Suicide inactivation of cytochrome P450 by midchain and terminal acetylenes. A mechanistic study of inactivation of a plant lauric acid omega-hydroxylase

Branched and linear fatty acid esters of hydroxy fatty acids (FAHFA) relevant to human health

Fatty acid esters of hydroxy fatty acids (FAHFAs) represent a complex lipid class that contains both signaling mediators and structural components of lipid biofilms in humans. The majority of endogenous FAHFAs share a common chemical architecture, characterized by an estolide bond that links the hydroxy fatty acid (HFA) backbone and the fatty acid (FA). Two structurally and functionally distinct FAHFA superfamilies are recognized based on the position of the estolide bond: omega-FAHFAs and in-chain branched FAHFAs. The existing variety of possible HFAs and FAs combined with the position of the estolide bond generates a vast quantity of unique structures identified in FAHFA families. In this review, we discuss the anti-diabetic and anti-inflammatory effects of branched FAHFAs and the role of omega-FAHFA-derived lipids as surfactants in the tear film lipid layer and dry eye disease. To emphasize potential pharmacological targets, we recapitulate the biosynthesis of the HFA backbone within the superfamilies together with the degradation pathways and the FAHFA regioisomer distribution in human and mouse adipose tissue. We propose a theoretical involvement of cytochrome P450 enzymes in the generation and degradation of saturated HFA backbones and present an overview of small-molecule inhibitors used in FAHFA research. The FAHFA lipid class is huge and largely unexplored. Besides the unknown biological effects of individual FAHFAs, also the enigmatic enzymatic machinery behind their synthesis could provide new therapeutic approaches for inflammatory metabolic or eye diseases. Therefore, understanding the mechanisms of (FA)HFA synthesis at the molecular level should be the next step in FAHFA research.

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.

Detecting N-myristoylation and S-acylation of host and pathogen proteins in plants using click chemistry

BACKGROUND

The plant plasma membrane is a key battleground in the war between plants and their pathogens. Plants detect the presence of pathogens at the plasma membrane using sensor proteins, many of which are targeted to this lipophilic locale by way of fatty acid modifications. Pathogens secrete effector proteins into the plant cell to suppress the plant's defense mechanisms. These effectors are able to access and interfere with the surveillance machinery at the plant plasma membrane by hijacking the host's fatty acylation apparatus. Despite the important involvement of protein fatty acylation in both plant immunity and pathogen virulence mechanisms, relatively little is known about the role of this modification during plant-pathogen interactions. This dearth in our understanding is due largely to the lack of methods to monitor protein fatty acid modifications in the plant cell.

RESULTS

We describe a rapid method to detect two major forms of fatty acylation, N-myristoylation and S-acylation, of candidate proteins using alkyne fatty acid analogs coupled with click chemistry. We applied our approach to confirm and decisively demonstrate that the archetypal pattern recognition receptor FLS2, the well-characterized pathogen effector AvrPto, and one of the best-studied intracellular resistance proteins, Pto, all undergo plant-mediated fatty acylation. In addition to providing a means to readily determine fatty acylation, particularly myristoylation, of candidate proteins, this method is amenable to a variety of expression systems. We demonstrate this using both Arabidopsis protoplasts and stable transgenic Arabidopsis plants and we leverage Agrobacterium-mediated transient expression in Nicotiana benthamiana leaves as a means for high-throughput evaluation of candidate proteins.

CONCLUSIONS

Protein fatty acylation is a targeting tactic employed by both plants and their pathogens. The metabolic labeling approach leveraging alkyne fatty acid analogs and click chemistry described here has the potential to provide mechanistic details of the molecular tactics used at the host plasma membrane in the battle between plants and pathogens.

Structural complex of sterol 14α-demethylase (CYP51) with 14α-methylenecyclopropyl-Delta7-24, 25-dihydrolanosterol

Sterol 14α-demethylase (CYP51) that catalyzes the removal of the 14α-methyl group from the sterol nucleus is an essential enzyme in sterol biosynthesis, a primary target for clinical and agricultural antifungal azoles and an emerging target for antitrypanosomal chemotherapy. Here, we present the crystal structure of Trypanosoma (T) brucei CYP51 in complex with the substrate analog 14α-methylenecyclopropyl-Δ7-24,25-dihydrolanosterol (MCP). This sterol binds tightly to all protozoan CYP51s and acts as a competitive inhibitor of F105-containing (plant-like) T. brucei and Leishmania (L) infantum orthologs, but it has a much stronger, mechanism-based inhibitory effect on I105-containing (animal/fungi-like) T. cruzi CYP51. Depicting substrate orientation in the conserved CYP51 binding cavity, the complex specifies the roles of the contact amino acid residues and sheds new light on CYP51 substrate specificity. It also provides an explanation for the effect of MCP on T. cruzi CYP51. Comparison with the ligand-free and azole-bound structures supports the notion of structural rigidity as the characteristic feature of the CYP51 substrate binding cavity, confirming the enzyme as an excellent candidate for structure-directed design of new drugs, including mechanism-based substrate analog inhibitors.

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.

Clinically important drug interactions potentially involving mechanism-based inhibition of cytochrome P450 3A4 and the role of therapeutic drug monitoring. Ther Drug Monit. 2007;29:687-710. [PMID: 18043468 DOI: 10.1097/ftd.0b013e31815c16f5]

Cytochrome P450 (CYP) 3A4 is the most abundant enzyme of CYPs in the liver and gut that metabolizes approximately 50% currently available drugs. A number of important drugs have been identified as substrates, inducers, and/or inhibitors of CYP3A4. The substrates of CYP3A4 considerably overlap with those of P-glycoprotein. Both CYP3A4 and P-glycoprotein are subject to inhibition and induction by a number of factors. Mechanism-based inhibition of CYP3A4 is characterized by NADPH-, time-, and concentration-dependent enzyme inactivation occurring when some xenobiotics or drugs are converted by CYPs to reactive metabolites. Such an inhibition of CYP3A4 is caused by chemical modification of the heme, the protein, or both as a result of covalent binding of modified heme to the protein. To date, the identified clinically important mechanism-based CYP3A4 inhibitors mainly include macrolide antibiotics (eg, clarithromycin and erythromycin), anti-HIV agents (eg, ritonavir and delavirdine), antidepressants (eg, fluoxetine and fluvoxamine), calcium channel blockers (eg, verapamil and diltiazem), steroids and their modulators (eg, gestodene and mifepristone), and several herbal and dietary components. The inactivation of CYP3A4 by drugs often causes unfavorable and long-lasting drug-drug interactions and probably fatal toxicity, depending on many factors associated with the enzyme, drugs, and the patients. Clinicians are encouraged to have a sound knowledge of drug-induced, mechanism-based CYP3A4 inhibition; take proper cautions, and perform close monitoring for possible drug interactions when using drugs that are mechanism-based CYP3A4 inhibitors. To minimize drug-drug interactions involving mechanism-based CYP3A4 inhibition, it is necessary to choose safe drug combination regimens, adjust drug dosages appropriately, and conduct therapeutic drug monitoring for drugs with narrow therapeutic indices.

The application of discovery toxicology and pathology towards the design of safer pharmaceutical lead candidates

Toxicity is a leading cause of attrition at all stages of the drug development process. The majority of safety-related attrition occurs preclinically, suggesting that approaches to identify 'predictable' preclinical safety liabilities earlier in the drug development process could lead to the design and/or selection of better drug candidates that have increased probabilities of becoming marketed drugs. In this Review, we discuss how the early application of preclinical safety assessment--both new molecular technologies as well as more established approaches such as standard repeat-dose rodent toxicology studies--can identify predictable safety issues earlier in the testing paradigm. The earlier identification of dose-limiting toxicities will provide chemists and toxicologists the opportunity to characterize the dose-limiting toxicities, determine structure-toxicity relationships and minimize or circumvent adverse safety liabilities.

CYP94A5, a new cytochrome P450 from Nicotiana tabacum is able to catalyze the oxidation of fatty acids to the omega-alcohol and to the corresponding diacid

A full length cDNA encoding a new cytochrome P450-dependent fatty acid hydroxylase (CYP94A5) was isolated from a tobacco cDNA library. CYP94A5 was expressed in S. cerevisiae strain WAT11 containing a P450 reductase from Arabidopsis thaliana necessary for catalytic activity of cytochrome P450 enzymes. When incubated for 10 min in presence of NADPH with microsomes of recombinant yeast, 9,10-epoxystearic acid was converted into one major metabolite identified by GC/MS as 18-hydroxy-9,10-epoxystearic acid. The kinetic parameters of the reaction were Km,app = 0.9 +/- 0.2 microM and Vmax,app = 27 +/- 1 nmol x min(-1) x nmol(-1) P450. Increasing the incubation time to 1 h led to the formation of a compound identified by GC/MS as 9,10-epoxy-octadecan-1,18-dioic acid. The diacid was also produced in microsomal incubations of 18-hydroxy-9,10-epoxystearic acid. Metabolites were not produced in incubations with microsomes of yeast transformed with a control plasmid lacking CYP94A5 and their production was inhibited by antibodies raised against the P450 reductase, demonstrating the involvement of CYP94A5 in the reactions. The present study describes a cytochrome P450 able to catalyze the complete set of reactions oxidizing a terminal methyl group to the corresponding carboxyl. This new fatty acid hydroxylase is enantioselective: after incubation of a synthetic racemic mixture of 9,10-epoxystearic acid, the chirality of the residual epoxide was 40/60 in favor of 9R,10S enantiomer. CYP94A5 also catalyzed the omega-hydroxylation of saturated and unsaturated fatty acids with aliphatic chain ranging from C12 to C18.

Induction and inactivation of a cytochrome P450 confering herbicide resistance in wheat seedlings

Cytochrome P450-dependent enzymes from wheat catalyze the oxidation of endogenous compounds (lauric and oleic acids) and of several herbicides (diclofop, chlortoluron, bentazon). Treatment of wheat seedlings with the safener, naphthalic anhydride and with phenobarbital increases dramatically several P450-dependent enzyme activities including diclofop and lauric acid hydroxylation. The parallel induction of lauric acid (omega-1)-hydroxylase and diclofop hydroxylase activities suggests that both compounds proceeds from the same or very similar forms of P450. To test whether either one or multiple P450 forms are involved in these oxidations, we have designed selective irreversible inhibitors of lauric acid (omega-1)-hydroxylase. Results of in vivo and in vitro experiments with acetylenic analogs of lauric acid (10- and 11-dodecynoic acids) strongly suggest that a single P450 catalyzes both laurate and diclofop hydroxylation. Treatment of wheat seedlings with these acetylenes results in a strong inhibition of the in vivo metabolism of diclofop although oxidation of chlortoluron and bentazon are not affected. Our results suggest that at least three distinct P450 forms are involved in the detoxification process of the three herbicides. Interestingly, we also demonstrate that herbicides themselves are potent inducers of the amount of total P450 and laurate/diclofop hydroxylase activies. This increased capacity of wheat to detoxify the herbicide through the induction of P450 enzymes seems to be for a large extend the mechanism which confers a tolerance on various herbicides.

Inhibitors of abscisic acid 8'-hydroxylase

Structural analogues of the phytohormone (+)-abscisic acid (ABA) have been synthesized and tested as inhibitors of the catabolic enzyme (+)-ABA 8'-hydroxylase. Assays employed microsomes from suspension-cultured corn cells. Four of the analogues [(+)-8'-acetylene-ABA, (+)-9'-propargyl-ABA, (-)-9'-propargyl-ABA, and (+)-9'-allyl-ABA] proved to be suicide substrates of ABA 8'-hydroxylase. For each suicide substrate, inactivation required NADPH, increased with time, and was blocked by addition of the natural substrate, (+)-ABA. The most effective suicide substrate was (+)-9'-propargyl-ABA (K(I) = 0.27 microM). Several analogues were competitive inhibitors of ABA 8'-hydroxylase, of which the most effective was (+)-8'-propargyl-ABA (K(i) = 1.1 microM). Enzymes in the microsomal extracts also hydroxylated (-)-ABA at the 7'-position at a low rate. This activity was not inhibited by the suicide substrates, showing that the 7'-hydroxylation of (-)-ABA was catalyzed by a different enzyme from that which catalyzed 8'-hydroxylation of (+)-ABA. Based on the results described, a simple model for the positioning of substrates in the active site of ABA 8'-hydroxylase is proposed. In a representative physiological assay, inhibition of Arabidopsis thaliana seed germination, (+)-9'-propargyl-ABA and (+)-8'-acetylene-ABA exhibited substantially stronger hormonal activity than (+)-ABA itself.

Effects of unsaturated side-chain analogs of tetrahydrocannabinol on cytochromes P450

The ability of unsaturated side-chain analogs of Delta(8)-tetrahydrocannabinol (THC) to selectively inactivate mouse hepatic cytochromes P450 3A11 and 2C29 was examined. THC side-chain analogs were preincubated with mouse hepatic microsomes and NADPH for various times before dilution and determination of Delta(9)-THC metabolism specific for P450s 3A11 and 2C29. THC-enyl analogs had little or no effect on P450 3A11 but inactivated P450 2C29 in a time-dependent manner, with approximately 50% inactivation observed after a 30-min preincubation. THC-ynyl analogs were less selective in their P450 inactivation but appeared to be more effective than their corresponding enyl analogs. THC-ynyl analogs inactivated P450s 3A11 and 2C29 in a time-dependent manner and could inactive 40-80% of their activities after a 30-min preincubation. The THC-ynyl analogs were nearly as effective as cannabidiol, a well-characterized inactivator of these mouse P450s. Despite their ability to inactivate P450 in vitro, neither the THC-enyl nor the THC-ynyl analogs were very effective after in vivo administration. Unsaturated side-chain THC analogs may be useful in the development of specific P450 inactivators.

Production in vitro by the cytochrome P450 CYP94A1 of major C18 cutin monomers and potential messengers in plant-pathogen interactions: enantioselectivity studies

The major C(18) cutin monomers are 18-hydroxy-9,10-epoxystearic and 9,10,18-trihydroxystearic acids. These compounds are also known messengers in plant-pathogen interactions. We have previously shown that their common precursor 9,10-epoxystearic acid was formed by the epoxidation of oleic acid in Vicia sativa microsomes (Pinot, Salaün, Bosch, Lesot, Mioskowski and Durst (1992) Biochem. Biophys. Res. Commun. 184, 183-193). Here we determine the chirality of the epoxide produced as (9R,10S) and (9S,10R) in the ratio 90:10 respectively. We further show that microsomes from yeast expressing the cytochrome P450 CYP94A1 are capable of hydroxylating the methyl terminus of 9,10-epoxystearic and 9,10-dihydroxystearic acids in the presence of NADPH to form the corresponding 18-hydroxy derivatives. The reactions were not catalysed by microsomes from yeast transformed with a void plasmid or in absence of NADPH. After incubation of a synthetic racemic mixture of 9,10-epoxystearic acid with microsomes of yeast expressing CYP94A1, the chirality of the residual epoxide was shifted to 66:34 in favour of the (9S,10R) enantiomer. Both enantiomers were incubated separately and V(max)/K(m) values of 16 and 3.42 ml/min per nmol of P450 for (9R, 10S) and (9S,10R) respectively were determined, demonstrating that CYP94A1 is enantioselective for the (9R,10S) enantiomer, which is preferentially formed in V. sativa microsomes. Compared with the epoxide, the diol 9,10-dihydroxystearic acid was a much poorer substrate for the omega-hydroxylase, with a measured V(max)/K(m) of 0.33 ml/min per nmol of P450. Our results indicate that the activity of CYP94A1 is strongly influenced by the stereochemistry of the 9, 10-epoxide and the nature of substituents on carbons 9 and 10, with V(max)/K(m) values for epoxide>>oleic acid>diol.

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.

Tissue-specific induction and inactivation of cytochrome P450 catalysing lauric acid hydroxylation in the sea bass, Dicentrarchus labrax

Microsomal cytochrome P450-dependent lauric acid hydroxylase activities were characterized in liver, kidney, and intestinal mucosa of the sea bass (Dicentrarchus labrax). Microsomes from these organs generated (omega-1)-hydroxylauric acid and a mixture of positional isomers including (omega)-, (omega-2)-, (omega-3)- and (omega-4)-hydroxylauric acids, which were identified by RP-HPLC and GC-MS analysis. Peroxisome proliferators, such as clofibrate and especially di(2-ethylhexyl) phthalate, increased kidney microsomal lauric acid hydroxylase activities. The synthesis of 11-hydroxylauric acid was enhanced 5.3-fold in kidney microsomes. Liver microsomal lauric acid hydroxylase activities were weakly affected and no significant induction was found in small intestine microsomes from clofibrate or di(2-ethylhexyl) phthalate-treated fish. The differences in lauric acid metabolisation and the tissue-specific induction by peroxisome proliferators suggest the involvement of several P450s in this reaction. Incubations of liver and kidney microsomes with lauric acid analogues (11- or 10-dodecynoic acids) resulted in a time- and concentration-dependent loss of lauric acid hydroxylase activities. The induction of these activities in fish by phthalates, which are widely-distributed environmental pollutants, may be taken into consideration for the development of new biomarkers.

Unusual propargylic oxidations catalyzed by chloroperoxidase

This paper describes a new chloroperoxidase oxidative activity, namely, propargylic oxidations. Under appropriate conditions, chloroperoxidase catalyzes the oxidation of a variety of 2-alkynes to aldehydes via alcohol intermediates. Both hydrogen peroxide and t-butyl hydroperoxide can serve as terminal oxidants. The triple bond in the substrate is usually untouched. A free radical mechanism is proposed for the initial hydroxylation step in the overall reaction.

Functional expression in yeast and characterization of a clofibrate-inducible plant cytochrome P-450 (CYP94A1) involved in cutin monomers synthesis

The chemical tagging of a cytochrome P-450-dependent lauric acid omega-hydroxylase from clofibrate-treated Vicia sativa seedlings with [1-14C]11-dodecynoic acid allowed the isolation of a full-length cDNA designated CYP94A1. We describe here the functional expression of this novel P-450 in two Saccharomyces cerevisiae strains overproducing their own NADPH-cytochrome P-450 reductase or a reductase from Arabidopsis thaliana. The results show a much higher efficiency of the yeast strain overproducing the plant reductase compared with the yeast strain overproducing its own reductase for expressing CYP94A1. The methyl end of saturated (from C-10 to C-16) and unsaturated (C18:1, C18:2 and C18:3) fatty acids was mainly oxidized by CYP94A1. Both E/Z and Z/E configurations of 9, 12-octadecadienoic acids were omega-hydroxylated. Lauric, myristic and linolenic acids were oxidized with the highest turnover rate (24 min-1). The strong regioselectivity of CYP94A1 was clearly shifted with sulphur-containing substrates, since both 9- and 11-thia laurate analogues were sulphoxidized. Similar to animal omega-hydroxylases, this plant enzyme was strongly induced by clofibrate treatment. Rapid CYP94A1 transcript accumulation was detected less than 20 min after exposure of seedlings to the hypolipidaemic drug. The involvement of CYP94A1 in the synthesis of cutin monomers and fatty acid detoxification is discussed.