The effect of iron to manganese substitution on microperoxidase 8 catalysed peroxidase and cytochrome P450 type of catalysis

This study describes the catalytic properties of manganese microperoxidase 8 [Mn(III)MP8] compared to iron microperoxidase 8 [Fe(III)MP8]. The mini-enzymes were tested for pH-dependent activity and operational stability in peroxidase-type conversions, using 2-methoxyphenol and 3,3'-dimethoxybenzidine, and in a cytochrome P450-like oxygen transfer reaction converting aniline to para-aminophenol. For the peroxidase type of conversions the Fe to Mn replacement resulted in a less than 10-fold decrease in the activity at optimal pH, whereas the aniline para-hydroxylation is reduced at least 30-fold. In addition it was observed that the peroxidase type of conversions are all fully blocked by ascorbate and that aniline para-hydroxylation by Fe(III)MP8 is increased by ascorbate whereas aniline para-hydroxylation by Mn(III)MP8 is inhibited by ascorbate. Altogether these results indicate that different types of reactive metal oxygen intermediates are involved in the various conversions. Compound I/II, scavenged by ascorbate, may be the reactive species responsible for the peroxidase reactions, the polymerization of aniline and (part of) the oxygen transfer to aniline in the absence of ascorbate. The para-hydroxylation of aniline by Fe(III)MP8, in the presence of ascorbate, must be mediated by another reactive iron-oxo species which could be the electrophilic metal(III) hydroperoxide anion of microperoxidase 8 [M(III)OOH MP8]. The lower oxidative potential of Mn, compared to Fe, may affect the reactivity of both compound I/II and the metal(III) hydroperoxide anion intermediate, explaining the differential effect of the Fe to Mn substitution on the pH-dependent behavior, the rate of catalysis and the operational stability of MP8.

Combined quantum mechanical and molecular mechanical reaction pathway calculation for aromatic hydroxylation by p-hydroxybenzoate-3-hydroxylase

The reaction pathway for the aromatic 3-hydroxylation of p-hydroxybenzoate by the reactive C4a-hydroperoxyflavin cofactor intermediate in p-hydroxybenzoate hydroxylase (PHBH) has been investigated by a combined quantum mechanical and molecular mechanical (QM/MM) method. A structural model for the C4a-hydroperoxyflavin intermediate in the PHBH reaction cycle was built on the basis of the crystal structure coordinates of the enzyme-substrate complex. A reaction pathway for the subsequent hydroxylation step was calculated by imposing a reaction coordinate that involves cleavage of the peroxide oxygen-oxygen bond and formation of the carbon-oxygen bond between the C3 atom of the substrate and the distal oxygen of the peroxide moiety of the cofactor. The geometric changes and the Mulliken charge distributions along the calculated reaction pathway are in line with an electrophilic aromatic substitution type of mechanism. The energy barrier of the calculated reaction is considerably lower when the substrate hydroxyl moiety is deprotonated, in comparison with the barrier found with a protonated hydroxyl moiety. This effect of the protonation state of the substrate on the calculated energy barrier supports experimental observations that deprotonation is required for hydroxylation of the substrate. A notable event in the calculated reaction pathway is a lengthening of the peroxide oxygen-oxygen bond at an intermediate stage. Further analysis of the reaction pathway indicates that this oxygen-oxygen bond elongation is accompanied by an increase in electrophilic reactivity on the distal oxygen of the peroxide moiety, which may assist the C-O bond formation in the reaction of the C4a-hydroperoxyflavin intermediate with the substrate. Analysis of the effect of individual active site residues on the reaction reveals a specific transition state stabilization by the backbone carbonyl moiety of Pro293. The crystal water 717 appears to drive the hydroxylation step through a stabilizing hydrogen bond interaction to the proximal oxygen of the C4a-hydroperoxyflavin intermediate, which increases in strength as the hydroperoxyflavin cofactor converts to the anionic (deprotonated) hydroxyflavin.

19F NMR study on the biodegradation of fluorophenols by various Rhodococcus species

Of all NMR observable isotopes 19F is the one perhaps most convenient for studies on biodegradation of environmental pollutants. The reasons underlying this potential of 19F NMR are discussed and illustrated on the basis of a study on the biodegradation of fluorophenols by four Rhodococcus strains. The results indicate marked differences between the biodegradation pathways of fluorophenols among the various Rhodococcus species. This holds not only for the level and nature of the fluorinated biodegradation pathway intermediates that accumulate, but also for the regioselectivity of the initial hydroxylation step. Several of the Rhodococcus species contain a phenol hydroxylase that catalyses the oxidative defluorination of ortho-fluorinated di- and trifluorophenols. Furthermore, it is illustrated how the 19F NMR technique can be used as a tool in the process of identification of an accumulated unknown metabolite, in this case most likely 5-fluoromaleylacetate. Altogether, the 19F NMR technique proved valid to obtain detailed information on the microbial biodegradation pathways of fluorinated organics, but also to provide information on the specificity of enzymes generally considered unstable and, for this reason, not much studied so far.

Quantitative structure activity relationships for the electron transfer reactions of Anabaena PCC 7119 ferredoxin-NADP+ oxidoreductase with nitrobenzene and nitrobenzimidazolone derivatives: mechanistic implications

The steady state single electron reduction of polynitroaromatics by ferredoxin-NADP+ oxidoreductase (EC 1.18.1.2) from cyanobacterium Anabaena PCC 7119 has been studied and quantitative structure activity relationships are described. The solubility of the polynitroaromatics as well as their reactivity towards ferredoxin-NADP+ oxidoreductase are markedly higher than those for previously studied mononitroaromatics and this enabled the independent measurement of the kinetic parameters-k(cat) and Km. Interestingly, the natural logarithm of the bimolecular rate constant, k(cat)/Km, and also the natural logarithm of k(cat) correlate with the calculated energy of the lowest unoccupied molecular orbital of the polynitroaromatic substrates. The minimal kinetic model in line with these quantitative structure activity relationships is a ping-pong mechanism which includes substrate binding equilibria in the second half reaction.

Antioxidant activities of carotenoids: quantitative relationships between theoretical calculations and experimental literature data

Quantitative structure activity relationships (QSARs) are described for the antioxidant activity of series of all-trans carotenoids. The antioxidant activity of the carotenoids is characterised by literature data for (i) their relative ability to scavenge the ABTS*+ radical cation, reflected by the so-called trolox equivalent antioxidant capacity (TEAC) value, (ii) their relative rate of oxidation by a range of free radicals, or (iii) their capacity to inhibit lipid peroxidation in multilamellar liposomes, leading to a decrease in formation of thiobarbituric acid reactive substances (TBARS). All these antioxidant values for radical scavenging action correlate quantitatively with computer-calculated ionisation potentials of the carotenoids. These correlations are observed both when the ionisation potential is calculated as the negative of the energy of the highest occupied molecular orbital (-E(HOMO)) of the molecule, or as the relative change in heat of formation (deltadeltaHF) upon the one-electron oxidation of the carotenoids. The calculations provide a theoretical assay able to characterise the intrinsic electron donating capacity of an antioxidant, in hydrophilic, hydrophobic or artificial membrane environment.

GSTP1-1 stereospecifically catalyzes glutathione conjugation of ethacrynic acid

Using 1H NMR two diastereoisomers of the ethacrynic acid glutathione conjugate (EASG) as well as ethacrynic acid (EA) could be distinguished and quantified individually. Chemically prepared EASG consists of equal amounts of both diastereoisomers. GSTP1-1 stereospecifically catalyzes formation of one of the diastereoisomers (A). The GSTP1-1 mutant C47S and GSTA1-1 preferentially form the same diastereoisomer of EASG as GSTP1-1. Glutathione conjugation of EA by GSTA1-2 and GSTA2-2 is not stereoselective. When human melanoma cells, expressing GSTP1-1, were exposed to ethacrynic acid, diastereoisomer A was the principal conjugate formed, indicating that even at physiological pH the enzyme catalyzed reaction dominates over the chemical conjugation.

Conversion of pentahalogenated phenols by microperoxidase-8/H2O2 to benzoquinone-type products

This study reports the microperoxidase-8 (MP8)/H2O2-catalyzed dehalogenation of pentafluorophenol and pentachlorophenol, compounds whose toxic effects and persistence in the environment are well documented. The primary products of this dehalogenation reaction appear to be the corresponding tetrahalo-p-benzoquinones. Under the conditions used, the fluorinated phenol and its intermediate products are more susceptible to degradation than the corresponding chlorinated analogue and its products. The main degradation products of tetrachloro-p-benzoquinone and tetrafluoro-p-benzoquinone were identified as trichlorohydroxy-p-benzoquinone and trifluorohydroxy-p-benzoquinone, respectively. This secondary conversion of tetrafluoro-p-benzoquinone and tetrachloro-p-benzoquinone was not mediated by MP8, but was driven by H2O2. Evidence is presented for a mechanism where H2O2 molecules and not hydroxide anions are the reactive nucleophilic species attacking the tetrahalo-p-benzoquinones. In addition to the formation of the trihalohydroxy-p-benzoquinones, the formation of adducts of the tetrahalo-p-benzoquinone products with ethanol, present in the incubation medium, was observed. The adduct from the reaction of tetrachloro-p-benzoquinone with ethanol was isolated and identified as trichloroethoxyquinone. Thus, the present paper describes a system in which the formation of tetrahalo-p-benzoquinone-type products by an oxidative heme-based catalyst could be unequivocally demonstrated.

Quantitative structure/activity relationship for the rate of conversion of C4-substituted catechols by catechol-1,2-dioxygenase from Pseudomonas putida (arvilla) C1

The influence of various C4/C5 substituents in catechol (1,2-dihydroxybenzene) derivatives on the overall rate of conversion by catechol-1,2-dioxygenase from Pseudomonas putida (arvilla) C1 was investigated. Using catechol, 4-methylcatechol, 4-fluorocatechol, 4-chlorocatechol, 4-bromocatechol, 4,5-difluorocatechol and 4-chloro-5-fluorocatechol, it could be demonstrated that substituents at the C4 and/or C5 position decrease the rate of conversion, from 62% (4-methylcatechol) down to 0.7% (4-chloro-5-fluorocatechol) of the activity with non-substituted catechol. The inhibition was reversible upon addition of excess catechol for all substrates tested. This indicates that the lower activities are neither due to irreversible inactivation of the enzyme nor to product inhibition. Based on the reaction mechanism proposed in the literature [Que, L. & Ho, R. Y. N. (1996) Chem. Rev. 96, 2606-2624], the nucleophilic reactivity of the catecholate was expected to be an essential characteristic for its conversion by catechol-1,2-dioxygenase. Therefore, the rates of conversion were compared with calculated energies of the highest occupied molecular orbital (E(HOMO)) of the substrates. A clear quantitative relationship (R>0.97) between the ln kcat and the calculated electronic parameter E(HOMO) was obtained. This indicates that the rate-limiting step of the reaction cycle is dependent on the nucleophilic reactivity of the substrate and not sterically hindered by the relatively large bromine or methyl substituents used in the present study. Possible steps in the reaction mechanism determining the overall rate at 20 degrees C are discussed.

Calculated ionisation potentials determine the oxidation of vanillin precursors by lignin peroxidase

In view of the biocatalytic production of vanillin, this research focused on the lignin peroxidase (LiP) catalysed oxidation of naturally occurring phenolic derivatives: O-methyl ethers, O-acetyl esters, and O-glucosyl ethers. The ionisation potential (IP) of a series of model compounds was calculated and compared to their experimental conversion by LiP, defining a relative IP threshold of approximately 9.0 eV. Based on this threshold value only the O-acetyl esters and glucosides of isoeugenol and coniferyl alcohol would be potential LiP substrates. Both O-acetyl esters were tested and were shown to be converted to O-acetyl vanillin in molar yields of 51.8 and 2.3%, respectively.

Molecular orbital study of porphyrin–substrate interactions in cytochrome P450 catalysed aromatic hydroxylation of substituted anilines

The reaction mechanism for the primary reaction step of the hydroxylation of 3-fluoro-6-methylaniline, attacked at different positions (oxygen attack across a C-C bond and direct attack at positions para and ortho with respect to the NH(2)-group) catalysed by a high-valent ferryl-oxo porphyrin a(2u)-cation complex with H(3)CS(-) as an axial ligand, has been investigated on the basis of electronic structure calculations in local spin-density approximation. Non-repulsive potential curves are obtained only in cases of direct attack at the para- and ortho-positions with respect to NH(2), but not for epoxide formation. Comparing the potential curves for the hydroxylation at the positions para and ortho to the NH(2)-group, an attack at the para-position is more likely. The relative orientation of the substrate towards the porphyrin is essentially determined by the interaction between the substituents of the substrate and the porphyrin. Consequently, different geometrical orientations of the substrate are obtained for hydroxylation at the para- and ortho-positions. In both cases of direct attack the substrate plane is not parallel to the porphyrin plane. The decisive role of sulphur in the hydroxylation is demonstrated by the participation of the S(3p)-orbitals in all molecular orbitals involved in the reaction.

5-Fluorouracil in colorectal cancer: rationale and clinical results of frequently used schedules

Colorectal cancer is one of the most frequent cancers in the western world. Approximately half of the patients will die of their disease because of metastases. The most active cytotoxic agent used to date is 5-fluorouracil (5-FU). However, clinical responses are achieved only in a minority of patients. Based on the current knowledge of the mechanism of action of 5-FU, many attempts have been made to improve the clinical results. These include the use of biochemical modulators and different methods of administration, and these are the subject of this review. Specifically, of five different modulators, i.e. leucovorin, methotrexate, interferon-alpha, N-(phosphonacetyl)-L-aspartate and trimetrexate glucuronate, the biochemical background and the clinical results obtained with these modulators are discussed. In order to get more insight, an overview of the 5-FU metabolism has been given. In addition, the different methods of systemic administration of 5-FU as well as possible mechanisms underlying 5-FU resistance are described.

Quantitative structure activity relationships for the conversion of nitrobenzimidazolones and nitrobenzimidazoles by DT-diaphorase: implications for the kinetic mechanism

Quantitative structure activity relationships (QSARs) for the conversion of nitrobenzimidazolones and nitrobenzimidazoles by rat liver DT-diaphorase (EC 1.6.99.2) are described. The parameter used for description of the QSARs is the energy of the lowest unoccupied molecular orbital (E(LUMO)) of the nitroaromatic compounds. Interestingly, correlations with E(LUMO) were observed for both the natural logarithm of kcat, but also for the natural logarithm of kcat/Km. The minimal kinetic model in line with these QSARs is a ping-pong mechanism that includes a substrate binding equilibrium in the second half reaction.

Occurrence of the NIH shift upon the cytochrome P450-catalyzed in vivo and in vitro aromatic ring hydroxylation of fluorobenzenes

The in vivo cytochrome P450-catalyzed aromatic hydroxylation of a series of fluorobenzenes was investigated with special emphasis on the importance of the fluorine NIH shift. The results obtained demonstrate a minor role for the NIH shift in the metabolism of the fluorobenzenes to phenolic metabolites in control male Wistar rats. These in vivo results could indicate that (1) the NIH shift is an inherently minor process for fluorine substituents or (2) it is a potentially significant process but the presumed epoxide that leads to formation of the NIH-shifted metabolite is lost to an alternative metabolic pathway. In contrast to the in vivo data, in vitro experiments showed a significant amount of an NIH-shifted metabolite for 1,4-difluorobenzene. This result eliminates the explanation that the NIH shift is an inherently minor process for fluorine substituents. Results of additional experiments presented in this paper show that the reduced tendency of fluorine-substituted benzenes to undergo an NIH shift in vivo can-at least in part-be ascribed to the possible existence of alternative pathways for metabolism of the epoxide, such as, for example, GSH conjugation, being more efficient for fluorinated than chlorinated arene oxides.

Oxygen exchange with water in heme-oxo intermediates during H2O2-driven oxygen incorporation in aromatic hydrocarbons catalyzed by microperoxidase-8

The present paper describes the oxygen incorporation into naphthalene and anthracene by H2O2-driven microperoxidase-8, forming alpha-naphthol and anthraquinone, respectively. Microperoxidase-8 is a minienzyme containing a histidinyl-coordinated Fe3+-protoporphyrin IX cofactor covalently attached to an eight-amino-acid peptide. Additional experiments were performed to investigate whether the reaction mechanism involved is like that of peroxidase and/or cytochrome P-450. A reaction pathway like that of cytochrome P-450 implies oxygen transfer to the substrate from the as yet uncharacterized iron-oxo species formed in the reaction of the heine cofactor with H2O2. In contrast, a peroxidase-type reaction chemistry involves reaction pathways proceeding by initial one-electron oxidation of, or H-abstraction from, the substrate, followed by incorporation of oxygen from sources other than the iron-oxo species, i.e. from other than H2O2. The results of the present study exclude Fenton-type chemistry and prove that the minicatalyst is able to catalyze the oxygen incorporation by both peroxidase and cytochrome P-450 types of reaction pathways, while exchange occurs between the high-valency iron-oxo species and H2O. The mechanistic implications of this exchange for cytochrome P-450 are discussed.

19F nuclear magnetic resonance as a tool to investigate microbial degradation of fluorophenols to fluorocatechols and fluoromuconates

A method was developed to study the biodegradation and oxidative biodehalogenation of fluorinated phenols by 19F nuclear magnetic resonance (NMR). Characterization of the 19F NMR spectra of metabolite profiles of a series of fluorophenols, converted by purified phenol hydroxylase, catechol 1,2-dioxygenase, and/or by the yeast-like fungus Exophiala jeanselmei, provided possibilities for identification of the 19F NMR chemical shift values of fluorinated catechol and muconate metabolites. As an example, the 19F NMR method thus defined was used to characterize the time-dependent metabolite profiles of various halophenols in either cell extracts or in incubations with whole cells of E. jeanselmei. The results obtained for these two systems are similar, except for the level of muconates observed. Altogether, the results of the present study describe a 19F NMR method which provides an efficient tool for elucidating the metabolic pathways for conversion of fluorine-containing phenols by microorganisms, with special emphasis on possibilities for biodehalogenation and detection of the type of fluorocatechols and fluoromuconates involved. In addition, the method provides possibilities for studying metabolic pathways in vivo in whole cells.

Metabolism of the insecticide teflubenzuron in rats

1. The metabolic fate of the insecticide teflubenzuron, orally dosed to the male Wistar rat, was investigated. Particular attention was paid to the metabolic fate of the benzoyl and aniline moiety after hydrolysis of the urea bridge. 2. The 0-48-h urinary and faecal metabolic patterns and recoveries showed that for a dose range of 4-53 mumol (1.5-20 mg) teflubenzuron, 90% of the dose was excreted in the faeces mainly in unmodified form, approximately 4.6% was absorbed from the lumen and excreted in the urine, and 5.4% was retained in the body. Metabolites excreted in the urine could be identified as benzoate and aniline derivatives originating from the two aromatic rings of teflubenzuron liberated from the parent molecule by hydrolysis of the urea bridge. 3. The amount of urinary benzoate-type metabolites was about eight times the amount of aniline-type metabolites, indicating significant differences in efficiency of urinary excretion of the benzoate moiety as compared with the aniline ring. 4. To investigate further the possible reason underlying this difference in urinary excretion efficiency between the two aromatic derivatives formed from teflubenzuron, dose-recovery studies of these aniline- and benzoate-type metabolites were performed. These studies confirmed the discrepancy observed between the urinary recovery of the benzoyl and the aniline moiety of teflubenzuron. 5. Additional results of the present study indicate that the above discrepancy can be explained by the fact that the benzoate derivative is excreted mainly in its unmetabolized form, whereas the aniline derivative needs additional phase I and II modifications before it can be excreted from the body, the former being a relatively slow reaction. Furthermore, conversion of the halogenated aniline derivative in phase I metabolism might result in a reactive benzoquinone-type or N-oxidized primary metabolite, which can be retained in the body due to reaction with cellular macromolecules.

MP8-dependent oxidative dehalogenation: evidence for the direct formation of 1,4-benzoquinone from 4-fluorophenol by a peroxidase-type of reaction pathway

The present study shows that MP8 in the presence of H2O2 is able to catalyze the rupture of the stable carbon-fluorine bond of 4-fluorophenol, used as a model substrate for the oxidative dehalogenation reaction. 1,4-Benzoquinone was shown to be the primary reaction product. It is also demonstrated that there was significant [18O] incorporation into the product, 1,4-benzoquinone, from 18O-labelled H2(18)O but not from H2(18)O2. This implies that water participates in the reaction mechanism, and acts as a source for the oxygen atom inserted into the product. It also suggests that the reaction is not a result of direct oxygen transfer from H2O2 through the heme catalyst to the product. Furthermore, ascorbic acid, known to efficiently block MP8-catalyzed peroxidase-type conversions, inhibits the MP8-dependent dehalogenation reaction, most likely because of its ability to reduce the phenoxy radical back to the parent substrate. This observation together with the above-mentioned incorporation of oxygen from the solvent into the benzoquinone product indicates that MP8 dehalogenates 4-fluorophenol and converts it to 1,4-benzoquinone in a peroxidase- and not a P-450-type of reaction mechanism. Overall, our results indicate that the oxidative dehalo genation of para-halogenated phenols, resulting in the formation of benzoquinones, is not specific only for cytochrome P-450 enzymes. Hemoproteins exhibiting peroxidase activity could also play a role in the metabolism of these xenobiotics, resulting in the formation of electrophilic reactive benzoquinone type metabolites.

Microperoxidase/H2O2-mediated alkoxylating dehalogenation of halophenol derivatives in alcoholic media

The results of this study report the H2O2-driven microperoxidase-8 (MP8)-catalyzed dehalogenation of halophenols such as 4-fluorophenol, 4-chlorophenol, 4-bromophenol, and 2-fluorophenol in alcoholic solvents. In methanol, the conversion of the para-halophenols and 2-fluorophenol to, respectively, 4-methoxyphenol and 2-methoxyphenol, as the major dehalogenated products is observed. In ethanol, 4-ethoxyphenol is the principal dehalogenated product formed from 4-fluorophenol. Two mechanisms are suggested for this MP8-dependent alkoxylating dehalogenation reaction. In one of these mechanisms the oxene resonant form of compound I of MP8 is suggested to react with methanol forming a cofactor-peroxide-alkyl intermediate. This intermediate reacts with the reactive pi-electrons of the substrate, leading to the formation of the alkoxyphenols and the release of the fluorine substituent as fluoride anion.

Influence of substituents in fluorobenzene derivatives on the cytochrome P450-catalyzed hydroxylation at the adjacent ortho aromatic carbon center

In a previous study, the in vivo cytochrome P450-catalyzed regioselectivity of aromatic ring hydroxylation for a series of (poly)fluorobenzenes could be quantitatively predicted by the calculated frontier orbital density distribution in the aromatic ring [Rietjens et al. (1993) Biochemistry 32, 4801-4812]. However, the relative small fluorine, its size almost comparable to a hydrogen, is not expected to influence the regioselectivity of aromatic hydroxylation due to steric hindrance. The aim of the present study was to investigate the influence of substituents larger than a hydrogen or fluorine on the possibilities for hydroxylation at the adjacent carbon center. First, the in vivo regioselectivity of aromatic ring hydroxylation of a series of C4-substituted fluorobenzenes was investigated. The results obtained demonstrate that a chlorine and cyano C4 substituent do not hamper hydroxylation at the positions ortho to the C4 carbon atom. For 4-chloro- and 4-cyanofluorobenzene, the observed regioselectivity of aromatic hydroxylation correlated with the regioselectivity predicted on the basis of the frontier orbital density distribution. In contrast, a bromine and iodine substituent affected the hydroxylation at the adjacent ortho carbon centers, reducing it to respectively 40 and 6% of the calculated intrinsic reactivity of the carbon centers. Additional experiments were performed to investigate whether the regioselectivity of the aromatic hydroxylation of the C4-substituted fluorobenzene model compounds was influenced by changes in the cytochrome P450 enzyme pattern. Results obtained demonstrate that for these relatively small substrates the regioselectivity of their hydroxylation was not significantly influenced by several cytochrome P450 inducers. This suggests that the active sites of the cytochromes P450 catalyzing the aromatic hydroxylation do not impose a stereoselective orientation of the aromatic rings with respect to the iron-oxo porphyrin reaction center. Thus, the working hypothesis for additional experiments was that the deviations for the regioselectivity of aromatic hydroxylation observed for 4-bromo- and 4-iodofluorobenzene may be ascribed to steric hindrance by the bromine and iodine substituents hampering the attack of the cytochrome P450 iron-oxo species on the adjacent carbon centers in the benzene derivative. This working hypothesis was further tested by investigating whether useful steric correction factors could be derived from the results obtained with the series C4-substituted fluorobenzenes. These correction factors should make it possible to correct calculated relative reactivities of carbon sites for steric hindrance by substituents positioned ortho with respect to the carbon to be hydroxylated. This will make it possible to better explain and predict the regioselectivities for other chlorine-, bromine-, iodine-, and cyano-containing fluorobenzenes. The in vivo regioselectivity of aromatic ring hydroxylation of a series of five chlorine-, bromine-, iodine-, or cyano-containing fluorobenzenes did not correlate with the noncorrected calculated reactivities (r = 0.49). However, upon correction of the calculated reactivity values by using the steric correction factors, a correlation between the observed and calculated regioselectivity for the substrates of the present study was obtained (r = 0.91). Together these results strongly indicate that for the fluorobenzenes studied the main factors directing the regioselectivity of their aromatic hydroxylation are (i) the nucleophilic chemical reactivity of the site to be hydroxylated and (ii) the steric influence of the substituent ortho with respect to the site of hydroxylation. This latter effect appears to be negligible for a fluorine, chlorine, and cyano substituent but significant for a bromine and iodine substituent.

Relationships between the regioselectivity of the hydroxylation of C4-substituted 2-fluoroaniline derivatives and their toxic endpoints

The in vitro and in vivo metabolic profiles of a series of C4-substituted 2-fluoroanilines were determined and compared to their capacity to induce methemoglobinemia and nephrotoxicity in male Wistar rats. Qualitative and quantitative relationships between the biotransformation and the toxic endpoint of the halogenated anilines were defined. The rate of in vitro N-hydroxylation of the aniline derivatives correlated with the capacity of the compounds to induce methemoglobinemia (r = 0.96). In the experiments on the nephrotoxicity, attention was focused on the relative importance of the C4- and C6-hydroxylated metabolites of the C4-substituted 2-fluoroanilines. In vivo, the formation of 4-aminophenol metabolites was demonstrated to vary in the opposite order as the formation of the 6-aminophenol metabolites. 1H-NMR urinalysis and characterization of a set of conventional biochemical urinary parameters revealed the occurrence of nephrotoxicity upon exposure to the aniline derivatives and were most consistent with damage at the proximal tubular site. Comparison of the extent of nephrotoxicity to the extent of formation of the 4-aminophenol and/or 6-aminophenol metabolites, respectively, indicates a predominant role for the C4-hydroxylation route, not the C6-hydroxylation route, in the induction of nephrotoxic effects. Thus, a qualitative relationship is observed for the extent of C4-hydroxylation of the aniline derivatives and the extent of their in vivo nephrotoxicity. In addition, comparison of the extent of 4-aminophenol formation and nephrotoxicity of both 2-fluoroaniline and 2,4-difluoroaniline pointed at a possible role for a bioactivation pathway through oxidative dehalogenation, resulting in direct formation of a 1,4-benzoquinoneimine as the primary metabolite in the case of 2,4-difluoroaniline. Altogether, it is concluded that a decrease in C4-hydroxylation in the series of aniline derivatives results in a metabolic switch to C6- and N-hydroxylation and, consequently, a shift in the type of toxic endpoint observed, i.e., from nephrotoxicity to methemoglobinemia.

The influence of the peptide chain on the kinetics and stability of microperoxidases

Microperoxidases with increasing lengths of the peptide attached to the heme moiety have been isolated after proteolytic digestion of horse-heart cytochrome c (microperoxidases 6, 8, and 11) and of cytochrome c550 from Thiobacillus versutus (microperoxidase 17). The different microperoxidases catalyze the H2O2-dependent para-hydroxylation of aniline relatively efficiently but are rapidly inactivated under turnover conditions. The horse-heart cytochrome-c-derived microperoxidases have identical values for Vmax but show a decrease of the K(m) for aniline and a higher stability when the attached peptide is longer. The kinetic constants obtained for microperoxidase 17, differ markedly from the microperoxidases derived from horse-heart cytochrome c. Possible factors underlying the observed differences are discussed.

5-Fluorouracil metabolite patterns in viable and necrotic tumor areas of murine colon carcinoma determined by 19F NMR spectroscopy

High-resolution 19F NMR spectroscopy at 9.4 T was used to study the difference in the metabolite pattern of 5-fluorouracil (5-FU) between viable and necrotic tissues of C38 murine colon tumors grown in C57BI/6 mice. Studies were performed on perchloric acid extracts of these tumor fractions after 5-FU treatment. The 19F nuclear magnetic resonance spectra exhibited resonances representing 5-FU, the catabolites alpha-fluoro-beta-ureidopropionic acid and alpha-fluoro-beta-alanine, as well as several fluoronucleotide anabolites. The absolute concentrations of anabolites and catabolites and the anabolite-to-catabolite ratio were significantly lower in the necrotic fraction than in the viable tumor fraction 50 min after administration of 5-FU, whereas the absolute concentration of 5-FU was the same. Therefore, in 5-FU metabolism studies with NMR spectroscopy, it is important to consider the necrotic contribution to the tumor volume.

Microperoxidase/H2O2-catalyzed aromatic hydroxylation proceeds by a cytochrome-P-450-type oxygen-transfer reaction mechanism

The mechanism of aromatic hydroxylation of aniline and phenol derivatives in a H2O2-driven microperoxidase-8(MP8)-catalyzed reaction was investigated. It was shown that the reaction was not inhibited by the addition of scavengers of superoxide anion or hydroxyl radicals, which demonstrates that the reaction mechanism differs from that of the aromatic hydroxylation catalyzed by a horseradish peroxidase/ dihydroxyfumarate system. Additional experiments with 18O-labelled H2 18O2 demonstrated that the oxygen incorporated into aniline to give 4-aminophenol originates from H2O2. Furthermore, it was found that the addition of ascorbic acid efficiently blocks all peroxidase-type reactions that can be catalyzed by the MP8/H2O2 system, but does not inhibit the aromatic hydroxylation of aniline and phenol derivatives. Together, these observations exclude reaction mechanisms for the aromatic hydroxylation that proceed through peroxidase-type mechanisms in which the oxygen incorporated into the substrate originates from O2 or H2O. The mechanism instead seems to proceed by an initial attack of the high-valent iron-oxo intermediate of MP8 on the pi-electrons of the aromatic ring of the substrate leading to product formation by a cytochrome-P-450-type of sigma-O-addition or oxygen-rebound mechanism. This implies that MP8, which has a histidyl and not a cysteinate fifth axial ligand, is able to react by a cytochrome-P-450-like oxygen-transfer reaction mechanism.

Influence of the halogen-substituent pattern of fluoronitrobenzenes on their biotransformation and capacity to induce methemoglobinemia

In the present study both the biotransformation patterns and the capacity to induce methemoglobinemia of a series of fluoronitrobenzenes were investigated. This was done to investigate to what extent variation in the number and position of the halogen substituents influence the metabolic fate of the fluoronitrobenzenes, thereby influencing their capacity to induce methemoglobinemia. The results obtained were compared to the effect of the fluorine substituent patterns on the calculated electronic characteristics and, thus, on the chemical reactivity of the fluoronitrobenzenes. Analysis of the in vivo metabolic profiles demonstrates a dependence of the extent of nitroreduction, of glutathione conjugation, and of aromatic hydroxylation with the pattern of halogen substitution. With an increasing number of fluorine substituents at electrophilic carbon centers, 24-hr urine recovery values decreased and fluoride anion elimination increased, due to increased reactivity of the fluoronitrobenzenes with cellular nucleophiles. In vitro studies even demonstrated a clear correlation between calculated parameters for the electrophilicity of the fluoronitrobenzenes and the natural logarithm of their rate of reaction with glutathione or with bovine serum albumin, taken as a model for cellular nucleophiles (r = 0.97 and r = 0.98, respectively). Increased possibilities for the conjugation of the fluoronitrobenzenes to cellular nucleophiles were accompanied by decreased contributions of nitroreduction and aromatic hydroxylation to the overall in vivo metabolite patterns, as well as by a decreased capacity of the fluoronitrobenzenes to induce methemoglobinemia. In vitro studies on the rates of nitroreduction of the various fluoronitrobenzenes by cecal microflora and rat liver microsomes revealed that the changes in the capacity of the fluoronitrobenzenes to induce methemoglobinemia were not due to differences in their intrinsic reactivity in the pathway of nitroreduction, leading to methemoglobinemia-inducing metabolites. Thus, the results of the present study clearly demonstrate that the number and position of fluorine substituents in the fluoronitrobenzenes influence the capacity of the fluoronitrobenzenes to induce methemoglobinemia, not because their intrinsic chemical reactivity for entering the nitroreduction pathway is influenced. The different methemoglobinemic capacity must rather result from differences in the inherent direct methemoglobinemic capacity and/or reactivity of the various toxic metabolites and/or from the fact that the halogen substituent pattern influences the electrophilic reactivity, thereby changing the possibilities for reactions of the nitrobenzenes with glutathione and, especially, other cellular nucleophiles. When the number of fluorine substituents increases, the electrophilicity of the fluoronitrobenzenes can become so high that glutathione conjugation is no longer able to compete efficiently with covalent binding of the fluoronitrobenzenes to cellular macromolecules. As a consequence, it can be suggested that with an increasing number of fluorine substituents at electrophilic carbon centers in a nitrobenzene derivative, a toxic end point of the nitrobenzene other than formation of methemoglobinemia can be foreseen.