Human drug metabolites are frequently biologically active, with many implications for human health. Pharmaceutical companies have become increasingly aware of the need to identify and test these metabolites. The P450 BM3 enzyme from Bacillus megaterium offers substantial advantages to the current methods of metabolite synthesis, as its soluble, catalytically self-sufficient nature, coupled with its high catalytic activity, make P450 BM3 ideal for engineering towards specificity for human drugs. The highly-active I401P BM3 mutant was characterized for its reactivity towards human drugs and for the development of a human P450-like metabolite profile. The I401P mutant exhibits binding to molecules including alkaloids, steroids, and azole drugs, along with many other compounds. I401P binds/oxidizes human CYP substrates, including alosetron, phenacetin, caffeine, nicotine and diclofenac. LC-MS product identification shows that I401P BM3 forms 4OH-diclofenac, the major human metabolite for diclofenac. I401P BM3 also produces nornicotine, the second major human metabolite of nicotine. I401P BM3 also forms theophylline, theobromine and paraxanthine, the three major human metabolites of caffeine. Thermostability (DSC) data show that the I401P mutation destabilizes the BM3 heme domain in both its substrate-free and substrate-bound forms. The I401P heme domain X-ray crystal structure reinforces previous structural observations that the Pro401 mutation causes the BM3 protein to adopt a high-spin, "substrate-bound" state, with a displaced heme iron axial water, producing a "catalytically primed" mutant with greater diversity in substrate selectivity. The destabilisation of the BM3 heme domain structure due to the Pro401 mutation increases conformational plasticity in this mutant, allowing it to function as a platform for future mutagenesis aimed at improved binding and metabolite yield from specific drug substrates. Further proline mutations (A330P, A330P/I401P and A82F/F87V/I401P) were examined for increased affinity for drug substrates. The A330P mutant shows no novel drug substrate specificity, despite its reported affinity for small molecules. The A330P/I401P double mutant demonstrates weak binding to WT BM3 and I401P substrates, but no synergistic effects were obtained by combining the two mutations. The double mutant exhibits very low solvent tolerance and significant structural destabilisation. DSC data confirms this, with the double mutant destabilising the BM3 heme domain by up to 20 °C. Initial work with the A82F/F87V/I401P mutant showed increased affinity for A82F/F87V- and I401P-type substrates, including diclofenac. LC-MS product analysis confirms that the A82F/F87V/I401P mutant oxidises diclofenac into its major human metabolite 4OH-diclofenac. These data indicate that human-like oxidation reactions are feasible with BM3 mutants. In this work, proline insertion mutants were generated that introduced novel affinity for biotechnologically relevant substrates. In particular the I401P mutant offers an excellent platform for future biotechnological engineering.

Pre-systemic elimination by local enzymatic degradation can play a key role in limiting the bioavailability of intranasally administered drugs. Despite remarkable advancement in the characterization of the nasal biotransformative enzymes, knowledge of the role of the nasal mucosa in limiting bioavailability of therapeutic agents is still inadequate. The aim of this work was to evaluate the expression and substrate biotransformation activity of cytochrome P450 enzymes in the nasal mucosa using bovine olfactory and respiratory explants as in vitro models. Gene expression and localization of major CYP450 isoforms in the nasal mucosa were examined using RT-PCR and immunohistochemistry. The bovine nasal mucosa showed abundant expression of CYP2A6 and 3A4 genes whereas 1A1, 1A2, 2C9, and 2C19 isoforms were expressed at much lower levels. The CYP450 proteins were observed to be present in the epithelial layer and in submucosal glandular cells. The diffusion of melatonin, a CYP1A2 substrate, and the appearance of 6-hydroxymelatonin, its primary metabolite, across bovine olfactory and respiratory explants was measured, and nasal olfactory and respiratory microsomal preparations were used to quantify the kinetic parameters for melatonin 6-hydroxylation. Results indicated that bovine olfactory and respiratory CYP450 isoforms were metabolically active towards melatonin metabolism, and the respiratory mucosa demonstrated the greatest melatonin 6-hydroxylation activity. Numerical simulations were used to probe the effects of the relative magnitudes of the permeability coefficient and enzymatic parameters on net substrate mass transfer across nasal mucosal tissues. The simulations indicated that the concentration gradient of the drug coupled with its permeability coefficient were the most significant factors controlling the transport of drugs across the mucosal tissue. Enzymatic degradation decreased the flux of drugs across the mucosa and had the greatest impact on low permeability compounds. The results from these studies show that the bovine nasal mucosa possesses significant metabolic activity, and the flux of a metabolically labile substrate across the nasal mucosa can be significantly reduced by its enzymatic degradation within the tissue. Use of kinetic modeling to characterize of the extent of biotransformation in the nasal mucosa enables the identification of metabolism-limited bioavailability of intranasally administered drug compounds.

Developmental exposure to aryl hydrocarbon receptor (AhR) agonists in fish causes severe defects in the cardiovascular system. However, the effects of acute AhR agonist exposure on the adult fish cardiovascular system as well as the genes mediating developmental AhR-induced deformities remain unclear. In this thesis, two studies were carried out to address these issues. Before experiments could begin, methods for quantitative real-time reverse transcriptase polymerase chain reaction (rtrt-PCR) as well as larval exposure and rearing were developed, validated, and optimized.<p> Following method development, a series of experiments was performed on adult zebrafish (<i>Danio rerio</i>) to assess how expression of cytochrome P450 (CYP) and cyclooxygenase (COX) enzyme mRNA in hepatic and vascular tissues is altered after intraperitoneal injection of AhR agonists benzo(a)pyrene (BaP) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alone and in combination with the purported AhR antagonists resveratrol (Res) or alpha-naphthoflavone (ANF). Both TCDD and BaP induced similar patterns of gene expression in arteries, although with different efficacies, and had slightly different effects in hepatic tissues. Resveratrol was generally without effect in all treatment groups and tissues with the exception of reducing TCDD-induced CYP1C2 in vascular tissues. In contrast, ANF antagonized TCDD- and BaP-induced changes, as well as reduced baseline gene expression in liver. However, in arteries, ANF alone acted as an agonist to increase expression of several of the genes investigated.<p> The second series of experiments involved zebrafish eggs aqueously exposed to BaP or TCDD alone and in combination with Res or ANF. Whole larvae CYP and COX isoform mRNA expression was quantified at 5 and 10 days post-fertilization (dpf), then correlated with developmental phenotype. Both TCDD and BaP caused concentration-dependent AhR-associated deformities with a significant increase in mortalities by 10 dpf and increased CYP1A mRNA expression, while TCDD alone decreased CYP1C2 expression. BaP/ANF co-exposure exhibited the highest rate of deformities and mortalities at both 5 and 10 dpf, caused marked alterations in cardiac and vascular morphology at 10 dpf, and increased CYP1A expression. Furthermore, ANF exhibited additive agonistic effects on gene expression with both BaP and TCDD. Correlation analyses revealed that gene expression at 5 dpf, but not 10 dpf, was strongly linked to abnormal cardiac and vascular phenotypes at 10 dpf with several genes related to cardiac development and one primary gene linked to vascular development.

Developmental exposure to aryl hydrocarbon receptor (AhR) agonists in fish causes severe defects in the cardiovascular system. However, the effects of acute AhR agonist exposure on the adult fish cardiovascular system as well as the genes mediating developmental AhR-induced deformities remain unclear. In this thesis, two studies were carried out to address these issues. Before experiments could begin, methods for quantitative real-time reverse transcriptase polymerase chain reaction (rtrt-PCR) as well as larval exposure and rearing were developed, validated, and optimized.<p> Following method development, a series of experiments was performed on adult zebrafish (<i>Danio rerio</i>) to assess how expression of cytochrome P450 (CYP) and cyclooxygenase (COX) enzyme mRNA in hepatic and vascular tissues is altered after intraperitoneal injection of AhR agonists benzo(a)pyrene (BaP) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alone and in combination with the purported AhR antagonists resveratrol (Res) or alpha-naphthoflavone (ANF). Both TCDD and BaP induced similar patterns of gene expression in arteries, although with different efficacies, and had slightly different effects in hepatic tissues. Resveratrol was generally without effect in all treatment groups and tissues with the exception of reducing TCDD-induced CYP1C2 in vascular tissues. In contrast, ANF antagonized TCDD- and BaP-induced changes, as well as reduced baseline gene expression in liver. However, in arteries, ANF alone acted as an agonist to increase expression of several of the genes investigated.<p> The second series of experiments involved zebrafish eggs aqueously exposed to BaP or TCDD alone and in combination with Res or ANF. Whole larvae CYP and COX isoform mRNA expression was quantified at 5 and 10 days post-fertilization (dpf), then correlated with developmental phenotype. Both TCDD and BaP caused concentration-dependent AhR-associated deformities with a significant increase in mortalities by 10 dpf and increased CYP1A mRNA expression, while TCDD alone decreased CYP1C2 expression. BaP/ANF co-exposure exhibited the highest rate of deformities and mortalities at both 5 and 10 dpf, caused marked alterations in cardiac and vascular morphology at 10 dpf, and increased CYP1A expression. Furthermore, ANF exhibited additive agonistic effects on gene expression with both BaP and TCDD. Correlation analyses revealed that gene expression at 5 dpf, but not 10 dpf, was strongly linked to abnormal cardiac and vascular phenotypes at 10 dpf with several genes related to cardiac development and one primary gene linked to vascular development.

Cyclosporine forms the cornerstone of therapy to prevent rejection after organ transplantation. However, the clinical use of the drug is compromised by a narrow therapeutic window and a wide inter- and intra-individual variation in metabolism. Cyclosporine is metabolised by the CYP3A4 isoenzymes in both the liver and intestine, while it has been reported that the metabolism of the drug can be inhibited by certain furocoumarin derivatives in grapefruit juice. Methoxsalen (8-methoxypsoralen) is a furocoumarin and a potent inhibitor of the cytochrome P450 system in both the liver and intestine. The study was conducted to investigate the possibility whether methoxsalen may inhibit the metabolism of cyclosporine and thereby increase the bioavailability of the drug. The interaction is of clinical relevance since both drugs are used in the treatment of psoriases. The study, conducted in 12 healthy male volunteers, was a three-way comparative bioavailability study with a wash out period of one week between treatments. The patients received 40 mg methoxsalen, 200 mg cyclosporine or a combination of the two on three separate occasions. Blood samples of 10 ml were collected by venupuncture at the following times: 0, 0.5, 1, 1.5, 2, 2.5, 3.4, 5,6, 8, 12 and 24 hours after drug administration. Methoxsalen was analysed by a high pressure liquid chromatograph method (HPLC) with UV detection (LOQ = 10 ng/ml), while cyclosporine was analysed using a fluorescence polarisation immunoassay (FPIA) technique. There was a statistical significant difference in AUCo-00 and Cmax ' for cyclosporine when methoxsalen was added to the drug regimen. When the methoxsalen levels were compared with those in the presence of cyclosporine, the levels were lower, although the difference was not statistical significant. We conclude that methoxsalen increase the levels of cyclosporine by inhibiting the P450 system enzymes in the liver and intestine. However, the absorption of methoxsalen is highly variable in the same individual which needs to be considered before this interaction can be regarded as being of any clinical relevance. / Thesis (M.Sc.(Pharmacology))--North-West University, Potchefstroom Campus, 2004.

Plant sesquiterpene synthases catalyze the conversion of the linear substrate farnesyl diphosphate, FPP, into a remarkable array of secondary metabolites. These secondary metabolites in turn mediate a number of important interactions between plants and their environment, such as plant-plant, plant-insect and plant-pathogen interactions. Given the relative biological importance of sesquiterpenes and their use in numerous practical applications, the current thesis was directed towards developing a better understanding of the mechanisms employed by sesquiterpene synthases in the biosynthesis of such a diverse class of compounds. Substrate preference for sesquiterpene synthases initially isolated from Nicotiana tabacum (TEAS), Hyoscyamus muticus (HPS) and Artemisia annuna (ADS) were optimized with regards to a divalent metal ion requirement. Surprisingly, careful titration with manganese stimulated bona fide synthase activity with the native 15-carbon substrate farnesyl diphopshate (FPP) as well as with the 10-carbon substrate geranyl diphosphate (GPP). Reaction product analysis suggested that the GPP could be used to investigate early steps in the catalytic cascade of these enzymes. To investigate how structural features of the sesquiterpene synthases translate into enzymatic traits, a series of substrate and active site residue contacts maps were developed and used in a comparative approach to identify residues that might direct product specificity. The role and contribution of several of these residues to catalysis and product specificity were subsequently tested by the creation of site-directed mutants. One series of mutants was demonstrated to change the reaction product to a novel sesquiterpene, 4-epi-eremophilene, and while another series successfully transmutated TEAS into a HPS-like enzyme. This is the first report of a rational redesign of product specificity for any terpene synthase. The contact map provides a basis for the prediction of specific configurations of amino acids that might be necessary for as yet uncharacterized sesquiterpene synthases from natural sources. This prediction was tested by the subsequent isolation and validation that valencene synthase, a synthase from citrus, did indeed have the amino acid configuration as predicted. Lastly, an in vitro system was developed for analyzing the interaction between sesquiterpene synthases and the corresponding terpene hydroxylase. Development of this in vitro system is presented as a new important tool in further defining those biochemical features giving rise to the biological diversity of sesquiterpenes.

Cyclosporine forms the cornerstone of therapy to prevent rejection after organ transplantation. However, the clinical use of the drug is compromised by a narrow therapeutic window and a wide inter- and intra-individual variation in metabolism. Cyclosporine is metabolised by the CYP3A4 isoenzymes in both the liver and intestine, while it has been reported that the metabolism of the drug can be inhibited by certain furocoumarin derivatives in grapefruit juice. Methoxsalen (8-methoxypsoralen) is a furocoumarin and a potent inhibitor of the cytochrome P450 system in both the liver and intestine. The study was conducted to investigate the possibility whether methoxsalen may inhibit the metabolism of cyclosporine and thereby increase the bioavailability of the drug. The interaction is of clinical relevance since both drugs are used in the treatment of psoriases. The study, conducted in 12 healthy male volunteers, was a three-way comparative bioavailability study with a wash out period of one week between treatments. The patients received 40 mg methoxsalen, 200 mg cyclosporine or a combination of the two on three separate occasions. Blood samples of 10 ml were collected by venupuncture at the following times: 0, 0.5, 1, 1.5, 2, 2.5, 3.4, 5,6, 8, 12 and 24 hours after drug administration. Methoxsalen was analysed by a high pressure liquid chromatograph method (HPLC) with UV detection (LOQ = 10 ng/ml), while cyclosporine was analysed using a fluorescence polarisation immunoassay (FPIA) technique. There was a statistical significant difference in AUCo-00 and Cmax ' for cyclosporine when methoxsalen was added to the drug regimen. When the methoxsalen levels were compared with those in the presence of cyclosporine, the levels were lower, although the difference was not statistical significant. We conclude that methoxsalen increase the levels of cyclosporine by inhibiting the P450 system enzymes in the liver and intestine. However, the absorption of methoxsalen is highly variable in the same individual which needs to be considered before this interaction can be regarded as being of any clinical relevance. / Thesis (M.Sc.(Pharmacology))--North-West University, Potchefstroom Campus, 2004.

A central theme within the concept of enzyme-directed bioreductive drug development is the potential to predict tumour response based on the profiling of enzymes involved in the bioreductive activation process. Mitomycin C (MMC) is the prototypical bioreductive drug that is reduced to active intermediates by several reductases including NAD(P)H:quinone oxidoreductase (NQO1) and NADPH cytochrome P450 reductase (P450R). The purpose of our study was to determine whether NQO1 and P450R protein expression in a panel of low-grade, human superficial bladder tumours correlates with clinical response to MMC. A retrospective clinical study was conducted in which the response to MMC of 92 bladder cancer patients was compared to the immunohistochemical expression of NQO1 and P450R protein in archived paraffin-embedded bladder tumour specimens. A broad spectrum of NQO1 protein levels exists in bladder tumours between individual patients, ranging from intense to no immunohistochemical staining. In contrast, levels of P450R were similar with most tumours having moderate to high levels. All patients were chemotherapy naïve prior to receiving MMC and clinical response was defined as the time to first recurrence. A poor correlation exists between clinical response and NQO1, P450R or the expression patterns of various combinations of the 2 proteins. The results of our study demonstrate that the clinical response of superficial bladder cancers to MMC cannot be predicted on the basis of NQO1 and/or P450R protein expression and suggest that other factors (other reductases or post DNA damage events) have a significant bearing on tumour response.

Thesis (M.Sc.)--Memorial University of Newfoundland, 1997. / Bibliography: leaves 50-51.

Neste trabalho foi investigado o potencial de modelos metaloporfirínicos em mimetizar a ação do citocromo P450 na oxidação de um herbicida, a atrazina. Foram utilizadas as metaloporfirinas comerciais de segunda geração solúveis em solvente orgânico, cloreto de 5,10,15,20-tetrakis(2,6-diclorofenil)porfirina metal(III) [M(TDCPP)Cl] e cloreto de 5,10,15,20-tetrakis(pentafluorofenil) porfirina metal(III) [M(TFPP)Cl] (metal = ferro e manganês), tanto em solução homogênea como suportadas em montmorilonita K-10 aminofuncionalizadas; e metaloporfirinas solúveis em água, como a cloreto de 5,10,15,20-tetrakis-(N-metil-4-piridil) porfirina ferro(III), [Fe(TMPy)Cl], e cloreto de [5,10,15,20-tetra(4-carboxifenil)porfirina] ferro(III), [Fe(TCPP)Cl]. Os oxidantes testados foram iodosilbenzeno, ácido metacloroperbenzóico e peróxido de hidrogênio em água, metanol e acetonitrila. Os produtos de oxidação da atrazina foram identificados por cromatografia líquida de alta eficiência (CLAE). Os resultados mostraram que as metaloporfirinas foram capazes de oxidar a atrazina, um herbicida com características de persistência no meio ambiente, e mimetizar a ação da enzima in vivo e in vitro com formação de dois metabólitos: DEA e DIA, resultado da N-desalquilação das cadeias etila e propila do substrato, respectivamente. O DEA correspondeu a um dos principais produtos da reação, e formou-se apenas traços de DIA, mostrando a preferência das metaloporfirinas em oxidar a cadeia etila da atrazina. Verificou-se também a formação de cinco produtos desconhecidos, sendo possível a identificação de apenas um deles por espectrometria de massas, devido à baixa concentração dos demais, o qual corresponde à formação de uma amida na cadeia etila da atrazina (COA). Esse composto correspondeu ao produto de maior rendimento na maioria das reações. O monitoramento das reações em diferentes intervalos de tempo e a variação nas condições reacionais mostraram que os principais produtos de oxidação do herbicida, DEA e COA, são formados por mecanismos independentes e por espécies catalíticas distintas. O DEA é formado via espécie ativa Me(V)OP [Mn(V)OP ou Fe(IV)OP+], enquanto o COA é originado via Me(IV)OP [Mn(IV)OP ou Fe(IV)OP]. Estudos de intermediários por UV-Vis e EPR mostraram que a espécie ferril predomina como intermediário de reação para os sistemas Fe(TFPP)Cl/ACN com os dois oxidantes, iodosilbenzeno e ácido metacloroperbenzóico. Para as metaloporfirinas Fe(TCPP)Cl e Fe(TMPy)Cl o estudo da oxidação do herbicida ficou comprometido devido à baixa solubilidade da atrazina em água, o que provocava sua precipitação e destruição do catalisador. Para as metaloporfirinas suportadas em montmorilonita K-10 aminofuncionalizada também não foi observada formação de produtos, resultado atribuído à dificuldade do substrato, considerado bastante inerte, atingir o sítio catalítico. Todos esses resultados mostraram o potencial de aplicação desses modelos biomiméticos em estudos que buscam elucidar o metabolismo de herbicidas in vivo, tendo em vista a dificuldade de se trabalhar com as enzimas in vitro, e resultaram na proposição de um esquema de reação da oxidação da atrazina catalisada pelas metaloporfirinas nas condições estudadas. / In this work we investigated the ability of metalloporphyrin model systems to mimic the action of cytochrome P450 in the oxidation of a herbicide, atrazine. To this end, we employed the second generation commercially available metalloporphyrins metal (III) 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin chloride [M(TDCPP)Cl] and metal (III) 5,10,15,20- tetrakis(pentafluorophenyl)porphyrin chloride [M(TFPP)Cl] (metal = iron or manganese), all soluble in organic solvents, as well as the water soluble metalloporphyrins iron (III) 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin chloride [Fe(TMPy)Cl] and iron (III) 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride [Fe(TCPP)Cl]. These metalloporphyrins were used both in homogeneous solution and supported on montmorillonite K-10. Iodosylbenzene, metachloroperbenzoic acid, and hydrogen peroxide were tested as oxidants, using one of the following reaction media: water, methanol, and acetonitrile. Products generated during atrazine oxidation were identified by high performance liquid chromatography. Our results show that the metalloporphyrins are able to oxidize atrazine, a highly persistent herbicide in the environment, as well as mimic the action of P450 enzymes both in vivo and in vitro, with formation of two metabolites, namely DEA and DIA, which result from the N-dealkylation of the ethyl and propyl chains of the substrate, respectively. We also detected the formation of five unknown products, and we were able to identify only one of them by means of mass spectrometry, which corresponds to the formation of an amide on the atrazine ethyl chain (COA) and was the compound obtained in highest yields in most of the reactions. The other four unknown products were obtained in very low concentrations, which prevented us from determining their structures. By monitoring the reactions at different time intervals and varying the reactional conditions, we were able to see that the main herbicide oxidation products, DEA and COA, are generated via distinct mechanisms and different active catalytic species. DEA is formed via the species Me(V)OP [Mn(V)OP or Fe(IV)OP.+], while COA results from the action of the species Me(IV)OP [Mn(IV)OP or Fe(IV)OP]. Studies of the reaction intermediates by UV-VIS and EPR showed that the ferryl species is the main reaction intermediate in the case of Fe(TFPP)Cl/ACN systems and the oxidants, iodosylbenzene and metachloroperbenzoic acid. Studies of herbicide oxidation were difficult to carry out in the case of the metalloporphyrins Fe(TCPP)Cl and Fe(TMPy)Cl due to the low solubility of atrazine in water, which led to its precipitation and catalyst destruction. With respect to the metalloporphyrins supported on montmorillonite K-10, no reaction products were obseved because of the difficult diffusion of the inert substrate into the catalytic site. All these results demonstrate the potential application of these biomimetic model systems in studies that pursue the elucidation of herbicide metabolism in vivo, especially when one bears in mind the difficulty in working with enzymes in vitro. Our data enabled the proposition of a scheme for metalloporphyrin-catalyzed atrazine oxidation under the conditions used herein.