CYP4 isozyme specificity and the relationship between omega-hydroxylation and terminal desaturation of valproic acid

Sex-Dependent Changes to the Intestinal and Hepatic Abundance of Drug Transporters and Metabolizing Enzymes in the SOD1G93A Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by death and dysfunction of motor neurons that result in a rapidly progressing loss of motor function. While there are some data on alterations at the blood-brain barrier (BBB) in ALS and their potential impact on CNS trafficking of drugs, little is reported on the impact of this disease on the expression of drug-handling proteins in the small intestine and liver. This may impact the dosing of the many medicines that individuals with ALS are prescribed. In the present study, a proteomic evaluation was performed on small intestine and liver samples from postnatal day 120 SOD1G93A mice (a model of familial ALS that harbors a human mutant form of superoxide dismutase 1) and wild-type (WT) littermates (n = 7/genotype/sex). Untargeted, quantitative proteomics was undertaken using either label-based [tandem mass tag (TMT)] or label-free [data-independent acquisition (DIA)] acquisition strategies on high-resolution mass spectrometric instrumentation. Copper chaperone for superoxide dismutase (CCS) was significantly higher in SOD1G93A samples compared to the WT samples for both sexes and tissues, therefore representing a potential biomarker for ALS in this mouse model. Relative to WT mice, male SOD1G93A mice had significantly different proteins (Padj < 0.05, |fold-change|>1.2) in the small intestine (male 22, female 1) and liver (male 140, female 3). This included an up-regulation of intestinal transporters for dietary glucose [solute carrier (SLC) SLC5A1] and cholesterol (Niemann-Pick c1-like 1), as well as for several drugs (e.g., SLC15A1), in the male SOD1G93A mice. There was both an up-regulation (e.g., SLCO2A1) and down-regulation (ammonium transporter rh type b) of transporters in the male SOD1G93A liver. In addition, there was both an up-regulation (e.g., phosphoenolpyruvate carboxykinase) and down-regulation (e.g., carboxylesterase 1) of metabolizing enzymes in the male SOD1G93A liver. This proteomic data set identified male-specific changes to key small intestinal and hepatic transporters and metabolizing enzymes that may have important implications for the bioavailability of nutrients and drugs in individuals with ALS.

Spotlight on CYP4B1

The mammalian cytochrome P450 monooxygenase CYP4B1 can bioactivate a wide range of xenobiotics, such as its defining/hallmark substrate 4-ipomeanol leading to tissue-specific toxicities. Similar to other members of the CYP4 family, CYP4B1 has the ability to hydroxylate fatty acids and fatty alcohols. Structural insights into the enigmatic role of CYP4B1 with functions in both, xenobiotic and endobiotic metabolism, as well as its unusual heme-binding characteristics are now possible by the recently solved crystal structures of native rabbit CYP4B1 and the p.E310A variant. Importantly, CYP4B1 does not play a major role in hepatic P450-catalyzed phase I drug metabolism due to its predominant extra-hepatic expression, mainly in the lung. In addition, no catalytic activity of human CYP4B1 has been observed owing to a unique substitution of an evolutionary strongly conserved proline 427 to serine. Nevertheless, association of CYP4B1 expression patterns with various cancers and potential roles in cancer development have been reported for the human enzyme. This review will summarize the current status of CYP4B1 research with a spotlight on its roles in the metabolism of endogenous and exogenous compounds, structural properties, and cancer association, as well as its potential application in suicide gene approaches for targeted cancer therapy.

Reactive metabolites of the anticonvulsant drugs and approaches to minimize the adverse drug reaction

Several generations of antiepileptic drugs (AEDs) are available in the market for the treatment of seizures, but these are amalgamated with acute to chronic side effects. The most common side effects of AEDs are dose-related, but some are idiosyncratic adverse drug reactions (ADRs) that transpire due to the formation of reactive metabolite (RM) after the bioactivation process. Because of the adverse reactions patients usually discontinue the medication in between the treatment. The AEDs such as valproic acid, lamotrigine, phenytoin etc., can be categorized under such types because they form the RM which may prevail with life-threatening adverse effects or immune-mediated reactions. Hepatotoxicity, teratogenicity, cutaneous hypersensitivity, dizziness, addiction, serum sickness reaction, renal calculi, metabolic acidosis are associated with the metabolites of drugs such as arene oxide, N-desmethyldiazepam, 2-(1-hydroxyethyl)-2-methylsuccinimide, 2-(sulphamoy1acetyl)-phenol, E-2-en-VPA and 4-en-VPA and carbamazepine-10,11-epoxide, etc. The major toxicities are associated with the moieties that are either capable of forming RM or the functional groups may itself be too reactive prior to the metabolism. These functional groups or fragment structures are typically known as structural alerts or toxicophores. Therefore, minimizing the bioactivation potential of lead structures in the early phases of drug discovery by a modification to low-risk drug molecules is a priority for the pharmaceutical companies. Additionally, excellent potency and pharmacokinetic (PK) behaviour help in ensuring that appropriate (low dose) candidate drugs progress into the development phase. The current review discusses about RMs in the anticonvulsant drugs along with their mechanism vis-a-vis research efforts that have been taken to minimize the toxic effects of AEDs therapy.

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato^3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an Ag^I-pyrrole (η²-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C₆H₄)₃C• and 1 gave k = 1.34(3) × 10³ M^-1 s^-1 at 23 °C and 2.2(2) M^-1 s^-1 at -60 °C, ΔH^⧧ = +9.8(3) kcal mol^-1, and ΔS^⧧ = -14(1) cal mol^-1 K^-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 10⁵ M^-1 s^-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F^- > Cl^- > OH^- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

Hydroxylation, Epoxidation, and Dehydrogenation of Capsaicin by a Microbial Promiscuous Cytochrome P450 105D7

Cytochrome P450 enzymes (P450s) are versatile biocatalysts, which insert a molecular oxygen into inactivated C-H bonds under mild conditions. CYP105D7 from Streptomyces avermitilis has been reported as a bacterial substrate-promiscuous P450 which catalyzes the hydroxylation of 1-deoxypentalenic acid, diclofenac, naringenin, compactin and steroids. In this study, CYP105D7 catalyzes hydroxylation, epoxidation and dehydrogenation of capsaicin, a pharmaceutical agent, revealing its functional diversity. The kinetic parameters of the CYP105D7 oxidation of capsaicin were determined as K_m =311.60±87.30 μM and k_cat =2.01±0.33 min^-1 . In addition, we conducted molecular docking, mutagenesis and substrate binding analysis, indicating that Arg81 plays crucial role in the capsaicin binding and catalysis. To our best knowledge, this study presents the first report to illustrate that capsaicin can be catalyzed by prokaryotic P450s.

Hydroxyl Transfer to Carbon Radicals by Mn(OH) vs Fe(OH) Corrole Complexes

The transfer of •OH from metal-hydroxo species to carbon radicals (R•) to give hydroxylated products (ROH) is a fundamental process in metal-mediated heme and nonheme C-H bond oxidations. This step, often referred to as the hydroxyl "rebound" step, is typically very fast, making direct study of this process challenging if not impossible. In this report, we describe the reactions of the synthetic models M(OH)(ttppc) (M = Fe (1), Mn (3); ttppc = 5,10,15-tris(2,4,6-triphenyl)phenyl corrolato^3-) with a series of triphenylmethyl carbon radical (R•) derivatives ((4-X-C₆H₄)₃C•; X = OMe, tBu, Ph, Cl, CN) to give the one-electron reduced M^III(ttppc) complexes and ROH products. Rate constants for 3 for the different radicals ranged from 11.4(1) to 58.4(2) M^-1 s^-1, as compared to those for 1, which fall between 0.74(2) and 357(4) M^-1 s^-1. Linear correlations for Hammett and Marcus plots for both Mn and Fe were observed, and the small magnitudes of the slopes for both correlations imply a concerted •OH transfer reaction for both metals. Eyring analyses of reactions for 1 and 3 with (4-X-C₆H₄)₃C• (X = tBu, CN) also give good linear correlations, and a comparison of the resulting activation parameters highlight the importance of entropy in these •OH transfer reactions. Density functional theory calculations of the reaction profiles show a concerted process with one transition state for all radicals investigated and help to explain the electronic features of the OH rebound process.

Impact of CYP2C19 and CYP2C9 gene polymorphisms on sodium valproate plasma concentration in patients with epilepsy

BACKGROUND

Valproic acid (VPA) is a broad spectrum anticonvulsant drug, which could be partially metabolised by cytochrome P450 (CYP) 2C9 and 2C19 enzymes. This study was designed to investigate the relationship between CYP2C19 and CYP2C9 gene polymorphisms and the plasma concentrations of VPA in subjects with epilepsy.

METHODS

Eighty-three subjects with epilepsy aged 18-92 years were enrolled in this study. All were treated with sustained-release VPA monotherapy. Based on the genotypes of CYP2C19 and the ability to metabolise substrates, the subjects were divided into poor metabolisers, intermediate metabolisers and extensive metabolisers. Sanger sequencing was used to detect the genotypic and allelic frequencies of CYP2C19 (*1, *2 and *3) and CYP2C9 (*13) of the patients. Automatic immunity analysis was used to find steady-state trough plasma concentrations of VPA. By adjusting the plasma concentrations of VPA with body weight and total daily dose of VPA, the concentration-to-dose ratio of VPA (CDRV) was obtained. Data were analysed using SPSS software.

RESULTS

The genetic frequencies of CYP2C19*2, CYP2C19*3 and CYP2C9*13 were 33.1%, 3.0% and 5.4%, respectively, among patients with epilepsy from Yunnan province, China who used VPA therapy. The CDRV was significantly lower in the CYP2C19 extensive metabolisers (3.33±1.78) than it was in the CYP2C19 intermediate metabolisers (4.45±1.42) and the CYP2C19 poor metabolizers (6.64±1.06). The CYP2C19*2 and CYP2C19*3 alleles were correlated with the plasma VPA concentration, while the CYP2C9*13 allele had no effect on the plasma VPA concentration (p=0.809).

CONCLUSIONS

The genetic polymorphisms of CYP2C19 significantly affect the VPA plasma concentration, and the dosage of VPA for intermediate and poor metabolisers could be lower than for extensive metabolisers. CYP2C9*13 carrier was not closely related to plasma concentrations of VPA in patients with epilepsy.

Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies

High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the Fe^IV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of Fe^IV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k₂ = 3.6(1)-1.21(3) × 10⁴ M^-1 s^-1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex, Mn^IV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for 1. The O-H bond dissociation energy (BDE) of the M^III(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol^-1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.

Noncovalent interactions dominate dynamic heme distortion in cytochrome P450 4B1

Cytochrome P450 4B1 (4B1) functions in both xenobiotic and endobiotic metabolism. An ester linkage between Glu-310 in 4B1 and the 5-methyl group of heme facilitates preferential hydroxylation of terminal (ω) methyl groups of hydrocarbons (HCs) and fatty acids compared with ω-1 sites bearing weaker C-H bonds. This preference is retained albeit diminished 4-fold for the E310A mutant, but the reason for this is unclear. Here, a crystal structure of the E310A-octane complex disclosed that noncovalent interactions maintain heme deformation in the absence of the ester linkage. Consistent with the lower symmetry of the heme, resonance Raman (RR) spectroscopy revealed large enhancements of RR peaks for high-spin HC complexes of 4B1 and the E310A mutant relative to P450 3A4. Whereas these enhancements were diminished in RR spectra of a low-spin 4B1-N-hydroxy-N'-(4-butyl-2-methylphenyl)formamidine complex, a crystal structure indicated that this inhibitor does not alter heme ruffling. RR spectra of Fe2+-CO HC complexes revealed larger effects of HC length in E310A than in 4B1, suggesting that reduced rigidity probably underlies increased E310A-catalyzed (ω-1)-hydroxylation. Diminished effects of the HC on the position of the Fe-CO stretching mode in 4B1 suggested that the ester linkage limits substrate access to the CO. Heme ruffling probably facilitates autocatalytic ester formation by reducing inhibitory coordination of Glu-310 with the heme iron. This also positions the 5-methyl for a reaction with the proposed glutamyl radical intermediate and potentially enhances oxo-ferryl intermediate reactivity for generation of the glutamyl radical to initiate ester bond formation and ω-hydroxylation.

A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn^V(O)(ttppc) (4) and Mn^III(H₂O)(ttppc) (1·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di- tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k₂ = 2.73(12) × 10⁴ M^-1 s^-1, which is ∼3 orders of magnitude larger than 4 ( k₂ = 17.4(1) M^-1 s^-1). Reaction of 3 with a series of para-substituted 2,6-di- tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k₂ = 510(10)-36(1.4) M^-1 s^-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes.

P450 catalysed dehydrogenation

Cytochrome P450s belong to a superfamily of enzymes that catalyse a wide variety of oxidative transformations. Hydroxylation is one the most thoroughly investigated of all identified P450-catalysed reactions whilst dehydrogenation has been relatively much less explored to date. P450-catalysed dehydrogenation is often found to occur with hydroxylation and thus, it was initially suspected to be a stepwise process consisting of hydroxylation and subsequent dehydration to yield the final olefin product. This theory has been proven to be invalid and the olefin was shown to be the direct product of a P450-catalysed reaction. This interesting reaction plays a vital role in the metabolism of xenobiotics and the biosynthesis of endogenous compounds, including a number of steroids. A number of well-known examples of P450 mediated dehydrogenation, including those in the metabolism of valproic acid, capsaicin and 3-methylindole and those in the biosynthesis of plant and fungal sterols are discussed in this review.

Oxidation of C18 Hydroxy-Polyunsaturated Fatty Acids to Epoxide or Ketone by Catalase-Related Hemoproteins Activated with Iodosylbenzene

Small catalase-related hemoproteins with a facility to react with fatty acid hydroperoxides were examined for their potential mono-oxygenase activity when activated using iodosylbenzene. The proteins tested were a Fusarium graminearum 41 kD catalase hemoprotein (Fg-cat, gene FGSG_02217), a Pseudomonas fluorescens Pfl01 catalase (37.5 kD, accession number WP_011333788.1), and a Mycobacterium avium ssp. paratuberculosis 33 kD catalase (gene MAP-2744c). 13-Hydroxy-octadecenoic acids (which are normally unreactive) were selected as substrates because these enzymes react specifically with the corresponding 13S-hydroperoxides (Pakhomova et al. 18:2559-2568, 5; Teder et al. 1862:706-715, 14). In the presence of iodosylbenzene Fg-cat converted 13S-hydroxy-fatty acids to two products: the 15,16-double bond of 13S-hydroxy α-linolenic acid was oxidized stereospecifically to the 15S,16R-cis-epoxide or the 13-hydroxyl was oxidized to the 13-ketone. Products were identified by UV, HPLC, LC-MS, NMR and by comparison with authentic standards prepared for this study. The Pfl01-cat displayed similar activity. MAP-2744c oxidized 13S-hydroxy-linoleic acid to the 13-ketone, and epoxidized the double bonds to form the 9,10-epoxy-13-hydroxy, 11,12-epoxy-13-hydroxy, and 9,10-epoxy-13-keto derivatives; equivalent transformations occurred with 9S-hydroxy-linoleic acid as substrate. In parallel incubations in the presence of iodosylbenzene, human catalase displayed no activity towards 13S-hydroxy-linoleic acid, as expected from the highly restricted access to its active site. The results indicated that with suitable transformation to Compound I, monooxygenase activity can be demonstrated by these catalase-related hemoproteins with tyrosine as the proximal heme ligand.

Cytotoxic 1-deoxysphingolipids are metabolized by a cytochrome P450-dependent pathway

The 1-deoxysphingolipids (1-deoxySLs) are atypical sphingolipids (SLs) that are formed when serine palmitoyltransferase condenses palmitoyl-CoA with alanine instead of serine during SL synthesis. The 1-deoxySLs are toxic to neurons and pancreatic β-cells. Pathologically elevated 1-deoxySLs cause the inherited neuropathy, hereditary sensory autonomic neuropathy type 1 (HSAN1), and are also found in T2D. Diabetic sensory polyneuropathy (DSN) and HSAN1 are clinically very similar, suggesting that 1-deoxySLs may be implicated in both pathologies. The 1-deoxySLs are considered to be dead-end metabolites, as they lack the C1-hydroxyl group, which is essential for the canonical degradation of SLs. Here, we report a previously unknown metabolic pathway, which is capable of degrading 1-deoxySLs. Using a variety of metabolic labeling approaches and high-resolution high-accuracy MS, we identified eight 1-deoxySL downstream metabolites, which appear to be formed by cytochrome P450 (CYP)4F enzymes. Comprehensive inhibition and induction of CYP4F enzymes blocked and stimulated, respectively, the formation of the downstream metabolites. Consequently, CYP4F enzymes might be novel therapeutic targets for the treatment of HSAN1 and DSN, as well as for the prevention of T2D.

Drug metabolism by cytochrome p450 enzymes: what distinguishes the pathways leading to substrate hydroxylation over desaturation?

Cytochrome P450 enzymes are highly versatile biological catalysts in our body that react with a broad range of substrates. Key functions in the liver include the metabolism of drugs and xenobiotics. One particular metabolic pathway that is poorly understood relates to the P450 activation of aliphatic groups leading to either hydroxylation or desaturation pathways. A DFT and QM/MM study has been carried out on the factors that determine the regioselectivity of aliphatic hydroxylation over desaturation of compounds by P450 isozymes. The calculations establish multistate reactivity patterns, whereby the product distributions differ on each of the spin-state surfaces; hence spin-selective product formation was found. The electronic and thermochemical factors that determine the bifurcation pathways were analysed and a model that predicts the regioselectivity of aliphatic hydroxylation over desaturation pathways was established from valence bond and molecular orbital theories. Thus, the difference in energy of the OH versus the OC bond formed and the π-conjugation energy determines the degree of desaturation products. In addition, environmental effects of the substrate binding pocket that affect the regioselectivities were identified. These studies imply that bioengineering P450 isozymes for desaturation reactions will have to include modifications in the substrate binding pocket to restrict the hydroxylation rebound reaction.

Identification of amino acid determinants in CYP4B1 for optimal catalytic processing of 4-ipomeanol

Mammalian CYP4B1 enzymes are cytochrome P450 mono-oxygenases that are responsible for the bioactivation of several exogenous pro-toxins including 4-ipomeanol (4-IPO). In contrast with the orthologous rabbit enzyme, we show here that native human CYP4B1 with a serine residue at position 427 is unable to bioactivate 4-IPO and does not cause cytotoxicity in HepG2 cells and primary human T-cells that overexpress these enzymes. We also demonstrate that a proline residue in the meander region at position 427 in human CYP4B1 and 422 in rabbit CYP4B1 is important for protein stability and rescues the 4-IPO bioactivation of the human enzyme, but is not essential for the catalytic activity of the rabbit CYP4B1 protein. Systematic substitution of native and p.S427P human CYP4B1 with peptide regions from the highly active rabbit enzyme reveals that 18 amino acids in the wild-type rabbit CYP4B1 protein are key for conferring high 4-IPO metabolizing activity. Introduction of 12 of the 18 amino acids that are also present at corresponding positions in other human CYP4 family members into the p.S427P human CYP4B1 protein results in a mutant human enzyme (P+12) that is as stable and as active as the rabbit wild-type CYP4B1 protein. These 12 mutations cluster in the predicted B-C loop through F-helix regions and reveal new amino acid regions important to P450 enzyme stability. Finally, by minimally re-engineering the human CYP4B1 enzyme for efficient activation of 4-IPO, we have developed a novel human suicide gene system that is a candidate for adoptive cellular therapies in humans.

Ab initio dynamics of the cytochrome P450 hydroxylation reaction

The iron(IV)-oxo porphyrin π-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis.

Endocannabinoid metabolism by cytochrome P450 monooxygenases

The endogenous cannabinoid system was first uncovered following studies of the recreational drug Cannabis sativa. It is now recognized as a vital network of signaling pathways that regulate several physiological processes. Following the initial discovery of the cannabinoid receptors 1 (CB1) and 2 (CB2), activated by Cannabis-derived analogs, many endogenous fatty acids termed "endocannabinoids" are now known to be partial agonists of the CB receptors. At present, the most thoroughly studied endocannabinoid signaling molecules are anandamide (AEA) and 2-arachidonylglycerol (2-AG), which are both derived from arachidonic acid. Both AEA and 2-AG are also substrates for the eicosanoid-synthesizing pathways, namely, certain cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) enzymes. In the past, research in the endocannabinoid field focused on the interaction of AEA and 2-AG with the COX and LOX enzymes, but accumulating evidence also points to the involvement of CYPs in modulating endocannabinoid signaling. The focus of this review is to explore the current understanding of CYP-mediated metabolism of endocannabinoids.

N-alkylprotoporphyrin formation and hepatic porphyria in dogs after administration of a new antiepileptic drug candidate: mechanism and species specificity

A new antiepileptic synaptic vesicle 2a (SV2a) ligand drug candidate was tested in 4-week oral toxicity studies in rat and dog. Brown pigment inclusions were found in the liver of high-dose dogs. The morphology of the deposits and the accompanying liver changes (increased plasma liver enzymes, increased total hepatic porphyrin level, decreased liver ferrochelatase activity, combined induction, and inactivation of cytochrome P-450 CYP2B11) suggested disruption of the heme biosynthetic cascade. None of these changes was seen in rat although this species was exposed to higher parent drug levels. Toxicokinetic analysis and in vitro metabolism assays in hepatocytes showed that dog is more prone to oxidize the drug candidate than rat. Mass spectrometry analysis of liver samples from treated dogs revealed an N-alkylprotoporphyrin adduct. The elucidation of its chemical structure suggested that the drug transforms into a reactive metabolite which is structurally related to a known reference porphyrogenic agent allylisopropylacetamide. That particular metabolite, primarily produced in dog but neither in rat nor in human, has the potential to alkylate the prosthetic heme of CYP. Overall, the data suggested that the drug candidate should not be porphyrogenic in human. This case study further exemplifies the species variability in the susceptibility to drug-induced porphyria.

Determination of drug toxicity using 3D spheroids constructed from an immortal human hepatocyte cell line

Numerous publications have documented that the immortal cells grown in three-dimensional (3D) cultures possess physiological behavior, which is more reminiscent of their parental organ than when the same cells are cultivated using classical two-dimensional (2D) culture techniques. The goal of this study was to investigate whether this observation could be extended to the determination of LD₅₀ values and whether 3D data could be correlated to in vivo observations. We developed a noninvasive means to estimate the amount of protein present in a 3D spheroid from it is planar area (± 21%) so that a precise dose can be provided in a manner similar to in vivo studies. This avoided correction of the actual dose given based on a protein determination after treatment (when some cells may have lysed). Conversion of published in vitro LC₅₀ data (mM) for six common drugs (acetaminophen, amiodarone, diclofenac, metformin, phenformin, and valproic acid) to LD₅₀ data (mg compound/mg cellular protein) showed that the variation in LD₅₀ values was generally less than that suggested by the original LC₅₀ data. Toxicological analysis of these six compounds in 3D spheroid culture (either published or presented here) demonstrated similar LD₅₀ values. Although in vitro 2D HepG2 data showed a poor correlation, the primary hepatocyte and 3D spheroid data resulted in a much higher degree of correlation with in vivo lethal blood plasma levels. These results corroborate that 3D hepatocyte cultures are significantly different from 2D cultures and are more representative of the liver in vivo.

[Relationship between plasma concentrations of valproic acid and hepatotoxicity in patients receiving high doses]

INTRODUCTION

Valproic acid (VPA) is an anticonvulsivant drug widely prescribed in the treatment of many forms of generalized epilepsy. In literature, the incidence of liver damage induced by AVP is 0.01%. It is potentialized by the combination therapy (phenobarbital, carbamazepine). Severe hepatotoxicity is rare and appears to be independent of dose and to cause a high mortality.

METHODS

The aim of our study was to evaluate the relationship between plasma concentrations of AVP and the occurrence of side effects especially hepatotoxicity in patients receiving high doses of AVP.

RESULTS

In this period, 425 plasmatic AVP monitoring were carried out in our laboratory. From 128 patients treated by high doses of AVP, only 73 were included in this study. Our work showed that adverse effects in epileptics under high doses of AVP was related to the association of the AVP with other antiepileptic in particular carbamazépine, phenobarbital and benzodiazepines rather than supra-therapeutic plasmatic concentrations of AVP. The association of AVP to major antiepileptics (carbamazépine and or phenobarbital) does not seem to generate an increase in the plasmatic concentration of AVP, which was not associated with a greater risque of adverse effects.

CONCLUSION

Consequently, clinical signs of liver toxicity may be present in AVP concentrations generally considered in the therapeutic range especially when used in high doses and or combined with antiepileptic drugs like phenobarbital or carbamazepine.

Mechanistic aspects of CYP74 allene oxide synthases and related cytochrome P450 enzymes

The existence of CYP5, CYP8A, and the CYP74 enzymes specialized for reaction with fatty acid peroxide substrates presents opportunities for a "different look" at the catalytic cycle of the cytochrome P450s. This review considers how the properties of the peroxide-metabolizing enzymes are distinctive, and how they tie in with those of the conventional monooxygenase enzymes. Some unusual reactions of each class have parallels in the other. As enzyme reactions and P450 structures emerge there will be possibilities for finding their special properties and edging this knowledge into the big picture.

Evidence for an ionic intermediate in the transformation of fatty acid hydroperoxide by a catalase-related allene oxide synthase from the Cyanobacterium Acaryochloris marina

Allene oxides are reactive epoxides biosynthesized from fatty acid hydroperoxides by specialized cytochrome P450s or by catalase-related hemoproteins. Here we cloned, expressed, and characterized a gene encoding a lipoxygenase-catalase/peroxidase fusion protein from Acaryochloris marina. We identified novel allene oxide synthase (AOS) activity and a by-product that provides evidence of the reaction mechanism. The fatty acids 18.4omega3 and 18.3omega3 are oxygenated to the 12R-hydroperoxide by the lipoxygenase domain and converted to the corresponding 12R,13-epoxy allene oxide by the catalase-related domain. Linoleic acid is oxygenated to its 9R-hydroperoxide and then, surprisingly, converted approximately 70% to an epoxyalcohol identified spectroscopically and by chemical synthesis as 9R,10S-epoxy-13S-hydroxyoctadeca-11E-enoic acid and only approximately 30% to the 9R,10-epoxy allene oxide. Experiments using oxygen-18-labeled 9R-hydroperoxide substrate and enzyme incubations conducted in H2(18)O indicated that approximately 72% of the oxygen in the epoxyalcohol 13S-hydroxyl arises from water, a finding that points to an ionic intermediate (epoxy allylic carbocation) during catalysis. AOS and epoxyalcohol synthase activities are mechanistically related, with a reacting intermediate undergoing a net hydrogen abstraction or hydroxylation, respectively. The existence of epoxy allylic carbocations in fatty acid transformations is widely implicated although for AOS reactions, without direct experimental support. Our findings place together in strong association the reactions of allene oxide synthesis and an ionic reaction intermediate in the AOS-catalyzed transformation.

Localization of CYP4B1 in the rat nasal cavity and analysis of CYPs as secreted proteins

CYP4B1 is highly expressed in rat nasal respiratory mucosa, and to a lesser extent in olfactory mucosa. Examination of high-power photomicrographs suggests that CYP4B1 may be a secreted protein, based on the fact that immunoreactivity appears to be present in the lumens of ducts of Bowman's glands (rather than intracellular localization, as we observed with an antibody recognizing CYP2F4) and in secretory granules in respiratory mucosa. Furthermore, anti-CYP4B1 immunoreactivity is present on the surface of both respiratory and olfactory mucosa. We used SignalP 3.0 analysis to ascertain the likelihood that rat CYP4B1 is a secreted protein. While this analysis does not suggest that rat CYP4B1 is a secreted protein, several other cytochrome P450 enzymes were predicted to be secreted proteins. The observation that multiple human cytochrome P450s appear to be secreted proteins helps to explain the appearance of anti-cytochrome P450 antigens in cases of human autoimmune liver diseases.

Open-label steady-state pharmacokinetic drug interaction study on co-administered quetiapine fumarate and divalproex sodium in patients with schizophrenia, schizoaffective disorder, or bipolar disorder

OBJECTIVE

To determine whether there is a pharmacokinetic drug interaction between quetiapine fumarate and divalproex sodium.

METHODS

The pharmacokinetics and short-term tolerability and safety of coadministered quetiapine and divalproex were examined in adults with schizophrenia/schizoaffective disorder (Cohort A) or bipolar disorder (Cohort B) in an open-label, parallel, 2-cohort drug-interaction study conducted at three centers in the United States. Cohort A was administered quetiapine (150 mg bid) prospectively for 13 days, with divalproex (500 mg bid) added on days 6-13. Cohort B was administered divalproex (500 mg bid) for 16 days, with quetiapine (150 mg bid) added on days 9-16. Quetiapine and valproic acid plasma concentration-time data over a 12-h steady-state dosing interval were used to determine C(max), T(max), C(min), area under the plasma concentration-time curve (AUC(tau)), and oral clearance (CL/F).

RESULTS

In Cohort A (n = 18), addition of divalproex did increase the C(max) of quetiapine by 17% but did not change AUC(tau). In Cohort B (n = 15), addition of quetiapine decreased both total valproic acid C(max) and AUC(tau) by 11%. No differences were observed in adverse events (AEs) with either quetiapine or divalproex monotherapy or their combination.

CONCLUSION

Combination therapy with quetiapine (150 mg bid) and divalproex (500 mg bid) resulted in small and statistically non-significant pharmacokinetic changes.

In Vitro Metabolic Characterization, Phenotyping, and Kinetic Studies of 9cUAB30, a Retinoid X Receptor-Specific Retinoid

The present study was conducted to compare the in vitro phase I and phase II metabolic profiles of (2E,4E,6Z,8E)-8-(3',4'-dihydro-1'(2'H)-naphthalen-1'-ylidene)-3,7-dimethyl-2,4,6-octatrienoic acid (9cUAB30) in human, rat, and dog microsomes and to characterize and identify the associated metabolic kinetics and specific isozymes from human liver microsomes (HLM) responsible for metabolism, respectively. Data from these experiments revealed that nine (M1-M9) phase I metabolites along with a single glucuronide conjugate were observed across the species investigated. With the exception of glucuronidation, no evidence of metabolism was detected for phase II enzymes (data not shown). Significant differences between species with regard to metabolic profile, stability, and gender were noted. For the eight phase I metabolites detected in HLM, the specific isozymes responsible for the biotransformations were CYP2C8, CYP2C9, and CYP2C19, with minor contributions from CYP1A2 and CYP2B6. For the glucuronide conjugate, UGT1A9 was the major catalyzing enzyme, with a minor contribution from UGT1A3. Kinetic analysis of eight of the detected metabolites indicated that four seemed to follow classical hyperbolic kinetics, whereas the remaining four showed evidence of either autoactivation or substrate inhibition.

Metabolism of capsaicinoids by P450 enzymes: a review of recent findings on reaction mechanisms, bio-activation, and detoxification processes

Capsaicinoids are botanical irritants present in chili peppers. Chili pepper extracts and capsaicinoids are common dietary constituents and important pharmaceutical agents. Use of these substances in modern consumer products and medicinal preparations occurs worldwide. Capsaicinoids are the principals of pepper spray self-defense weapons and several over-the-counter pain treatments as well as the active component of many dietary supplements. Capsaicinoids interact with the capsaicin receptor (a.k.a., VR1 or TRPV1) to produce acute pain and cough as well as long-term analgesia. Capsaicinoids are also toxic to many cells via TRPV1-dependent and independent mechanisms. Chemical modifications to capsaicinoids by P450 enzymes decreases their potency at TRPV1 and reduces the pharmacological and toxicological phenomena associated with TRPV1 stimulation. Metabolism of capsaicinoids by P450 enzymes also produces reactive electrophiles capable of modifying biological macromolecules. This review highlights data describing specific mechanisms by which P450 enzymes convert the capsaicinoids to novel products and explores the relationship between capsaicinoid metabolism and its effects on capsaicinoid pharmacology and toxicology.

CYP4B1: an enigmatic P450 at the interface between xenobiotic and endobiotic metabolism

CYP4B1 belongs to the mammalian CYP4 enzyme family that also includes CYP4A, 4F, 4V, 4X, and 4Z subfamilies. CYP4B1 shares with other CYP4 proteins a capacity to omega-hydroxylate medium-chain fatty acids, which may be related to an endogenous role for the enzyme. CYP4B1 also participates in the metabolism of certain xenobiotics that are protoxic, including valproic acid, 3-methylindole, 4-ipomeanol, 3-methoxy-4-aminoazobenzene, and numerous aromatic amines. Although these compounds have little in common structurally or chemically, their metabolism by CYP4B1 leads to tissue-specific toxicities in several experimental animals. The bioactivation capabilities of rabbit CYP4B1 have also attracted attention in the cancer community and form the basis of a potential therapeutic strategy involving prodrug activation by the CYP4B1 transgene. The metabolic capabilities of human CYP4B1 are less clear due to difficulties in heterologous expression and existence of alternatively spliced products. Also, many CYP4B1 enzymes covalently bind their heme, a posttranslational modification unique to the CYP4 family of P450s, but common to the mammalian peroxidases. These varied characteristics render CYP4B1 an interesting and enigmatic investigational target.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Transfection of Cytochrome P4504B1 into the Cornea Increases Angiogenic Activity of the Limbal Vessels

Injury to the ocular surface induces the production of the corneal epithelial-derived 12-hydroxyeicosatetrienoic acid (12-HETrE), which exhibits stereospecific potent inflammatory and angiogenic properties and is formed by a cytochrome P450 (P450) enzyme, CYP4B1. We have cloned the rabbit corneal CYP4B1 into the expression plasmid pIRES2-enhanced green fluorescent protein (EGFP) and examined the effect of CYP4B1 overexpression on corneal inflammation in vivo and limbal vessel sprouting ex vivo. Cultured rabbit corneal epithelial cells transfected with pIRES2-EGFP-CYP4B1 metabolized arachidonic acid to 12-HETrE at a rate five times higher than that of pIRES2-EGFP-transfected cells (3.53 +/- 0.08 versus 0.62 +/- 0.10 nmol/h/10(6) cells; mean +/- S.E.M., n = 6, p < 0.05), indicating a functional expression of the CYP4B1. Injection of either plasmid into the rabbit cornea resulted in EGFP fluorescence in the corneal epithelium. However, corneal neovascularization, as measured by the length of penetrating blood vessels, was significantly greater in the corneas of eyes transfected with the pIRES2-CYP4B1 compared with pIRES2-EGFP. Corneal-limbal explants from eyes transfected with pIRES2-CYP4B1 showed a marked angiogenic activity (46 +/- 10 versus 12 +/- 3 mm capillary length, n = 6, p < 0.05), which correlated with increased levels of 12-HETrE, the CYP4B1-derived angiogenic 12-hydroxyeicosanoid (0.93 +/- 0.18 versus 0.15 +/- 0.02 pmol/explant, n = 6, p < 0.05), and was inhibited (76 +/- 5%) by the P450 inhibitor 17-octadecynoic acid. The results further implicate the corneal CYP4B1 as a component of the inflammatory and angiogenic cascade initiated by injury to the ocular surface and raise the possibility of a new therapeutic target for preventing corneal neovascularization, namely, the CYP4B1-12-HETrE system.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Molecular mechanisms regulating human CYP4B1 lung-selective expression

Lung-selective cytochrome P450 expression is well recognized; however, little is known regarding regulatory mechanisms. To address this knowledge gap, transient expression of CYP4B1/luciferase constructs was used to identify a proximal, positively acting regulatory element, position -139 to -45, that functioned in all cells examined; a negatively acting element, position -457 to -216, that only functioned in HepG2 hepatoblastoma cells; and a distal, positively acting element, position -1087 to -1008, that functioned in A549 or BEAS-2B lung-derived cells but not HepG2 cells or 293 kidney-derived cells. Competitive electrophoretic mobility shift assays further localized specific A549, but not HepG2, nuclear protein binding to two sites within the distal element, CYP4B1 position -1052 to -1042 and -1026 to -1008. Several potential lung-selective transcription factor recognition sequences were identified within these elements. However, attempts to identify specific factor(s) were unsuccessful. In contrast, in vitro DNA/protein binding assays combined with transient expression and mutagenesis studies identified two functional Sephadex protein/Krüppel-like factor families of transcription factor sites within the proximal element (position -118 to -114 and position -77 to -73) that bound both Sephadex protein 1 (Sp1) and Sephadex protein 3 (Sp3) in vitro. Furthermore, Sp1-dependent synergistic regulation was observed in A549 cells involving the proximal and distal regulatory elements. Chromatin immunoprecipitation assays demonstrated binding of neither Sp1 nor Sp3 to the CYP4B1 proximal element in human liver tissue, whereas selective Sp1 binding was observed in human lung tissue. Thus, the composite findings are consistent with both the proximal Sp1 elements and the distal regulatory element acting to synergistically control CYP4B1 lung-selective expression.

Structural and enzymatic parameters that determine alkyl dehydrogenation/hydroxylation of capsaicinoids by cytochrome p450 enzymes

Previous studies on the metabolism of capsaicinoids, natural products isolated from chili peppers, demonstrated the production of unique macrocyclic, alkyl dehydrogenated, omega-, and omega-1-hydroxylated products. This study investigated the structural and enzymatic parameters that direct selective alkyl dehydrogenation and hydroxylation of capsaicinoids, using a variety of structurally related capsaicinoid analogs and cytochrome P450 (P450) enzymes. CYP2C9 preferentially catalyzed alkyl dehydrogenation, whereas CYP2E1 and 3A4 catalyzed omega- and omega-1-hydroxylation, respectively. Analysis of incubations containing various P450s and structural variants of capsaicin by liquid chromatography-tandem mass spectrometry demonstrated similarities in the rate of capsaicinoid metabolism, but marked differences in the metabolite profiles. Production of macrocyclic and omega-1-hydroxylated metabolites from the various capsaicinoids was dependent on the structure of the alkyl terminus and P450 enzyme. A tertiary carbon at the omega-1 position, coupled to an adjacent unsaturated bond at the omega-2,3 position, enhanced the formation of the macrocyclic and dehydrogenated metabolites and were requisite structural features for omega-1-hydroxylated product formation. Conversely, substrates lacking these structural features were efficiently oxidized to the omega-hydroxylated metabolite. These data were consistent with our hypothesis that metabolism of the alkyl portion of capsaicinoids was governed, in part, by the stability and propensity to form an intermediate radical and a carbocation, and a direct interaction between the alkyl terminus and the heme of many P450 enzymes. These results provided valuable insights into potential mechanisms by which P450s metabolize capsaicinoids and highlight critical chemical features that may also govern the metabolism of structurally related compounds including fatty acids, monoter-penes, and isoprenoids.

Dose-related pharmacokinetics and pharmacodynamics of valproate in the elderly

Valproate exhibits a complex pharmacokinetic profile due to concentration-dependent protein binding and clearance. It has been shown that the protein binding of valproate decreases as the serum concentration increases in a young adult population. Furthermore, the percentage of protein binding is lower in the elderly compared with young adults at comparable low therapeutic serum concentrations. The extent of valproate protein binding at higher concentrations in the elderly has not been described. Studies conducted in the elderly have found unbound valproate clearance to be decreased compared with younger adults, although these changes in clearance have not been evaluated at higher therapeutic serum concentrations. We evaluated the pharmacokinetics of valproate (protein binding and clearance) across a wide dosage range in the elderly and measured the impact of this on drug-related side effects using a single-blind within-subject study design in 6 healthy elderly volunteers (aged 65-76 years). Steady-state total and unbound serum valproate concentrations were assessed at 3 doses: 500, 1000, and 1500 mg/d. As doses and valproate serum concentrations increased, the unbound fraction (10.0%, 13.0%, 17.4%) and total clearance (4.8, 6.0, 6.7 mL/h/kg) increased, respectively. Unbound clearance decreased (49.4, 45.8, 39.4 mL/h/kg) with increasing valproate serum concentrations. Drug-induced CNS effects and nausea severity scores correlated with total and unbound serum valproate concentrations. Significant dose-dependent changes in valproate pharmacokinetics were observed in the elderly.

Characterization of pulmonary CYP4B2, specific catalyst of methyl oxidation of 3-methylindole

The selective toxicity of chemicals to lung tissues is predominantly mediated by the selective expression of certain pulmonary cytochrome P450 enzymes. This report describes the purification, cloning, and characterization of a unique enzyme, CYP4B2, from goat lung. The purified P450 enzyme was isolated by multistep ion exchange chromatography to electrophoretic homogeneity with an apparent molecular mass of 55,000 Da. Western blotting studies demonstrated that CYP4B enzymes were selectively expressed in lung tissues of rabbits, rats, and mice. Two cDNAs, CYP4B2 and CYP4B2v, were cloned from goat lung tissue. CYP4B2 was predicted to be 511 amino acids and approximately 82% similar to the four known CYP4B1 proteins. Concurrently, a variant of the known human CYP4B1 cDNA, that contained a S207 insertion, was cloned from human lung tissue. The modified recombinant goat CYP4B2 was expressed in Escherichia coli and the enzyme catalyzed the N-hydroxylation of the prototypical substrate 2AF. CYP4B2 preferentially dehydrogenated, rather than hydroxylated, the pneumotoxicant 3-methylindole (3MI) (V(max) = 4.61 versus 0.83 nmol/nmol of P450/min, respectively). To investigate the relevance of covalent heme binding of CYP4 enzymes in CYP4B2-mediated metabolism of 3MI, a site-directed mutant (CYP4B2/A315E) was evaluated. The mutation had little effect on the V(max) of either dehydrogenation or hydroxylation but increased the K(m), which decreased the catalytic efficiency (V/K) for 3MI. The A315E mutation shifted the absorbance maximum of the enzyme from 448 to 451 nm, suggesting that the electron density of the heme was altered. These results demonstrate that CYP4B2 is highly specific for methyl group oxidation of 3MI, without formation of ring-oxidized metabolites, and seems to be predominately responsible for the highly organ-specific toxicity of 3MI in goats.

Genotyping and site-directed mutagenesis of a cytochrome P450 meander Pro-X-Arg motif critical to CYP4B1 catalysis

CYP4B1 isoforms from rodents and other common laboratory animals are involved in the bioactivation of a range of protoxins, including 2-aminofluorene, 4-ipomeanol, and valproic acid. However, an earlier study provided evidence for a human allele encoding a nonfunctional CYP4B1 enzyme due to a Pro427Ser transversion in the meander region of the protein. In the present study, the CYP4B1 gene from several racial groups, Caucasians, African-Americans, and Hispanics, and from six nonhuman primate species was genotyped using a PCR-Hinf1 restriction enzyme fragment length polymorphism assay or by direct sequencing. All human populations examined were found to possess only the Ser allele at codon 427 ((1279)TCT) and all of the nonhuman primate species possessed only the Pro (CCT) allele. Therefore, an inactivating (1279)C-->T mutation in the human CYP4B1 gene likely arose following divergence of the Homo and Pan clades. Amino acid sequence alignments revealed further that this key Pro residue is located two amino acid residues N-terminal to the distal Arg of a Glu-Arg-Arg triad thought to participate in heme binding and/or redox partner interactions. Mutation of the corresponding Arg424 residue in rabbit CYP4B1 to Leu, but not His, resulted in a loss of lauric acid hydroxylase activity and ability to generate a reduced-CO binding spectrum. These data provide additional evidence for the importance of this meander region Pro-X-Arg motif in CYP4B1 heme binding and catalytic function.

Genetic polymorphism of the human cytochrome P450 CYP4B1: evidence for a non-functional allelic variant

In the present study, we report the first systematic investigation of polymorphism in the human CYP4B1 gene. Using a strategy based on single-strand conformation polymorphism analysis of PCR products (PCR-SSCP), we analyzed the twelve exons of the gene, as well as their 5'- and 3'- proximal flanking sequences, in DNA samples from 190 French Caucasians. In addition to the wild-type CYP4B1* allele (CYP4B1*1), four variants, namely CYP4B1*2, *3, *4 and *5, were characterized. The CYP4B1*3, *4 and *5 alleles encode missense mutations Arg173Trp, Ser322Gly and Met331Ile, respectively. The fourth variant, CYP4B1*2, harbors three missense mutations (Met331Ile, Arg340Cys and Arg375Cys) and a double nucleotide deletion (AT881-882del) that causes a frameshift and premature stop codon in the second third of the coding sequence of the gene. This latter mutation can be assumed to lead to the synthesis of a severely truncated protein and, therefore, probably contributes to interindividual variability of CYP4B1 expression and enzymatic activity. In order to investigate the extent of the CYP4B1*2 allele in a large population, a rapid genotyping test, based on restriction analysis of PCR products, was developed and applied to 2082 French Caucasians. Forty-two subjects were found homozygous for the AT881-882 deletion, which suggests that about 2% of individuals should be unable to develop metabolic reactions mediated by CYP4B1. Given the relatively high frequency and the functional consequences of the CYP4B1*2 allele, associations between CYP4B1 polymorphism and certain pathological processes should be considered.

Methsuximide reduces valproic acid serum levels

The authors investigated whether methsuximide affects serum levels of valproic acid. Pre-morning-dose serum valproic acid levels were measured in 17 patients (12 male, 5 female; age range, 8-19; mean, 14.5 years) who either started or stopped taking methsuximide but whose dose of valproate and other medication remained unchanged. Four of these patients both started and stopped taking methsuximide. For the whole group the mean valproic acid level (+/- standard error) while not taking methsuximide was 81.9 +/- 5.3 mg/L and while taking methsuximide was 55.7 +/- 4.3 mg/L. The difference between the means was highly significant (paired t test, p < 0.001). The mean valproic acid serum level before taking methsuximide was 85.4 +/- 4.5 mg/L (14 patients), which decreased to 58.2 +/- 4.8 mg/L while taking methsuximide (difference highly significant, p < 0.001). In the eight patients who stopped taking methsuximide the mean serum level increased from 49.8 +/- 7.5 mg/L to 71.7 +/- 8.5 mg/L (difference significant p = 0.025). Because methsuximide reduces valproic acid serum levels, it may be necessary to increase the valproate dose when methsuximide is added or reduce the valproate dose when methsuximide therapy stops, to avoid loss of seizure control or valproate toxicity respectively.

In vitro evaluation of valproic acid as an inhibitor of human cytochrome P450 isoforms: preferential inhibition of cytochrome P450 2C9 (CYP2C9)

AIMS

To evaluate the potency and specificity of valproic acid as an inhibitor of the activity of different human CYP isoforms in liver microsomes.

METHODS

Using pooled human liver microsomes, the effects of valproic acid on seven CYP isoform specific marker reactions were measured: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), tolbutamide hydroxylase (CYP2C9), S-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1) and midazolam 1'-hydroxylase (CYP3A4).

RESULTS

Valproic acid competitively inhibited CYP2C9 activity with a Ki value of 600 microM. In addition, valproic acid slightly inhibited CYP2C19 activity (Ki = 8553 microM, mixed inhibition) and CYP3A4 activity (Ki = 7975 microM, competitive inhibition). The inhibition of CYP2A6 activity by valproic acid was time-, concentration- and NADPH-dependent (KI = 9150 microM, Kinact=0.048 min(-1)), consistent with mechanism-based inhibition of CYP2A6. However, minimal inhibition of CYP1A2, CYP2D6 and CYP2E1 activities was observed.

CONCLUSIONS

Valproic acid inhibits the activity of CYP2C9 at clinically relevant concentrations in human liver microsomes. Inhibition of CYP2C9 can explain some of the effects of valproic acid on the pharmacokinetics of other drugs, such as phenytoin. Co-administration of high doses of valproic acid with drugs that are primarily metabolized by CYP2C9 may result in significant drug interactions.

Differential effect of valproate and its Delta2- and Delta4-unsaturated metabolites, on the beta-oxidation rate of long-chain and medium-chain fatty acids

Overall fatty acid oxidation rates were investigated in rat hepatocytes using [9,10-3H]-palmitic, [9,10-3H]-oleic, [9,10-3H]-myristic and [2,3-3H]-phenylpropionic acids. The effect of both valproate (VPA) (0-10 mM) and two of its unsaturated metabolites, Delta2(E)-VPA and Delta4-VPA (0-10 mM), on the overall 3H2O production rate was studied. The results give evidence of a general inhibitory effect of VPA on the beta-oxidation rate of all the tested substrates. Similar effects were observed with both VPA metabolites but these effects appeared to be dependent on the chain length of the substrate. When the effect on the oxidation of the medium-chain fatty acid 3-phenylpropionate (PPA) was studied, Delta2(E)-VPA at 0.5 mM caused a 94% inhibition of the overall beta-oxidation rate. However, with long-chain substrates, 0.5 mM Delta(4)-VPA was a more potent inhibitor (20-30% of control activity) than 0.5 mM Delta(2E)-VPA (60-80% of control activity). Our results suggest that VPA and/or its metabolites inhibit fatty acyl-CoA metabolism within the mitochondrion by two different mechanisms. The first mechanism involves CoASH sequestration, which affects the oxidation rate of all fatty acids with different chain length. The second mechanism is more specific in nature and involves selective inhibition of particular enzymes implicated in fatty acid beta-oxidation.

Valproate metabolism during valproate-associated hepatotoxicity in a surviving adult patient

The plasma profiles of valproate (VPA), its beta-oxidation metabolites E-2-en-VPA and 3-oxo-VPA and its terminal desaturation metabolite 4-en-VPA, have been measured in a patient receiving NaVPA 1000 mg twice per day from early in the course of serious hepatotoxicity and for 2 weeks after the drug was stopped. Concurrent profiles of liver, renal and haematological function parameters were available. Relative to concurrent plasma VPA concentrations, E-2-en-VPA concentrations were not different to those of the VPA-treated epileptic population at any stage of the illness, whereas 3-oxo-VPA concentrations relative to concurrent VPA concentrations were abnormally high early in the toxicity, abnormally low at its peak (3-5 days later), and comfortably within normal limits for the treated epileptic population late in the recovery phase (9-13 days from the onset). When measurable, plasma 4-en-VPA concentrations were not elevated. The elimination half-life of VPA during the recovery phase was 100 h, which is some 6-12 times greater than values reported for this parameter in normal patients. These data clearly define, in this patient, a link between idiosyncratic VPA-associated hepatotoxicity at its onset and peak and the later stages of VPA beta-oxidation. Whether the beta-oxidation abnormalities are causative or a consequence of an as yet undefined defect is unknown. In this patient, 4-en-VPA was unlikely to have been involved in the pathogenesis of the toxicity.