Effect of phenethyl isothiocyanate on microsomal N-nitrosodimethylamine metabolism and other monooxygenase activities

A comprehensive review of cytochrome P450 2E1 for xenobiotic metabolism

Cytochrome P450 2E1 (CYP2E1) plays a vital role in drug-induced hepatotoxicity and cancers (e.g. lung and bladder cancer), since it is responsible for metabolizing a number of medications and environmental toxins to reactive intermediate metabolites. CYP2E1 was recently found to be the highest expressed CYP enzyme in human livers using a proteomics approach, and CYP2E1-related toxicity is strongly associated with its protein level that shows significant inter-individual variability related to ethnicity, age, and sex. Furthermore, the expression of CYP2E1 demonstrates regulation by extensive genetic polymorphism, endogenous hormones, cytokines, xenobiotics, and varying pathological states. Over the past decade, the knowledge of pharmacology, toxicology, and biology about CYP2E1 has grown remarkably, but the research progress has yet to be summarized. This study presents a timely systematic review on CYP2E1's xenobiotic metabolism, genetic polymorphism, and inhibitors, with the focus on their clinical relevance for the efficacy and toxicity of various CYP2E1 substrates. Moreover, several knowledge gaps have been identified towards fully understanding the potential interactions among different CYP2E1 substrates in clinical settings. Through in-depth analyses of these knowns and unknowns, we expect this review will aid in future drug development and improve management of CYP2E1 related clinical toxicity.

A principal mechanism for the cancer chemopreventive activity of phenethyl isothiocyanate is modulation of carcinogen metabolism

Isothiocyanates are small molecules characterized by high chemical reactivity that allows them to interact readily with cellular constituents eliciting a plethora of biological activities. They are present exclusively in cruciferous vegetables, as glucosinolates, the intake of which has been associated with cancer chemoprevention. When the physical structure of these vegetables is disturbed, e.g. during mastication, the enzyme myrosinase is released and converts the glucosinolates to isothiocyanates (R-N=C=S), where R can be aliphatic or aromatic. Although sulforaphane, an aliphatic isothiocyanate, has received most attention worldwide, the most extensively studied aromatic isothiocyanate is phenethyl isothiocyanate (PEITC), and there are substantial differences in biological activity between the two sub-classes. In animal cancer models, PEITC effectively antagonized the carcinogenicity of chemicals, especially nitrosocompounds. A principal mechanism of their action is to protect the integrity of DNA by decreasing the levels of the genotoxic metabolites of chemical carcinogens. Extensive studies established that PEITC modulates the metabolism of the tobacco-specific carcinogenic nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) by inhibiting its cytochrome P450-mediated bioactivation. Moreover, PEITC is a potent inducer of detoxification enzymes such as quinone reductase, glutathione S-transferase and glucuronosyl transferase. PEITC is rapidly absorbed and is characterized by a large bioavailability; Cmax concentrations achieved in plasma after dietary intake are sufficient to modulate carcinogen metabolism. PEITC is primarily metabolized by glutathione conjugation and is excreted in the urine and bile as the mercapturate. The ability of PEITC to perturb carcinogen metabolism through modulation of cytochrome P450 and phase II detoxification enzymes is comprehensively and critically reviewed.

Pharmacokinetics and pharmacodynamics of phenethyl isothiocyanate: implications in breast cancer prevention

Phenethyl isothiocyanate (PEITC)-a naturally occurring isothiocyanate in cruciferous vegetables-has been extensively studied as a chemopreventive agent in several preclinical species and in humans. Pharmacokinetic features of unchanged PEITC are (I) linear and first-order absorption, (II) high protein binding and capacity-limited tissue distribution, and (III) reversible metabolism and capacity-limited hepatic elimination. Membrane transport of PEITC is mediated by BCRP, multidrug resistance-associated protein (MRP) 1, and MRP2 transporters belonging to the ATP-binding-cassette (ABC) family. PEITC is metabolized by glutathione S-transferase (GST) in the liver, with the glutathione conjugate of PEITC undergoing further conversion to mercapturic acid by N-acetyl transferase in rats and humans. PEITC modulates the activity and expression of numerous phase I and phase II drug-metabolizing enzymes and can inhibit the metabolism of procarcinogens to form carcinogens and increase carcinogen elimination. In recent years, several in vitro and in vivo studies have elucidated molecular mechanisms underlying the pharmacodynamics of PEITC in breast cancer that include cancer cell apoptosis by upregulation of apoptotic genes, cell cycle arrest at G2/M phase by generation of reactive oxygen species and depletion of intracellular glutathione, downregulation of the estrogen receptor, decrease in sensitivity to estrogen, and inhibition of tumor metastasis. Inhibition of angiogenesis is one of the recently reported mechanisms of breast cancer prevention by PEITC. Complex pharmacokinetics and pharmacodynamics of PEITC necessitate a systems-biology approach in parallel with PK/PD modeling to develop PEITC as a therapeutic agent for treating cancers.

Inhibitory potency of 4-carbon alkanes and alkenes toward CYP2E1 activity

CYP2E1 has been implicated in the bioactivation of many small molecules into reactive metabolites which form adducts with proteins and DNA, and thus a better understanding of the molecular determinants of its selectivity are critical for accurate toxicological predictions. In this study, we determined the potency of inhibition of human CYP2E1 for various 4-carbon alkanes, alkenes and alcohols. In addition, known CYP2E1 substrates and inhibitors including 4-methylpyrazole, aniline, and dimethylnitrosamine were included to determine their relative potencies. Of the 1,3-butadiene-derived metabolites studied, 3,4-epoxy-1-butene was the strongest inhibitor with an IC50 of 110 μM compared to 1700 μM and 6600 μM for 1,2-butenediol and 1,2:3,4-diepoxybutane, respectively. Compared to known inhibitors, inhibitory potency of 3,4-epoxy-1-butene is between 4-methylpyrazole (IC50 = 1.8 μM) and dimethylnitrosamine (IC50 = 230 μM). All three butadiene metabolites inhibit CYP2E1 activity through a simple competitive mechanism. Among the 4-carbon compounds studied, the presence and location of polar groups seems to influence inhibitory potency. To further examine this notion, the investigation was extended to include structurally and chemically similar analogues, including propylene oxide and various butane alcohols. Those results demonstrated preferential recognition of CYP2E1 toward the type and location of polar and hydrophobic structural elements. Taken together, CYP2E1 metabolism may be modified in vivo by exposure to 4-carbon compounds, such as drugs, and nutritional constituents, a finding that highlights the complexity of exposure to mixtures.

Repeated oral administration modulates the pharmacokinetic behavior of the chemopreventive agent phenethyl isothiocyanate in rats

The principal objective of this study was to evaluate whether repeated oral administration influences the pharmacokinetic behavior of the chemopreventive agent phenethyl isothiocyanate (PEITC) in rat. Animals were treated orally with 0.5, 1.0 and 5.0 mg/kg of the isothiocyanate for 4 days, and plasma levels at various times post-administration were determined by LC/MS after the first and last day. To determine absolute bioavailability, a group of animals was treated with a single (0.5 mg/kg) intravenous dose of PEITC. Following single oral dose administration, PEITC was rapidly absorbed, peak plasma concentrations being attained within the hour, and achieved an absolute bioavailability of 77%, but displayed dose-dependent pharmacokinetics, with bioavailability decreasing and clearance increasing moderately with dose; C(max) values did not rise proportionately to the dose and volume of distribution increased. At the higher doses of 1.0 and 5.0 mg/kg, repeated administration led to higher PEITC plasma C(max) concentrations and decreased plasma clearance of the isothiocyanate leading to enhanced bioavailability.

Genetic polymorphism in CYP2E1: Population distribution of CYP2E1 activity

Cytochrome P-450 2E1 (CYP2E1) is a key enzyme in the metabolic activation of a variety of toxicants including nitrosamines, benzene, vinyl chloride, and halogenated solvents such as trichloroethylene. CYP2E1 is also one of the enzymes that metabolizes ethanol to acetaldehyde, and is induced by recent ethanol ingestion. There is evidence that interindividual variability in the expression and functional activity of this cytochrome (CYP) may be considerable. Genetic polymorphisms in CYP2E1 were identified and linked to altered susceptibility to hepatic cirrhosis induced by ethanol and esophageal and other cancers in some epidemiological studies. Therefore, it is important to evaluate how such polymorphisms affect CYP2E1 function and whether it is possible to construct a population distribution of CYP2E1 activity based upon the known effects of these polymorphisms and their frequency in the population. This analysis is part of the genetic polymorphism database project described in the lead article in this series and followed the approach described in that article (Ginsberg et al., 2009, this issue). Review of the literature found that there are a variety of CYP2E1 variant alleles but the functional significance of these variants is still unclear. Some, but not all, studies suggest that several upstream 5' flanking mutations affect gene expression and response to inducers such as ethanol or obesity. None of the coding-region variants consistently affects enzyme function. Part of the reason for conflicting evidence regarding genotype effect on phenotype may be due to the wide variety of exposures such as ethanol or dietary factors and physiological factors including body weight or diabetes that modulate CYP2E1 expression. In conclusion, evidence is too limited to support the development of a population distribution of CYP2E1 enzyme activity based upon genotypes. Health risk assessments may best rely upon data reporting interindividual variability in CYP2E1 function for input into physiologically based pharmacokinetic (PBPK) models involving CYP2E1 substrates.

Tissue differences in the modulation of rat cytochromes P450 and phase II conjugation systems by dietary doses of phenethyl isothiocyanate

Rats were fed diets supplemented with phenethyl isothiocyanate (PEITC) at 0.06 (low dose, dietary intake level), 0.6 (medium dose) and 6.0 micromole/g (high dose), and xenobiotic-metabolising enzymes were monitored in liver, lung and kidney. At the low dose, inhibition of the hepatic O-dealkylation of ethoxy- and methoxyresorufin was noted, whereas at the high dose increases in the O-depentylation of pentoxyresorufin and O-debenzylation of benzyloxyquinoline were observed, whereas p-nitrophenol hydroxylase was inhibited. Hepatic bioactivation of 2-amino-3-methylimidazo-[4,5-f]quinoline to mutagens was not influenced by the PEITC-treatment. In the lung, at the high dose, ethoxyresorufin dealkylation was elevated and that of pentoxyresorufin suppressed; no significant changes were seen in the kidney. Quinone reductase was markedly elevated at all doses in liver, but the lung enzyme was refractive whereas in the kidney a modest rise was observed at the high dose. Hepatic glutathione S-transferase activity was stimulated by PEITC-treatment, but no effect was evident in the lung or kidney. It is concluded that the effects of PEITC on xenobiotic-metabolising systems are dose- and tissue-dependent, with the liver being the most sensitive and the lung generally resistant. Increased detoxication rather than cytochrome P450 inhibition is the likely mechanism of the chemopreventive activity of PEITC.

Modulation of N-nitrosomethylbenzylamine metabolism by black raspberries in the esophagus and liver of Fischer 344 rats

Dietary freeze-dried black raspberries (BRBs) inhibit N-nitrosomethylbenzylamine (NMBA)-induced tumorigenesis in the Fischer 344 rat esophagus. To determine the mechanistic basis of the anti-initiating effects of BRBs, NMBA metabolism was studied in esophageal explant cultures and in liver microsomes taken from rats fed with AIN-76A diet or AIN-76A diet containing 5% or 10% BRBs. Five percent and 10% dietary BRBs inhibited NMBA metabolism in explants (26% and 20%) and in microsomes (22% and 28%), but the inhibition was not dose dependent. To identify active inhibitory component(s) in BRBs, esophageal explants and liver microsomes from control rats were treated in vitro with an ethanol extract of BRBs or with individual components of BRBs [ellagic acid (EA) and two anthocyanins (cyanidin-3-glucoside and cyanidin-3-rutinoside)]. NMBA metabolism in explants was inhibited maximally by cyanidin-3-rutinoside (47%) followed by EA (33%), cyanidin-3-glucoside (23%), and the extract (11%). Similarly, in liver microsomes, the inhibition was maximal by cyanidin-3-rutinoside (47%) followed by EA (33%) and cyanidin-3-glucoside (32%). Phenylethylisothiocyanate (PEITC), a potent inhibitor of NMBA tumorigenesis in rat esophagus, was a stronger inhibitor of NMBA metabolism in vivo and in vitro than BRBs or their components. Dietary BRBs and PEITC induced glutathione S-transferase activity in the liver.

Enzymatic mechanisms of ethanol oxidation in the brain

BACKGROUND

The exact enzymatic mechanisms of ethanol oxidation in the brain are still unclear. The catalase-mediated oxidation of ethanol was demonstrated in rat brain using incubation of brain homogenates with catalase inhibitors. The role of the alcohol dehydrogenase (ADH) or cytochrome P450-dependent system in this process is possible, but has not been confirmed. The objective of the study was to determine the contribution of the different enzymatic pathways to ethanol oxidation in brain homogenates from mice and rats.

METHODS

Three approaches were used to investigate the enzymatic mechanisms of ethanol oxidation in the brain of rats and mice: (1) preincubation of brain homogenates with inhibitors of the ethanol-metabolizing enzymes (catalase, CYP2E1, ADH, and ALDH); (2) utilization of mice with genetic deficiency in ethanol-metabolizing enzymes (catalase, CYP2E1, or both enzymes); and (3) determination of ethanol oxidation in brain subcellular fractions known to have differential activity of ethanol-metabolizing enzymes. The ethanol-derived acetaldehyde (AC) and acetate were determined in brain samples by gas chromatography.

RESULTS

The catalase inhibitors sodium azide (5 mM) and aminotriazole (5 mM) as well as CYP2E1 inhibitors diallyl sulfide (2 mM) and beta-phenethyl isothiocyanate (0.1 mM) lowered significantly the accumulation of the ethanol-derived AC and acetate in brain homogenates. The ADH inhibitor 4-methyl pyrazole (5 mM) significantly decreased the acetate but not the AC accumulation. Ethanol-derived AC accumulation in brain homogenates of acatalasemic mice was 47% of the control value, 91% in CYP2E1-null mice, and 24% in double mutants (with deficiency of both catalase and CYP2E1). The highest levels of ethanol oxidation were found in microsomal and peroxisomal subcellular brain fractions, where CYP2E1 and catalase are located, respectively.

CONCLUSIONS

Catalase is the key enzyme of ethanol oxidation in the brain of rodents: it may be responsible for about 60% of the process. CYP2E1 plays an important role in ethanol oxidation in the rodent brains. Alcohol dehydrogenase plays a minor role, if any, in this process. Aldehyde dehydrogenase plays the crucial role in the further oxidation of ethanol-derived AC in the brain homogenates.

Herbal bioactivation: the good, the bad and the ugly

It has been well established that the formation of reactive metabolites of drugs is associated with drug toxicity. Similarly, there are accumulating data suggesting the role of the formation of reactive metabolites/intermediates through bioactivation in herbal toxicity and carcinogenicity. It has been hypothesized that the resultant reactive metabolites following herbal bioactivation covalently bind to cellular proteins and DNA, leading to toxicity via multiple mechanisms such as direct cytotoxicity, oncogene activation, and hypersensitivity reactions. This is exemplified by aristolochic acids present in Aristolochia spp, undergoing reduction of the nitro group by hepatic cytochrome P450 (CYP1A1/2) or peroxidases in extrahepatic tissues to reactive cyclic nitrenium ion. The latter was capable of reacting with DNA and proteins, resulting in activation of H-ras oncogene, gene mutation and finally carcinogenesis. Other examples are pulegone present in essential oils from many mint species; and teucrin A, a diterpenoid found in germander (Teuchrium chamaedrys) used as an adjuvant to slimming diets. Extensive pulegone metabolism generated p-cresol that was a glutathione depletory, and the furan ring of the diterpenoids in germander was oxidized by CYP3A4 to reactive epoxide which reacts with proteins such as CYP3A and epoxide hydrolase. On the other hand, some herbal/dietary constituents were shown to form reactive intermediates capable of irreversibly inhibiting various CYPs. The resultant metabolites lead to CYP inactivation by chemical modification of the heme, the apoprotein, or both as a result of covalent binding of modified heme to the apoprotein. Some examples include bergamottin, a furanocoumarin of grapefruit juice; capsaicin from chili peppers; glabridin, an isoflavan from licorice root; isothiocyanates found in all cruciferous vegetables; oleuropein rich in olive oil; dially sulfone found in garlic; and resveratrol, a constituent of red wine. CYPs have been known to metabolize more than 95% therapeutic drugs and activate a number of procarcinogens as well. Therefore, mechanism-based inhibition of CYPs may provide an explanation for some reported herb-drug interactions and chemopreventive activity of herbs. Due to the wide use and easy availability of herbal medicines, there is increasing concern about herbal toxicity. The safety and quality of herbal medicine should be ensured through greater research, pharmacovigilance, greater regulatory control and better communication between patients and health professionals.

An investigation of dopaminergic metabolites in the striatum and in the substantia nigra in vivo utilising radiolabelled L-DOPA and high performance liquid chromatography: a new approach in the search for transmitter metabolites

Although the major routes of dopamine metabolism seem to be established, at least in terminal regions such as the striatum, it is important to search for previously unknown metabolites and to investigate the relevance of previously suggested minor alternative pathways. An urgent issue is to verify and quantify the transformation of dopamine to putative toxic species, another is to further explore metabolism of dopamine located in cell bodies/dendrites, e.g. in the substantia nigra. We have developed a new method in order to widen the search for alternative metabolites of dopamine. The method is based on systemic injection of tritiated L-DOPA to rats in vivo. Brain tissue was homogenised and centrifuged and the resulting supernatant fractioned following passage through a liquid chromatography system. The radioactivity of each fraction was measured using a scintillation system. By identifying fractions containing major catecholamines and metabolites, according to a standard solution, novel metabolites can be searched for in the remaining fractions. It was possible to obtain sufficient radioactivity in separate fractions of supernatant of homogenised tissue, even from such a small brain nucleus as substantia nigra. Radioactivity was obtained in those fractions that contained the major catecholamines and their metabolites, as well as in other fractions where it may represent previously unknown metabolites of L-DOPA/dopamine. The method was used to evaluate the possibility that cytochrome P450 2E1 is involved in the metabolism of dopamine in the substantia nigra. Significant changes in the radioactivity pattern were induced by inhibition of the enzyme but conclusions about whether cytochrome P450 2E1 is involved in the metabolism of dopamine or not requires further studies. The method can be used to study the metabolism of dopamine and can be extended, by using other radiolabelled precursors, also to evaluate metabolism of other transmitters, e.g. serotonin.

Narcotic drugs change the expression of cytochrome P450 2E1 and 2C6 and other activities of carcinogen-metabolizing enzymes in the liver of male mice

Drug-metabolizing enzymes play a great role in the bioactivation and also detoxification of zenobiotics and carcinogens such as N-nitrosamines and polycyclic aromatic hydrocarbons (PAHs). Therefore, the present study was undertaken to investigate the effect of narcotic drugs such as cannabis (hashish) and diacetylmorphine (heroin) on the activity of N-nitrosodimethylamine N-demethylase I [NDMA-dI], arylhydrocarbon [benzo(a)pyerne] hydroxylase [AHH], cytochrome P450 (CYP), cytochrome b(5), NADPH-cytochrome c reductase, glutathione-S-transferase, and levels of glutathione and thiobarbituric acid-reactive substances (TBARS). In addition, the present study showed the influence of hashish and heroin after single (24 h) and repeated-dose treatments (4 consecutive days) on the expression of cytochrome P450 2E1 (CYP 2E1) and cytochrome P450 2C6 (CYP 2C6). The expression of CYP 2E1 was slightly induced after single-dose and markedly induced after repeated dose-treatments of mice with hashish (10 mg kg(-1) body weight). Contrarily, heroin markedly induced the expression of CYP 2C6 after single-dose and potentially reduced this expression after repeated-dose treatments. It is believed that N-nitrosamines are activated principally by CYP 2E1 and in support of this, the activity of NDMA-dI was found to be increased after single- and repeated-dose treatments of mice with hashish by 23 and 41%, respectively. In addition, single- and repeated-dose treatments of mice with hashish increased: (1) the total hepatic content of CYP by 112 and 206%, respectively; (2) AHH activity by 110 and 165%, respectively; (3) NADPH-cytochrome c reductase activity by 21 and 98%, respectively; (4) and glutathione level by 81 and 173%, respectively. Also, single-dose treatments of mice with heroin increased the total hepatic content of CYP, AHH, NADPH-cytochrome c reductase, and glutathione level by 126, 72, 39, 205%, respectively. However, repeated dose-treatments of mice with heroin did not change such activities except cytochrome c reductase activity increased by 20%. Interestingly, the level of free radicals, TBARS, was potentially decreased after single or repeated-dose treatments with either hashish or heroin. It is clear from this study that the effects of hashish are different from those of heroin on the above mentioned enzymes particularly after repeated dose treatments. It is concluded that hashish induced the expression of CYP 2E1 and other carcinogen-metabolizing enzymes activities, and this induction could potentiate the deleterious effects of N-nitrosamines and aromatic hydrocarbons, e.g. benzo(a)pyrene, upon the liver and probably other organs. Such alterations may also change the therapeutic actions of other drugs, which are primarily metabolized by the P450 system, when administered to peoples using hashish or heroin.

Drug–phytochemical interactions

Changes in dietary habits favouring diets rich in fruits and vegetables, and a meteoric rise in the consumption of dietary supplements and herbal products have substantially increased human exposure to phytochemicals. It is, therefore, not surprising that diet and herbal remedies can modulate drug-metabolising enzyme systems, such as cytochromes P450, leading to clinically relevant drug-phytochemical interactions. Phytochemicals have the potential to both elevate and suppress cytochrome P450 activity. Such effects are more likely to occur in the intestine, where high concentrations of phytochemicals may be achieved, and alteration in cytochrome P450 activity will influence, in particular, the fate of drugs that are subject to extensive first-pass metabolism as a result of intestinal cytochrome P450-mediated biotransformation. Moreover, it is becoming increasingly apparent that phytochemicals can also influence the pharmacological activity of drugs by modifying their absorption characteristics through interaction with drug transporters. Clearly, phytochemicals have the potential to alter the effectiveness of drugs, either impairing or exaggerating their pharmacological activity.

Pharmacokinetic interactions between herbal remedies and medicinal drugs

1. The use of herbal products to treat a wide range of conditions is rising rapidly, leading to increased intake of phytochemicals. Recent studies revealed potentially fatal interactions between herbal remedies and traditional drugs. 2. In transplant patients, self-medication with St John's wort (Hypericum perforatum) has led to a drop in plasma levels of the immunosuppressant drug cyclosporine, causing tissue rejection. 3. Intake of St John's wort increases the expression of intestinal P-glycoprotein and the expression of CYP3A4 in the liver and intestine. The combined up-regulation in intestinal P-glycoprotein and hepatic and intestinal CYP3A4 impairs the absorption and stimulates the metabolism of cyclosporine, leading to subtherapeutic plasma levels. The St John's wort component, hyperforin, contributes to the induction of CYP3A4. 4. St John's wort also enhances the metabolism of other CYP3A4 substrates including the protease inhibitors indinavir and nevirapine, oral contraceptives, and tricyclic antidepressants such as amitriptyline. 5. Other herbal remedies with the potential to modulate cytochrome P450 activity and thus participate in interactions with conventional drugs include Milk thistle, Angelica dahurica, ginseng, garlic preparations, Danshen and liquorice. 6. Herbal products are currently not subject to the rigorous testing indispensable for conventional drugs. However, if potential drug interactions are to be predicted, it is essential that the ability of herbal products to interfere with drug-metabolizing enzyme systems is fully established.

Changes in the expression of cytochrome P450 isozymes and related carcinogen metabolizing enzyme activities in Schistosoma mansoni-infected mice

Mixed-function oxidase enzymes metabolize most xenobiotic agents. Western blotting was used to investigate the effect of Schistosoma mansoni infection on the expression of various cytochrome P450 (CYP) isozymes and specific enzyme assays to study related metabolic functions in mouse liver microsomes. Male BK-TO mice were infected with 200 cercariae per mouse and their livers were assayed at 6, 15, 30 and 45 days post-infection (p.i.) and compared with appropriately matched controls. The expression of each of the CYP isozymes (1A1, 2B1/2, 2C6, and 4A) was either unaffected or transiently increased up to 30 days post-infection. By 45 days, a significant loss of signal was observed, particularly for CYP 1A1 and 2B1 /2 where no signal could be detected. Evidence supporting these findings was obtained from enzyme assays specific for particular CYP isozymes. The activity of ethoxyresorufin O-deethylase (CYP 1A1) was reduced by 97% and that of pentoxyresorufin O-depentylase (CYP 2B1 /2) by 96% at 45 days p.i. Similarly, the activity of ethoxycoumarin hydroxylase was progressively reduced over the period under study. It is believed that N-nitrosamines are activated principally by N-nitrosodimethylamine N-demethylase I which was significantly increased at both 30 and 45 days p.i. To further investigate metabolic competency following S. mansoni infection, the in vitro binding of benzo(a)pyrene metabolites to DNA was measured, using isolated liver microsomes to activate benzo(a)pyrene. Benzo(a)-pyrene-DNA adduct formation was markedly increased at 6,15 and 30 days with a maximum at 15 days, but decreased at 45 days p.i. It was concluded that S. mansoni infection changes the expression of different CYP isozymes and also the activity of phase I drug-metabolizing enzymes at different periods of infection and may thus change the liver's capacity to activate or detoxify many endogenous and exogenous compounds. Such alterations may also change the therapeutic actions of drugs that are primarily metabolized by the P450 system, when administered to patients with schistosomiasis.

Pharmacokinetics of ethanol: a review of the methodology

This paper reviews current concepts on tools for studying the pharmacokinetics of alcohol. It has been known that ethanol metabolism occurs mainly in the liver via alcohol dehydrogenase and an accessory microsomal pathway. The contribution of each pathway has been examined by administration of metabolic inhibitors. The role of gastric alcohol dehydrogenase in the first-pass effects of ethanol has been speculative and may be relatively low. Some pharmacokinetic approaches with mathematical models have elucidated the role of gastric alcohol dehydorgenase, hepatic alcohol dehydrogenase and cytochrome P450 2E1 in ethanol elimination. The scale-up of ethanol elimination kinetics has enabled extrapolation from animal models to human kinetics. The clarification of the pharmacokinetics of ethanol is very important for estimating the effects of ethanol on biological events.

Mechanisms of carcinogenesis inhibition by isothiocyanates

Epidemiological studies have suggested that frequent consumption of cruciferous vegetables is associated with a decreased risk in various types of cancer. Cruciferous vegetables are commonly consumed foods that contain organosulfur compounds known as isothiocyanates. These compounds are potent inhibitors of chemically induced carcinogenesis in animals. Extensive work has been conducted to elucidate the mechanisms involved in the inhibition of carcinogenesis by isothiocyanates. These mechanisms include blocking the metabolic activation of the carcinogens by way of altering the enzymes involved in the process, induction of detoxification enzymes and induction of apoptosis. Since their mode of action is selective, the enzyme composition of the tissue and the inhibition or induction of the enzymes by the isothiocyanates will influence their chemopreventive activities. Isothiocyanates may potentially be beneficial in protecting against human carcinogenesis.

Inhibition of methyl-n-amylnitrosamine hydroxylation by diallyl sulfide and phenethylisothiocyanate in the rat

Formation of the stable 2-, 3-, and 4-hydroxy derivatives of methyl-n-amylnitrosamine (MNAN) probably reflects cytochrome P-450-catalyzed activation of MNAN by 1-hydroxylation. Here we studied inhibition of the oxidation of MNAN to hydroxy-MNANs (HO-MNANs) by freshly excised tissues from MRC-Wistar rats treated with the vegetable-derived chemicals diallyl sulfide (DAS) and phenethylisothiocyanate (PEITC). Rats were gavaged with DAS (200 mg/kg), PEITC (163 mg/kg), or vehicle (corn oil) alone. After various times, the rats were killed, the esophagus, nasal mucosa, and liver were removed, and the tissues/tissue slices were incubated for two hours with 23 microM MNAN. HO-MNAN formation was measured by gas chromatography-thermal energy analysis. Significant (p < 0.01) 72-75%, 40%, and 44% inhibitions of total HO-MNAN formation were observed for nasal mucosa removed at 3-18 hours, for esophagus at 18 hours, and for liver at 3 hours, respectively, after gavage of DAS. Significant (p < 0.03) 46-75% inhibition of HO-MNAN formations was observed for the esophagus at 2-24 hours after gavage of PEITC. In disposition studies, rats were treated with DAS (200 mg/kg) in corn oil and sacrificed after various intervals. DAS was determined by gas chromatography of tissue homogenate extracts. After gavage of DAS, its total recovery from all tissues studied was 27% of the dose after 45 minutes and 15-19% after 90 and 180 minutes, with > 80% of the recovered DAS in the stomach contents. Up to 2% per tissue of the recovered DAS was found in the stomach wall, liver, and blood. After intraperitoneal injection of DAS, < or = 2% of the dose was recovered in the blood and < or = 0.7% in the liver. Hence, gavage of DAS and PEITC significantly inhibited HO-MNAN formation for up to 18 and 24 hours, respectively, whereas DAS was > 80% metabolized 90 minutes after its gavage. These findings suggest that long-lasting inhibitors or their metabolites, or inactivation of P-450 enzymes, were responsible for the persistence of inhibition of MNAN metabolism.

Inhibition of cytochrome P450 2E1 induces an increase in extracellular dopamine in rat substantia nigra: a new metabolic pathway?

We presented data previously on dopamine (DA) synthesis and catabolism in the rat substantia nigra (SN) suggesting that a substantial part of the synthesized DA in this brain part is metabolized by unknown nonclassical metabolic pathways. On the basis of that a relatively high density of cytochrome P450 2E1 (CYP 2E1) has been detected in rat SN the aim of the present study was to investigate the possibility that this enzyme is involved in the metabolism of DA. Systemic administration of either phenylethyl isothiocyanate (100 mg/kg ip), diethyldithiocarbamate (500 mg/kg, ip) or diallyl sulfide (200 mg/kg, sc or ip), three different inhibitors of cytochrome P450 2E1, induced an increase of the extracellular DA concentration in the SN, measured with microdialysis in awake rats, by 130%, 90%, and 35%, respectively. A tendency to increased concentrations of the classical DA metabolites in the dialysate from the SN was also observed in some experiments. In the striatum, no profound effects were induced by the drugs on the concentrations of DA or its metabolites. The results show that CYP 2E1 activity affects dopaminergic neurotransmission in the SN, possibly by participating in DA metabolism. Other mechanisms, such as the influence on the DA transporter or the release process cannot, however, be ruled out.

Effect of organosulfur compounds from garlic and cruciferous vegetables on drug metabolism enzymes

The frequent consumption of cruciferous vegetables and garlic is associated with several health benefits. These foods contain organosulfur compounds that are known to affect the biotransformation of xenobiotics, and therefore can influence the toxicity and carcinogenicity of environmental chemicals. In this article, we review the effects of isothiocyanates and diallyl sulfide on xenobiotic metabolism and the enzymes involved in the process. Isothiocyanates and diallyl sulfide can modulate the levels of phase I and phase II drug-metabolizing enzymes by affecting the transcriptional rates of their genes, the turnover rates of specific mRNAs or enzymes, or the enzyme activity. These compounds are not general enzyme inhibitors or inducers. They elicit selectivity in their mode of action. Elucidating the mechanisms involved in the alteration of drug-metabolizing enzymes by isothiocyanates and diallyl sulfide will increase our understanding of their possible effects on the biotransformation of drugs as well as the potential beneficial or detrimental effects of these organosulfur compounds.

Effect of Siamese cassia leaves on the activities of chemical carcinogen metabolizing enzymes and on mammary gland carcinogenesis in the rat

Male Wistar rats were fed AIN-76 semipurified diet or diet containing 5% ground lyophilized Siamese cassia leaves for 2 weeks before sacrifice. Hepatic S9 fractions were prepared and assayed for the level of cytochrome P450 (P450), the activities of monooxygenase, i.e., aniline hydroxylase (ANH), aminopyrine-N-demethylase (AMD) as well as the capacity to metabolically activate the mutagenicities of aflatoxin B(1) (AFB(1)) and benzo(a)pyrene (B(a)P). In addition, the activities of detoxificating enzymes such as glutathione-S-transferase (GST) and UDP-glucuronyltransferase (UGT) were also measured. It was found that feeding of Siamese cassia leaves significantly reduced the activities of hepatic ANH and AMD as well as the capacity to activate the mutagenicity of AFB(1) towards Salmonella typhimurium TA100, being 31, 73 and 41% of control group, respectively. It also slightly decreased, but not significantly, the capacity to activate the mutagenicity of B(a)P towards S. typhimurium YG1029. On the other hand, however, the activities of both GST and UGT were markedly increased in those animals, being 250 and 220% of control animals. The anticarcinogenic potential of Siamese cassia leaves was also investigated in female Sprague Dawley rats treated with 9,10-dimethyl-1,2-benzanthracene (DMBA). The animals were fed control diet or diet containing ground lyophilized Siamese cassia leaves 2 weeks prior to and 1 week after intragastrically administration of DMBA, and then they were placed on a pellet diet for additional 25 weeks. Interestingly, it was found that feeding of diet containing 2.5 and 4% Siamese cassia leaves resulted in a significant decrease in the multiplicity of mammary gland tumors as well as a slight delay of the onset of tumor development. The incidence of tumors in the group fed 4% Siamese cassia leaves, but not in the 2.5% group, was lowered, although not significantly, than that of control group. The results in the present study therefore demonstrated that Siamese cassia leaves possess phase II enzyme inducing property as well as the ability to reduce some phase I enzyme activities in rat liver. This Thai vegetable also exhibit cancer chemopreventive potential, at least against DMBA-induced mammary gland carcinogenesis which may be partly due to phase II inducing capacity as well as phase I inhibitory activity.

Production of DNA strand breaks by N-nitrosodimethylamine and 2-amino-3-methylimidazo[4,5-f]quinoline in THLE cells expressing human CYP isoenzymes and inhibition by sulforaphane

The production of DNA strand breaks by N-nitrosodimethylamine (NDMA) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) has been observed in T5-2E1 (expressing human CYP2E1) and T5-1A2 (expressing human CYP1A2) human liver cells respectively, using the Comet assay. Responses were statistically significant (P<0.05) and concentration dependent (0.01-1 microg ml-1 NDMA and 0.1-10 microg ml-1 IQ) and were not observed in T5-neo cells devoid of cytochrome P450 activity. Sulforaphane (1-isothiocyanate-4-methylsulfinylbutane) (0.1-10 microM) gave a marked inhibition of DNA strand breakage by these carcinogens (P<0.05 linear regression). This was seen in the absence of cytotoxicity and in the absence of an inhibition of H2O2-induced DNA strand breakage. The ability of sulforaphane to inhibit both CYP2E1 and CYP1A2-mediated genotoxicity therefore is relevant to human isoforms of these enzymes and may contribute to a chemopreventative activity.

Mechanism-based inactivation of cytochromes P450 2E1 and 2B1 by 5-phenyl-1-pentyne

A series of acetylenic compounds whose structures were based on "P450 2E1-like" substrates was investigated for their ability to cause inactivation of P450 2E1-dependent p-nitrophenol hydroxylation. The most effective compound with liver microsomes from pyridine-treated rats or with rabbit P450 2E1 in a reconstituted system was 5-phenyl-1-pentyne. The inactivation of purified 2B1, 2E1, a truncated 2E1 lacking amino acids 3-29, 2E1(Delta3-29), or a truncated 2E1 in which threonine 303 was replaced with alanine, 2E1(Delta3-29, T303A), in a reconstituted system by 5-phenyl-1-pentyne was NADPH- and time-dependent and followed pseudo-first-order kinetics. The maximal rate constants for inactivation, the concentrations that gave half-maximal inactivation (KI), and the partition ratios (the number of 5-phenylvaleric acid molecules formed/inactivation event) were determined with each P450. The KI values for 2B1 and 2E1(Delta3-29, T303A) were twice those for 2E1 and 2E1(Delta3-29), and the partition ratios for 2B1 and 2E1(Delta3-29, T303A) were 5-10 times greater than those of 2E1 or 2E1(Delta3-29). During the incubation of P450 2E1 with 5-phenyl-1-pentyne, the loss of P450 as determined by the reduced-CO difference spectra was equal to the loss of catalytic activity. The formation of a heme adduct was demonstrated by HPLC analysis of reaction mixtures containing 5-[3H]phenyl-1-pentyne. HPLC analysis with diode-array detection showed that the Soret region of the proposed heme adduct was different from that of the unmodified heme. The HPLC peak containing the proposed heme adduct was further analyzed by matrix-assisted laser desorption ionization mass spectrometry, and the resulting peaks could result from the addition of a 2-oxo-5-phenylpentyl group to the heme.

Effects of neem flowers, Thai and Chinese bitter gourd fruits and sweet basil leaves on hepatic monooxygenases and glutathione S-transferase activities, and in vitro metabolic activation of chemical carcinogens in rats

The objectives of this study were to determine the effects of feeding of four vegetables commonly consumed in Thailand, namely, flowers of the neem tree (Azadirachta indica var. siamensis), fruits of Thai and the Chinese bitter gourd (Momordica charantia Linn.) and leaves of sweet basil (Ocimum basilicum Linn) on the levels of phase I enzymes, which include cytochrome P450 (P450), aniline hydroxylase (ANH) and aminopyrine-N-demethylase (AMD) as well as the capacity to activate the mutagenicities of aflatoxin B1 (AFB1) and benzo[a]pyrene (BaP), and to induce the phase II enzymes [i.e. glutathione S-transferase (GST)] in rat liver. It was found that feeding of the diets containing 12.5% neem flowers and Thai bitter gourd fruits for 2 weeks strongly enhanced GST activity, 2.7- and 1.6- fold of the pair-fed control values, respectively, while resulting in a marked reduction of the levels of most phase I reactions. Fruits of the Chinese bitter gourd, which is in the same species as Thai bitter gourd, had no effect on GST activity but decreased AMD activity and the in vitro metabolic activation of AFB1 and BaP. On the other hand, however, dietary sweet basil leaves caused a significant increase in the levels of both GST and all phase I enzymes. Results in the present study clearly demonstrate that neem flowers and Thai bitter gourd fruits contain monofunctional phase II enzyme inducers and compounds capable of repressing some monooxygenases, especially those involved in the metabolic activation of chemical carcinogens, while sweet basil leaves contain compounds, probably bifunctional inducers, capable of inducing both phase I and phase II enzymes and Chinese bitter gourd fruits contain only compounds capable of repressing some monooxygenases. These results therefore suggest that neem flowers and Thai bitter gourd fruits may possess chemopreventive potential, while those of Chinese bitter gourd fruits and sweet basil leaves are uncertain.

Isothiocyanates and plant polyphenols as inhibitors of lung and esophageal cancer

A group of arylalkyl isothiocyanates were tested for their abilities to inhibit tumorigenicity and DNA methylation induced by both the tobacco-specific nitrosamine, NNK, in A/J mouse lung and the esophageal-specific carcinogen, NMBA, in F344 rat esophagus. In addition, ellagic acid was tested for its ability to inhibit NMBA-induced esophageal tumorigenesis. In the strain A lung tumor model, PEITC effectively inhibited NNK-induced lung tumors at a dose of 5 micromol, but was not inhibitory at lower doses. PPITC, PBITC, PPeITC, and PHITC were all considerably more potent inhibitors of NNK lung tumorigenesis than PEITC, and PHITC was the most potent inhibitor of all. Thus, in the strain A lung tumor model, there was a trend of increased inhibitory efficacy among arylalkyl isothiocyanates with increased alkyl chain length. In the F344 rat esophageal tumor model, PPITC was clearly more potent than PEITC, BITC and PBITC had little inhibitory effect on esophageal tumorigenesis, and in a separate experiment, PHITC actually enhanced esophageal tumorigenesis. Thus, the structure-activity relationships for inhibition of tumorigenesis by arylalkyl isothiocyanates were considerably different in the two animal models. However, the effects of the isothiocyanates on tumorigenesis were well-correlated to their effects on DNA adduct formation in either model. The most likely mechanism of inhibition of tumorigenesis by these isothiocyanates is via inhibition of the cytochrome p450 enzymes responsible for activation of NNK in mouse lung or NMBA in rat esophagus. Ellagic acid was an effective inhibitor of esophageal tumorigenesis, although not as potent as PEITC or PPITC. Like the isothiocyanates, ellagic acid inhibits cytochrome p450-mediated activation of NMBA.

A review of mechanisms underlying anticarcinogenicity by brassica vegetables

The mechanisms by which brassica vegetables might decrease the risk of cancer are reviewed in this paper. Brassicas, including all types of cabbages, broccoli, cauliflower and Brussels sprouts, may be protective against cancer due to their relatively high glucosinolate content. Glucosinolates are usually broken down through hydrolysis catalyzed by myrosinase, an enzyme that is released from damaged plant cells. Some of the hydrolysis products, viz. indoles and isothiocyanates, are able to influence phase 1 and phase 2 biotransformation enzyme activities, thereby possibly influencing several processes related to chemical carcinogenesis, e.g. the metabolism, DNA-binding and mutagenic activity of promutagens. A reducing effect on tumor formation has been shown in rats and mice. The anticarcinogenic action of isothiocyanates and indoles depends upon many factors, such as the test system, the target tissue, the type of carcinogen challenge and the anticarcinogenic compound, their dosage, as well as the timing of the treatment. Most evidence concerning anticarcinogenic effects of glucosinolate hydrolysis products and brassica vegetables has come from studies in animals. Animal studies are invaluable in identifying and testing potential anticarcinogens. In addition, studies carried out in humans using high but still realistic human consumption levels of indoles and brassica vegetables have shown putative positive effects on health.

Mechanism of enhancement of esophageal tumorigenesis by 6-phenylhexyl isothiocyanate

6-Phenylhexyl isothiocyanate (PHITC) enhances esophageal tumorigenesis induced by the carcinogen N-nitrosomethylbenzylamine (NMBA) in rats while its shorter chain analog, phenethyl isothiocyanate (PEITC), inhibits NMBA-induced esophageal tumorigenesis. A significant increase in O6-methylguanine levels in esophageal DNA at 72 h after NMBA administration to rats pretreated with PHITC suggested that PHITC might enhance NMBA metabolic activation or inhibit DNA repair. To test this hypothesis, groups of 20 rats were administered PEITC or PHITC at concentrations of 0, 1.0, or 2.5 mmol/kg in modified AIN-76A diet for 2 weeks. The esophagi were removed from rats, stripped, split, and maintained in HEPES buffered saline (HBS) for assays of NMBA metabolism (n = 5 per group) or were snap frozen for DNA repair assays (n = 15 per group). The principal metabolites of NMBA produced by esophageal explants were: two unidentified peaks, benzyl alcohol (at 4 h only), and benzoic acid. Esophageal explants from PEITC-treated animals showed a significantly decreased ability to metabolize NMBA as expected. PHITC-treated animals showed a slight inhibition in the formation of most NMBA-related metabolites, rather than an overall increase in NMBA activation. This inhibition was less than that observed with PEITC. No inhibitory effects were observed on O6-alkylguanine transferase (AGT) activity in the esophagi of rats treated with 1.0 micromol/g or 2.5 micromol/g PHITC. Thus, effects of PHITC on esophageal metabolism and DNA repair do not account for the enhancement of NMBA tumorigenicity by PHITC.

Decrease of plasma and urinary oxidative metabolites of acetaminophen after consumption of watercress by human volunteers

To investigate the effect of the consumption of watercress (Nasturtium officinale R. Br.), a cruciferous vegetable, on acetaminophen metabolism, the pharmacokinetics of acetaminophen and its metabolites were studied in a crossover trial of human volunteers. A single oral dose of acetaminophen (1 gm) was given 10 hours after ingestion of watercress homogenates (50 gm). In comparison with acetaminophen only, the ingestion of watercress resulted in a significant reduction in the area under the plasma cysteine acetaminophen (Cys-acetaminophen) concentration-time curve and in the peak plasma Cys-acetaminophen concentration by 28% +/- 3% and by 21% +/- 4% (mean +/- SE; n = 7; p < 0.005), respectively. Correspondingly, the Cys-acetaminophen formation rate constant and Cys-acetaminophen formation fraction were decreased by 55% +/- 9% and 52% +/- 7% (p < 0.01), respectively. Consistent with the results obtained from the plasma, the total urinary excretion of Cys-acetaminophen in 24 hours was also reduced. A decrease of mercapturate acetaminophen, a Cys-acetaminophen metabolite, was also shown in the plasma and urine samples. However, the plasma pharmacokinetic processes and the urinary excretions of acetaminophen, acetaminophen glucuronide, and acetaminophen sulfate were not altered significantly by the watercress treatment. These results suggest that the consumption of watercress causes a decrease in the levels of oxidative metabolites of acetaminophen, probably due to inhibition of oxidative metabolism of this drug.

Effects of Benomyl on Cytochrome P450 Monooxygenase Systems in the Hep G2 Cell Line

Changes in the cytochrome P450 monooxygenase system were investigated in Hep G2 cells treated for 24 hours with various concentrations of benomyl. Decreases in both benzo[a]pyrene hydroxylase (AHH) and ethoxyresorufin deethylase (EROD), markers of the P4501A1 isoenzyme, were noted. Ethoxycoumarin deethylase (ECOD), a marker of the P4502B1 isoenzyme, showed a dose-dependent increase. Characterisation by SDS-polyacryl-amide gel electrophoresis of Hep G2 cell microsomal proteins revealed a decrease in the polypeptide bands at 55.5kD (P4501A1) and 48kD (P4501A1 and P4502B1) and an increase in the polypeptide band at 52kD (P4502B1). Benomyl induced a decrease in cytochrome P4501A1 and an increase in cytochrome P4502B1 in Hep G2 cells, as indicated by variations in AHH, EROD and ECOD activity, and by characterisation of microsomal proteins by SDS-polyacryl-amide gel electrophoresis.

A highly sensitive tool for the assay of cytochrome P450 enzyme activity in rat, dog and man. Direct fluorescence monitoring of the deethylation of 7-ethoxy-4-trifluoromethylcoumarin

The O-deethylation of 7-ethoxy-4-trifluoromethylcoumarin (EFC) by liver microsomes has been assessed as a method for monitoring the activity of cytochrome P450. The principle advantage of this substrate is the formation of a fluorescent product 7-hydroxy-4-trifluoromethylcoumarin (HFC) which can be assayed directly in the reaction medium. For rat microsomes the deethylated product was confirmed as the main metabolite, the reaction rate was linear with respect to both time and microsomal protein concentration and was independent of small changes in the added co-factors. A linear formation rate for the deethylated metabolite was also confirmed with dog and human microsomes. The intra-assay precision for rat, dog and human microsomes was 3, 5 and 4%, respectively. Hanes transformations of the dog and human data showed two phases, in contrast to a linear decline seen for the rat. Hybrid parameters for Vmax and Km, calculated from the apparently linear portions of these curves, gave interday SD for the Vmax of rat, dog and man of 2, 14 and 4%, respectively, and approximately 15% for the Km in all species. The Vmax in rat, dog and human microsomes was 1.4 +/- 0.2, 4.3 +/- 1.5 and 0.9 +/- 0.5 nmol HFC/min/nmol P450, respectively. The Km was 11.0 +/- 3.1, 67 +/- 19 and 6.8 +/- 2.5 microM, respectively. Direct evidence that at least two isoenzymes (cytochrome P450 1A2 and 2E1) metabolize EFC was obtained by experiments with competitive, suicide and immuno-inhibitors. Compared with ethoxycoumarin, the involvement of P450 2E1 in O-deethylation seemed similar in the rat. In conclusion, EFC provides a straightforward and reproducible assay for microsomal enzyme activity, requiring at most 25 pmol/mL of cytochrome P450.

Effects of isothiocyanates on tumorigenesis by benzo[a]pyrene in murine tumor models

Previous studies have shown that benzyl isothiocyanate (BITC) inhibited lung tumorigenesis induced in A/J mice by benzo[a]pyrene (BaP), but other experiments using a somewhat different protocol demonstrated that phenethyl isothiocyanate (PEITC) had no effect on lung tumorigenesis induced by BaP in this strain. In contrast, PEITC but not BITC had been shown to inhibit lung tumorigenesis induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in A/J mice. Therefore, one goal of this study was to directly compare the chemopreventive activities of BITC and PEITC on BaP-induced lung tumorigenesis in A/J mice. In the same experiment we also compared the tumorigenic activities of BaP and NNK. Either BITC or PEITC was administered by gavage 15 min before gavage of BaP. This regimen was carried out three times at 2-week intervals, and the mice were sacrificed 26 weeks after the first treatment. As assessed by tumor multiplicity, BITC but not PEITC significantly inhibited lung tumorigenesis by BaP, whereas PEITC but not BITC significantly inhibited forestomach tumorigenesis. Comparison of the tumorigenic activities of NNK and BaP demonstrated that NNK was about ten times more potent than BaP as a lung tumorigen, while BaP but not NNK induced forestomach tumors. In a second set of experiments we evaluated the effects of isothiocyanates on the mouse skin tumor-initiating activity of BaP. The isothiocyanates tested were BITC, PEITC, 6-phenylhexyl isothiocyanate (PHITC) and a series of isothiocyanates structurally related to polynuclear aromatic hydrocarbons: 9-phenanthryl isothiocyanate (9-PhenITC), 9-phenanthrylmethyl isothiocyanate (9-PhenMeITC), 6-chrysenyl isothiocyanate (6-ChrysITC) and 6-benzo[a]pyrenyl isothiocyanate (6-BaPITC). None of the isothiocyanates inhibited tumor development by BaP, and three of them--PHITC, 9-PhenITC and 9- PhenMeITC--enhanced skin tumor multiplicity. Taken together with available literature data, the results of this study suggest that different isothiocyanates selectively inhibit cytochrome P450 enzymes involved in the metabolic activation or detoxification of BaP and therefore have differing effects on BaP tumorigenesis.

Dose-related inhibition by dietary phenethyl isothiocyanate of esophageal tumorigenesis and DNA methylation induced by N-nitrosomethylbenzylamine in rats

The purpose of this investigation was to establish a dose response for the effects of dietary phenethyl isothiocyanate (PEITC) on N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumorigenesis and DNA methylation. Groups of 13-27 rats were randomly assigned to AIN-76A diets containing 0, 0.325, 0.75, 1.5 or 3.0 mumol PEITC/g. Two weeks later, rats were administered NMBA subcutaneously at a dose of 0.5 mg/kg once a week for 15 weeks. Animals were maintained on control or experimental diets for an additional 8 weeks and were terminated at week 25 of the experiment. No significant effects on weight gain or food intake were noted for any of the experimental diets when compared with control values. Animals receiving only NMBA developed 9.3 +/- 0.9 tumors/rat, with an incidence of 100%. Dietary PEITC at concentrations of 0.75, 1.5 and 3.0 mumol/g inhibited NMBA-induced esophageal tumor multiplicity by 39%, 90% and 100%, respectively. Esophageal tumor incidence in these groups was reduced by 0%, 40% and 100%, respectively. The 0.325 mumol/g PEITC diet did not significantly affect NMBA-induced esophageal tumorigenesis. These results indicate that the minimum inhibitory dietary concentration of PEITC is between 0.325 and 0.75 mumol/g. Groups of 20 rats were assigned to diets containing 0-3.0 mumol PEITC/g for two weeks as described above, and then sacrificed 24 hours after administration of [3H-methyl]NMBA. The esophageal DNA was isolated, purified, hydrolyzed, and analyzed by HPLC. PEITC inhibited DNA methylation in a dose-dependent manner, as was found in the tumor bioassay. The inhibition of tumor incidence was highly correlated with the percentage inhibition of either 7-methylguanine or O6-methylguanine. These latter results suggest that the inhibitory activity of PEITC in this model is manifested, at least in part, during the functional equivalent of tumor initiation.

The clastogenic effects of isothiocyanates

Four isothiocyanates (ITCs), three of which are commonly found in the human diet, were tested for the ability to induce chromosome aberrations in an SV40-transformed Indian muntjac cell line. Whilst allyl ITC was found to be inactive the other three (benzyl ITC, phenethyl ITC and phenyl ITC) were found to be significant inducers of chromosome damage in the absence of any metabolic activation. Given that experimental data have demonstrated that ITCs can also protect laboratory animals from the induction of tumours by model carcinogens it is proposed that the presence of ITCs in the human diet may, potentially, have both beneficial and harmful consequences depending on the levels consumed.

A review of the pharmacokinetics and pharmacodynamics of disulfiram and its metabolites

After ingestion, disulfiram (DSF) is rapidly converted, probably in the stomach, to its bis (diethyldithiocarbamato) copper complex. Consequently, absorption and distribution via the gastrointestinal mucosa into the blood might involve both the parent drug and its copper complex. In the blood, both compounds are rapidly degraded to form diethyldithiocarbamic acid (DDC), which is unstable and is further degraded to form diethylamine and carbon disulphide. DDC is also a substrate of phase II metabolism, which involves formation of diethyldithiomethylcarbamate (Me-DDC) and the glucuronic acid of DDC. Me-DDC also undergoes oxidative biotransformation to diethylthiomethylcarbamate (Me-DTC), which is further oxidized to its corresponding sulphoxide and sulphone metabolites. Me-DTC may to act as a suicide inhibitor with a preference for the mitochondrial low Km isozyme of aldehyde dehydrogenases (ALDH 1), whereas the two S-oxidized metabolites, especially the sulfone metabolite, are more potent inhibitors not only of ALDH 1, but also of the cytosolic high Km isozyme of ALDH (ALDH 2). The inhibitory reaction between the enzyme and each of the three metabolites is characterized by a covalent adduct formation, probably with the cysteine residue at the active site of the enzymes. The adduct formed is nonreducible at a physiological concentration of glutathione, and inactivation in the presence of this endogenous tripeptide was increased by action in vitro of the sulphoxide and sulphone metabolites. Those findings are all in concordance with the in vivo observations made on DSF. In human volunteers treated with increasing doses of DSF and challenged with ethanol between each of the dosage periods, the mean plasma concentrations of Me-DTC at steady state were proportional to the DSF doses given. There was also a close relationship between increased oxidative metabolic formation of Me-DTC, high oxidative formation of acetaldehyde, and the full complements of a valid disulfiram ethanol reaction (DER). Consequently, Me-DTC in plasma may not only serve as a marker of the oxidative metabolic function of the liver, but also of the therapeutic effectiveness of the treatment in subjects at steady state. Obviously, there is a need for individual dose-titration regimens. In patients with alcohol-related severe hepatocellular damage, the oxidative P 450 catalyzed formation of the Me-DTC and probably also of its sulfoxide and sulphone metabolites is impaired, and thus inactivation of ALDH activity in the liver appears to be delayed or even completely absent. The consequence for the patient may be an insufficient DER.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of cytochrome P-450IIE1 in N-nitroso-N-methylaniline induced hepatocyte cytotoxicity

1. The cytotoxicity of N-nitrosomethylaniline (NMA) towards hepatocytes isolated from rats was prevented by acetone or ethanol (inhibitors for cytochrome P-450IIE1) but not by metyrapone or SKF525A (inhibitors for cytochrome P-450IIB1/2). Various alcohols, secondary ketones and isothiocyanates that induced cytochrome P-450IIE1 were also found to be protective. Various aromatic and chlorinated hydrocarbon solvents that are substrates or inducers of cytochrome P-450IIE1 also prevented NMA cytotoxicity. Nitrogen-containing heterocycles that induced cytochrome P-450IIE1 were less effective. Further evidence that cytochrome P-450IIE1 was responsible for the activation of NMA was the marked increase in hepatocyte susceptibility if hepatocytes from pyrazole-induced rats were used. 2. NMA was more cytotoxic to hepatocytes isolated from phenobarbital-pretreated rats than uninduced rats. However, metyrapone now prevented and SKF525A delayed the cytotoxicity whereas ethanol, acetone, allyl isocyanate, isoniazid or trichloroethylene had no effect on the susceptibility of phenobarbital-induced hepatocytes. Furthermore, microsomes isolated from phenobarbital-pretreated rats had higher NMA-N-demethylase activity which was more inhibited by metyrapone and SKF525A than that of uninduced microsomal activity. By contrast the N-demethylase activity of phenobarbital induced microsomes was more resistant to acetone, ethanol, hexanal, trichloroethylene and toluene than uninduced microsome. 3. The above results suggest that cytochrome P-450IIE1 catalyses the cytotoxic activation of NMA in normal or pyrazole-induced hepatocytes whereas cytochrome P-450IIB1/2 is responsible for cytotoxicity in phenobarbital-induced hepatocytes.

Effect of frequency of isothiocyanate administration on inhibition of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced pulmonary adenoma formation in A/J mice

The abilities of phenethyl isothiocyanate (PEITC) and 6-phenylhexyl isothiocyanate (PHITC) to inhibit 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenicity, when administered by a standard four-dose protocol or by a single-dose protocol, were determined. Corn oil or isothiocyanates were administered once or for four consecutive days by gavage, with the final (or single) administration of corn oil or inhibitor occurring 2 h prior to a single i.p. injection of NNK (10 mumol/mouse). Sixteen weeks following NNK administration, the experiment was terminated and pulmonary adenomas were quantitated. Pretreatment with PEITC at a dose of 5 mumol or PHITC at a dose of 0.2 mumol resulted in significant reductions of tumor multiplicity compared to control, regardless of whether each isothiocyanate was administered once or four times. For both isothiocyanates, there were no statistically significant differences between dosing frequencies in inhibitory effects on tumor multiplicities and tumor incidences. Furthermore, the results achieved following a single pretreatment with either isothiocyanate were in good agreement with previous results obtained utilizing the four-dose protocol. Thus, it appears that most or all of the inhibitory potential of the four-dose protocol is due to the final dose of isothiocyanate.

Effects of disulfiram on hepatic P450IIE1, other microsomal enzymes, and hepatotoxicity in rats

Disulfiram, widely used in avoidance therapy for alcohol abuse, has been shown to have protective effects against chemically induced toxicity and carcinogenesis. The purpose of this work was to elucidate the biochemical mechanisms of this protective action by examining its effects on cytochrome P450IIE1 and other related microsomal enzyme activities. When a dose of disulfiram was given intragastrically to rats, a very rapid decrease of N-nitrosodimethylamine (NDMA) demethylase activity, possibly due to the inactivation of P450IIE1, was seen. The loss of P450IIE1 protein from the microsomal membrane was observed at 18 hr after receiving disulfiram, but not within the first 5 hr after the treatment. P450IIB1, on the other hand, was induced markedly between 15 and 72 hr after the disulfiram treatment. The treatment, however, caused only moderate changes in some other P450 isozymes. Carbon disulfide, a putative metabolite of disulfiram, produced similar effects on P450IIE1, but with shorter duration. Carbon disulfide, however, did not induce P450IIB1. Diethyldithiocarbamate, a reductive product of disulfiram, was an inhibitor of P450IIE1 activity in vitro, and upon preincubation with microsomes, it produced an NADPH-dependent inactivation of NDMA demethylase activity. The results suggest that this or other metabolites of disulfiram are inhibitors of P450IIE1 and are responsible for the inactivation of P450IIE1 in vivo. Hepatotoxicity of NDMA or CCI4 in rats was blocked by pretreatment with disulfiram. The present work demonstrates that P450IIE1 was inhibited and inactivated by disulfiram, and this mechanism can account for many of the reported inhibitory actions of disulfiram against chemically induced toxicity and carcinogenesis.