Determinants of protein modification versus heme alkylation: inactivation of cytochrome P450 1A1 by 1-ethynylpyrene and phenylacetylene

Improved methods for the detection of heme and protoporphyrin IX adducts and quantification of heme B from cytochrome P450 containing systems

Heme B is a critical prosthetic group for the function of numerous proteins including the cytochrome P450 (CYP) family of enzymes. CYP enzymes are involved in the metabolism of endogenous and xenobiotic molecules that are of central interest in drug development. Formation of reactive metabolites by CYPs can lead to heme modification and destruction of the enzyme. The structure of the adducted heme can provide key information on the mechanism of inactivation, which is of great interest during preclinical drug discovery. Historically, techniques to extract the modified heme or protoporphyrin IX species involved harsh extraction conditions and esterification of propionate groups to aid chromatography. We have developed a simplified extraction method and LC/MS chromatography system that does not require derivatization to quantify heme B and identify modified heme B species from multiple CYP-containing matrices. The method uses mass defect filter triggered data dependent MS2 scans to rapidly identify heme and protoporphyrin IX adducts. These methods may also be useful for the analysis of other heme variants and hemoproteins.

Mechanism-Based Inactivation of Cytochrome P450 Enzymes: Computational Insights

Mechanism-based inactivation (MBI) refers to the metabolic bioactivation of a xenobiotic by cytochrome P450s to a highly reactive intermediate which subsequently binds to the enzyme and leads to the quasi-irreversible or irreversible inhibition. Xenobiotics, mainly drugs with specific functional units, are the major sources of MBI. Two possible consequences of MBI by medicinal compounds are drug-drug interaction and severe toxicity that are observed and highlighted by clinical experiments. Today almost all of these latent functional groups (e.g., thiophene, furan, alkylamines, etc.) are known, and their features and mechanisms of action, owing to the vast experimental and theoretical studies, are determined. In the past decade, molecular modeling techniques, mostly density functional theory, have revealed the most feasible mechanism that a drug undergoes by P450 enzymes to generate a highly reactive intermediate. In this review, we provide a comprehensive and detailed picture of computational advances toward the elucidation of the activation mechanisms of various known groups with MBI activity. To this aim, we briefly describe the computational concepts to carry out and analyze the mechanistic investigations, and then, we summarize the studies on compounds with known inhibition activity including thiophene, furan, alkylamines, terminal acetylene, etc. This study can be reference literature for both theoretical and experimental (bio)chemists in several different fields including rational drug design, the process of toxicity prevention, and the discovery of novel inhibitors and catalysts.

Acetylene Group, Friend or Foe in Medicinal Chemistry

The use of an acetylene (ethynyl) group in medicinal chemistry coincides with the launch of the Journal of Medicinal Chemistry in 1959. Since then, the acetylene group has been broadly exploited in drug discovery and development. As a result, it has become recognized as a privileged structural feature for targeting a wide range of therapeutic target proteins, including MAO, tyrosine kinases, BACE1, steroid receptors, mGlu5 receptors, FFA1/GPR40, and HIV-1 RT. Furthermore, a terminal alkyne functionality is frequently introduced in chemical biology probes as a click handle to identify molecular targets and to assess target engagement. This Perspective is divided into three parts encompassing: (1) the physicochemical properties of the ethynyl group, (2) the advantages and disadvantages of the ethynyl group in medicinal chemistry, and (3) the impact of the ethynyl group on chemical biology approaches.

Acetylenes: cytochrome P450 oxidation and mechanism-based enzyme inactivation

The oxidation of carbon-carbon triple bonds by cytochrome P450 produces ketene metabolites that are hydrolyzed to acetic acid derivatives or are trapped by nucleophiles. In the special case of 17α-ethynyl sterols, D-ring expansion and de-ethynylation have been observed as competing pathways. The oxidation of acetylenic groups is also associated with mechanism-based inactivation of cytochrome P450 enzymes. One mechanism for this inactivation is reaction of the ketene metabolite with cytochrome P450 residues essential for substrate binding or catalysis. However, in the case of monosubstituted acetylenes, inactivation can also occur by addition of the oxidized acetylenic function to a nitrogen of the heme prosthetic group. This addition reaction is not mediated by the ketene metabolite, but rather occurs during oxygen transfer to the triple bond. In some instances, a detectable intermediate is formed that is most consistent with a ketocarbene-iron heme complex. This complex can progress to the N-alkylated heme or revert back to the unmodified enzyme. The ketocarbene complex may intervene in the formation of all the N-alkyl heme adducts, but is normally too unstable to be detected.

Formation of Both Heme and Apoprotein Adducts Contributes to the Mechanism-Based Inactivation of Human CYP2J2 by 17α-Ethynylestradiol

17α-Ethynylestradiol (EE), a major component of many oral contraceptives, affects the activities of a number of the human cytochrome P450 (P450) enzymes. Here, we characterized the effect of EE on CYP2J2, a major human P450 isoform that participates in metabolism of arachidonic acid. EE inactivated the hydroxyebastine carboxylation activity of CYP2J2 in a reconstituted system. The loss of activity is time and concentration dependent and requires NADPH. The KI and kinact values for the inactivation were 3.6 μM and 0.08 minute-1, respectively. Inactivation of CYP2J2 by EE was due to formation of a heme adduct as well as an apoprotein adduct. Mass spectral analysis of CYP2J2 partially inactivated by EE showed two distinct protein masses in the deconvoluted spectrum that exhibited a mass difference of approximately 312 Da, which is equivalent to the sum of the mass of EE and one oxygen atom. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed a heme adduct with MH+ ion at m/z 875.5, corresponding to alkylation of an iron-depleted prosthetic heme by EE plus one oxygen atom. The reactive intermediate responsible for covalently modifying both the prosthetic heme and apoprotein was characterized by trapping with glutathione (GSH). LC-MS/MS analysis revealed two GSH conjugate isomers with MH+ ions at m/z 620, which were formed by reaction between GSH and EE with the oxygen being added to either the internal or terminal carbon of the ethynyl moiety. High-pressure liquid chromatography analysis revealed that three other major metabolites were formed during EE metabolism by CYP2J2.

Heme Modification Contributes to the Mechanism-Based Inactivation of Human Cytochrome P450 2J2 by Two Terminal Acetylenic Compounds

The mechanism-based inactivation of human CYP2J2 by three terminal acetylenic compounds: N-(methylsulfonyl)-6-(2-propargyloxyphenyl)hexanamide (MS), 17-octadecynoic acid (OD), and danazol (DZ) was investigated. The loss of hydroxyebastine (OHEB) carboxylation activity in a reconstituted system was time- and concentration-dependent and required NADPH for MS and OD, but not DZ. The kinetic constants for the mechanism-based inactivation of OHEB carboxylation activity were: KI of 6.1 μM and kinact of 0.22 min-1 for MS and KI of 2.5 μM and kinact of 0.05 min-1 for OD. The partition ratios for MS and OD were ∼10 and ∼20, respectively. Inactivation of CYP2J2 by MS or OD resulted in a loss of the native heme spectrum and a similar decrease in the reduced CO difference spectrum. A heme adduct was observed in the MS-inactivated CYP2J2. The possible reactive metabolite which covalently modified the prosthetic heme was characterized by analysis of the glutathione conjugates formed by MS or OD following oxygenation of the ethynyl moiety. Liquid chromatography-mass spectrometry showed that inactivation by MS or OD did not lead to modification of apoprotein. Interaction of CYP2J2 with DZ produced a type II binding spectrum with a Ks of 2.8 μM and the IC50 for loss of OHEB carboxylation activity was 0.18 μM. In conclusion, heme modification by MS and OD was responsible for the mechanism-based inactivation of CYP2J2. The results suggest that the ethynyl moiety of MS and OD faces the heme iron, whereas the isoxazole ring of DZ is preferentially oriented toward the heme iron of CYP2J2.

Cytochrome P450 family 1 inhibitors and structure-activity relationships

With the widespread use of O-alkoxyresorufin dealkylation assays since the 1990s, thousands of inhibitors of cytochrome P450 family 1 enzymes (P450s 1A1, 1A2, and 1B1) have been identified and studied. Generally, planar polycyclic molecules such as polycyclic aromatic hydrocarbons, stilbenoids, and flavonoids are considered to potentially be effective inhibitors of these enzymes, however, the details of the structure-activity relationships and selectivity of these inhibitors are still ambiguous. In this review, we thoroughly discuss the selectivity of many representative P450 family 1 inhibitors reported in the past 20 years through a meta-analysis.

Cytochrome p450 architecture and cysteine nucleophile placement impact raloxifene-mediated mechanism-based inactivation

The propensity for cytochrome P450 (P450) enzymes to bioactivate xenobiotics is governed by the inherent chemistry of the xenobiotic itself and the active site architecture of the P450 enzyme(s). Accessible nucleophiles in the active site or egress channels of the P450 enzyme have the potential of sequestering reactive metabolites through covalent modification, thereby limiting their exposure to other proteins. Raloxifene, a drug known to undergo CYP3A-mediated reactive metabolite formation and time-dependent inhibition in vitro, was used to explore the potential for bioactivation and enzyme inactivation of additional P450 enzymes (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A5). Every P450 tested except CYP2E1 was capable of raloxifene bioactivation, based on glutathione adduct formation. However, raloxifene-mediated time-dependent inhibition only occurred in CYP2C8 and CYP3A4. Comparable inactivation kinetics were achieved with K(I) and k(inact) values of 0.26 μM and 0.10 min(-1) and 0.81 μM and 0.20 min(-1) for CYP2C8 and CYP3A4, respectively. Proteolytic digests of CYP2C8 and CYP3A4 Supersomes revealed adducts to Cys225 and Cys239 for CYP2C8 and CYP3A4, respectively. For each P450 enzyme, proposed substrate/metabolite access channels were mapped and active site cysteines were identified, which revealed that only CYP2C8 and CYP3A4 possess accessible cysteine residues near the active site cavities, a result consistent with the observed kinetics. The combined data suggest that the extent of bioactivation across P450 enzymes does not correlate with P450 inactivation. In addition, multiple factors contribute to the ability of reactive metabolites to form apo-adducts with P450 enzymes.

Pivotal role of water in terminating enzymatic function: a density functional theory study of the mechanism-based inactivation of cytochromes P450

The importance of the mechanism-based inactivation (MBI) of enzymes, which has a variety of physiological effects and therapeutic implications, has been garnering appreciation. Density functional theory calculations were undertaken to gain a clear understanding of the MBI of a cytochrome P450 enzyme (CYP2B4) by tert-butylphenylacetylene (tBPA). The results of calculations suggest that, in accordance with previous proposals, the reaction proceeds via a ketene-type metabolic intermediate. Once an oxoiron(IV) porphyryn π-cation radical intermediate (compound I) of P450 is generated at the heme reaction site, ketene formation is facile, as the terminal acetylene of tBPA can form a C-O bond with the oxo unit of compound I with a relatively low reaction barrier (14.1 kcal/mol). Unexpectedly, it was found that the ketene-type intermediate was not very reactive. Its reaction with the hydroxyl group of a threonine (Thr302) to form an ester bond required a substantial barrier (38.2 kcal/mol). The high barrier disfavored the mechanism by which these species react directly. However, the introduction of a water molecule in the reaction center led to its active participation in the reaction. The water was capable of donating its proton to the tBPA molecule, while accepting the proton of threonine. This water-mediated mechanism lowered the reaction barrier for the formation of an ester bond by about 20 kcal/mol. Therefore, our study suggests that a water molecule, which can easily gain access to the threonine residue through the proton-relay channel, plays a critical role in enhancing the covalent modification of threonine by terminal acetylene compounds. Another type of MBI by acetylenes, N-alkylation of the heme prosthetic group, was less favorable than the threonine modification pathway.

7-Ethynylcoumarins: selective inhibitors of human cytochrome P450s 1A1 and 1A2

To discover new selective mechanism-based P450 inhibitors, eight 7-ethynylcoumarin derivatives were prepared through a facile two-step synthetic route. Cytochrome P450 activity assays indicated that introduction of functional groups in the backbone of coumarin could enhance the inhibition activities toward P450s 1A1 and 1A2, providing good selectivity against P450s 2A6 and 2B1. The most potent product 7-ethynyl-3,4,8-trimethylcoumarin (7ETMC) showed IC(50) values of 0.46 μM and 0.50 μM for P450s 1A1 and 1A2 in the first six minutes, respectively, and did not show any inhibition activity for P450s 2A6 and 2B1 even at the dose of 50 μM. All of the inhibitors except 7-ethynyl-3-methyl-4-phenylcoumarin (7E3M4PC) showed mechanism-based inhibition of P450s 1A1 and 1A2. In order to explain this mechanistic difference in inhibitory activities, X-ray crystallography data were used to study the difference in conformation between 7E3M4PC and the other compounds studied. Docking simulations indicated that the binding orientations and affinities resulted in different behaviors of the inhibitors on P450 1A2. Specifically, 7E3M4PC with its two-plane structure fits into the P450 1A2's active site cavity with an orientation leading to no reactive binding, causing it to act as a competitive inhibitor.

Thr302 is the site for the covalent modification of human cytochrome P450 2B6 leading to mechanism-based inactivation by tert-butylphenylacetylene

The mechanism-based inactivation of human CYP2B6 by tert-butylphenylacetylene (BPA) in the reconstituted system was investigated. The inactivation of CYP2B6 by BPA is time-, concentration-, and NADPH-dependent and exhibits a K(I) of 2.8 μM, a k(inact) of 0.7 min(-1), and a t(1/2) of 1 min. The partition ratio is ∼5. Unlike CYP2B1 and CYP2B4, in addition to the formation of an apoprotein adduct and a glutathione conjugate, a small heme adduct was observed when CYP2B6 was incubated with BPA. The mass increase of the adducted apoprotein and GSH conjugate is 174 Da, equivalent to the mass of one molecule of BPA plus one oxygen atom. To identify the adducted residue, BPA-inactivated CYP2B6 was digested with trypsin, and the digest was then analyzed by liquid chromatography-tandem mass spectrometry. A mass shift of 174 Da was used for the SEQUEST database search, and the identity of the modified residue was confirmed by MS/MS fragmentation of the modified peptide. Two residues, Lys274 and Thr302, were identified as having been modified. Further mutagenesis studies have demonstrated that the residue that is modified to result in inactivation is Thr302, not Lys274. Docking studies show that in the enzyme-substrate complex, Thr302 is in close contact with the triple bond of BPA with a distance of 3.8 Å between the terminal carbon of BPA and the oxygen in the hydroxyl group of Thr302. In conclusion, Thr302 of CYP2B6 is covalently modified by a reactive metabolite of BPA, and this modification is responsible for the mechanism-based inactivation.

Substituted imidazole of 5-fluoro-2-[4-[(2-phenyl-1H-imidazol-5-yl)methyl]-1-piperazinyl]pyrimidine Inactivates cytochrome P450 2D6 by protein adduction

5-Fluoro-2-[4-[(2-phenyl-1H-imidazol-5-yl)methyl]-1-piperazinyl]pyrimidine (SCH 66712) is a potent mechanism-based inactivator of human cytochrome P450 2D6 that displays type I binding spectra with a K(s) of 0.39 ± 0.10 μM. The partition ratio is ~3, indicating potent inactivation that addition of exogenous nucleophiles does not prevent. Within 15 min of incubation with SCH 66712 and NADPH, ∼90% of CYP2D6 activity is lost with only ~20% loss in ability to bind CO and ~25% loss of native heme over the same time. The stoichiometry of binding to the protein was 1.2:1. SDS-polyacrylamide gel electrophoresis with Western blotting and autoradiography analyses of CYP2D6 after incubations with radiolabeled SCH 66712 further support the presence of a protein adduct. Metabolites of SCH 66712 detected by mass spectrometry indicate that the phenyl group on the imidazole ring of SCH 66712 is one site of oxidation by CYP2D6 and could lead to methylene quinone formation. Three other metabolites were also observed. For understanding the metabolic pathway that leads to CYP2D6 inactivation, metabolism studies with CYP2C9 and CYP2C19 were performed because neither of these enzymes is significantly inhibited by SCH 66712. The metabolites formed by CYP2C9 and CYP2C19 are the same as those seen with CYP2D6, although in different abundance. Modeling studies with CYP2D6 revealed potential roles of various active site residues in the oxidation of SCH 66712 and inactivation of CYP2D6 and showed that the phenyl group of SCH 66712 is positioned at 2.2 Å from the heme iron.

Analysis of the oxidation of short chain alkynes by flavocytochrome P450 BM3

Bacillus megaterium flavocytochrome P450 BM3 (BM3) is a high activity fatty acid hydroxylase, formed by the fusion of soluble cytochrome P450 and cytochrome P450 reductase modules. Short chain (C6, C8) alkynes were shown to be substrates for BM3, with productive outcomes (i.e. alkyne hydroxylation) dependent on position of the carbon-carbon triple bond in the molecule. Wild-type P450 BM3 catalyses ω-3 hydroxylation of both 1-hexyne and 1-octyne, but is suicidally inactivated in NADPH-dependent turnover with non-terminal alkynes. A F87G mutant of P450 BM3 also undergoes turnover-dependent heme destruction with the terminal alkynes, pointing to a key role for Phe87 in controlling regioselectivity of alkyne oxidation. The terminal alkynes access the BM3 heme active site led by the acetylene functional group, since hydroxylated products are not observed near the opposite end of the molecules. For both 1-hexyne and 1-octyne, the predominant enantiomeric product formed (up to ∼90%) is the (S)-(-)-1-alkyn-3-ol form. Wild-type P450 BM3 is shown to be an effective oxidase catalyst of terminal alkynes, with strict regioselectivity of oxidation and potential biotechnological applications. The absence of measurable octanoic or hexanoic acid products from oxidation of the relevant 1-alkynes is also consistent with previous studies suggesting that removal of the phenyl group in the F87G mutant does not lead to significant levels of ω-oxidation of alkyl chain substrates.

Targeting of the highly conserved threonine 302 residue of cytochromes P450 2B family during mechanism-based inactivation by aryl acetylenes

Cytochromes P450 (CYPs or P450s) contain a highly conserved threonine residue in the active site, which is referred to as Thr302 in the amino acid sequence of CYP2B4. Extensive biochemical and crystallographic studies have established that this Thr302 plays a critical role in activating molecular oxygen to generate Compound I, a putative iron(IV)-oxo porphyrin cation radical, that carries out the preliminary oxygenation of CYP substrates. Because of its proximity to the center of the P450 active site, this Thr302 is susceptible to mechanism-based inactivation under certain conditions. In this article, we review recent studies on the mechanism-based inactivation of three mammalian P450s in the 2B family, CYP2B1 (rat), 2B4 (rabbit) and 2B6 (human) by tert-butylphenylacetylene (tBPA). These studies showed that tBPA is a potent mechanism-based inactivator of CYP2B1, 2B4 and 2B6 with high k(inact)/K(I) ratios (0.23-2.3min(-1)μM(-1)) and low partition ratios (0-5). Furthermore, mechanistic studies revealed that tBPA inactivates these three CYP2B enzymes through the formation of a single ester adduct with the Thr302 in the active site. These inhibitory properties of tBPA allowed the preparation of a modified CYP2B4 where the Thr302 was covalently and stoichiometrically labeled by a reactive intermediate of tBPA in quantities large enough to permit spectroscopic and crystallographic studies of the consequences of covalent modification of Thr302. Molecular modeling studies revealed a unique binding mode of tBPA in the active site that may shed light on the potency of this inhibition. The results from these studies may serve as a basis for designing more specific and potent inhibitors for P450s by targeting this highly conserved threonine residue which is present in the active sites of most mammalian P450s.

In silico studies of polyaromatic hydrocarbon inhibitors of cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1

A computational study was undertaken to understand the nature of binding and the structural features that play a significant role in the binding of arylacetylene molecules to cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1. Nine polycyclic arylacetylenes determined to be mechanism-based P450 enzyme inhibitors were studied. The lack of polar substituents in these compounds causes them to be incapable of hydrogen bonding to the polar protein residues. The four P450 enzymes of interest all have phenylalanine residues in the binding pocket for potential pi-pi interactions with the aromatic rings of the inhibitors. The inhibition potency of these arylacetylenes toward P450s 1A1 and 2B1 showed a dependence on the proximity of the inhibitor's triple bond to the prosthetic heme Fe of the enzyme. In P450 enzyme 1A2, the inhibitor's potency showed more dependence on the pi-pi interactions of the inhibitor's ring systems with the phenylalanine residues of the protein, with the proximity of the inhibitor triple bond to the heme Fe weighing in as the second most important factor. The results suggest that maximizing the pi-pi interactions with phenylalanine residues in the binding pocket and optimum proximity of the acetylene moiety to the heme Fe will provide for a substantial increase in the potency of the polyaromatic hydrocarbon mechanism-based inhibitors. A fine balance of these two aspects of binding coupled with attention to supplementing hydrophobic interactions could address potency and selectivity issues for these inhibitors.

tert-Butylphenylacetylene is a potent mechanism-based inactivator of cytochrome P450 2B4: inhibition of cytochrome P450 catalysis by steric hindrance

We have demonstrated that 4-(tert-butyl)-phenylacetylene (tBPA) is a potent mechanism-based inactivator for cytochrome P450 2B4 (P450 2B4) in the reconstituted system. It inactivates P450 2B4 in a NADPH- and time-dependent manner with a K(I) of 0.44 microM and k(inact) of 0.12 min(-1). The partition ratio was approximately zero, indicating that inactivation occurs without the reactive intermediate leaving the active site. Liquid chromatography-mass spectrometry analyses revealed that tBPA forms a protein adduct with a 1:1 stoichiometry. Peptide mapping of the tBPA-modified protein provides evidence that tBPA is covalently bound to Thr302. This is consistent with results of molecular modeling that show the terminal carbon of the acetylenic group is only 3.65 A away from Thr302. To characterize the effect of covalent modification of Thr302, tBPA-modified P450 2B4 was purified to homogeneity from the reconstituted system. The Soret band of tBPA-modified protein is red-shifted by 5 to 422 nm compared with unmodified protein. Benzphetamine binding to the modified P450 2B4 causes no spin shift, indicating that substrate binding and/or the heme environment has been altered by covalently bound tBPA. Cytochrome P450 reductase reduces the unmodified and tBPA-modified P450s at approximately the same rate. However, addition of benzphetamine stimulates the rate of reduction of unmodified P450 2B4 by approximately 20-fold but only marginally stimulates reduction of the tBPA-modified protein. This large discrepancy in the stimulation of the first electron transfer by benzphetamine strongly suggests that the impairment of P450 catalysis is due to inhibition of benzphetamine binding to the tBPA-modified P450 2B4.

Mechanism-based inactivation of CYP2B1 and its F-helix mutant by two tert-butyl acetylenic compounds: covalent modification of prosthetic heme versus apoprotein

The mechanism-based inactivation of cytochrome CYP2B1 [wild type (WT)] and its Thr205 to Ala mutant (T205A) by tert-butylphenylacetylene (BPA) and tert-butyl 1-methyl-2-propynyl ether (BMP) in the reconstituted system was investigated. The inactivation of WT by BPA exhibited a k(inact)/K(I) value of 1343 min(-1)mM(-1) and a partition ratio of 1. The inactivation of WT by BMP exhibited a k(inact)/K(I) value of 33 min(-1)mM(-1) and a partition ratio of 10. Liquid chromatography/tandem mass spectrometry analysis (LC/MS/MS) of the WT revealed 1) inactivation by BPA resulted in the formation of a protein adduct with a mass increase equivalent to the mass of BPA plus one oxygen atom, and 2) inactivation by BMP resulted in the formation of multiple heme adducts that all exhibited a mass increase equivalent to BMP plus one oxygen atom. LC/MS/MS analysis indicated the formation of glutathione (GSH) conjugates by the reaction of GSH with the ethynyl moiety of BMP or BPA with the oxygen being added to the internal or terminal carbon. For the inactivation of T205A by BPA and BMP, the k(inact)/K(I) values were suppressed by 100- and 4-fold, respectively, and the partition ratios were increased 9- and 3.5-fold, respectively. Only one major heme adduct was detected following the inactivation of the T205A by BMP. These results show that the Thr205 in the F-helix plays an important role in the efficiency of the mechanism-based inactivation of CYP2B1 by BPA and BMP. Homology modeling and substrate docking studies were presented to facilitate the interpretation of the experimental results.

Human cytochrome P450 1A2 involvement in the formation of reactive metabolites from a species-specific hepatotoxic pyrazolopyrimidine derivative, 5-n-butyl-7-(3,4,5-trimethoxybenzoylamino)pyrazolo[1,5-a]pyrimidine

5-n-Butyl-7-(3,4,5-trimethoxybenzoylamino)pyrazolo[1,5-a]pyrimidine) (OT-7100) is a pyrazolopyrimidine derivative with potential analgesic effects. Exclusively limited elevations in serum levels of aspirate- and alanine-aminotransferase were abnormally observed in a clinical study, in contrast to no toxicological potential to experimental animals. The aim of this study was to clarify the mechanism responsible for species-specific hepatotoxicity of this model compound. OT-7100 was primarily metabolized to a carboxylic acid derivative and an amino derivative (5-n-butyl-pyrazolo[1,5-a]pyrimidine, M-5) by hydrolysis in humans and rats. In human liver, pyrazolo[1,5-a]pyrimidine derivative M-5 was further metabolized to mainly M-23OH (a C-3-position hydroxyl derivative, 3-hydroxy-5-n-butyl-pyrazolo[1,5-a]pyrimidine). Studies with recombinant cytochrome P450s (P450s), correlation analysis using a panel of human liver microsomes as well as immunoinhibition with anti-P450 antibodies collectively suggested that human liver microsomal P450 1A2 preferentially metabolized M-5 to predominantly M-23OH. Human liver microsomes were capable of activating M-5 to a covalently bound metabolite faster than rat liver microsomes: reduced glutathione prevented the bindings. A cysteine adduct derivative of M-23OH at the C-6-position was structurally confirmed. On the contrary, rat liver microsomal P450 1A2 could metabolize M-5 to equally M-23OH, M-22OH (a C-6-position hydroxyl derivative, 6-hydroxy-5-n-butyl-pyrazolo[1,5-a]pyrimidine), or an unknown metabolite. These results suggest that differences in the regiospecific metabolic function of human and rat P450 1A2 would be responsible for the human-specific metabolic activation of the primary metabolite of OT-7100 to a proximate form. It is presumed that hepatotoxicity associated with OT-7100 could be likely related to the formation of a human-specific reactive metabolite from M-23OH. OT-7100 activation by inducible P450 1A2 may therefore exhibit marked individual differences.

Metabolic activation of mifepristone [RU486; 17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-propynyl)-estra-4,9-dien-3-one] by mammalian cytochromes P450 and the mechanism-based inactivation of human CYP2B6

Mifepristone [RU486; 17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-propynyl)-estra-4,9-dien-3-one] inactivates CYP2B6 in the reconstituted system in a mechanism-based manner. The loss of 7-ethoxy-4-(trifluoromethyl)-coumarin deethylation activity of CYP2B6 is concentration- and time-dependent. The inactivation requires NADPH and is irreversible. The concentration of inactivator required to give the half-maximal rate of inactivation is 2.8 microM, and the maximal rate constant for inactivation at a saturating concentration of the inactivator is 0.07 min(-1). Incubation of CYP2B6 with 20 microM RU486 for 15 min resulted in 61% loss of catalytic activity, 60% loss of the reduced cytochrome P450 (P450)-CO complex, and a 40% loss of native heme. The partition ratio is approximately 5, and the stoichiometry of binding is approximately 0.6 mol RU486/mol P450 inactivated. SDS-polyacrylamide gel electrophoresis and high-pressure liquid chromatography analysis showed that [(3)H]RU486 was irreversibly bound to CYP2B6 apoprotein. RU486 is metabolized to form three major metabolites and bioactivated to give reactive intermediates by purified P450s in the reconstituted system. After incubation of RU486 with the purified P450s and liver microsomes from rats and humans in the presence of glutathione (GSH) and NADPH, GSH conjugates with MH(+) ions at m/z 769, 753, and 751 were detected by liquid chromatography-tandem mass spectrometry. Two GSH conjugates with MH(+) ions at m/z 753 are formed from the reaction of GSH with RU486. The adducts are formed after addition of an activated oxygen to the carbon-carbon triple bond of the propynyl moiety. This suggests that oxirene intermediates may be involved in the mechanism of inactivation. It seems that the potential for drug-drug interactions of RU486 may not be limited only to CYP3A4 and should also be evaluated for drugs metabolized primarily by CYP2B6, such as bupropion and efavirenz.

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.

The inactivation of cytochrome P450 3A5 by 17alpha-ethynylestradiol is cytochrome b5-dependent: metabolic activation of the ethynyl moiety leads to the formation of glutathione conjugates, a heme adduct, and covalent binding to the apoprotein

17Alpha-ethynylestradiol (EE) inactivates cytochrome P450 3A5 (3A5) in the reconstituted system in a mechanism-based manner. The inactivation is dependent on NADPH, and it is irreversible. The inactivation of 3A5 by EE is also dependent on cytochrome b5 (b5). The values for the K(I) and k(inact) of the 7-benzyloxy-4-(trifluoromethyl)coumarin O-debenzylation activity of 3A5 are 26 microM and 0.06 min(-1), respectively. Incubation of 3A5 with EE resulted in a 62% loss of catalytic activity, 60% loss in the reduced CO difference spectrum, and 40% decrease in native heme with the formation of a heme adduct. The partition ratio was approximately 25, and the stoichiometry of binding was approximately 0.3 mol of EE metabolite bound/mol of P450 inactivated. Four major metabolites were formed during the metabolism of EE by 3A5. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [3H]EE was irreversibly bound to 3A5 apoprotein. Liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) revealed that two glutathione (GSH) conjugates with m/z values of 620 were formed only in the presence of b5. These two conjugates are formed from the reaction of GSH with the ethynyl group with the oxygen being inserted into either the internal or terminal carbon. A heme adduct with the ion at m/z 927 and two dipyrrole adducts with ions at m/z 579 were detected by LC-MS/MS analysis. In conclusion, 3A5 can activate EE to a 17alpha-oxirene-related reactive species that can then partition the oxygen between the internal and terminal carbons of the ethynyl group to form heme and apoprotein adducts, resulting in the inactivation of P450 3A5.

Calibration of the channel that determines the omega-hydroxylation regiospecificity of cytochrome P4504A1: catalytic oxidation of 12-HALODOdecanoic acids

The fatty acid omega-hydroxylation regiospecificity of CYP4 enzymes may result from presentation of the terminal carbon to the oxidizing species via a narrow channel that restricts access to the other carbon atoms. To test this hypothesis, the oxidation of 12-iodo-, 12-bromo-, and 12-chlorododecanoic acids by recombinant CYP4A1 has been examined. Although all three 12-halododecanoic acids bind to CYP4A1 with similar dissociation constants, the 12-chloro and 12-bromo fatty acids are oxidized to 12-hydroxydodecanoic acid and 12-oxododecanoic acid, whereas the 12-iodo analogue is very poorly oxidized. Incubations in H(2)(18)(2)O show that the 12-hydroxydodecanoic acid oxygen derives from water, whereas that in the aldehyde derives from O(2). The alcohol thus arises from oxidation of the halide to an oxohalonium species that is hydrolyzed by water, whereas the aldehyde arises by a conventional carbon hydroxylation-elimination mechanism. No irreversible inactivation of CYP4A1 is observed during 12-halododecanoic acid oxidation. Control experiments show that CYP2E1, which has an omega-1 regiospecificity, primarily oxidizes 12-halododecanoic acids to the omega-aldehyde rather than alcohol product. Incubation of CYP4A1 with 12,12-[(2)H](2)-12-chlorododecanoic acid causes a 2-3-fold increase in halogen versus carbon oxidation. The fact that the order of substrate oxidation (Br > Cl >> I) approximates the inverse of the intrinsic oxidizability of the halogen atoms is consistent with presentation of the halide terminus via a channel that accommodates the chloride and bromide but not iodide atoms, which implies an effective channel diameter greater than 3.90 Angstroms but smaller than 4.30 Angstroms.

The grapefruit juice effect is not limited to cytochrome P450 (P450) 3A4: evidence for bergamottin-dependent inactivation, heme destruction, and covalent binding to protein in P450s 2B6 and 3A5

Bergamottin (BG), a component of grapefruit juice, is a mechanism-based inactivator of cytochromes P450 (P450) 2B6 and 3A5 in the reconstituted system. The inactivation of both P450s was NADPH-dependent and irreversible. The kinetic constants for the inactivation of the 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of P450 2B6 were: K(I), 5 microM; k(inact) 0.09 min(-1); and t1/2, 8 min. The kinetic constants obtained for the inactivation of the testosterone 6beta-hydroxylation activity of P450 3A5 were: K(I), 20 microM; k(inact) 0.045 min(-1); and t1/2, 15 min. Incubations of P450s 2B6 and 3A5 with 20 microM BG at 37 degrees C for 20 min resulted in an approximately 60% loss in the catalytic activity that was accompanied by a significant loss in intact heme and a similar decrease in the reduced CO difference spectrum. The extrapolated partition ratios for BG with P450s 2B6 and 3A5 were approximately 2 and approximately 20, respectively. Liquid chromatography-mass spectroscopy analysis of the BG-inactivated samples showed that the mass of the inactivated apoprotein had increased by approximately 388 Da for both P450 2B6 and P450 3A5. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [14C]BG was irreversibly bound to the apoprotein in the BG-inactivated samples. The stoichiometry of binding was approximately 0.5 mol BG metabolite/mol of each P450 inactivated. High-pressure liquid chromatography analysis of the metabolites of BG showed that P450 2B6 generated two major metabolites, whereas P450 3A5 generated three additional metabolites. Two of metabolites were identified as 6',7'-dihydroxybergamottin and bergaptol.

Metabolic activation of troglitazone: identification of a reactive metabolite and mechanisms involved

Troglitazone (TGZ), the first glitazone used for the treatment of type II diabetes mellitus and removed from the market for liver toxicity, was shown to bind covalently to microsomal protein and glutathione (GSH) following activation by cytochrome P450 (P450). The covalent binding of (14)C-TGZ in dexamethasone-induced rat liver microsomes was NADPH-dependent and required the active form of P450; it was completely inhibited by ketoconazole (10 microM) and GSH (4 mM). The covalent binding in P450 3A4 Supersomes (9.2 nmol of TGZ Eq/nmol P450) was greater than that with P450 1A2 (0.7), 2C8 (3.7), 2C19 (1.4), 2E1 (0.6), and 2D6 (1.1) and 3A5 (3.0). The covalent binding in liver microsomes from rats pretreated with dexamethasone (5.3 nmol of TGZ Eq bound/nmol P450) was greater than that from rats pretreated with vehicle (3.5), beta-naphthoflavone (0.4), phenobarbital (1.1), or pyridine (2.5). A TGZ-GSH adduct was detected by liquid chromatography-tandem mass spectrometry and radioactivity detection with a deprotonated quasi-molecular ion [M-H](-) at m/z 745, with fragment ions at m/z 438 (deprotonated TGZ moiety), and at m/z 306 (deprotonated GSH moiety). The TGZ-GSH adduct was determined to be 5-glutathionyl-5-[4-(6-hydroxy-2,5,7,8-tetramethylchroman-2-ylmethoxy)benzyl]-thiazolidine-2,4-dione based on collision-induced dissociation fragmentation, and one- and two-dimensional NMR analysis of the isolated adduct. The synthetic 5-hydroxy TGZ and the benzylidene derivative of TGZ did not react with GSH or GSH ethyl ester. The mechanisms for metabolic activation of TGZ may involve an ultimate reactive sulfonium ion which could be formed from an initial sulfoxide followed by a formal Pummerer rearrangement, or a C5 thiazolidinedione radical or a sulfur cation radical.

Mechanism-based inactivation of cytochrome P450 2B6 by a novel terminal acetylene inhibitor

N-(3,5-Dichloro-4-pyridyl)-3-(cyclopentyloxy)-4-methoxybenzamide (DCMB) is a known marker substrate for cytochrome p450 2B6. Based on the chemical template of DCMB, a novel terminal acetylene compound, N-(3,5-dichloro-4-pyridyl)-4-methoxy-3-(prop-2-ynyloxy)benzamide (TA) was synthesized and evaluated as a mechanism-based inactivator of p450 2B6. The pseudo first-order inactivation of expressed p450 2B6 by TA was both substrate and time-dependent. The kinetics of inhibition resulted in a maximal rate constant (k(inactivation)) of 0.09 min(-1) and an apparent K(I) of 5.1 microM. Incubation of expressed p450 2B6 with TA and NADPH resulted in a 68% loss in enzyme activity and a concurrent 62% loss in the formation of a reduced carbon monoxide complex, suggesting that heme destruction is the primary mode of enzyme inactivation. Enzyme inactivation of p450 2B6 was not reduced by the presence of 10 mM glutathione and was protected by incubation of excess DCMB with TA. The production of the carboxylic acid metabolite, N-(3,5-Dichloro-4-pyridyl)-3-(2-carboxyethoxy)-4-methoxybenzamide (TA-COOH), during the incubation of TA with 2B6 suggests that inactivation proceeds through a ketene intermediate. For 2B6 inactivation, the partition ratio was approximately 1.5 nmol TA-COOH formed/nmol P450 inactivated. Finally, TA was evaluated for mechanism-based inactivation of p450 3A4, 2C9, 2C19, 2D6, and 2E1 using human liver microsomes. In addition to 2B6, p450 2C forms were also found to be sensitive to TA-mediated inactivation, suggesting that subtle changes in the O-alkyl chain of the parent may be critical for the selectivity of enzyme inactivation.

Mechanism-based inactivation of cytochrome P450 3A4 by 17 alpha-ethynylestradiol: evidence for heme destruction and covalent binding to protein

17 alpha-Ethynylestradiol (EE), a major constituent of many oral contraceptives, inactivated the testosterone 6 beta-hydroxylation activity of purified P450 3A4 reconstituted with phospholipid and NADPH-cytochrome P450 reductase in a mechanism-based manner. The inactivation of P450 3A4 followed pseudo first order kinetics and was dependent on NADPH. The values for the K(I) and k(inact) were 18 microM and 0.04 min(-1), respectively, and the t(1/2) was 16 min. Incubation of 50 microM EE with P450 3A4 at 37 degrees C for 30 min resulted in a 67% loss of testosterone 6 beta-hydroxylation activity accompanied by a 35% loss of the spectral absorbance of the native protein at 415 nm and a 70% loss of the spectrally detectable P450-CO complex. The inactivation of P450 3A4 by EE was irreversible. Testosterone, an alternate substrate, was able to protect P450 3A4 from EE-dependent inactivation. The partition ratio was approximately 50. The stoichiometry of binding was approximately 1.3 nmol of an EE metabolite bound per nmol of P450 3A4 inactivated. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [(3)H]EE was irreversibly bound to the P450 3A4 apoprotein. After extensive dialysis of the [(3)H]EE inactivated samples, high-pressure liquid chromatography (HPLC) analysis demonstrated that the inactivation resulting from EE metabolism led to the destruction of approximately half the heme with the concomitant generation of modified heme and EE-labeled heme fragments and produced covalently radiolabeled P450 3A4 apoprotein. Electrospray mass spectrometry demonstrated that the fraction corresponding to the major radiolabeled product of EE metabolism has a mass (M - H)(-) of 479 Da. HPLC and gas chromatography-mass spectometry analyses revealed that EE metabolism by P450 3A4 generated one major metabolite, 2-hydroxyethynylestradiol, and at least three additional metabolites. In conclusion, our results demonstrate that EE is an effective mechanism-based inactivator of P450 3A4 and that the mechanism of inactivation involves not only heme destruction, but also the irreversible modification of the apoprotein at the active site.

Mechanism-based inhibition of rat liver microsomal diazepam C3-hydroxylase by mifepristone associated with loss of spectrally detectable cytochrome P450

Since initial studies with the steroids norethindrone and ethynylestradiol, reported by White and Muller-Eberhard in 1977 (Biochem. J. 166, 57-64), there has been continuing interest in xenobiotics that bear terminal or sub-terminal acetylenic groups which can cause catalysis-dependent inhibition of CYP monooxygenases associated either with loss of prosthetic group heme or protein adduct formation. Mifepristone is a synthetic steroid bearing a propyne substitution on carbon 17 and this suggested to us that it may act as a mechanism-based inhibitor of the CYP isoforms responsible for its metabolism. In human and rat liver, CYP3A isoforms have been implicated in mifepristone clearance and mifepristone administration to rats has also been shown to induce CYP3A enzymes and the associated diazepam C3-hydroxylase activity (Cheesman, Mason and Reilly, J. Steroid Biochem. Mol. Biol., 58, 1996, 447-454). With microsomes prepared from the livers of untreated female rats and others in which diazepam C3-hydroxylase has been induced, we show here that mifepristone can cause catalysis-dependent inhibition of this monooxygenase. In addition, incubation of microsomes with mifepristone in the presence, but not in the absence, of NADPH caused loss of spectrally detectable cytochrome P450. These results suggest that heme adduct formation may result from mifepristone metabolism by CYP3A monooxygenases which undergo self-catalysed irreversible inactivation with this drug as substrate. Since mifepristone administration in vivo is able also to cause induction of the synthesis of hepatic CYP3A apoprotein, mifepristone may have the potential in human medicine for complex interactions with other co-administered drugs which are also substrates for CYP3A monooxygenases.

Mechanism-based inactivation of cytochromes P450 2E1 and 2B1 by 5-phenyl-1-pentyne

A series of acetylenic compounds whose structures were based on "P450 2E1-like" substrates was investigated for their ability to cause inactivation of P450 2E1-dependent p-nitrophenol hydroxylation. The most effective compound with liver microsomes from pyridine-treated rats or with rabbit P450 2E1 in a reconstituted system was 5-phenyl-1-pentyne. The inactivation of purified 2B1, 2E1, a truncated 2E1 lacking amino acids 3-29, 2E1(Delta3-29), or a truncated 2E1 in which threonine 303 was replaced with alanine, 2E1(Delta3-29, T303A), in a reconstituted system by 5-phenyl-1-pentyne was NADPH- and time-dependent and followed pseudo-first-order kinetics. The maximal rate constants for inactivation, the concentrations that gave half-maximal inactivation (KI), and the partition ratios (the number of 5-phenylvaleric acid molecules formed/inactivation event) were determined with each P450. The KI values for 2B1 and 2E1(Delta3-29, T303A) were twice those for 2E1 and 2E1(Delta3-29), and the partition ratios for 2B1 and 2E1(Delta3-29, T303A) were 5-10 times greater than those of 2E1 or 2E1(Delta3-29). During the incubation of P450 2E1 with 5-phenyl-1-pentyne, the loss of P450 as determined by the reduced-CO difference spectra was equal to the loss of catalytic activity. The formation of a heme adduct was demonstrated by HPLC analysis of reaction mixtures containing 5-[3H]phenyl-1-pentyne. HPLC analysis with diode-array detection showed that the Soret region of the proposed heme adduct was different from that of the unmodified heme. The HPLC peak containing the proposed heme adduct was further analyzed by matrix-assisted laser desorption ionization mass spectrometry, and the resulting peaks could result from the addition of a 2-oxo-5-phenylpentyl group to the heme.

Molecular modelling of CYP1A subfamily members based on an alignment with CYP102: rationalization of CYP1A substrate specificity in terms of active site amino acid residues

1. Using a novel amino acid sequence alignment, proteins of the CYP1A subfamily have been produced from the CYP102 crystal structure template via residue replacement and energy minimization procedures. 2. Known substrates and inhibitors of CYP1A1 and CYP1A2 are shown to fit their respective active sites via key interactions with complementary amino acid residues. Substrates used in the modelling studies include: caffeine, PhIP, oestradiol, 2,4- and 2,5-diaminotoluenes, Glu-P-1, phenacetin, acetanilide, 7-methoxy and 7-ethoxyresorufins, 11-methyl cyclopenta[a]phenanthren-17-one, 7-ethoxycoumarin, aflatoxin B1, benzo[a]pyrene, benzo[a]pyrene-7,8-diol and 1'-hydroxy 3-methylcholanthrene. 3. A number of aspects relating to CYP1A substrate specificity and metabolism can be explained in terms of the enzyme models, as it is found that key interactions with active site amino acid residues direct CYP1A-mediated metabolism in the known positions.

Rat liver microsomal cytochrome P450-dependent oxidation of 3,5-disubstituted analogues of paracetamol

1. The cytochrome P450-dependent binding of paracetamol and a series of 3,5-disubstituted paracetamol analogues (R = -F, -Cl, -Br, -I, -CH3, -C2H5, -iC3H7) have been determined with beta-naphthoflavone (beta NF)-induced rat liver microsomes and produced reverse type I spectral changes. Ks,app varied from 0.14 mM for 3,5-diiC3H7-paracetamol to 2.8 mM for paracetamol. 2. All seven analogues underwent rat liver microsomal cytochrome P450-dependent oxidation, as reflected by the formation of GSSG in the presence of GSH. The GSSG-formation was increased in all cases upon pretreatment of rats by beta-naphthoflavone (beta NF) and was generally decreased upon pretreatment by phenobarbital (PB). 3. Rat liver microsomal cytochrome P450 as well as horseradish peroxidase catalysed the formation of 3,5-disubstituted NAPQI analogues from the corresponding parent compounds, as identified by UV-spectrophotometry of the NAPQI analogues and by GC/MS detection of the following GSH-conjugates: 2-glutathione-S-yl-3,5-dimethyl-1,4-dihydroxybenzene, 2-glutathione-S-yl-3,5-dichloro-paracetamol, and 2-glutathione-S-yl-3,5-dibromo-paracetamol. 4. In liver microsomal (beta NF-induced) incubations, apparent K(m) values, as determined for the cytochrome P450 catalysis-dependent oxidation of GSH, for seven 3,5-disubstituted paracetamol analogues (R = -F, -Cl, -Br, -I, -CH3, -C2H5, iC3H7) varied from 0.07 to 0.64 mM. Paracetamol exhibited an apparent K(m) of 0.73 mM. Apparent Vmax values for the cytochrome P450 catalysis dependent oxidation of GSH varied from 0.66 nmol min-1 mg-1 protein for paracetamol to 3.0 nmol min-1 mg-1 protein for 3,5-dimethyl-paracetamol.

Photoaffinity labeling of cytochrome P450 2B4: capture of active site heme ligands by a photocarbene

Spiro[adamantane-2,2'-diazirine], which produces adamantyl carbene upon photolysis, binds tightly to P450 2B4 (KS = 3.2 microM), giving a normal substrate binding difference spectrum. Irradiation of 2-[3H]adamantane diazirine at 365 nm in the presence of native, ferric P450 2B4 resulted in first-order photolysis (t1/2 = 1.8 min). The main product was 2-[3H]adamantanol, with about 6% of the radioactivity covalently bound to P450 2B4. With the ferrous carbonyl form of P450 2B4, 2-adamantanol production decreased and protein labeling increased to 12%. When ferric cyanide 2B4 was used, 2-adamantanecarbonitrile was formed in addition to 2-adamantanol. The nitrile appears to have resulted from capture of the iron-bound cyanide ligand by the carbene. The use of multiple cycles of photolysis increased the percentage of protein labeling to 76%. Photolabeling was inhibited by known 2B4 substrates and inhibitors. Also, N-demethylation of benzphetamine and generation of a substrate binding difference spectrum by benzphetamine were both inhibited stoichiometrically with the fraction of radiolabeled protein. The labeled protein was permanently converted to the high-spin state, as indicated by the characteristic change in the absorbance spectrum, demonstrating irreversible occupation of the substrate binding site by the adamantyl residue. Mild acid hydrolysis of radiolabeled 2B4 at the five Asp-Pro bonds generated a 2-kDa peptide which carried 78% of the radioactivity. These results are interpreted as the result of the active site carbene reacting by three competing pathways: capture of the heme sixth ligand to yield either 2-adamantanol or 2-adamantanecarbonitrile, capture of an unbound active site water molecule to yield adamantanol, and covalent attachment to a protein residue. Thus, the P450 2B4 active site appears to contain at least one unbound water molecule in addition to the heme aquo sixth ligand, even when substrate is present.