Showing papers in "Drug Metabolism Reviews in 1999"

Structural and functional studies of udp-glucuronosyltransferases*

Abstract: UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Oxidant mechanisms in toxic acute renal failure

Abstract: Over the last decade, there is accumulating evidence for a role of reactive oxygen metabolites in the pathogenesis of a variety of renal diseases, including gentamicin, glycerol, cisplatin, and cyclosporine A models of toxic acute renal failure. Gentamicin has been shown both in in vitro and in vivo studies to enhance the generation of reactive oxygen metabolites. Iron is important in models of tissue injury, presumably because it is capable of catalyzing free-radical formation. Gentamicin has been shown to cause release of iron from renal cortical mitochondria. Scavengers of reactive oxygen metabolites as well as iron chelators provide protection in gentamicin-induced nephrotoxicity. In glycerol-induced acute renal failure, an animal model of rhabdomyolysis, there is enhanced generation of hydrogen peroxide, and scavengers of reactive oxygen metabolites and iron chelators provide protection. Although the dogma is that the myoglobin is the source of iron, recent studies suggest that cytochrome P450 may be an important source of iron in this model. In addition, there are marked alterations in antioxidant defenses, such as glutathione, as well as changes in heme oxygenase. Several recent in vitro and in vivo studies indicate an important role of reactive oxygen metabolites in cisplatin-induced nephrotoxicity. Thus, catalytic iron is increased both in vitro and in vivo by cisplatin, and iron chelators as well as hydroxyl radical scavengers have been shown to be protective. Recent studies indicate that cytochrome P450 may also be an important source of the catalytic iron in cisplatin nephrotoxicity. Cyclosporine A has been shown to enhance generation of hydrogen peroxide in vitro and enhance lipid peroxidation in vitro and in vivo. Antioxidants have been shown to be protective in cyclosporine A nephrotoxicity. This collective body of evidence suggests an important role for reactive oxygen metabolites in toxic acute renal failure and may provide therapeutic opportunities of preventing or treating acute renal failure in humans.

The role of cyp2b6 in human xenobiotic metabolism

Abstract: The cytochromes P450 (CYP) comprise a large multigene family of heme-thiolate proteins which are of considerable importance in the metabolism of xenobiotics and endobiotics. Over 500 CYP genes have...

The isoprostanes: unique prostaglandin-like products of free-radical-initiated lipid peroxidation*

Abstract: The discovery of IsoPs as products of nonenzymatic lipid peroxidation has opened up new areas of investigation regarding the role of free radicals in human physiology and pathophysiology. The quantification of IsoPs as markers of oxidative stress status appears to be an important advance in our ability to explore the role of free radicals in the pathogenesis of human disease. An important need in the field of free-radical medicine is information regarding the clinical pharmacology of antioxidant agents. Because of the evidence implicating free radicals in the pathogenesis of a number of human diseases, large clinical trials are planned or underway to assess whether antioxidants can either prevent the development or ameliorate the pathology of certain human disorders. However, data regarding the most effective doses and combination of antioxidant agents to use in these clinical trials is lacking. As mentioned previously, administration of antioxidants suppresses the formation of IsoPs, even in normal individuals. Thus, measurement of IsoPs may provide a valuable approach to define the clinical pharmacology of antioxidants. In addition to being markers of oxidative stress, several IsoPs possess potent biological activity. The availability of additional IsoPs in synthetic form should broaden our knowledge concerning the role of these molecules as mediators of oxidant stress. Despite the fact that considerable information has been obtained since the initial report of the discovery of IsoPs [6], much remains to be understood about these molecules. With continued research in this area, we believe that much new information will emerge that will open up additional important new areas for future investigation.

Interspecies differences in cancer susceptibility and toxicity

Abstract: One of the most complex challenges to the toxicologist represents extrapolation from laboratory animals to humans. In this article, we review interspecies differences in metabolism and toxicity of heterocyclic amines, aflatoxin B1, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and related compounds, endocrine disrupters, polycyclic aromatic hydrocarbons, tamoxifen, and digitoxin. As far as possible, extrapolations to human toxicity and carcinogenicity are performed. Humans may be more susceptible to the carcinogenic effect of heterocyclic amines than monkeys, rats, and mice. Especially, individuals with high CYP1A2 and 3A4 activities and the rapid acetylator phenotype may be expected to have an increased risk. Striking interspecies variation in susceptibility to aflatoxin B1 carcinogenesis is known, with rats representing the most sensitive and mice the most resistant species, refractory to dietary levels three orders of magnitude higher than rats. An efficient conjugation with glutathione, catalyzed by glutathione S-transferase mYc, confers aflatoxin B1 resistance to mice. Extremely large interspecies differences in TCDD-induced toxicity are known. The guinea pig is the most susceptible mammal known, with an LD50 in the range 1-2 micrograms TCDD/kg, whereas the hamster is the most resistant species with an LD50 greater than 3000 micrograms/kg. A number of experts have pointed out to the fact that humans appear to be less sensitive to TCDD than most laboratory animals. Human exposure to background levels of TCDD is not likely to cause an incremental cancer risk. A clear cause--effect relationship has been shown between environmental endocrine-disrupting contaminants and adverse health effects in wildlife, whereas the effects seem to be less critical for humans. Studies on DNA adduct formation and metabolism of the nonsteroidal antiestrogen tamoxifen indicate that rats and mice are orders of magnitude more susceptible than humans.

P450 subfamily cyp2j and their role in the bioactivation of arachidonic acid in extrahepatic tissues

Abstract: Historically, there has been intense interest in P450 metabolic oxidation, peroxidation, and reduction of xenobiotics. More recently, there has been a growing appreciation for the role of P450s in the oxidation of lipophilic endobiotics, such as bile acids, fat-soluble vitamins, and eicosanoids. This review details the emerging CYP2J subfamily of P450s and their role as catalysts of arachidonic acid metabolism.

Genetic epidemiology of environmental toxicity and cancer susceptibility: human allelic polymorphisms in drug-metabolizing enzyme genes, their functional importance, and nomenclature issues.

Abstract: Pharmacogenetics is the study of idiosyncratic drug responses that have an hereditary basis and usually reflect differences in drug-metabolizing enzymes (DMEs) and the receptors that control DME levels. The purpose of this review is to provide a brief overview of recent findings concerning more than a dozen clinically important polymorphisms and to emphasize the need to standardize the nomenclature of these alleles in each polymorphism, as quickly as possible. This nomenclature system should be consistent with the Human Gene Nomenclature Guidelines. Because DMEs have existed before divergence of prokaryotes and eukaryotes more than 2 billion years ago, it is clear that DME genes first must have evolved for critical life functions and that, more recently in animals, DME genes expanded to include the role of detoxification of dietary products, evolving plant metabolites, and, of course, pharmaceutical drugs. Many human DME polymorphisms are relevant to clinical problems in that they represent the basis of risk factors in the development of cancer, toxicity, and other diseases associated with drug, chemical, or dietary exposure. The study of the relationship among human genetic polymorphisms, cancer susceptibility, toxicity, and environmental exposure is a new and exciting area of research--which will undoubtedly have increasingly important implications for risk assessment and the prevention, early diagnosis, and intervention of clinical disease.

Arylamine n-acetyltransferase activity in man

Abstract: Hepatic arylamine N-acetyltransferase 2 (NAT2) is one of most frequently phenotyped polymorphic drug-metabolizing enzymes. Both human N-acetyltransferases, NAT1 and NAT2, act as major enzymes in th...

FK506 and the role of the immunophilin FKBP-52 in nerve regeneration.

Abstract: In summary, FKBP-12 does not mediate the neurite outgrowth-promoting properties of neuroimmunophilin ligands (e.g., FK506). Instead, the neurotrophic properties of neuroimmunophilin ligands (FK506) and steroid hormones are mediated by disruption of steroid-receptor complexes. It remains unclear which component mediates neurite outgrowth, although the most likely candidates are FKBP-52, hsp-90, and p23 [42]. Regardless of the underlying mechanism involved, the FKBP-52 antibody data reveal that it should be possible to design, based on the structure of FK506, non-FKBP-12-binding (nonimmunosuppressant) compounds selective for FKBP-52 and test these new libraries for their ability to augment nerve regeneration. It may also be possible to exploit the structure of geldanamycin to develop a new class of hsp-90-binding compounds for use in nerve regeneration.

Food toxins, ampa receptors, and motor neuron diseases.

Abstract: Environmental chemicals involved in the etiology of human neurodegenerative disorders are challenging to identify. Described here is research designed to determine the etiology and molecular pathog...

Peroxynitrite inactivates prostacyclin synthase by heme–thiolate-catalyzed tyrosine nitration

Abstract: Previous work has shown a sensitive inhibition of prostacyclin synthase activity by peroxynitrite as well as by superoxide in the presence of NO donors Neither superoxide nor NO alone nor decomposed peroxynitrite is effective The inhibition of activity was paralleled by a nitration of a tyrosine residue and both could be prevented by a stable substrate analog The same IC50 value for peroxynitrite was also found for the cellular prostacyclin activity in endothelial and kidney mesangial cells, indicating that the antioxidant potential of the cell cannot prevent the inactivation Aortic tissue shows a co-localization of prostacyclin synthase and nitrotyrosine staining after treatment of the tissue with 1 μM peroxynitrite It can be speculated that this pathway of enzyme nitration is of pathophysiological significance

Cytochrome p450-based cancer gene therapy: recent advances and future prospects

Abstract: Cytochrome P450-based cancer gene therapy is a novel prodrug activation strategy for cancer treatment that has substantial potential for improving the safety and efficacy of cancer chemotherapeutics. The primary goal of this strategy is to selectively increase tumor cell exposure to cytotoxic drug metabolites generated locally by a prodrug-activating P450 enzyme. This strategy has been exemplified for the alkylating agents cyclophosphamide and ifosfamide, which are bioactivated by select P450 enzymes whose expression is generally high in liver and deficient in tumor cells. Transduction of tumors with a prodrug-activating P450 gene, followed by prodrug treatment, greatly increases intratumoral formation of activated drug metabolites. This leads to more efficient killing of the transduced tumor cells without a significant increase in host toxicity. P450 gene therapy is accompanied by substantial bystander cytotoxicity which greatly enhances the therapeutic effect by extending it to nearby tumor cells not tr...

Modulation of drug metabolism in infectious and inflammatory diseases

Abstract: During inflammation, expression of various P450 genes is modulated differentially. Both the downregulation of some P450s and the induction of others may be the result of a complex interaction involving inflammatory cytokines, stress hormones, and metabolic perturbations. Our results suggest that NO is not an important mediator for the downregulation of CYP2C11 in rat liver.

Duplication, multiduplication, and amplification of genes encoding drug-metabolizing enzymes: evolutionary, toxicological, and clinical pharmacological aspects.

Abstract: Genetic polymorphism of drug-metabolizing enzymes is well established and now encompasses more than 20 different phase 1 and phase 2 enzymes. Important interethnic differences are apparent in the d...

Functions and transcriptional regulation of pah-inducible human udp-glucuronosyl-transferases

Abstract: It is a pleasure to contribute to the symposium in honor of Professor Alfred Hildebrandt at the occasion of his 60th birthday In his thesis, Professor Hildebrandt was one of the first collaborator

Damage and repair of nerve cell DNA in toxic stress.

Abstract: It is generally agreed that ALS/PDC is triggered by a disappearing environmental factor peculiar to the lifestyle of people of the western Pacific (i.e., Guam, Irian Jaya, Indonesia, and the Kii Peninsula of Japan). A strong candidate is the cycad plant genotoxin cycasin, the beta-D-glucoside of methylazoxymethanol (MAM). We propose that prenatal or postnatal exposure to low levels of cycasin/MAM may damage neuronal DNA, compromise DNA repair, perturb neuronal gene expression, and irreversibly alter cell function to precipitate a slowly evolving disease ("slow-toxin" hypothesis). In support of our hypothesis, we have demonstrated the following: 1. DNA from postmitotic rodent central nervous system neurons is particularly sensitive to damage by MAM. 2. MAM reduces DNA repair in human and rodent neurons, whereas DNA-repair inhibitors potentiate MAM-induced DNA damage and toxicity in mature rodent nervous tissue. 3. Human neurons (SY5Y neuroblastoma) that are deficient in DNA repair are susceptible to MAM-induced cytotoxicity and DNA damage, whereas overexpression of DNA repair in similar cells is protective. 4. MAM alters gene expression in SY5Y human neuroblastoma cells and, in the presence of DNA damage and reduced DNA repair, enhances glutamate-modulated expression of tau mRNA in rat primary neurons; the corresponding protein (TAU) is elevated in ALS/PDC and Alzheimer's disease. These findings support a direct relationship between MAM-induced DNA damage and neurotoxicity and suggest the genotoxin may operate in a similar manner in vivo. More broadly, a combination of genotoxin-induced DNA damage (via exogenous and/or endogenous agents) and disturbed DNA repair may be important contributing factors in the slow and progressive degeneration of neurons that is characteristic of sporadic neurodegenerative disease. Preliminary studies demonstrate that DNA repair is reduced in the brain of subjects with western Pacific ALS/PDC, ALS, and Alzheimer's disease, which would increase the susceptibility of brain tissue to DNA damage by endogenous/exogenous genotoxins. Interindividual differences in the extent of prior exposure to DNA-damaging agents and/or the efficiency of its repair might produce population variety in the rate of damage accumulation and explain the susceptibility of certain individuals to sporadic neurodegenerative disease. Studies are underway using DNA-repair proficient and deficient neuronal cell cultures and mutant mice to explore gene-environment interplay with respect to MAM treatment, DNA damage, and DNA repair, and the age-related appearance of neurobehavioral and neuropathological compromise.

Kinetic and chemical mechanism of epoxide hydrolase.

Abstract: Epoxide hydrolases (EC 3.3.2.3) catalyze the trans-antiperiplanar addition of water to epoxides and arene oxides generated in the oxidation of unsaturated hydrocarbons [1][2][3]. As a consequence, ...

Role of Kupffer cells in peroxisome proliferator-induced hepatocyte proliferation.

Abstract: Peroxisome proliferators are a group of structurally diverse compounds that cause an increase in both the number and size of peroxisomes, elevate rates of cell proliferation, and cause liver cancer...

Frontier orbitals in chemical and biological activity: quantitative relationships and mechanistic implications*

Abstract: The frontier orbital theory of chemical reactivity was originally outlined by Fukui in the early 1950s (summarized in Ref. [1]) and was further developed by Klopman and Hudson [2][3][4] and by Sale...

Merits and limitations of recombinant models for the study of human P450-mediated drug metabolism and toxicity: an intralaboratory comparison.

Abstract: A wide variety of pharmacological and toxicological properties of drugs are determined by cytochrome P450-mediated metabolism. Characterization of these pathways and of the P450 isoenzymes involved constitutes an essential part of drug development. Similarly, because P450s are catalyzing the toxication and detoxication of environmental pollutants, an understanding of these reactions facilitates risk assessment in environmental toxicology. Recently, a variety of recombinant expression systems has been employed to study the role of human P450s in these reactions. These include insect, bacterial, yeast, and mammalian models. As these were developed and characterized by different laboratories, evaluation of their merits and limitations is inherently difficult. To resolve this problem, we have established and characterized the latter three systems and present the key results here. In general, the catalytic properties of P450 isozymes in the various models were rather similar. However, taking technical considerations into account as well as the high level of functional expression of P450s achieved in bacteria make this system ideally suited for drug metabolism research, including the generation of milligram quantities of metabolites for structural determinations. For toxicological studies, however, expression of P450s in mammalian cells was most appropriate. This is exemplified here by studies into the role of human P450s in the activation and inactivation of chemotherapeutic drugs.

Trichloroethylene activates cd4+ t cells: potential role in an autoimmune response*

Abstract: Trichloroethylene is an industrial solvent and has become a major environmental contaminant. Autoimmune-prone MRL +/+ mice were treated for up to 22 weeks with trichloroethylene in the drinking water (0, 2.5, and 5.0 mg/mL) in order to study the immunoregulatory effects of this environmental toxicant. After only 4 weeks of treatment, trichloroethylene was shown to promote the expansion of CD4+ T cells that expressed a memory/activation phenotype (i.e., CD44hi CD45RBlo) and secreted high levels of IFN-gamma, but not IL-4. In addition, trichloroethylene treatment accelerated the development of an autoimmune response in the MRL +/+ mice as evidenced by an earlier appearance of antinuclear antibodies and increased levels of total IgG2a. MRL +/+ mice treated with trichloroethylene for 22 weeks also contained antibodies specific for trichloroethylene adducts, suggesting the activation of trichloroethylene-specific T cells. The results suggest that trichloroethylene can stimulate antigen nonspecific as well as specific T cells that are capable of promoting autoimmunity in genetically predisposed individuals.

Extrapolating in vitro data on drug metabolism to in vivo pharmacokinetics: evaluation of the pharmacokinetic interaction between amitriptyline and fluoxetine.

Abstract: Recently, models have been proposed to extrapolate in vitro data on the influence of inhibitors on drug metabolism to in vivo decrement in drug clearance. Many factors influence drug clearance such as age, gender, habits, diet, environment, liver disease, heredity, and other drugs. In vitro investigation of hepatic cytochrome P450 activity has generally centered on genetic influences and interactions with other drugs. This group of enzymes is involved in many, although not all, drug interactions. The interaction of amitriptyline and fluoxetine is an example. Of the different in vitro paradigms, interaction studies utilizing human liver microsomal preparations have proved to be the most generally applicable for in vitro scaling models. Assuming Michaelis—Menten conditions and applying nonlinear regression, a hybrid inhibition constant (Ki) can be generated that allows classification of the inhibitory potency of an inhibitor toward a specific reaction. This constant is largely independent of the substrate c...

Regulation of endoplasmic reticulum biogenesis in response to cytochrome P450 overproduction.

Abstract: Most of the eukaryotic cytochrome P450 forms are integral membrane proteins of the endoplasmic reticulum (ER). They are targeted to the ER and inserted into the membrane by uncleavable signal-ancho...

Kinetics of hydrolysis and reaction of aflatoxin B1 exo-8,9-epoxide and relevance to toxicity and detoxication.

Abstract: Aflatoxin (AF) toxicity is a serious health risk in several parts of the world. Aflatoxin B1 (AFB1) was originally discovered in studies on the acute toxicity of moldy grain to turkeys. Subsequentl...

Cytochrome p450-null mice

Abstract: In 1995, the first report on the establishment of a cytochrome P450 (P450)-null mouse line appeared [1]. Since then, several other mouse lines lacking P450s have been produced. The total number of ...

The mechanism of bioactivation of N-nitrosodiethanolamine.

Abstract: N-Nitrosodiethanolamine (NDELA) (Structure 1), is one of the most widespread nitrosamines in the human environment because it forms from diethanolamine, triethanolamine, and various derivatives the...

Role of canonical glucocorticoid responsive elements in modulating expression of genes regulated by the arylhydrocarbon receptor.

Abstract: A number of genes involved in foreign compound metabolism are inducibleby polycyclic aromatic hydrocarbons and the study of their molecular regulationis an active area of research. The prototypical...

Metabolism-based drug interactions involving oral azole antifungals in humans.

Abstract: AUC:Area under the plasma concentration–time curveCmax:Maximum plasma concentrationCmin:Minimum plasma concentrationCss:Plasma concentration at steady stateCl:Total clearance from plasmaClH:Hepatic...