Identification of 17-alpha-ethynylestradiol-modified active site peptides and glutathione conjugates formed during metabolism and inactivation of P450s 2B1 and 2B6

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.

Biomimetic trapping cocktail to screen reactive metabolites: use of an amino acid and DNA motif mixture as light/heavy isotope pairs differing in mass shift

Candidate drugs that can be metabolically transformed into reactive electrophilic products, such as epoxides, quinones, and nitroso compounds, are of special concern because subsequent covalent binding to bio-macromolecules can cause adverse drug reactions, such as allergic reactions, hepatotoxicity, and genotoxicity. Several strategies have been reported for screening reactive metabolites, such as a covalent binding assay with radioisotope-labeled drugs and a trapping method followed by LC-MS/MS analyses. Of these, a trapping method using glutathione is the most common, especially at the early stage of drug development. However, the cysteine of glutathione is not the only nucleophilic site in vivo; lysine, histidine, arginine, and DNA bases are also nucleophilic. Indeed, the glutathione trapping method tends to overlook several types of reactive metabolites, such as aldehydes, acylglucuronides, and nitroso compounds. Here, we introduce an alternate way for screening reactive metabolites as follows: A mixture of the light and heavy isotopes of simplified amino acid motifs and a DNA motif is used as a biomimetic trapping cocktail. This mixture consists of [²H₀]/[²H₃]-1-methylguanidine (arginine motif, Δ 3 Da), [²H₀]/[²H₄]-2-mercaptoethanol (cysteine motif, Δ 4 Da), [²H₀]/[²H₅]-4-methylimidazole (histidine motif, Δ 5 Da), [²H₀]/[²H₉]-n-butylamine (lysine motif, Δ 9 Da), and [¹³C₀,¹⁵N₀]/[¹³C₁,¹⁵N₂]-2'-deoxyguanosine (DNA motif, Δ 3 Da). Mass tag triggered data-dependent acquisition is used to find the characteristic doublet peaks, followed by specific identification of the light isotope peak using MS/MS. Forty-two model drugs were examined using an in vitro microsome experiment to validate the strategy. Graphical abstract Biomimetic trapping cocktail to screen reactive metabolites.

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 K_I and k_inact 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: K_I of 6.1 μM and k_inact of 0.22 min^-1 for MS and K_I of 2.5 μM and k_inact 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 K_s 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.

Inhibition of Human Cytochrome P450 2c8-catalyzed Amodiaquine N-desethylation: Effect of Five Traditionally and Commonly Used Herbs

Background:

In Southeast Asia and many parts of the world, herbal products are increasingly used in parallel with modern medicine.

Objective:

This study aimed to investigate the effects of herbs commonly used in Southeast Asia on activity of cytochrome P450 2C8 (CYP2C8), an important human hepatic enzyme in drug metabolism.

Materials and Methods:

The selected herbs, such as Eurycoma longifolia Jack (ELJ), Labisia pumila (LP), Echinacea purpurea (EP), Andrographis paniculata (AP), and Ginkgo biloba (GB), were subjected to inhibition studies using an in vitro CYP2C8 activity marker, amodiaquine N-desethylase assay. Inhibition parameters, inhibitory concentration 50% (IC₅₀), and K_i values were determined to study the potency and mode of inhibition.

Results:

All herbs inhibited CYP2C8 with the following order of potency: LP > ELJ > GB > AP > EP. LP and ELJ inhibited potently at K_i's of 2 and 4 times the K_i of quercetin, the positive control. The inhibition by LP was uncompetitive in nature as compared to competitive or mixed type inhibition observed with other herbs. GB exhibited moderate inhibitory effect at a K_i6 times larger than quercetin K_i. AP and EP, on the other hand, showed only weak inhibition.

Conclusion:

The herbs we chose represented the more commonly used herbs in Southeast Asia where collision of tradition and modernization in healthcare, if not properly managed, may lead to therapeutic misadventures. We conclude that concurrent consumption of some herbs, in particular, LP and ELJ, may have relevance in drug-herb interactions via CYP2C8 inhibition in vivo.

SUMMARY

Herbs are increasingly used in parallel with modern medicines nowadays. In this study five commonly used herbs in Southeast Asia region, ELJ, LP, EP, AP and GB, were investigated for their in vitro inhibitory potency on CYP2C8, an important drug-metaboliz-ing human hepatic enzyme. All herbs inhibited CYP2C8 activity marker, amodiaquine N-desethylation, with potency order of LP > ELJ > GB >AP > EP. LP, ELJ and GB exhibited K_i values of 2, 4 and 6 times the K_i of quercetin, the positive control, indicating potent to moderate degree of enzyme inhibition. AP and EP, on the other hand, showed only weak inhibition. In summary, concurrent consumption of some herbs especially LP and ELJ may have relevance in drug-herb interactions via CYP2C8 inhibition in vivo.

Abbreviations Used: AQ: Amodiaquine, AP: Andrographis paniculata, CYP: Cytochrome P450, DEAQ: Desethylamodiaquine, EP: Echinacea purpurea, ELJ: Eurycoma longifolia Jack, GB: Ginkgo biloba, K_i: Inhibition constant, LP: Labisia pumila, Vmax: Maximal velocity, K_m: Michaelis-Menten constant.

Post-acquisition analysis of untargeted accurate mass quadrupole time-of-flight MS(E) data for multiple collision-induced neutral losses and fragment ions of glutathione conjugates

RATIONALE

Analytical methods to assess glutathione (GSH) conjugate formation based on mass spectrometry usually take advantage of the specific fragmentation behavior of the glutathione moiety. However, most methods used for GSH adduct screening monitor only one specific neutral loss or one fragment ion, even though the peptide moiety of GSH adducts shows a number of other specific neutral fragments and fragment ions which can be used for identification.

METHODS

Nine reference drugs well known to form GSH adducts were incubated with human liver microsomes. Mass spectrometric analysis was performed with a quadrupole time-of-flight mass spectrometer in untargeted accurate mass MS(E) mode. The data analysis and evaluation was achieved in an automated approach with software to extract and identify GSH conjugates based on the presence of multiple collision-induced neutral losses and fragment ions specific for glutathione conjugates in the high-energy MS spectra.

RESULTS

In total 42 GSH adducts were identified. Eight (18%) adducts did not show the neutral loss of 129 but were identified based on the appearance of other GSH-specific neutral losses or fragment ions. In high-energy MS(E) spectra the GSH-specific fragment ions of m/z 308 and 179 as well as the neutral loss of 275 Da were complementary to the commonly used neutral loss of 129 Da. Further, one abundant (yet unpublished) GSH conjugate of troglitazone formed in human liver microsomes was found.

CONCLUSIONS

A software-aided approach was developed to reliably retrieve GSH adduct formation data out of untargeted complex full scan QTOFMS(E) data in a fast and efficient way. The present approach to detect and analyze multiple collision-induced neutral losses and fragment ions of glutathione conjugates in untargeted MS(E) data might be applicable to higher throughput to assess reactive metabolite formation in drug discovery.

The effect of ritonavir on human CYP2B6 catalytic activity: heme modification contributes to the mechanism-based inactivation of CYP2B6 and CYP3A4 by ritonavir

The mechanism-based inactivation of human CYP2B6 by ritonavir (RTV) in a reconstituted system was investigated. The inactivation is time, concentration, and NADPH dependent and exhibits a K(I) of 0.9 μM, a k(inact) of 0.05 min⁻¹, and a partition ratio of approximately 3. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that the protonated molecular ion of RTV exhibits an m/z at 721 and its two major metabolites are an oxidation product with MH⁺ at m/z 737 and a deacylated product with MH⁺ at m/z 580. Inactivation of CYP2B6 by incubation with 10 μM RTV for 10 min resulted in an approximately 50% loss of catalytic activity and native heme, but no modification of the apoprotein was observed. RTV was found to be a potent mixed-type reversible inhibitor (K(i) = 0.33 μM) and a type II ligand (spectral dissociation constant-K(s) = 0.85 μM) of CYP2B6. Although previous studies have demonstrated that RTV is a potent mechanism-based inactivator of CYP3A4, the molecular mechanism responsible for the inactivation has not been determined. Here, we provide evidence that RTV inactivation of CYP3A4 is due to heme destruction with the formation of a heme-protein adduct. Similar to CYP2B6, there is no significant modification of the apoprotein. Furthermore, LC-MS/MS analysis revealed that both CYP3A4 and human liver microsomes form an RTV-glutathione conjugate having a MH⁺ at m/z 858 during metabolism of RTV, suggesting the formation of an isocyanate intermediate leading to formation of the conjugate.

Impact of combined oral contraceptives containing ethinylestradiol on the liver microsomal metabolism

OBJECTIVES

To check whether currently used combined oral contraceptives (COCs) containing ethinylestradiol (EE) affect the liver microsomal metabolism.

METHODS

(13)C-methacetin breath test ((13)C-MBT) - a sensitive non-invasive probe of cytochrome P-450 1A2 activity - was performed in 15 women on day 14, 15, 16, 17 or 18 of intake of their COC (containing EE), and between day 1 and 5 during the withdrawal bleeding, as well as in nine women not using hormonal contraception during the luteal phase of their cycle (between the 17th and the 23rd day), and between day 1 and 5 during menstruation.

RESULTS

The maximum breath (13)C elimination was significantly lower during the phase of intake of contraceptive pills than during withdrawal bleeding: 31.5 ± 2.2 %/h vs. 38.2 ± 1.9 %/h (p = 0.0045), whereas the time to reach it was similar on the two study days: 21.2 ± 1.2 min vs. 21.0 ± 1.1 min. Between the 27th and the 180th min of observation the cumulative breath (13)C elimination was statistically significantly lower during intake of the pill than during withdrawal bleeding. No significant menstrual cycle phase-dependent fluctuations in the results of the (13)C- methacetin breath test were observed in the control group.

CONCLUSION

COCs containing EE markedly inhibit hepatic microsomal function. This phenomenon must be taken into consideration when interpreting results of (13)C-MBT.

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.

From known knowns to known unknowns: predicting in vivo drug metabolites

'It is better to be useful than perfect'. This review attempts to critically cover and assess the currently available approaches and tools to answer the crucial question: Is it possible (and if it is, to what extent is it possible) to predict in vivo metabolites and their abundances on the basis of in vitro and preclinical animal studies? In preclinical drug development, it is possible to produce metabolite patterns from a candidate drug by virtual means (i.e., in silico models), but these are not yet validated. However, they may be useful to cover the potential range of metabolites. In vitro metabolite patterns and apparent relative abundances are produced by various in vitro systems employing tissue preparations (mainly liver) and in most cases using liquid chromatography-mass spectrometry analytical techniques for tentative identification. The pattern of the metabolites produced depends on the enzyme source; the most comprehensive source of drug-metabolizing enzymes is cultured human hepatocytes, followed by liver homogenate fortified with appropriate cofactors. For specific purposes, such as the identification of metabolizing enzyme(s), recombinant enzymes can be used. Metabolite data from animal in vitro and in vivo experiments, despite known species differences, may help pinpoint metabolites that are not apparently produced in in vitro human systems, or suggest alternative experimental approaches. The range of metabolites detected provides clues regarding the enzymes attacking the molecule under study. We also discuss established approaches to identify the major enzymes. The last question, regarding reliability and robustness of metabolite extrapolations from in vitro to in vivo, both qualitatively and quantitatively, cannot be easily answered. There are a number of examples in the literature suggesting that extrapolations are generally useful, but there are only a few systematic and comprehensive studies to validate in vitro-in vivo extrapolations. In conclusion, extrapolation from preclinical metabolite data to the in vivo situation is certainly useful, but it is not known to what extent.

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.

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.

Role of reactive metabolites in drug-induced hepatotoxicity

Drugs are generally converted to biologically inactive forms and eliminated from the body, principally by hepatic metabolism. However, certain drugs undergo biotransformation to metabolites that can interfere with cellular functions through their intrinsic chemical reactivity towards glutathione, leading to thiol depletion, and functionally critical macromolecules, resulting in reversible modification, irreversible adduct formation, and irreversible loss of activity. There is now a great deal of evidence which shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, such as acetaminophen, tamoxifen, isoniazid, and amodiaquine. The main theme of this article is to review the evidence for chemically reactive metabolites being initiating factors for the multiple downstream biological events culminating in toxicity. The major objectives are to understand those idiosyncratic hepatotoxicities thought to be caused by chemically reactive metabolites and to define the role of toxic metabolites.

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.

Modification of serine 360 by a reactive intermediate of 17-alpha-ethynylestradiol results in mechanism-based inactivation of cytochrome P450s 2B1 and 2B6

17-alpha-Ethynylestradiol (17EE) is a mechanism-based inactivator of P450 2B1 and P450 2B6 in the reconstituted monooxygenase system. The loss in enzymatic activity was due to the binding of a reactive intermediate of 17EE to the apoprotein. P450 2B1 and P450 2B6 were inactivated by 17EE and digested with trypsin. The peptides obtained following digestion with trypsin of 17EE-inactivated P450 2B1 and P450 2B6 were separated by liquid chromatography and analyzed by ESI-MS. Adducted peptides exhibiting an increase in mass consistent with the addition of the mass of the reactive intermediate of 17EE were identified for each enzyme. Analysis of these modified peptides by ESI-MS/MS and precursor ion scanning facilitated the identification of the Ser360 in both enzymes as a site that had been adducted by a reactive intermediate of 17EE. A P450 2B1 mutant where Ser360 was replaced by alanine was constructed, expressed, and purified. Activity and inactivation studies indicated that mutation of the Ser360 residue to alanine did not prevent inactivation of the mutant enzyme by 17EE. These observations suggest that Ser360 is not critical for the catalytic function of these P450s. Spectral binding studies of the 17EE-inactivated P450 2B1 and P450 2B6 indicated that modification of the enzymes by the reactive intermediate of 17EE resulted in an enzyme that was no longer capable of binding substrates. These results suggest that the inactivation by 17EE may be due to modification of an amino acid residue in the substrate access channel near the point of entry into the active site.

Mechanism-based inactivation of human cytochromes p450s: experimental characterization, reactive intermediates, and clinical implications

The P450 type cytochromes are responsible for the metabolism of a wide variety of xenobiotics and endogenous compounds. Although P450-catalyzed reactions are generally thought to lead to detoxication of xenobiotics, the reactions can also produce reactive intermediates that can react with cellular macromolecules leading to toxicity or that can react with the P450s that form them leading to irreversible (i.e., mechanism-based) inactivation. This perspective describes the fundamentals of mechanism-based inactivation as it pertains to P450 enzymes. The experimental approaches used to characterize mechanism-based inactivators are discussed, and the criteria required for a compound to be classified as a mechanism-based inactivator are outlined. The kinetic scheme for mechanism-based inactivation and the calculation of the relevant kinetic constants that describe a particular inactivation event are presented. The structural aspects and important functional groups of several classes of molecules that have been found to impart mechanism-based inactivation upon metabolism by P450s such as acetylenes, thiol-containing compounds that include isothiocyanates, thiazolidinediones, and thiophenes, arylamines, quinones, furanocoumarins, and cyclic tertiary amines are described. Emphasis throughout this perspective is placed on more recent findings with human P450s where the site of modification, whether it be the apoprotein or the heme moiety, and, at least in part, the identity of the reactive intermediate responsible for the loss in P450 activity are known or inferred. Recent advances in trapping procedures as well as new methods for identification of reactive intermediates are presented. A variety of clinically important drugs that act as mechanism-based inactivators of P450s are discussed. The irreversible inactivation of human P450s by these drugs has the potential for causing serious drug-drug interactions that may have severe toxicological effects. The clinical significance of inactivating human P450s for improving drug efficacy as well as drug safety is discussed along with the potential for exploiting mechanism-based inactivators of P450s for therapeutic benefits.

Identification of Cytochrome P450 3A4 Modification Site with Reactive Metabolite Using Linear Ion Trap-Fourier Transform Mass Spectrometry

Covalent binding of reactive metabolites to cytochrome P450s (P450s) often causes their mechanism-based inactivation (MBI), resulting in drug-drug interactions or toxicity. The detection and identification of the P450 sites to which reactive metabolites bind would elucidate MBI mechanisms. We describe a proteomic approach using nano-LC/linear ion trap-Fourier transform ion cyclotron resonance (FTICR) mass spectrometry to characterize the binding of a reactive metabolite of raloxifene, which is a known P450 3A4 inhibitor, to the P450 3A4 isozyme. LTQ-FT analyses revealed that the metabolic reaction of raloxifene in a reconstituted P450 3A4 system formed a reactive metabolite adduct to P450 3A4 apoprotein, accompanied by a mass shift of 471 Da relative to intact P450 3A4 apoprotein. The reaction mixtures were digested with trypsin, and then the tryptic digests were analyzed by nano-LC-MS/MS. This technique revealed that VWGFYDGQQPVLAITDPDMIK (position 71-91) was a tryptic peptide modified by the reactive metabolite derived from raloxifene. The site of adduction with the reactive metabolite was further postulated to be the nucleophilic OH group of Tyr-75 of P450 3A4. A proteomic approach using LTQ-FT can yield direct information on the P450 3A4 modification site without radiolabeled compounds. In addition, this information can elucidate mechanisms involved in the covalent binding of reactive metabolites and the inactivation of P450 3A4.

Mutation of a single residue (K262R) in P450 2B6 leads to loss of mechanism-based inactivation by phencyclidine

Human cytochrome P450 (P450) 2B6 plays an important role in the metabolism of many drugs used in the clinic, and it has been shown to be highly polymorphic and inducible by a variety of substrates. The metabolism of phencyclidine (PCP) by P450 2B6 results in mechanism-based inactivation of the enzyme. We investigated the effects of a naturally occurring mutation of P450 2B6 where a lysine 262 is changed to an arginine (K262R) on PCP metabolism and mechanism-based inactivation of 2B6 by PCP. The K262R mutant retained the 7-ethoxy-4-trifluoromethylcoumarin O-deethylation activity when it was incubated with PCP and NADPH in the reconstituted system, whereas the wild-type enzyme was readily inactivated by PCP. Spectral binding studies showed that PCP was reversibly bound in the active site of the K262R mutant with slightly higher affinity (156 muM) compared with the wild-type 2B6 (397 muM). In addition, all the metabolites of PCP (M1-M8) that were formed by the wild-type enzyme were also formed by the K262R mutant. Although the K262R mutant metabolized PCP to give similar metabolite profiles, the overall rate of metabolite formation was lower than the wild-type enzyme. A reactive intermediate of PCP was formed by wild-type P450 2B6 and trapped with glutathione (GSH). However, no GSH conjugates were detected from incubations with the K262R mutant. These data suggest that the lysine 262 residue plays an important role in the formation of a reactive intermediate of PCP that leads to the mechanism-based inactivation of P450 2B6.

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.

Structure-function analysis of cytochromes P450 2B

In the last 4 years, breakthroughs were made in the field of P450 2B (CYP2B) structure-function through determination of one ligand-free and two inhibitor-bound X-ray crystal structures of CYP2B4, which revealed many of the structural features required for binding ligands of different size and shape. Large conformational changes of several plastic regions of CYP2B4 can dramatically reshape the active site of the enzyme to fit the size and shape of the bound ligand without perturbing the overall P450 fold. Solution biophysical studies using isothermal titration calorimetry (ITC) have revealed the large difference in the thermodynamic parameters of CYP2B4 in binding inhibitors of different ring chemistry and side chains. Other studies have revealed that the effects of site-specific mutations on steady-state kinetic parameters and mechanism-based inactivation are often substrate dependent. These findings agree with the structural data that the enzymes adopt different conformations to bind various ligands. Thus, the substrate specificity of an individual enzyme is determined not only by active site residues but also non-active site residues that modulate conformational changes that are important for substrate access and rearrangement of the active site to accommodate the bound substrate.