The catalytic mechanism of cytochrome P-450. Spin-trapping evidence for one-electron substrate oxidation

Oxidative N-Dealkylation of N,N-Dimethylanilines by Non-Heme Manganese Catalysts

Non-heme manganese(II) complexes [(IndH)MnIICl2] (1) and [(N4Py*)MnII(CH3CN)](ClO4)2 (2) with tridentate isoindoline and pentadentate polypyridyl ligands (IndH = 1,3-bis(2′-pyridylimino)isoindoline; N4Py* = N,N-bis(2-pyridylmethyl)-1,2- di(2-pyridyl)ethylamine) proved to be suitable to catalyze the oxidative demethylation of N,N-dimethylaniline (DMA) with various oxidants such as tert-butyl hydroperoxide (TBHP), peracetic acid (PAA), and meta-chloroperoxybenzoic acid (mCPBA), resulting N-methylaniline (MA) as a main product with N-methylformanilide (MFA) as a result of a free-radical chain process under air. The effect of electron-donating and electron-withdrawing substituents on the aromatic ring on the relative reactivity of the substrates and on the product composition (MA/MFA) was also studied and showed a significant impact on the catalytic N-demethylation reaction. Based on the Hammett correlation with ρ = −0.38 (PAA), −0.45 (mCPBA), and −0.63 (TBHP) for 1 and ρ = −0.38 (PAA) and −0.37 (mCPBA) for 2, an electrophilic intermediate is suggested as the key oxidant. Furthermore, the spectral investigation (UV-Vis) resulted in direct evidence for the formation of a high-valent oxomanganese(IV) and a transient radical cation intermediate, p-Me-DMA•+, suggesting that the initial step in the manganese-catalyzed oxidations is a fast electron-transfer between the amine and the high valent oxometal species. The mechanisms of the subsequent steps are discussed.

μ-Oxo-bis[(octacosafluoro-meso-tetraphenylporphyrinato)iron(iii)] - synthesis, crystal structure, and catalytic activity in oxidation reactions

We describe the synthesis and X-ray crystal structure of μ-oxo-bis[(octacosafluoro-meso-tetraphenylporphyrinato)iron(iii)] [(FeTPPF₂₈)₂O]. This novel iron complex is an efficient catalyst for oxidative biaryl coupling reactions of diarylamines and carbazoles. The asymmetric oxidative coupling in the presence of an axially chiral biaryl phosphoric acid as co-catalyst provides the 2,2'-bis(arylamino)-1,1'-biaryl in 96% ee. The Wacker-type oxidation of alkenes to the corresponding ketones with (FeTPPF₂₈)₂O as catalyst in the presence of phenylsilane proceeds at room temperature with air as the terminal oxidant. For internal and aliphatic alkenes increased ketone/alcohol product ratios were obtained.

N-Methyl Protoporphyrin IX: An Understudied Porphyrin

N-Methyl protoporphyrin IX (NmePPIX) is a derivative of protoporphyrin IX (PPIX) and the lattice of heme. Certain xenobiotics strongly induce NmePPIX production in the liver. The existence of endogenous NmePPIX in untreated animal liver has also been reported. The detailed mechanisms of NmePPIX biosynthesis remain unclear, but cytochrome P450 enzymes are thought to be critical in xenobiotic-induced NmePPIX production. High levels of NmePPIX cause PPIX accumulation because NmePPIX is a potent inhibitor (K_i = 7 nM) of ferrochelatase, the last enzyme in the heme biosynthesis pathway that converts PPIX to heme. NmePPIX is also involved in several other physiological processes, including inhibition of nitric oxide production and promotion of lamin aggregation. Compared to the two well-characterized porphyrins, PPIX and heme, NmePPIX is understudied regarding the mechanism of formation, fate, and physiological functions. This Review summarizes the current understanding of NmePPIX and provides perspectives on areas of future research on NmePPIX.

Detection and structural analysis of lipid-derived radicals in vitro and in vivo

Lipids can be oxidized by reactive oxygen species, resulting in lipid peroxidation and the formation of reactive metabolites such as lipid-derived electrophiles. These products have been reported to induce inflammation, angiogenesis, and ferroptosis. Lipid peroxidation can produce many different products, each of which performs a different function, and which can be challenging to detect in vivo. The initial products of lipid oxidation are lipid-derived radicals, which can cause extensive chain reactions leading to lipid peroxidation. Hence, the ability to detect lipid radicals may provide information about this important class of molecules and the mechanism by which they cause cellular and tissue damage in a wide range of oxidative conditions. In this review, we report recent scientific advances in the detection of lipid-derived radicals in vitro and in cultured cells. We also introduce the possibility of visualization and structural analysis of lipid-derived radicals generated not only in in cells but also in animal tissue samples from oxidative disease models, using fluorescence-based lipid radicals' detection probes. We anticipate that the various innovative techniques summarized in this paper will be applied and further developed to clarify the role of lipid peroxidation in the pathogenesis of oxidative stress-associated diseases.

A closer look into NADPH oxidase inhibitors: Validation and insight into their mechanism of action

NADPH-oxidases (NOXs) purposefully produce reactive-oxygen-species (ROS) and are found in most kingdoms of life. The seven human NOXs are each characterized by a specific expression profile and a fine regulation to spatio-temporally tune ROS concentration in cells and tissues. One of the best known roles for NOXs is in host protection against pathogens but ROS themselves are important second messengers involved in tissue regeneration and the modulation of pathways that induce and sustain cell proliferation. As such, NOXs are attractive pharmacological targets in immunomodulation, fibrosis and cancer. We have studied an extensive number of available NOX inhibitors, with the specific aim to identify bona fide ligands versus ROS-scavenging molecules. Accordingly, we have established a comprehensive platform of biochemical and biophysical assays. Most of the investigated small molecules revealed ROS-scavenging and/or assay-interfering properties to various degrees. A few compounds, however, were also demonstrated to directly engage one or more NOX enzymes. Diphenylene iodonium was found to react with the NOXs' flavin and heme prosthetic groups to form stable adducts. We also discovered that two compounds, VAS2870 and VAS3947, inhibit NOXs through the covalent alkylation of a cysteine residue. Importantly, the amino acid involved in covalent binding was found to reside in the dehydrogenase domain, where the nicotinamide ring of NADPH is bound. This work can serve as a springboard to guide further development of bona fide ligands with either agonistic or antagonistic properties toward NOXs.

One small step for cytochrome P450 in its catalytic cycle, one giant leap for enzymology

The intermediates operating in the cytochrome P450 catalytic cycle have been investigated for more than half a century, fascinating many enzymologists. Each intermediate has its unique role to carry out diverse oxidations. Natural time course of the catalytic cycle is quite fast, hence, not all of the reactive intermediates could be isolated during physiological catalysis. Different high-valent iron intermediates have been proposed as primary oxidants: the candidates are compound 0 (Cpd 0, [FeOOH][Formula: see text]P450) and compound I (Cpd I, Fe(IV)[Formula: see text]O por[Formula: see text]P450). Among them, the role of Cpd I in hydroxylation is fairly well understood due the discovery of the peroxide shunt. This review endeavors to put the outstanding research efforts conducted to isolate and characterize the intermediates together. In addition to spectral features of each intermediate in the catalytic cycle, the oxidizing powers of Cpd 0 and Cpd I will be discussed along with most recent scientific findings.

3-Carboxypyridinium Chlorochromate (CPCC): An Inexpensive and Convenient Reagent for Efficient Oxidation of Hantzsch 1,4-dihydropyridines

A variety of Hantzsch 1,4-dihydropyridines are oxidized to their corresponding pyridines in excellent yields by 3-carboxypyridinium chlorochromate.

Cytochrome P450 research and The Journal of Biological Chemistry

In honor of the 100th birthday of Dr. Herbert Tabor, JBC's Editor-in-Chief for 40 years, I will review here JBC's extensive coverage of the field of cytochrome P450 (P450) research. Research on the reactions catalyzed by these enzymes was published in JBC before it was even realized that they were P450s, i.e. they have a "pigment" with an absorption maximum at 450 nm. After the P450 pigment discovery, reported in JBC in 1962, the journal proceeded to publish the methods for measuring P450 activities and many seminal findings. Since then, the P450 field has grown extensively, with significant progress in characterizing these enzymes, including structural features, catalytic mechanisms, regulation, and many other aspects of P450 biochemistry. JBC has been the most influential journal in the P450 field. As with many other research areas, Dr. Tabor deserves a great deal of the credit for significantly advancing this burgeoning and important topic of research.

Mechanisms of Cytochrome P450-Catalyzed Oxidations

Enzymes are complex biological catalysts and are critical to life. Most oxidations of chemicals are catalyzed by cytochrome P450 (P450, CYP) enzymes, which generally utilize mixed-function oxidase stoichiometry, utilizing pyridine nucleotides as electron donors: NAD(P)H + O₂ + R → NAD(P)⁺ + RO + H₂O (where R is a carbon substrate and RO is an oxidized product). The catalysis of oxidations is largely understood in the context of the heme iron-oxygen complex generally referred to as Compound I, formally FeO³⁺, whose basis was in peroxidase chemistry. Many X-ray crystal structures of P450s are now available (≥ 822 structures from ≥146 different P450s) and have helped in understanding catalytic specificity. In addition to hydroxylations, P450s catalyze more complex oxidations, including C-C bond formation and cleavage. Enzymes derived from P450s by directed evolution can even catalyze more unusual reactions, e.g. cyclopropanation. Current P450 questions under investigation include the potential role of the intermediate Compound 0 (formally Fe^III-O₂ ^-) in catalysis of some reactions, the roles of high- and low-spin forms of Compound I, the mechanism of desaturation, the roles of open and closed structures of P450s in catalysis, the extent of processivity in multi-step oxidations, and the role of the accessory protein cytochrome b ₅. More global questions include exactly how structure drives function, prediction of catalysis, and roles of multiple protein conformations.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Suicide Inhibition of Cytochrome P450 Enzymes by Cyclopropylamines via a Ring-Opening Mechanism: Proton-Coupled Electron Transfer Makes a Difference

N-benzyl-N-cyclopropylamine (BCA) has been attracting great interests for decades for its partial suicide inactivation role to cytochrome P450 (P450) via a ring-opening mechanism besides acting as a role of normal substrates. Understanding the mechanism of such partial inactivation is vital to the clinical drug design. Thus, density functional theoretical (DFT) calculations were carried out on such P450-catalyzed reactions, not only on the metabolic pathway, but on the ring-opening inactivation one. Our theoretical results demonstrated that, in the metabolic pathway, besides the normal carbinolamine, an unexpected enamine was formed via the dual hydrogen abstraction (DHA) process, in which the competition between rotation of the H-abstracted substrate radical and the rotation of hydroxyl group of the protonated Cpd II moiety plays a significant role in product branch; In the inactivation pathway, the well-noted single electron transfer (SET) mechanism-involved process was invalidated for its high energy barrier, a proton-coupled electron transfer [PCET(ET)] mechanism plays a role. Our results are consistent with other related theoretical works on heteroatom-hydrogen (X-H, X = O, N) activation and revealed new features. The revealed mechanisms will play a positive role in relative drug design.

Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Cytochrome P450 (P450) enzymes oxidize xenobiotics into chemically reactive metabolites or intermediates as well as into stable metabolites. If the reactivity of the product is very high, it binds to a catalytic site or sites of the enzyme itself and inactivates it. This phenomenon is referred to as mechanism-based inactivation. Many clinically important drugs are mechanism-based inactivators that include macrolide antibiotics, calcium channel blockers, and selective serotonin uptake inhibitors, but are not always structurally and pharmacologically related. The inactivation of P450s during drug therapy results in serious drug interactions, since irreversibility of the binding allows enzyme inhibition to be prolonged after elimination of the causal drug. The inhibition of the metabolism of drugs with narrow therapeutic indexes, such as terfenadine and astemizole, leads to toxicities. On the other hand, the fate of P450s after the inactivation and the toxicological consequences remains to be elucidated, while it has been suggested that P450s modified and degraded are involved in some forms of tissue toxicity. Porphyrinogenic drugs, such as griseofulvin, cause mechanism-based heme inactivation, leading to formation of ferrochelatase-inhibitory N-alkylated protoporphyrins and resulting in porphyria. Involvement of P450-derived free heme in halothane-induced hepatotoxicity and catalytic iron in cisplatin-induced nephrotoxicity has also been suggested. Autoantibodies against P450s have been found in hepatitis following administration of tienilic acid and dihydralazine. Tienilic acid is activated by and covalently bound to CYP2C9, and the neoantigens thus formed activate immune systems, resulting in the formation of an autoantibodydirected against CYP2C9, named anti-liver/kidney microsomal autoantibody type 2, whereas the pathological role of the autoantibodies in drug-induced hepatitis remains largely unknown.

Theoretical study of N-demethylation of substituted N,N-dimethylanilines by cytochrome P450: the mechanistic significance of kinetic isotope effect profiles

The mechanism of N-demethylation of N,N-dimethylanilines (DMAs) by cytochrome P450, a highly debated topic in mechanistic bioinorganic chemistry (Karki, S. B.; Dinnocenczo, J. P.; Jones, J. P.; Korzekwa, K. R. J. Am. Chem. Soc. 1995, 117, 3657), is studied here using DFT calculations of the reactions of the active species of the enzyme, Compound I (Cpd I), with four para-(H, Cl, CN, NO2) substituted DMAs. The calculations resolve mechanistic controversies, offer a consistent mechanistic view, and reveal the following features: (a) the reaction pathways involve C-H hydroxylation by Cpd I followed by a nonenzymatic carbinolamine decomposition. (b) C-H hydroxylation is initiated by a hydrogen atom transfer (HAT) step that possesses a "polar" character. As such, the HAT energy barriers correlate with the energy level of the HOMO of the DMAs. (c) The series exhibits a switch from spin-selective reactivity for DMA and p-Cl-DMA to two-state reactivity, with low- and high-spin states, for p-CN-DMA and p-NO2-DMA. (d) The computed kinetic isotope effect profiles (KIEPs) for these scenarios match the experimentally determined KIEPs. Theory further shows that the KIEs and TS structures vary in a manner predicted by the Melander-Westheimer postulate: as the substituent becomes more electron withdrawing, the TS is shifted to a later position along the H-transfer coordinate and the corresponding KIEs increases. (e) The generated carbinolaniline can readily dissociate from the heme and decomposes in a nonenzymatic environment, which involves water assisted proton shift.

Metabolic aromatization of N-alkyl-1,2,3,4-tetrahydroquinoline substructures to quinolinium by human liver microsomes and horseradish peroxidase

Metabolic aromatization of xenobiotics is an unusual reaction with some documented examples. For instance, the oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine to the neurotoxic pyridinium ion metabolite 1-methyl-4-phenylpyridinium by monoamine oxidase (MAO) B in the brain has been of interest to a number of investigators. It has also been reported that although the aromatization of N-methyl-tetrahydroisoquinoline occurs with MAO B, the metabolism does not proceed for its isomer, N-methyl-tetrahydroquinoline, by the same enzyme. The aromatization of an N-alkyl-tetrahydroquinoline substructure was identified during in vitro metabolite profiling of compound A, which was designed as a potent renin inhibitor for the treatment of hypertension. The N-alkylquinolinium metabolite of compound A was identified by liquid chromatography-tandem mass spectrometry of human liver microsomal incubates and proton NMR of the isolated metabolite. Further in vitro metabolism studies with a commercially available chemical (compound B), containing the same substructure, also generated an N-alkylquinolinium metabolite. In vitro cytochrome P450 (P450) reaction phenotyping of compound A revealed that the metabolism was catalyzed exclusively by CYP3A4. Although compound B was a substrate for several P450 isoforms, its quinolinium metabolite was also generated predominantly by CYP3A4. Neither compound A nor compound B was a substrate of MAOs. The quinolinium metabolites were readily produced by horseradish peroxidase, suggesting that aromatization of the N-alkyltetrahydroquinoline could occur via a mechanism involving single electron transfer from nitrogen. Although dihydro intermediates from the tetrahydroquinoline substrates were not observed in the formation of quinolinium metabolites, cyanide trapping results indicated the occurrence of iminium intermediates.

Model Electrochemical-Mass Spectrometric Studies of the Cytochrome P450-Catalyzed Oxidations of Cyclic Tertiary Allylamines

Single-electron transfer and hydrogen atom transfer pathways have been proposed to account for the cytochrome P450-catalyzed alpha-carbon oxidations of amines. With the aid of electrochemistry-electrospray ionization mass spectrometry, the electrochemical potentials required for the one-electron oxidations of N-methyl- and selected N-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridinyl derivatives and the chemical fates of the resulting aminyl radical cations have been investigated. Comparison of the results of these studies with those observed in the corresponding enzyme catalyzed oxidations suggests that aminyl radical cations are not obligatory intermediates in the cytochrome P450-catalyzed alpha-carbon oxidations of this class of substrates.

Studies on the cytochrome P450 catalyzed oxidation of 13C labeled 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine by 13C NMR

A recent study from Hanzlik's laboratory (J. Am. Chem. Soc. 2002, 124, 8268) has provided compelling evidence of a hydrogen atom transfer pathway for the cytochrome P450-catalyzed oxidative N-decyclopropylation of N-cyclopropyl-N-methylaniline. In the present paper, we report an analogous pathway for the oxidative decyclopropylation of a 13C-labeled 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridinyl substrate. Three 13C-enriched metabolites were characterized: (1) a diastereomeric pair of N-cyclopropyl-N-oxides; (2) the N-cyclopropylpyridinium species; and (3) cyclopropanone hydrate. These results extend the hydrogen atom transfer pathway to include aliphatic tertiary amine substrates. Consideration of all of the available evidence, however, leads us to conclude that the cytochrome P450-catalyzed alpha-carbon oxidations of cyclopropylamines may proceed via both the single electron and hydrogen atom transfer pathways.

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.

Synthesis and Aromatization of Hantzsch 1,4-Dihydropyridines under Microwave Irradiation. An Overview

Domestic microwave ovens as well as laboratory reactors have been successfully employed to prepare dialkyl 1,4-dihydropyridine-3,5-dicarboxylates and to induce the synthesis of the corresponding aromatic derivatives. In that latter particular case, unexpected results have been reported.

Alteration of the heme prosthetic group of neuronal nitric-oxide synthase during inactivation by N(G)-amino-L-arginine in vitro and in vivo

It is established that N(G)-amino-L-arginine (NAA) is a metabolism-based inactivator of all three major nitric-oxide synthase (NOS) isoforms. The mechanism by which this inactivation occurs, however, is not well understood. In the current study, we discovered that inactivation of the neuronal isoform of NOS (nNOS) by NAA in vitro results in covalent alteration of the heme prosthetic group, in part, to products that contain an intact porphyrin ring and are either dissociable from or irreversibly bound to the protein. The alteration of the heme is concomitant with the loss of nNOS activity. Studies with nNOS containing a 14C-labeled prosthetic heme moiety indicate that the major dissociable product and the irreversibly bound heme adduct account for 21 and 28%, respectively, of the heme that is altered. Mass spectral analysis of the major dissociable product gave a molecular ion of m/z 775.3 that is consistent with the mass of an adduct of heme and NAA minus a hydrazine group. Peptide mapping of the irreversibly bound heme adduct indicates that the heme is bound to a residue in the oxygenase domain of nNOS. We show for the first time that metabolism-based inactivation of nNOS occurs in vivo as highly similar heme products are formed. Because inactivation and alteration may trigger ubiquitination and proteasomal degradation of nNOS, NAA may be a useful biochemical tool for the study of these basic regulatory processes.

Microsomal P450-catalyzed N-dealkylation of N,N-dialkylanilines: evidence for a C(alpha)-H abstraction mechanism

The early proposal that P450-catalyzed N-dealkylation of N,N-dialkylamines proceeds through a single-electron-transfer (SET) mechanism was later challenged in favor of the C(alpha)-H abstraction mechanism. In the present study, a series of N-alkyl-N-cyclopropyl-p-chloroaniline probes have been used to examine whether the P450-catalyzed N-dealkylations proceed through a C(alpha)-H abstraction and/or a SET mechanism, using phenobarbital-induced rat liver microsomal P450 enzymes as a model system. While the findings are highly consistent with a C(alpha)-H abstraction mechanism, further experimental evidence may be necessary to completely rule out the SET mechanism.

Isolated perfused rat hearts release secondary free radicals during ischemia reperfusion injury: cardiovascular effects of the spin trap alpha-phenyl N-tert-butylnitrone

Free radicals produced during myocardial post-ischemic reperfusion are aggravating factors for functional disturbances and cellular injury. The aim of our work was to investigate the significance of the secondary free radical release during non ischemic perfusion and post-ischemic reperfusion and to evaluate the cardiovascular effects of the spin trap used. For that purpose, isolated perfused rat hearts underwent 0, 20, 30 or 60 min of a total ischemia, followed by 30 min of reperfusion. The spin trap: alpha-phenyl N-tert-butylnitrone (PBN) was used (3 mM). Functional parameters were recorded and samples of coronary effluents were collected and analyzed using Electron Paramagnetic Resonance (EPR) to identify and quantify the amount of spin adducts produced. During non ischemic perfusion, almost undetectable levels of free radical release were observed. Conversely, a large and long-lasting (30 min) release of spin adducts was detected from the onset of reperfusion. The free radical species were identified as alkyl and alkoxyl radicals with amounts reaching 40 times the pre-ischemic values. On the other hand, PBN showed a cardioprotective effect, allowing a significant reduction of rhythm disturbances and a better post-ischemic recovery for the hearts which were submitted to 20 min of ischemia. When the duration of ischemia increased, the protective effects of PBN disappeared and toxic effects became more important. Our results have therefore confirmed the antioxidant and protective properties of a spin trap agent such as PBN. Moreover, we demonstrated that the persistent post-ischemic dysfunction was associated with a sustained production and release of free radical species.

Evidence for a 1-electron oxidation mechanism in N-dealkylation of N,N-dialkylanilines by cytochrome P450 2B1. Kinetic hydrogen isotope effects, linear free energy relationships, comparisons with horseradish peroxidase, and studies with oxygen surrogates

Many enzymes catalyze N-dealkylations of alkylamines, including cytochrome P450 (P450) and peroxidase enzymes. Peroxidases, exemplified by horseradish peroxidase (HRP), are generally accepted to catalyze N-dealkylations via 1-electron transfer processes. Several lines of evidence also support a 1-electron mechanism for many P450 reactions, although this view has been questioned in light of reported trends for kinetic hydrogen isotope effects for N-demethylation with a series of 4-substituted N,N-dimethylanilines. No continuous trend for an increase of isotope effects with the electronic parameters of para-substitution was seen for the P450 2B1-catalyzed reactions in this study. The larger value seen with the 4-nitro derivative is consistent with a shift in mechanism due to either a reversible electron transfer step preceding deprotonation or to a hydrogen atom abstraction mechanism. With HRP, the trend is to lower isotope effects with para electron-withdrawing substituents, due to an apparent shift in rate-limiting steps. Biomimetic model high-valent porphyrins showed reduction rates with variously 4-substituted N,N-dialkylanilines that were consistent with a positively charged intermediate; such relationships were not seen for anisole O-demethylation with P450 2B1. In contrast to the case with the NADPH-supported P450 reactions, high deuterium isotope effects ( approximately 7) were seen in the N-dealkylations supported by the oxygen surrogate iodosylbenzene. With iodosylbenzene, colored aminium radicals were observed in the oxidations of aminopyrine, N,N-dimethyl-4-aminothioanisole, and 4-methoxy-N,N-dimethylaniline. With the latter compound, a substantial intermolecular deuterium isotope effect was observed for N-demethylation. In the N-dealkylation of N-ethyl,N-methylaniline by P450 2B1 (NADPH-supported), the ratio of N-demethylation to N-deethylation was 16. Although it is probably possible for P450s to catalyze amine N-dealkylations via hydrogen atom abstraction when such a course is electronically or sterically favored, we interpret the evidence to favor a 1-electron pathway with N,N-dialkylamines with P450 2B1 as well as HRP and several biomimetic models.

Identification of radical adducts formed in the reactions of unsaturated fatty acids with soybean lipoxygenase using continuous flow fast atom bombardment with tandem mass spectrometry

Structures of alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) radical adducts formed in the reactions of soybean lipoxygenase with linoleic acid, linolenic acid, and arachidonic acid were determined using continuous flow fast atom bombardment (CF-FAB) combined with tandem mass spectrometry. The radical adducts of these lipoxygenase-dependent reactions were: n-octanoic acid radical, 12,13-dihydroxylinoleic acid radical, 12,13-epoxylinoleic acid radical, and n-pentyl radical from linoleic acid; n-octanoic acid radical, ethyl radical, and cis/trans and/or positional isomers (1- and 3-pentenyl) of pentenyl radical from linolenic acid; and 14,15-epoxyarachidonic acid radical and n-pentyl radical from arachidonic acid. Of these radical adducts, the n-octanoic acid radical from linoleic and linolenic acid, the ethyl radical from linolenic acid, and the 12,13-dihydroxylinoleic acid radical are identified for the first time in the reactions of soybean lipoxygenase. Thus the CF-FAB combined with tandem mass spectrometry employed here, by which both radical adducts and their fragment ions can be detected, is shown to be a powerful tool in the structural identification of free radicals.

On the mechanism of amine oxidations by P450

1. Experimental data previously used to support an electron/proton transfer mechanism for oxidative dealkylation of amines by P450 are critically analysed with the conclusion that the mechanistic evidence is indecisive. 2. A new mechanistic criterion recently proposed to distinguish between electron/proton transfer and hydrogen atom transfer mechanisms is discussed. It is based on isotope effect profiles determined for the deprotonation of a series of para-substituted N-methyl-N-trideuteriomethyl)aniline cation radicals by pyridine and for hydrogen atom abstraction from the corresponding neutral amines by the tert-butoxyl radical. These reactions model the steps proposed in the two P450 mechanisms. 3. Isotope effect profiles measured for the demethylation of substituted NN-bis(dideuteriomethyl)anilines by four different forms of P450 were found to be experimentally indistinguishable from the hydrogen atom transfer profile, and distinctly different from the cation radical deprotonation profile. This provides strong evidence that P450 oxidatively dealkylates the amines by a hydrogen atom transfer mechanism and not by an electron/proton transfer mechanism.

Radical cation intermediates in N-dealkylation reactions

1. A number of mechanistic possibilities exist for P450-catalysed N-dealkylation and have been considered over the years, including C- and N-hydroxylation and sequential electron transfer (SET). With peroxidases the evidence strongly favours SET and free radicals can be detected. Any mechanism must account for lack of incorporation of label from H218O into product by P450s and the high kinetic deuterium isotope effects that are seen in N-dealkylation reactions catalysed by peroxidases but not P450s. 2. Several lines of evidence support a role for SET in P450 amine oxidations, including Hammett analysis, products of dihydropyridine oxidations, and products of mechanism-based inhibition by strained cycloalkylamines. 3. The hypothesis was considered that the P450s act via base catalysis to deprotonate the aminium radical generated by SET, since the pKa has been estimated to be approximately 9. Dihydropyridine aminium radicals have low pKa (< 4) and are generally considered to have considerable kinetic acidity. None of the haemoproteins under consideration (including the peroxidases and haemoglobin) showed high kinetic hydrogen isotope effects for the oxidation of [4-2H]- or [4-3H]-labelled 1,4-dihydropyridines. These results are consonant with the view that P450s catalyse the deprotonation of N,N-dialkylaniline aminium radicals. 4. Since low isotope effects were seen with biomimetic metalloporphyrin models as well as P450s, the deprotonation is attributed to the (FeO)2+ entity, expected to be a strong base, and not the apoprotein. Thus, the FeO moiety of peroxidases is shielded, consistent with evidence by others that SET occurs through the porphyrin edge. Both P450s and peroxidases catalysed the oxidative N-demethylation of aminopyrine and N,N-dimethylaminothioanisole; however, only the peroxidases generated the stable coloured aminium radicals. 5. The rates of N-demethylation of variously para-substituted N,N-dimethylanilines can be used to undertake Hammett or Marcus analysis. The former yields rho = -0.6 and the latter an apparent E1/2 of approximately 1.8 for the formal (FeO)3+ entity of P4502B1. 6. Even in the oxidation of N,N-dialkylanilines, a finite rate of N-oxidation is seen (approximately 0.1% of N-dealkylation). The simplest paradigm has N-oxygenation and N-dealkylation both proceeding from a common aminium radical intermediate.

Interpretations of cytochrome P450 mechanisms from kinetic studies

The catalytic mechanism of cytochrome P450 (P450) enzymes has generally been understood in terms of a classic cycle in which electron donation is often limiting and catalysis is understood in terms of hydrogen abstraction and rapid oxygen rebound. In the course of detailed investigations with kinetic hydrogen isotope effects we have studied two systems in which somewhat unusual isotope effects have been interpreted in terms of modifications of the general paradigm. The low isotope effects observed for N-demethylation reactions are in contrast to high values seen with P450-catalyzed C-hydroxylation and peroxidase-catalyzed N-demethylation and are consonant with a role for the P450 FeO2+ entity in base-catalyzed deprotonation of an aminium radical. With P450 2E1, kinetic deuterium isotope effects are seen on the apparent Km for the substrate (increased) but not on Vmax. The results are interpreted in terms of a mechanism where C-H bond cleavage is sensitive to deuterium substitution but a step following this is rate-limiting. This step may be product release.

Evidence for mechanism-based inactivation of rat and chick embryo hepatic cytochrome P4501A and P4503A by dihydropyridines, sydnones, and dihydroquinolines

Rat hepatic P4501A1 and 3A1/2 have been shown previously to be targets for mechanism-based inactivation by the 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC), namely, 4-ethyl DDC and 4-isopropyl DDC. In this study we have shown that rat hepatic P4501A and P4503A are targets for mechanism-based inactivation by the sydnones, 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS). The dihydroquinoline, 2,4-diethyl-2-methyl-1,2-dihydroquinoline (DMDQ), caused mechanism-based inactivation of rat hepatic P4501A but not of P4503A. The P4501A isozyme(s) of chick embryo liver was found to share the ability of rat liver P4501A to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, the sydnones, TTMS and PEMS, and the dihydroquinoline, DMDQ. A P4503A-like isozyme of chick embryo liver shared the ability of the rat liver P4503A isozyme(s) to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, and the sydnone, TTMS, but not of the sydnone PEMS. The dihydropyridine, DDC, was found to serve as a mechanism-based inactivator of the chick embryo P4501A isozyme(s), but not of the P4503A isozyme(s), in contrast to its previously reported inactivity with both the rat hepatic P4501A1 and 3A1/2 isozymes.

Microbial models of mammalian metabolism: involvement of cytochrome P450 in the N-demethylation of N-methylcarbazole by Cunninghamella echinulata

1. As previously reported (Yang and Davis 1992), N-methylcarbazole (NMC) is converted to N-hydroxymethylcarbazole (NHMC), and 3-hydroxy-N-hydroxymethylcarbazole (3-OH-NHMC), two relatively stable carbinolamine metabolites by the fungus Cunninghamella echinulata (ATCC 9244). Decomposition of these two carbinolamines yields the corresponding dealkylated metabolites, carbazole and 3-hydroxycarbazole. In the present study, the possible involvement of cytochrome P450 in the requisite N-alkyl hydroxylation reaction was examined. 2. Carbon monoxide, a classical P450 inhibitor, markedly inhibited the formation of NHMC, as did potassium cyanide. 1-Benzylimidazole, piperonyl butoxide and SKF-525A inhibited the formation of both NHMC and 3-OH-NHMC, while beta-naphthoflavone (5,6-benzoflavone) induced their formation. 3. The source of the oxygen atom in the metabolite NHMC was examined by GC/MS analysis of NHMC formed during incubation of NMC in H218O-enriched medium which resulted in no incorporation of labelled oxygen into the metabolite. 4. An intermolecular isotope effect was not observed for the formation of NHMC suggesting that C-H bond cleavage is not a rate limiting step in the formation of this metabolite under the conditions examined. 5. It was concluded that P450 enzymes may be involved in the N-demethylation of NMC catalyzed by this fungal model of mammalian metabolism, and provides further support for biochemical and mechanistic parallels between mammalian metabolism and microbial systems catalyzing phase-1 biotransformations.

Generalized cytochrome P450-mediated oxidation and oxygenation reactions in aromatic substrates with activated N-H, O-H, C-H, or S-H substituents

1. The general mechanism of metabolic oxidation of substrates by cytochromes P450 (P450s) appears to consist of sequential one-electron oxidation steps rather than of a single concerted transfer of activated oxygen species from P450 to substrates. 2. In case of the acetanilides paracetamol (PAR), phenacetin (PHEN), and 4-chloro-acetanilide (4-CLAA), the first one-electron oxidation step consists of a hydrogen abstraction from the acetylamino nitrogen and/or from the other side-chain substituent on the aromatic ring. The substrate radicals thus formed delocalize their spin and the respective reactive centres of the substrate radical recombine with a P450 iron-bound hydroxyl radical to either yield oxygenated metabolites, or undergo a second hydrogen abstraction forming dehydrogenated products. By this mechanism, the formation of all known oxidative metabolites of PAR, PHEN, and 4-ClAA can be explained. Furthermore, this mechanism is consistent with all available experimental data on [18O]PAR/PHEN, [2H]PAR, and [14C]PHEN. 3. The oxidative metabolic reactions proposed for the acetanilides PAR, PHEN, and 4-ClAA are used to generalize P450-mediated oxidations of these and other acetanilides, such as analogues of PAR and 2-N-acetyl-aminofluorene. 4. A further generalization of the hydrogen abstraction, spin delocalization, radical recombination concept is derived for other aromatic substrates with abstractable hydrogen atoms, notably those with activated N-H, O-H, C-H, or S-H bonds directly attached to the aromatic nucleus.

Formation of N-methyl protoporphyrin in chemically-induced protoporphyria. Studies with a novel porphyrogenic agent

1-[4-(3-Acetyl-2,4,6-trimethylphenyl)-2,6-cyclohexanedionyl]-O-eth yl propionaldehyde oxime (for short ATMP) is a novel porphyrogenic agent causing hepatic protoporphyria in the mouse. Mice given a single dose of the drug showed 24 h later a 70% inhibition of liver ferrochelatase and marked accumulation of protoporphyrin. These changes were not seen in similarly treated rats, guinea pigs, hamsters or chick embryos. A green pigment was isolated from the liver of mice treated with ATMP and identified by its electronic absorption spectrum and chromatographic properties on HPLC as N-methyl protoporphyrin. The ATMP pigment markedly inhibited the enzyme ferrochelatase in vitro, thus supporting its identification as N-methyl protoporphyrin. Two inhibitors of liver cytochrome P450, compound SKF 525-A and piperonyl butoxide, when given before ATMP, afforded protection against ATMP-induced porphyria and production of N-methyl protoporphyrin, suggesting a role of cytochrome P450 in the induction of the metabolic disorder. The most likely interpretation for these findings is therefore that ATMP is metabolized in the mouse to a reactive species, which in turn alkylates the haem moiety of liver cytochrome P450, thus producing N-methyl protoporphyrin. This inhibits ferrochelatase and, as a secondary response, protoporphyrin accumulates. This pathway of metabolism to the postulated reactive metabolite presumably does not occur to a significant extent in the other species examined and hence is the likely basis for the species difference in protoporphyria.

Biological methane activation involves the intermediacy of carbon-centered radicals

The spin-trapping technique has demonstrated that carbon-centered radicals are produced during soluble-methane-monooxygenase catalysis of the hydroxylation of several different types of substrate. The resulting spin-adducts were identified from the hyperfine splitting constants in their EPR spectra. Isotopic labelling showed unequivocally that the trapped radicals were derived from substrate. The carbon-centered substrate radicals are believed to result from hydrogen-atom abstraction by a ferryl species in a cytochrome-P-450-like mechanism. No hydroxy radical nor an oxygen-based radical of any kind was detected in any of the spin-trapping experiments.

Thin-layer chromatographic separation of the ferrochelatase-inhibitory ring A and ring B regioisomers of N-ethylprotoporphyrin from a mixture of the four regioisomers

When N-alkylprotoporphyrins are prepared synthetically or biologically, a mixture of four regioisomers is obtained. For our studies, separation of the potent ferrochelatase-inhibitory ring A (NA) and ring B (NB) regioisomers from the ring C (NC) and ring D (ND) regioisomers of low potency is required. Previously this separation required two successive high-performance liquid chromatography procedures. We now report that the separation of the zinc complexes of the NA and NB regioisomers from the NC and ND regioisomers can be achieved by a rapid and inexpensive thin-layer chromatography procedure.

The role of an endogenous nonheme iron in microsomal redox reactions

Microsomal membranes contain a nonheme iron which serves in vitro for the peroxidation of unsaturated lipids or the oxidation of several other chemicals. These redox reactions are reviewed in light of a recent identification of two or more iron-binding proteins in the microsomal milieu. Indirect evidence that the microsomal iron might serve in vivo for the synthesis of heme iron is also presented and discussed. Consistent with this, the newly identified iron proteins not only participate in redox reactions but also release their bound iron upon incubation with certain intermediates of heme synthesis.

Application of the EPR spin-trapping technique to the detection of radicals produced in vivo during inhalation exposure of rats to ozone

Ozone is known to induce lipid peroxidation of lung tissue, although no direct evidence of free radical formation has been reported. We have used the electron paramagnetic resonance (EPR) spin-trapping technique to search for free radicals produced in vivo by ozone exposure. The spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) was administered ip to male Sprague-Dawley rats. The rats were then exposed for 2 hr to either 0, 0.5, 1.0, 1.5, or 2.0 ppm ozone with 8% CO2 to increase their respiratory rate. A six-line 4-POBN/radical spin adduct signal (aN = 15.02 G and a beta H = 3.27 G) was detected by EPR spectroscopy in lipid extracts from lungs of rats treated with 4-POBN and then exposed to ozone. Only a weak signal was observed in the corresponding solution from rats exposed to 0 ppm ozone (air with CO2 only). The concentration of the radical adduct increased as a function of ozone concentration. After administration of 4-POBN, rats were exposed for either 0.5, 1.0, 2.0, or 4.0 hr to either 0 or 2.0 ppm ozone (with CO2). The radical adduct concentration of the ozone-exposed groups at exposure times of 2.0 and 4.0 hr was significantly different from that of the corresponding air control groups. A correlation was observed between the radical adduct concentration and the lung weight/body weight ratio. These results demonstrate that ozone induces the production of free radicals in rat lungs during inhalation exposure and that radical production may be involved in the induction of pulmonary toxicity by ozone. This is the first direct evidence for ozone-induced free radical production in vivo.

Differential apoprotein loss of rat liver cytochromes P450 after their inactivation by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine: a case for distinct proteolytic mechanisms?

Suicide inactivation of hepatic cytochrome P450 (P450) enzymes 2C11, 2C6, and 3A1/A2 by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP) in intact rats results in prosthetic heme destruction, albeit by apparently distinct mechanisms. Such heme destruction is now shown to be associated with the loss of immunochemically detectable apoprotein of P450s 2C11 and 3A but with little of that of P450 2C6, in spite of their comparable DDEP-mediated functional inactivation. The loss of a approximately 50-kDa hepatic microsomal protein band along with the immunoreactive P450 3A loss strengthens the concept that such an in vivo loss indeed reflects proteolysis of the DDEP-inactivated P450. Furthermore, this propensity of DDEP-inactivated P450s 3A for proteolysis appears to correlate with the relative degree of prosthetic heme alkylation of their apoprotein rather than their functional inactivation per se. Thus, rapid degradation of apoP450s 3A was seen after DDEP treatment, which promoted extensive irreversible heme binding to apoP450s 3A, but not after exposure to allylisopropylacetamide (AIA), which inactivates these isozymes comparably, but induced minimal apoP450 3A heme alkylation. In addition, differences were observed in the relative sensitivities of proteolysis of DDEP-inactivated P450s 2C11 and 3A to hemin, which largely prevented the DDEP-induced proteolytic loss of P450 2C11 but apparently failed to prevent the loss of DDEP-inactivated P450s 3A, when coadministered with DDEP. This differential hemin sensitivity of the proteolysis of DDEP-inactivated P450 2C11, coupled with the observation that immunochemically detectable P450 2C11 loss occurs after its inactivation by both AIA and DDEP, provides compelling support for the existence of distinct proteolytic pathways for individual suicidally inactivated P450s.