Differential inactivation of plant lauric acid omega- and in-chain-hydroxylases by terminally unsaturated fatty acids

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.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

Evidence for Singlet Oxygen Involvement in Rat and Human Cytochrome P450-dependent Substrate Oxidations

Recently, we proposed that singlet oxygen ((1)O(2)) plays an essential role in microsomal cytochrome P450 (P450)-dependent p-hydroxylation of aniline and O-deethylation of 7-ethoxycoumarin. We then examined whether the role of (1)O(2) is general in the P450-dependent substrate oxidations. In the present study, we examined omega- and (omega-1)-hydroxylations of lauric acid, O-demethylation of p-nitroanisole, and N-demethylation of aminopyrine in rat liver microsomes. The addition of beta-carotene and NaN(3) significantly suppressed these reactions in a concentration-dependent manner, and (1)O(2) during the reactions was detected by ESR spin-trapping using 2,2,6,6-tetramethyl-4-piperidone (TMPD) as a (1)O(2)-spin trapping reagent, where the addition of (1)O(2) quenchers, SKF-525A as a P450 inhibitor, or p-nitroanisole decreased ESR signal intensities due to TMPD-(1)O(2) adduct. Next, we examined the effect of (1)O(2) quenchers on P450-dependent reactions in the human liver microsomes, and (1)O(2) was also indicated to be an active species in substrate hydroxylations and dealkylations such as nifedipine oxidation by CYP3A4. On the basis of the results, we concluded that (1)O(2) is an essentially important active oxygen species in both rat and human P450-dependent substrate oxidations.

Cloning and functional characterization of CYP94A2, a medium chain fatty acid hydroxylase from Vicia sativa

A full length cDNA encoding a new cytochrome P450-dependent fatty acid hydroxylase (CYP94A2) was isolated from a Vicia sativa library. CYP94A2 displays 58% sequence identity with CYP94A1, a fatty acid omega-hydroxylase isolated from the same material. Heterologous expression of CYP94A2 in Saccharomyces cerevisiae yeast strain WAT11 shows that it catalyses the hydroxylation of myristic (C14) acid with a K(m(app)) of 4.0 microM and a turnover rate number of 80 min(-1). In addition, lauric (C12) and palmitic (C16) acids were hydroxylated at a ten-fold lower rate, while C18 fatty acids were not oxidized. Remarkably, the regiospecificity of hydroxylation is different for the C12, C14, and C16 fatty acids and appears to be correlated with the length of the carbon chain. Northern blot analysis showed a low level of constitutive expression of CYP94A2 in V. sativa seedlings. In contrast to CYP94A1, transcript accumulation of CYP94A2 was only weakly enhanced in seedlings treated with clofibrate or methyl jasmonate, indicating that both substrate range and gene regulation of the two fatty acid hydroxylases are different.

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.

Induction of cytochrome P450 activities in Drosophila melanogaster strains susceptible or resistant to insecticides

We analysed Drosophila melanogaster cytochrome P450s (P450) through the measurements of four enzymatic activities: ethoxycoumarin-O-deethylase, ethoxyresorufin-O-deethylase, lauric acid hydroxylation, and testosterone hydroxylation. We did these measurements in two Drosophila strains: one is susceptible to insecticides (Cantons) and the other is resistant to insecticides by enhanced P450 activities (RDDTR). In addition, we also treated the flies with eight chemicals (beta-naphtoflavone, benzo-alpha-pyrene, 3-methylcholanthrene, phenobarbital, aminopyrine, rifampicin, prochloraz, and clofibrate) known to induces genes from the families CYP1, CYP2, CYP3, CYP4, and CYP6. Metabolisation of all the substrates by P450 from flies microsomes was observed. The chemicals had different effects on these activities, ranging from induction to inhibition. The effects of these chemicals varied with the strains as most of them were ineffective on the RDDTR strain. The results showed that P450-dependent activities are numerous in Drosophila. Regulation features of these activities are complex. The availability of mutant strains as RDDTR should allow fundamental studies of P450 in insects.

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

Incubation of Vicia sativa microsomes, containing cytochrome P450-dependent lauric acid omega-hydroxylase (omega-LAH), with [1-(14)C]11-dodecynoic acid (11-DDYA) generates a major metabolite characterized as 1,12-dodecandioic acid. In addition to time- and concentration-dependent inactivation of lauric acid and 11-DDYA oxidation, irreversible binding of 11-DDYA (200 pmol of 11-DDYA bound/mg of microsomal protein) at a saturating concentration of 11-DDYA was observed. SDS-polyacrylamide gel electrophoresis analysis showed that 30% of the label was associated with several protein bands of about 53 kDa. The presence of beta-mercaptoethanol in the incubate reduces 1,12-dodecandioic acid formation and leads to a polar metabolite resulting from the interaction of oxidized 11-DDYA with the nucleophile. Although the alkylation of proteins was reduced, the lauric acid omega-hydroxylase activity was not restored, suggesting an active site-directed inactivation mechanism. Similar results were obtained when reconstituted mixtures of cytochrome P450 from family CYP4A from rabbit liver were incubated with 11-DDYA. In contrast, both 11- and 10-DDYA resulted in covalent labeling of the cytochrome P450 2B4 protein and irreversible inhibition of activity. These results demonstrate that acetylenic analogues of substrate are efficient mechanism-based inhibitors and that a correlation between the position of the acetylenic bond in the inhibitor and the regiochemistry of cytochromes P450 oxygenation is essential for enzyme inactivation.

Fate of a terminal olefin with Drosophila microsomes and its inhibitory effects on some P-450 dependent activities

In vitro bioassays were used to analyze the metabolism of the 11-dodecenoic acid (11-DDNA) by microsomes prepared from Drosophila melanogaster RalDDTR strain. 11-DDNA is metabolized to 11,12-epoxylauric acid (epoxyLA) in a NADPH-dependent way. The microsomal production of epoxyLA reaches a plateau very quickly, suggesting the occurrence of an enzyme inactivation process. After incubation of microsomes with (1-14C)11-DDNA, three proteins of Mr approximately 50 kDa were labeled. 11-DDNA inhibits the microsomal metabolism of lauric acid and 7-ethoxycoumarin in a time and NADPH-dependent process. An inhibition of metabolites generated from DDT and testosterone was also obtained but at higher concentrations. These results are discussed according to the fact that RalDDTR is an insecticide resistant strain characterized as a high metabolizer of the insecticide DDT and also of lauric acid, testosterone, and ethoxycoumarin.

Cytochrome P450-dependent oxidation of fatty acids

Cytochrome P450-dependent monooxygenases from plants catalyse in-chain and omega hydroxylation as well as epoxidation of medium- and long-chain fatty acids. Recent research efforts have clarified that there are multiple forms of cytochrome P450 involved in these reactions, each of which possesses distinguishable substrate specificity. The biological roles of these distinct P450 forms are poorly understood. However, evidence suggests that some may play an important role in the biosynthesis of plant cuticles. We review current knowledge on the induction and inhibition of activities as well as the regio- and stereo-specificity of the distinct forms so far characterised.

Involvement of cytochrome P450 3A enzyme family in the major metabolic pathways of toremifene in human liver microsomes

The anti-estrogen toremifen-Fc-1157a or 4-chloro-1,2-diphenyl-1-[4-[2(N,N-dimethylamino)ethoxy]-phenyl]-1- butene is now used for the treatment of breast cancer. This drug is extensively metabolized by cytochrome P450 dependent hepatic mixed function oxidase in man, yielding mainly the N-demethyl-(DMTOR), 4-hydroxy-(4OH-TOR) and deamino-hydroxy-(TOR III) toremifene metabolites. The specific forms of cytochrome P450 involved in these oxidation reactions were examined in 32 human liver microsomal preparations previously characterized with respect to their contents of several known P450 enzymes. Toremifene was demethylated with an apparent Km of 124 microM while it was hydroxylated with an apparent Km of 139 microM. The metabolic rates were 71 +/- 56, 13 +/- 9 and 15 +/- 4 pmol/min/mg microsomal protein, respectively, for DMTOR, 4-OH-TOR and TOR III. The N-demethylation activity was strongly correlated with estradiol 2-hydroxylation (r = 0.75), nifedipine oxidation (r = 0.86), tamoxifen N-demethylation (r = 0.73), testosterone 6 beta-hydroxylation (r = 0.78) and erythromycin N-demethylation (r = 0.84), all these monooxygenase activities known to be supported by CYP3A4 isoform. Furthermore, the CYP3A content of liver microsomal samples, measured by western blot analysis using a monoclonal anti-human CYP3A4 antibody, was strongly correlated with DMTOR formation (r = 0.80). Compounds such as cyclosporin, triacetyl-oleandomycin and testosterone inhibited the N-demethylation of toremifene metabolism at 80, 89 and 56% vs control, respectively, while the formation of TOR III was inhibited at 78, 82 and 73% vs control and the 4-hydroxylation pathway was inhibited no more than about 50% vs control. Prior incubation of microsomes with 100 microM gestodene, known to be a selective mechanism-based inhibitor of CYP3A4 in the presence of NADPH, led to 76 +/- 6 and 76 +/- 5% (N = 5 samples) reductions in the N-demethylation and formation of TOR III, respectively. Polyclonal antibody directed against human CYP3A enzymes inhibited formation of DMTOR and TOR III by 60 and 46%, respectively. The metabolism of toremifene was not activated by alpha-naphthoflavone. Finally, the use of yeasts genetically engineered for expression of human P4501A1, 1A2, 2C9 and 3A4 allowed us to demonstrate that DMTOR and TOR III formations are mediated by P4501A and 3A4 enzymes and by contrast these enzymes are not involved in the 4-hydroxylation pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Regioselectivity of a plant lauric acid omega hydroxylase. Omega hydroxylation of cis and trans unsaturated lauric acid analogs and epoxygenation of the terminal olefin by plant cytochrome P-450

The study of the stereochemistry and regioselectivity of plant fatty acid hydroxylases is hampered by the difficulty to purify plant cytochrome P-450 enzymes. To provide an alternative, we have now defined an experimental plant system which expresses only one hydroxylase activity towards lauric acid: microsomes from clofibrate-induced Vicia sativa seedlings hydroxylate this fatty acid exclusively at the methyl terminus. To explore the catalytic capabilities of this laurate oxidase, a series of 1-14C-radiolabeled unsaturated lauric acid analogs (7-, 8-, 9-, 10- and 11-dodecenoic acids) were synthesized. Microsomes from clofibrate induced Vicia sativa seedling catalyzed the omega-oxidation of the lauric acid analogs in the presence of O2 and NADPH. The cis and trans forms of the four in-chain unsaturated analogs of lauric acid were 12-hydroxylated with similar efficiency. The terminal olefin was readily converted to the epoxide with only marginal autocatalytic inactivation of the enzyme. The formation of each metabolite was inhibited to the same extend when microsomes were incubated in presence of carbon monoxide or a suicide-substrate for omega LAH, suggesting that a single cytochrome P-450 isoenzyme from Vicia sativa microsomes is able to omega hydroxylate lauric acid and in-chain unsaturated analogs, and to epoxygenate 11-dodecenoic acid.

Involvement of lauric acid hydroxylase in the activation of beta-substituted nitrosamines

The mutagenicity of N-nitrosobis (2-hydroxypropyl) amine (BHP), N-nitrosobis(2-oxopropyl)amine (BOP) and N-nitroso-(2-hydroxy-propyl) (2-oxopropyl) amine (HPOP) was measured in V79 cells. Hepatocytes, used to metabolize (activate) the nitrosamines, were isolated from untreated Syrian hamsters (control) and hamsters treated with clofibrate (CLO) or dehydroepiandrosterone (DHEA) in vivo. BHP and HPOP mutagenicity increased 3- and 2-fold when hepatocytes from CLO- and DHEA-treated hamsters were used. BOP mutagenicity did not increase. 10-Undecynoic acid, a lauric acid hydroxylase inhibitor, inhibited the increase in BHP and HPOP mutagenicity by 80-90% but did not affect that of BOP. Antimycin A1, a fatty acyl coenzyme A beta-oxidase inhibitor did not affect the mutagenicity of these nitrosamines. Lauric acid hydroxylase, probably omega-1 hydroxylase (cytochrome P-450 IVA2), appears to be involved in the activation of BHP and HPOP.