114 followed by phase partitioning and anion exchange chromatography. The purified membrane LOX preparation consisted of a single major band with an apparent molecular mass of 97 kDa after sodium dodecyl sulfate polyacrylamide gel electrophoresis. The results obtained using intact chloroplasts indicate that the enzyme is not localized in the stroma. When the enzyme reacts with linoleic acid, it produces a single peak, which comigrates with standard 9-hydroperoxy-octadecadienoic acid. A physiological role for this chloroplastic LOX is proposed.

Human term placental lipoxygenase-mediated N-demethylation of phenothiazines and insecticides in the presence of linoleic acid

This study investigated the hypothesis that human term placental lipoxygenase (HTPLO) and soybean lipoxygenase (SLO) are capable of mediating N-demethylation of selected phenothiazines and insecticides in the presence of linoleic acid (LA). In addition to being LA dependent, the N-demethylation reaction mediated by HTPLO and SLO was limited by incubation time, pH of the medium, concentration of the enzyme and the substrate. Using Nash reagent to monitor formaldehyde production, the specific activity for LA-dependent N-demethylation of chlorpromazine, a model phenothiazine, was determined to be 1.7+/-0.3 nmoles/min/mg HTPLO. Besides chlorpromazine, N-demethylation of promazine, promethazine and trimeprazine was also observed. The insecticide, aminocarb, displayed a specific activity of 2.2+/-0.3 nmoles/min/mg HTPLO for N-demethylation. Other insecticides, namely chlordimeform, dicrotophos and zectran, were oxidized in a similar manner. As compared with HTPLO, the rates of N-demethylation of phenothiazines and insecticides mediated by SLO were higher. Classical inhibitors of lipoxygenase, as well as antioxidants and free radical scavengers, caused a dose-dependent reduction in the production of formaldehyde from chlorpromazine and aminocarb by HTPLO. These results clearly demonstrate the ability of polyunsaturated free fatty acids to support N-demethylation of xenobiotics via the lipoxygenase pathway.

Lipoxygenase-mediated hydrogen peroxide-dependent N-demethylation of N,N-dimethylaniline and related compounds

To date, studies of xenobiotic N-demethylation have focused on heme-proteins such as P450 and peroxidases. In this study we investigated the ability of non-heme iron proteins, namely soybean lipoxygenase (SLO) and human term placental lipoxygenase (HTPLO) to mediate N-demethylation of N,N-dimethylaniline (DMA) and related compounds in the presence of hydrogen peroxide. In addition to being hydrogen peroxide dependent, the reaction was also dependent on incubation time, concentration of enzyme and DMA and the pH of the medium. Using Nash reagent to estimate formaldehyde production, we determined the specific activity for SLO mediated N-demethylation of DMA to be 200 + 18 nmol HCHO/min per mg protein or 23 +/- 2 nmol/min per nmol of enzyme, while that of HTPLO was 33 +/- 4 nmol HCHO/min per mg protein. Nordihydroguaiaretic acid (NDGA), a classical inhibitor of lipoxygenase (LO), as well as antioxidants and free radical reducing agents, caused a marked reduction in the rate of production of formaldehyde from DMA by SLO. Besides N,N-dimethylaniline, N-methylaniline, N,N,N',N'-tetramethylbenzidine, N,N-dimethyl-p-phenylenediamine, N,N-dimethyl-3-nitroaniline and N,N-dimethyl-p-toluidine were also demethylated by SLO. The formation of a DMA N-oxide was not detected. Preliminary experiments suggested SLO-mediated hydrogen peroxide-dependent S-dealkylation of methiocarb or O-dealkylation of 4-nitroanisole does not occur.

A simple and efficient method for hemoglobin removal from mammalian tissue cytosol by zinc sulfate and its application to the study of lipoxygenase

A simple and efficient method is described to remove hemoglobin (Hb) from human term placental cytosol to study dioxygenase and co-oxidase activities of lipoxygenase. In the untreated samples, 70%-80% of the linoleic acid-dependent dioxygenase and co-oxidase activities were found to be associated with the pseudo-lipoxygenase activity of Hb. Zinc sulfate (0.5 mM) precipitated >97% of the Hb present in the cytosol. The dioxygenase activity of the ZnSO4 treated cytosol exhibited a Vmax value of 313 nmoles linoleic acid hydroperoxide formed/min/mg protein and a K(M) of 1.4 mM for linoleic acid. The ZnSO4 treated cytosol displayed co-oxidase activity toward benzidine, dimethoxybenzidine, guaiacol, pyrogallol, tetramethylbenzidine and tetramethyl-p-phenylenediamine. Nordihydroguaiaretic acid, 5,8,11-eicosatriynoic acid, butylated hydroxyanisole, butylated hydroxytoluene and gossypol caused concentration dependent inhibition of dioxygenase and co-oxidase activities. These results suggest ZnSO4 precipitation of Hb from cytosol does not alter the functional characteristics of the human term placental lipoxygenase.

Lipoxygenase-mediated biotransformation of p-aminophenol in the presence of glutathione: possible conjugate formation

This study tested a hypothesis that soybean lipoxygenase (SLO), a model enzyme, may be capable of generating a glutathione (GSH) conjugate(s) from p-aminophenol (PAP). Horseradish peroxidase was employed as a positive control. GSH depletion or an increase in the absorption at 327 nm with time due to GS-PAP formation was used to quantitate the reaction. The rate of GS-PAP formation was dependent on the incubation time and the amount of SLO and exhibited Km values of 0.44 and 0.71 mM for PAP and H2O2, respectively. Classical inhibitors of lipoxygenase and free radical scavengers markedly decreased the rate of GS-PAP formation in a concentration-dependent manner. PAP-dependent GSH depletion from the reaction medium occurred at a rate of 2.37 +/- 0.18 micromol/min/mg protein. Collectively, the results suggest that lipoxygenase pathway may be involved in the enzymatic formation of GSH conjugate(s) from PAP.

Hydroperoxidase activity of lipoxygenase in the presence of cyclodextrins

The oxidation of xenobiotics by the hydroperoxidase activity of lipoxygenase in the presence of cyclodextrins was studied. These produced an inhibitory effect on xenobiotics oxidation, based on their degree of hydrophobicity and the charge (isoproterenol < 4-methyl-catechol (4MC) < 4-tert-butylcatechol (TBC) < 4-tert-octylcatechol (TOC)). This inhibitory effect was due to the complexation of xenobiotics in the hydrophobic cavity of cyclodextrins. The complexation constant Kc was calculated by nonlinear regression of the inhibition curves obtained in the presence of cyclodextrins, and the values obtained were 400, 16,250, and 35,127 M-1 for 4MC, TBC, and TOC, respectively. The validity of these values was checked at different points of the Michaelis-Menten saturation curve, and a sigmoidal inhibition curve was obtained at the saturating concentration of the o-diphenol, TBC, with no change in the Kc value. This demonstrates the validity of the equations used to calculate Kc for the complete range of the Michaelis-Menten equation.

Hydroperoxidative oxidation of diethylstilbestrol by lipoxygenase

The oxidation of diethylstilbestrol (DES), a synthetic carcinogenic estrogen, by the hydroperoxidase activity of lipoxygenase was studied. Lipoxygenase catalyzes the oxidation of DES to its corresponding DES quinone to yield free radical species intermediates (DES semiquinone and DES quinone), which are associated with the adverse effects of this synthetic estrogen. The reaction was dependent on enzyme, DES, and hydrogen peroxide concentrations. Due to the low degree of water solubility of DES, the enzyme works in a range of DES concentrations below K(m). The enzyme presents a high affinity for hydrogen peroxide (5.7 microM), and produces substrate inhibition (Ksi = 2.5 mM). This study is the first demonstration that this reaction, which is known to be catalyzed by a variety of enzymes, including peroxidases, is also catalyzed by lipoxygenase.

A kinetic study of the one-electron oxidation of Trolox C by the hydroperoxidase activity of lipoxygenase

The oxidation of Trolox C (a vitamin E analog) by the hydroperoxidase activity of lipoxygenase was studied. Trolox C was oxidized to its corresponding phenoxyl radical in the presence of hydrogen peroxide, evolving through a ketodiene intermediate to the Trolox C quinone. The H2O2/Trolox C quinone molar ratio was 1.0. The overall reaction followed an enzymatic-chemical second-order system and involved a substrate regeneration mechanism. From the equations derived from this mechanism, the dismutation constant of the Trolox C radical was evaluated by non-linear regression as 4 x 10(5) M(-1) x s(-1). The accumulation curve of Trolox C quinone was found to be linear, with no lag period, and dependent on enzyme concentration. No phenoxyl radical was detected when the reaction was carried out in the presence of ascorbate. This synergistic reaction between the Trolox C radical and ascorbate was quantitative and depended on the respective concentrations of enzyme, Trolox C and hydrogen peroxide. The results presented in this paper suggest that the diferences observed in the kinetic behaviour of monophenols (one-electron donors) and diphenols (two-electron donors) stem from the fact that the latter evolve directly into ferric form without taking the slow pathway once the steady state is reached, whereas the monophenols are always forced take the slow way, even in the steady state. This peroxidative oxidation of a vitamin E analog by the hydroperoxidase activity of lipoxygenase together with the oxidation produced by dioxygenase activity suggests that lipoxygenase might be a key enzyme in destroying the lipophilic antioxidant barrier against the reactive oxygen species in membranes.

Oxidation of aminopyrine by the hydroperoxidase activity of lipoxygenase: a new proposed mechanism of N-demethylation

The oxidation of aminopyrine, an N-alkyl aromatic amine, by the hydroperoxidase activity of lipoxygenase was studied. Aminopyrine gave rise to a purple color in the presence of H2O2 and lipoxygenase, the color being proportional to the aminopyrine radical cation. The H2O2/aminopyrine radical cation molar ratio was 0.5. The overall reaction was considered as an enzymic-chemical second order mechanism with substrate regeneration. From the equations, the apparent constant of the radical cation's decomposition (k'app) was evaluated under different experimental conditions. It was found to be inversely proportional to the proton concentration but unaffected by the concentration of aminopyrine. These results suggest a new comprehensive mechanism for N-demethylation, which takes into account the described presence of both nitrogen- and carbon-centered radicals and the marked effect of pH on the stability of the radical cation.

Protection by different agents against inactivation of lipoxygenase by hydrogen peroxide

H2O2 is a potent inactivator of lipoxygenase. In this paper, the ability of different agents [mannitol, oleic, stearic and linoleic acid, n-butanol, and hydroperoxy octadecadienoic acid (HPOD)] to prevent the inactivation of tomato lipoxygenase by hydrogen peroxide has been studied. The involvement of OH' in the inactivation process is suggested by the ability of mannitol to prevent the loss of activity. This radical would be produced by reaction of H2O2 with the Fe(II) lipoxygenase. The most effective protection was displayed by HPOD, the product of the reaction of lipoxygenase with linoleic acid. This result could be explained by the conversion of the native enzyme into the Fe(III) lipoxygenase in the presence of HPOD; the Fe(III) enzyme is not able to react with H2O2 and no OH' will be produced. The protective effect obtained with oleic and stearic acid could be explained by an occupation of the active center by these inhibitors. The enzyme would not transform them, but their presence would hamper the conversion of H2O2 in OH' and limit the damage in the active center.

Hydroperoxidase activity of lipoxygenase: a kinetic study of isoproterenol oxidation

The hydroperoxidase activity of soybean lipoxygenase, a non-heme protein, oxidized isoproterenol using H2O2 at pH 6.0. This oxidation was enzymatic, since neither heat-denaturated enzyme or iron ions in the presence of H2O2 produced an increase in absorbance. The initial rate was not linear and showed a characteristic lag period whose length depended on the enzyme and substrate concentration. The lag was decreased if the enzyme and isoproterenol concentration were increased, whereas it increased if the H2O2 concentration was increased. Lipoxygenase showed the typical low specificity for electron donor characteristic of this hydroperoxidase activity (26 mM), but a high affinity for H2O2 (94 microM), although with substrate inhibition (ksi = 3.6 mM). The chemical intermediates produced during the oxidation of isoproterenol were characterized in order to determine the origin of the lag period. A plausible kinetic mechanism is proposed to explain the observed lag period and inhibition by high concentrations of H2O2.