Relationship between cytochrome P450 2E1 and acetone catabolism in rats as studied with diallyl sulfide as an inhibitor

Evidence-based capacity of natural cytochrome enzyme inhibitors to increase the effectivity of antineoplastic drugs

Cytochrome (CYP) enzymes catalyze the metabolism of numerous exogenous and endogenous substrates in cancer therapy leading to significant drug interactions due to their metabolizing effect. CYP enzymes play an important role in the metabolism of essential anticancer medications. They are shown to be overexpressed in tumor cells at numerous locations in the body. This overexpression could be a result of lifestyle factors, presence of hereditary variants of CYP (Bio individuality) and multi-drug resistance. This finding has sparked an interest in using CYP inhibitors to lower their metabolizing activity as a result facilitating anti-cancer medications to have a therapeutic impact. As a result of the cytotoxic nature of synthetic enzyme inhibitors and the increased prevalence of herbal medication, natural CYP inhibitors have been identified as an excellent way to inhibit overexpression sighting their tendency to show less cytotoxicity, lesser adverse drug reactions and enhanced bioavailability. Nonetheless, their effect of lowering the hindrance caused in chemotherapy due to CYP enzymes remains unexploited to its fullest. It has been observed that there is a substantial decrease in first pass metabolism and increase in intestinal absorption of chemotherapeutic drugs like paclitaxel when administered along with flavonoids which help suppress certain specific cytochrome enzymes which play a role in paclitaxel metabolism. This review elaborates on the role and scope of phytochemicals in primary, secondary and tertiary care and how targeted prevention of cancer could be a breakthrough in the field of chemotherapy and oncology. This opens up a whole new area of research for delivery of these natural inhibitors along with anticancer drugs with the help of liposomes, micelles, nanoparticles, the usage of liquid biopsy analysis, artificial intelligence in medicine, risk assessment tools, multi-omics and multi-parametric analysis. Further, the site of action, mechanisms, metabolites involved, experimental models, doses and observations of two natural compounds, quercetin & thymoquinone, and two plant extracts, liquorice & garlic on CYP enzymes have been summarized.

Alcoholic Liver Disease: Alcohol Metabolism, Cascade of Molecular Mechanisms, Cellular Targets, and Clinical Aspects

Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

Porcine cytochrome 2A19 and 2E1

Cytochrome P450 (CYP) is a major group of enzymes, which conduct Phase I metabolism. Among commonly used animal models, the pig has been suggested as the most suitable model for investigating drug metabolism in human beings. Moreover, porcine CYP2A19 and CYP2E1 are responsible for the biotransformation of both endogenous and exogenous compounds such as 3‐methylindole (skatole), sex hormones and food compounds. However, little is known about the regulation of porcine CYP2A19 and CYP2E1. In this MiniReview, we summarise the current knowledge about the regulation of porcine CYP2A19 and CYP2E1 by environmental, biological and dietary factors. Finally, we reflect on the need for further research, to clarify the interaction between active feed components and the porcine CYP system.

Combination treatment of epilepsy with ketogenic diet and concurrent pharmacological inhibition of cytochrome P450 2E1

While most epileptic patients respond to treatment with existing antiepileptic drugs, there remains a considerable number of patients in whom these drugs do not suffice. Such patients, particularly children, are often treated using the ketogenic diet. This diet imposes a strict limit on carbohydrates; while providing for adequate protein, most of the calories are supplied as triacylglycerol, much of which is metabolized to ketone bodies. Animal experiments have provided evidence that the anticonvulsant effect of the ketogenic diet is mediated by acetone and correlates with blood acetone levels. Acetone can be converted in vivo to glucose via acetol and pyruvate; the initial conversion to acetol is catalyzed by cytochrome P450 2E1 (CYP2E1). When CYP2E1 knockout mice are subjected to starvation to induce ketogenesis, they develop blood acetone levels much higher than those observed in wild-type mice. Similarly, pharmacological inhibition of CYP2E1 significantly increases blood acetone levels in rat and man. Taken together, these observations suggest that pharmacological inhibition of CYP2E1 has the potential to significantly increase the antiepileptic effect of the ketogenic diet. With patients that respond insufficiently to the diet alone, increased acetone levels may improve response. With patients who respond sufficiently to the diet, CYP2E1 inhibitors might allow a relaxation of the fairly severe diet regimen and so improve compliance and quality of life. An existing inhibitor of CYP2E1 is the drug disulfiram. This drug also inhibits the enzyme aldehyde dehydrogenase, which functions in alcohol degradation, and in this capacity has long been used in the treatment of alcohol addiction. Disulfiram inhibits CYP2E1 at conventional therapeutic dosages and increases blood acetone levels in humans and animals. It should therefore be a viable candidate for the proposed drug/diet combination treatment.

Knockdown of Cytochrome P450 2E1 Inhibits Oxidative Stress and Apoptosis in the cTnT R141W Dilated Cardiomyopathy Transgenic Mice

Cytochrome P450 2E1 (CYP2E1) is a cytochrome P450 enzyme that catalyzes the metabolism of toxic substrates. CYP2E1 is upregulated in heart disease, including the dilated cardiomyopathy (DCM) mouse model. Here, knockdown of CYP2E1 significantly ameliorated the dilated left ventricle, thin wall, and dysfunctional contraction in the cTnT R141W and adriamycin-induced DCM mouse models. Interstitial fibrosis, poorly organized myofibrils, and swollen mitochondria with loss of cristae were improved in the myocardium of α-myosin heavy chain (MHC)-cTnT R141W ×CYP2E1-silence double-transgenic mice when compared with the cTnT R141W transgenic mice. Oxidative stress, the activation of caspase 3 and caspase 9, the release of cytochrome c , and the apoptosis in the myocardium were significantly decreased in double-transgenic mice compared with the cTnT R141W transgenic mice. In summary, the expression of CYP2E1 is upregulated in heart disease and might be induced by hypoxemia in cardiomyopathy. The overexpression of CYP2E1 can enhance the metabolism of endogenous ketones to meet the energy demand of the heart in certain disease states, but the overexpression of CYP2E1 can also increase oxidative stress and apoptosis in the DCM heart. Knockdown or downregulation of CYP2E1 might be a therapeutic strategy to control the development of DCM after mutations of cTnT R141W or other factors, because DCM is the third most common cause of heart failure and the most frequent cause of heart transplantation.

Harmful and beneficial effects of organic monosulfides, disulfides, and polysulfides in animals and humans

Many organic sulfides (mono-, di-, and polysulfides) are present in our environment. Simple derivatives are produced by some plants and animals, while complex sulfides are secondary metabolites of several genera of bacteria and fungi. Sulfides play an important role in the smell and taste of food, and many such compounds are used as food flavorings. Some sulfides are toxic, and there is evidence that such toxicity is caused by the ability of these substances to generate reactive oxygen species. Some sulfides, however, have been shown to protect against toxicants and carcinogens. These beneficial effects are believed to involve, at least in part, the ability of sulfides to inhibit the enzymatic activation of pro-toxicants and to increase tissue activities of enzymes that protect against electrophiles. Some sulfides also have potential as cancer chemotherapeutics. In this review, the toxic and beneficial effects of sulfides in animals are described, and the possible value of sulfides in cancer chemoprotection and cancer chemotherapy is discussed.

Chemopreventive effects of tolfenamic acid against esophageal tumorigenesis in rats

The primary objective of this study is to identify small molecules that target critical transcription factors for potential application in the chemoprevention of esophageal cancer. Specificity proteins (Sp) play a critical role in the growth and metastasis of several malignancies including esophageal cancer. Researchers at the M. D. Anderson Cancer Center Orlando Cancer Research Institute have reported previously that tolfenamic acid (TA) inhibits cancer cell proliferation and tumor growth through the degradation of Sp1, Sp3, and Sp4. We evaluated the chemopreventive properties of TA against esophageal tumorigenesis in N-nitrosomethylbenzylamine (NMBA)-induced murine tumor model. Fischer-344 rats were treated with NMBA (0.5 mg/kg s.c. 3 times a week) for 5 weeks to initiate the tumor formation, and then treated with 50 mg/kg TA from week 6 through week 25. Tumor incidence, tumor multiplicity (number of papilloma per rat), and tumor volume were evaluated after 25 weeks. All rats in the control group that received only NMBA developed lesions (100% incidence), while the TA-treated group showed significantly lower (33%) tumor incidence and tumor multiplicity. Furthermore, the tumor volume was significantly diminished in the TA-treated group when compared with the control group. Using small molecules such as TA to target key transcription factors associated with tumorigenesis for the prevention of esophageal malignancies is a new and promising strategy. Results of the current study provide evidence that TA, when given orally after tumor initiation, can significantly suppress tumorigenesis induced by carcinogenic nitrosamines in rats. These appealing results demonstrate that TA may potentially serve as an effective chemopreventive agent in patient populations vulnerable to esophageal cancer.

Disposition and metabolism of dipropyl disulphide in vivo in rat

The metabolism of dipropyl disulphide (DPDS), a sulphur compound from onion, was investigated in vivo in the rat. A single dose (200 mg kg(-1)) was administered by gastric intubation and the time courses of DPDS and its metabolites were followed over 48 h by gas chromatography coupled with mass spectrometry in the stomach, intestine, liver, and blood. DPDS was detected in the stomach where it was transformed into propyl mercaptan, whereas the liver contained only traces of DPDS and none at all in the other examined organs. The metabolites methylpropyl sulphide, methylpropyl sulphoxide (MPSO), and methylpropyl sulphone (MPSO2) were sequentially formed in the liver. The route of elimination from the liver seemed to be mainly via the blood. The bile also participated in the excretory process, but only for MPSO2. The pharmacokinetic parameters were determined for all of the above compounds. Whereas the bioavailability of DPDS was very low (0.008 h mM), the areas under the curve were higher for the S-oxidized metabolites MPSO and MPSO2, i.e. 9.64 and 24.15 h mM, respectively. The half-lives for DPDS and its metabolites varied between 2.0 and 8.25 h, except for MPSO2, which had a half-life of 29.6 h. MPSO2 was the most abundant and persistent of these metabolites.

A ketogenic diet and diallyl sulfide do not elevate afterdischarge thresholds in adult kindled rats

INTRODUCTION

Acetone has been shown to have broad-spectrum anticonvulsant actions in animal seizure models and has been hypothesized to play a role in the anticonvulsant mechanism of the ketogenic diet (KD). The present study examined the ability of a KD to elevate amygdaloid afterdischarge thresholds (ADT) in fully kindled rats. The effects of the KD were studied in the presence and absence of diallyl sulfide (DAS), an inhibitor of acetone metabolism.

METHODS

Twenty-four adult male rats were kindled to 30 stage 5 seizures. Afterdischarge thresholds (ADT) were determined. Subjects were then administered one of the following diets: (1) KD+V (vehicle; KD+V); (2) KD+DAS; (3) control diet+V (CD+V); (4) CD+DAS. They were stimulated every second day. Blood sampling was performed every second day--on non-stimulating days--to determine levels of glucose, beta-hydroxybutyrate, acetoacetate, and acetone. After 20 days, ADTs were re-determined.

RESULTS

Blood acetone concentrations were significantly higher in the KD+DAS group as compared to the other groups, although they did not reach "therapeutic levels". None of the treatments, however, elevated ADTs.

CONCLUSIONS

The KD was unable to elevate amygdaloid ADTs in fully kindled rats. Although subjects in the KD+DAS group achieved significant elevations of blood acetone, these concentrations (e.g. 0.2 mM) were much lower than those (>2.0 mM) previously shown to confer anticonvulsant activity. There appears to be large difference between humans and rats in their ability to produce elevated blood acetone levels on the KD. These data suggest that adult rats are not ideal subjects for modeling the anticonvulsant actions of the KD.

Seleno-compounds in garlic and onion

Garlic (Allium sativum) and onion (Allium cepa) are widely known for their biological properties but are far from having revealed all of their secrets even if the compounds involved in the biological mechanisms, flavenols, sulphur and seleno compounds have been identified. The beneficial effect of garlic on health including protection against cardiovascular diseases and cancers results from all of these compounds although their individual involvement is complex. Garlic and onion, broccoli, wild leek, have the ability to accumulate the selenium (Se) from soil. These Se-enriched plants present a greater protection against carcinogenesis than the common plants and two Se-compounds possessing anti-cancer activity have been identified: Se-methyl selenocysteine and gamma-glutamyl-Se-methyl selenocysteine. However, several Se-compounds from Se-enriched garlic or onion remain unidentified. The techniques for the detection of Se-species are numerous but few methods are able to identify the detected compounds. The very small quantities of Se-compounds present and the clear lack of standards do not make their analysis straightforward, particularly for non-enriched samples. Over the last 10 or so years development of the synthesis of Se-compounds and the use of GC-AED or EC/HPLC-ICP-MS have shown considerable possibilities. These techniques have allowed advances in the identification of Se-compounds, some of which are analogues of S-compounds in plants and yeasts. When these techniques are coupled to EC/HPLC-APCI-MS-MS, they provide a lot of information about the Se-biosynthesis in garlic. This has allowed the preferential formation of methylated compounds in Se-biochemistry to be identified, in contrast to the sulphur biochemistry of the Allium spp. in which compounds containing propenylic groups predominate. This review focuses on the recent advances in the analytical methods of Se-compounds in garlic and onion and particular attention is given to the biological properties of Se-species identified in Se-enriched plants.

Analytical method for appreciation of garlic therapeutic potential and for validation of a new formulation

The consumption of garlic reduces the risk of cardiovascular disease and cancer, S-allylcysteine sulfoxide (alliin), allicin (DATi), diallyl disulfide (DADS), S-allylcysteine (SAC) and several storage dipeptides are the organo-sulphur compounds (OSC) involved in the protective mechanism of garlic against cardiovascular disorders and carcinogenesis. Thus it is very interesting to quantify simultaneously all these compounds in different garlic powders obtained in several cultural conditions. The quantification of OSC by a new ion-pair HPLC method allowed showing the general sulphur-dependence positive effect of garlic on cardiovascular disorder and carcinogenesis and the variable specific activity of each implicated OSC. The screening of 11 garlic tablets proposed on the market showed the variability and particularly the differential instability of each OSC. From these results, a new garlic tablet was realised and each step was controlled by this method. This analytical method proved to be a very powerful tool for the understanding of the garlic protective mechanism against cancer and cardiovascular diseases and the development and quality control of garlic tablets.

The discovery of the microsomal ethanol oxidizing system and its physiologic and pathologic role

Oxidation of ethanol via alcohol dehydrogenase (ADH) explains various metabolic effects of ethanol but does not account for the tolerance. This fact, as well as the discovery of the proliferation of the smooth endoplasmic reticulum (SER) after chronic alcohol consumption, suggested the existence of an additional pathway which was then described by Lieber and DeCarli, namely the microsomal ethanol oxidizing system (MEOS), involving cytochrome P450. The existence of this system was initially challenged but the effect of ethanol on liver microsomes was confirmed by Remmer and his group. After chronic ethanol consumption, the activity of the MEOS increases, with an associated rise in cytochrome P450, especially CYP2E1, most conclusively shown in alcohol dehydrogenase negative deer mice. There is also cross-induction of the metabolism of other drugs, resulting in drug tolerance. Furthermore, the conversion of hepatotoxic agents to toxic metabolites increases, which explains the enhanced susceptibility of alcoholics to the adverse effects of various xenobiotics, including industrial solvents. CYP2E1 also activates some commonly used drugs (such as acetaminophen) to their toxic metabolites, and promotes carcinogenesis. In addition, catabolism of retinol is accelerated resulting in its depletion. Contrasting with the stimulating effects of chronic consumption, acute ethanol intake inhibits the metabolism of other drugs. Moreover, metabolism by CYP2E1 results in a significant release of free radicals which, in turn, diminishes reduced glutathione (GSH) and other defense systems against oxidative stress which plays a major pathogenic role in alcoholic liver disease. CYP1A2 and CYP3A4, two other perivenular P450s, also sustain the metabolism of ethanol, thereby contributing to MEOS activity and possibly liver injury. CYP2E1 has also a physiologic role which comprises gluconeogenesis from ketones, oxidation of fatty acids, and detoxification of xenobiotics other than ethanol. Excess of these physiological substrates (such as seen in obesity and diabetes) also leads to CYP2E1 induction and nonalcoholic fatty liver disease (NAFLD), which includes nonalcoholic fatty liver and nonalcoholic steatohepatitis (NASH), with pathological lesions similar to those observed in alcoholic steatohepatitis. Increases of CYP2E1 and its mRNA prevail in the perivenular zone, the area of maximal liver damage. CYP2E1 up-regulation was also demonstrated in obese patients as well as in rat models of obesity and NASH. Furthermore, NASH is increasingly recognized as a precursor to more severe liver disease, sometimes evolving into "cryptogenic" cirrhosis. The prevalence of NAFLD averages 20% and that of NASH 2% to 3% in the general population, making these conditions the most common liver diseases in the United States. Considering the pathogenic role that up-regulation of CYP2E1 also plays in alcoholic liver disease (vide supra), it is apparent that a major therapeutic challenge is now to find a way to control this toxic process. CYP2E1 inhibitors oppose alcohol-induced liver damage, but heretofore available compounds are too toxic for clinical use. Recently, however, polyenylphosphatidylcholine (PPC), an innocuous mixture of polyunsaturated phosphatidylcholines extracted from soybeans (and its active component dilinoleoylphosphatidylcholine), were discovered to decrease CYP2E1 activity. PPC also opposes hepatic oxidative stress and fibrosis. It is now being tested clinically.

Allicin and allicin-derived garlic compounds increase breath acetone through allyl methyl sulfide: use in measuring allicin bioavailability

Progress in establishing systemic pharmacological effects for fresh, crushed garlic (Allium sativum L) in humans has been hindered by (1) the inability to measure allicin bioavailability, (2) lack of direct evidence that allicin has significant systemic activity at doses of garlic normally consumed, and (3) lack of a model for an acute effect. We have addressed these problems by quantifying the increases in breath acetone and breath allyl methyl sulfide (AMS). The area under the 48 h curve was measured in humans after consumption of standardized garlic preparations, allicin, and allicin-derived compounds, at the equivalent of 7 g of crushed garlic. It was shown that the allyl thiosulfinates (mainly allicin) are solely responsible for breath AMS and increased breath acetone. Diallyl trisulfide, diallyl disulfide, ajoene, and S-allylmercaptocysteine, at isomolar dithioallyl, showed the same quantitative effects as allicin. Consumption of AMS at isomolar allyl also gave the same effects as allicin, indicating that AMS is the main metabolite of allicin and is an active metabolite. In conclusion, allicin and allicin-derived compounds are rapidly metabolized to AMS, a compound which stimulates the production of acetone and which can be used to measure the bioavailability of allicin and, hence, the ability of garlic supplements to represent fresh garlic.

Inhibition by allyl sulfides and crushed garlic of O6-methylguanine formation in liver DNA of dimethylnitrosamine-treated rats

Garlic consumption is linked with lower incidences of certain cancers perhaps because garlic-derived allyl sulfides inhibit nitrosamine activation by cytochrome P450s. To help evaluate this view, effects of allyl sulfides on O6-methylguanine (O6MG) levels were examined in liver of rats injected with 20 mg/kg of liver carcinogen dimethylnitrosamine (DMN) and killed 3 h later. DNA was isolated and hydrolyzed, and O6MG/guanine ratios were determined by HPLC-fluorescence. Mean inhibition of O6MG formation fell from 89% for 200 to 33% for 12 mg diallyl sulfide (DAS) per kilogram gavaged 18 h before DMN injection. Gavage of DAS 3 or 6 h (instead of 18 h) before DMN injection significantly reduced inhibitions. Mean inhibitions for diallyl disulfide, diallyl sulfoxide, and diallyl sulfone (75-100 mg/kg) gavaged 18 h before DMN were 39%, 72%, and 82%. In lung and kidney, DAS produced mean inhibitions of 98% and 74% compared with 89% in liver. When methylnitrosourea was injected instead of DMN, neither DAS nor DADS inhibited O6MG formation in liver DNA. Feeding 2.5% garlic for 7 days inhibited DMN-induced O6MG formation in liver DNA by 46%, similar to that expected from the estimated yield of allyl sulfides from garlic. Hence, dosing with DAS or feeding garlic may be useful chemopreventive strategies against nitrosamine-induced cancers.

Metabolism of alcohol

Most tissues of the body contain enzymes capable of ethanol oxidation or nonoxidative metabolism, but significant activity occurs only in the liver and, to a lesser extent, in the stomach. Hence, medical consequences are predominant in these organs. In the liver, ethanol oxidation generates an excess of reducing equivalents, primarily as NADH, causing hepatotoxicity. An additional system, containing cytochromes P-450 inducible by chronic alcohol feeding, was demonstrated in liver microsomes and found to be a major cause of hepatotoxicity.

Bacterial acetone carboxylase is a manganese-dependent metalloenzyme

Bacterial acetone carboxylase catalyzes the ATP-dependent carboxylation of acetone to acetoacetate with the concomitant production of AMP and two inorganic phosphates. The importance of manganese in Rhodobacter capsulatus acetone carboxylase has been established through a combination of physiological, biochemical, and spectroscopic studies. Depletion of manganese from the R. capsulatus growth medium resulted in inhibition of acetone-dependent but not malate-dependent cell growth. Under normal growth conditions (0.5 microm Mn2+ in medium), growth with acetone as the carbon source resulted in a 4-fold increase in intracellular protein-bound manganese over malate-grown cells and the appearance of a Mn2+ EPR signal centered at g = 2 that was absent in malate-grown cells. Acetone carboxylase purified from cells grown with 50 microm Mn2+ had a 1.6-fold higher specific activity and 1.9-fold higher manganese content than cells grown with 0.5 microm Mn2+, consistently yielding a stoichiometry of 1.9 manganese/alpha2beta2gamma2 multimer, or 0.95 manganese/alphabetagamma protomer. Manganese in acetone carboxylase was tightly bound and not removed upon dialysis against various metal ion chelators. The addition of acetone to malate-grown cells grown in medium depleted of manganese resulted in the high level synthesis of acetone carboxylase (15-20% soluble protein), which, upon purification, exhibited 7% of the activity and 6% of the manganese content of the enzyme purified from acetone-grown cells. EPR analysis of purified acetone carboxylase indicates the presence of a mononuclear Mn2+ center, with possible spin coupling of two mononuclear sites. The addition of Mg.ATP or Mg.AMP resulted in EPR spectral changes, whereas the addition of acetone, CO2, inorganic phosphate, and acetoacetate did not perturb the EPR. These studies demonstrate that manganese is essential for acetone carboxylation and suggest a role for manganese in nucleotide binding and activation.

Role of CYP2E1 in ketone-stimulated insulin release in pancreatic B-cells

The role of CYP2E1 in ketone-stimulated insulin release was investigated using isolated pancreatic islets of Langerhans and two mammalian insulin secreting pancreatic beta-cell lines engineered to stably express human CYP2E1 (designated BRIN BD11h2E1 and INS-1h2E1). Isolated rat pancreatic islets were shown to express the CYP2E1 isoform which was inducible by pretreatment of animals with acetone. The cDNA encoded CYP2E1 was expressed and inducible in the engineered cells as shown by Western blotting. The transfected protein was enzymatically active in the heterologous cells as determined by p-nitrophenol hydroxylation rates (0.176 +/- 0.08 vs. 0.341 +/- 0.08 nmol/min/mg microsomal protein in BRIN BD11 control cells and BRIN BD11h2E1 cells respectively, P < 0.001; 0.204 +/- 0.03 vs. 0.633 +/- 0.102 nmol/min/mg microsomal protein in INS-1 and INS-1h2E1, respectively, P < 0.001). Cultivation of CYP2E1 expressing BRIN BD11h2E1 and INS-1h2E1 cells in 40 mM ethanol increased the rate of p-nitrophenol hydroxylation (0.968 +/- 0.09 nmol/min/mg microsomal protein, P < 0.001 and 0.846 +/- 0.103 nmol/min/mg microsomal protein, P < 0.001, respectively) providing further evidence that the heterologous protein is inducible. Cultivation of control cells with ethanol had no observable effect (0.186 +/- 0.05 and 0.195 +/- 0.03 in BRIN BD11 and INS-1, respectively). These cell lines also express NADPH-cytochrome P450 reductase protein which was enzymatically active (0.632 +/- 0.023 in parental BRIN BD11 vs. 0.657 +/- 0.066 without ethanol and 0.824 +/- 0.014 nmol/min/mg microsomal protein with ethanol in BRIN BD11h2E1, P < 0.05; and 1.568 +/- 0.118 in parental INS-1 vs. 1.607 +/- 0.093 without ethanol and 1.805 +/- 0.066 nmol/min/mg microsomal protein with ethanol in INS-1h2E1, P < 0.05) thereby providing a functional cytochrome P450 system. The insulin secretory response of control cell lines and islets was similar to cell lines and islets which had been chemically pretreated, to induce CYP2E1 expression, in response to known nutrient secretagogues. However, insulin output was significantly higher in pretreated islets (1.3-fold, P < 0.05) and CYP2E1 expressing cell lines (BRIN BD11h2E1 2.3-fold, P < 0.001; INS1-1h2E1 1.6-fold, P < 0.001) when stimulated with the ketone 3-hydroxybutyrate than control islets and parental cell lines respectively. Similar acute exposure to acetoacetate enhanced insulin secretion by 1.3-fold (P < 0.05) in pretreated islets, 2.6-fold (P < 0.001) in ethanol pretreated BRIN BD11h2E1 and 1.4-fold (P < 0.001) in ethanol pretreated INS-1h2E1 cells compared to the respective control islets or ethanol pretreated control parental cells. Therefore, these studies highlight a possible role for CYP2E1 in pancreatic cell dysfunction.

Organosulfur compounds from alliaceae in the prevention of human pathologies

A strong association between elevated plasma low-density-lipoprotein (LDL) and the development of cardiovascular diseases (CVD) has been established. Oxidation of LDL (Ox-LDL) promotes vascular dysfunction, enhances the production and release of inflammatory mediators such as reactive oxygen species and contribute to the initiation and progression of atherosclerosis. In addition, Ox-LDL enhances the production and release of tumor necrosis factor (TNF-alpha), interleukin (IL)-6, arachidonic acid metabolites and nitric oxide (NO) that are responsible for various human pathologies including cancer. Organosulfur compounds (OSC) from alliaceae modulate the glutathione (GSH) redox cycle and inhibits NFkappa-B activation in human T cells. Furthermore, OSC bioactivities include antioxidant, antibacterial, anticarcinogenic, antiatherogenic, immunostimulatory, and liver protection potential.

Spontaneous oxidative stress and liver tumors in mice lacking methionine adenosyltransferase 1A

In mammals, methionine metabolism occurs mainly in the liver via methionine adenosyltransferase-catalyzed conversion to S-adenosylmethionine. Of the two genes that encode methionine adenosyltransferase(MAT1Aand MAT2A), MAT1A is mainly expressed in adult liver whereas MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic S-adenosylmethionine content and hyperplasia and spontaneously develop nonalcoholic steatohepatitis. In this study, we examined whether chronic hepatic S-adenosylmethionine deficiency generates oxidative stress and predisposes to injury and malignant transformation. Differential gene expression in MAT1A knockout mice was analyzed following the criteria of the Gene Ontology Consortium. Susceptibility of MAT1A knockout mice to CCl4-induced hepatotoxicity and malignant transformation was determined in 3- and 18-month-old mice, respectively. Analysis of gene expression profiles revealed an abnormal expression of genes involved in the metabolism of lipids and carbohydrates in MAT1A knockout mice, a situation that is reminiscent of that found in diabetes, obesity, and other conditions associated with nonalcoholic steatohepatitis. This aberrant expression of metabolic genes in the knockout mice was associated with hyperglycemia, increased hepatic CYP2E1 and UCP2 expression and triglyceride levels, and reduced hepatic glutathione content. The knockout animals have increased lipid peroxidation and enhanced sensitivity to CCl4-induced liver damage, which was largely due to increased CYP2E1 expression because diallyl sulfide, an inhibitor of CYP2E1, prevented CCl4-induced liver injury. Hepatocellular carcinoma developed in more than half of the knockout mice by 18 months of age. Taken together, our findings define a critical role for S-adenosylmethionine in maintaining normal hepatic function and tumorigenesis of the liver.

Biochemical, molecular, and genetic analyses of the acetone carboxylases from Xanthobacter autotrophicus strain Py2 and Rhodobacter capsulatus strain B10

Acetone carboxylase is the key enzyme of bacterial acetone metabolism, catalyzing the condensation of acetone and CO(2) to form acetoacetate. In this study, the acetone carboxylase of the purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus was purified to homogeneity and compared to that of Xanthobacter autotrophicus strain Py2, the only other organism from which an acetone carboxylase has been purified. The biochemical properties of the enzymes were virtually indistinguishable, with identical subunit compositions (alpha(2)beta(2)gamma(2) multimers of 85-, 78-, and 20-kDa subunits), reaction stoichiometries (CH(3)COCH(3) + CO(2) + ATP-->CH(3)COCH(2)COO(-) + H(+) + AMP + 2P(i)), and kinetic properties (K(m) for acetone, 8 microM; k(cat) = 45 min(-1)). Both enzymes were expressed to high levels (17 to 25% of soluble protein) in cells grown with acetone as the carbon source but were not present at detectable levels in cells grown with other carbon sources. The genes encoding the acetone carboxylase subunits were identified by transposon mutagenesis of X. autotrophicus and sequence analysis of the R. capsulatus genome and were found to be clustered in similar operons consisting of the genes acxA (beta subunit), acxB (alpha subunit), and acxC (gamma subunit). Transposon mutagenesis of X. autotrophicus revealed a requirement of sigma(54) and a sigma(54)-dependent transcriptional activator (AcxR) for acetone-dependent growth and acetone carboxylase gene expression. A potential sigma(54)-dependent promoter 122 bp upstream of X. autotrophicus acxABC was identified. An AcxR gene homolog was identified 127 bp upstream of acxA in R. capsulatus, but this activator lacked key features of sigma(54)-dependent activators, and the associated acxABC lacked an apparent sigma(54)-dependent promoter, suggesting that sigma(54) is not required for expression of acxABC in R. capsulatus. These studies reveal a conserved strategy of ATP-dependent acetone carboxylation and the involvement of transcriptional enhancers in acetone carboxylase gene expression in gram-negative acetone-utilizing bacteria.

Mechanisms by which garlic and allyl sulfur compounds suppress carcinogen bioactivation. Garlic and carcinogenesis

Overall, a host of studies provides compelling evidence that garlic and its organic allyl sulfur components are effective inhibitors of the cancer process. These studies reveal that the benefits of garlic are not limited to a specific species, to a particular tissue, or to a specific carcinogen. Several mechanisms are likely to account for this protection. Notable among these is a depression in nitrosamine formation and a reduction in carcinogen bioactivation. The benefits provided by garlic must be viewed as part of the entire diet, since several dietary constituents can influence the degree of protection. More than one compound is responsible for the anticancer properties associated with garlic. Future research should focus on how genetic variability and daily environmental factors influence the anticancer benefits attributed to garlic and its allyl sulfur components.

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 circadian rhythmicity of carbon tetrachloride hepatotoxicity

The toxicity of carbon tetrachloride (CCl(4)) and certain other chemicals varies over a 24-h period. Because the metabolism of some drugs follows a diurnal rhythm, it was decided to investigate whether the hepatic metabolic activation of CCl(4) was rhythmic and coincided in time with maximum susceptibility to CCl(4) hepatotoxicity. A related objective was to test the hypothesis that abstinence from food during the sleep cycle results in lipolysis and formation of acetone, which participates in induction of liver microsomal cytochrome P450IIE1 (CYP2E1), resulting in a diurnal increase in CCl(4) metabolic activation and acute liver injury. Groups of fed and fasted male Sprague-Dawley rats were given a single oral dose of 800 mg of CCl(4)/kg at 2- to 4-h intervals over a 24-h period. Serum enzyme activities, measured 24 h post dosing as indices of acute liver injury, exhibited distinct maxima in both fed and fasted animals dosed with CCl(4) near the beginning of their dark/active cycle. Blood acetone, hepatic CYP2E1 activity, and covalent binding of (14)CCl(4)/metabolites to hepatic microsomal proteins in untreated rats fed ad libitum followed circadian rhythms similar to that of susceptibility to CCl(4). Parallel fluctuations of greater amplitude were seen in rats fasted for 24 h. Hepatic glutathione levels were lowest at the time of greatest susceptibility to CCl(4). Acetone dose-response experiments showed high correlations between blood acetone levels, CYP2E1 induction, and CCl(4)-induced liver injury. Pretreatment with diallyl sulfide suppressed CYP2E1 and abolished the circadian rhythmicity of susceptibility to CCl(4). These findings provide additional support for acetone's physiological role in CYP2E1 induction and for CYP2E1's role in modulating CCl(4) chronotoxicity in rats.

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.

The effects of alcohol and diallyl sulphide on CYP2E1 activity in humans: a phenotyping study using chlorzoxazone

The effects of acute administration of dietary levels of ethanol and the garlic oil extract, diallyl sulphide (DAS), on cytochrome P450 2E1 (CYP2E1) activity in volunteers were studied using the selective probe substrate, chlorzoxazone (CZX). The ratio of the CZX metabolite 6- hydroxychlorzoxazone (6-OHCZX) to CZX was taken to indicate CYP2E1 activity. The mean differences between the baseline and DAS-treated (0.2 mg/kg) CYP2E1 activities were significantly different (two-tailed p value = 0.0242, n = 8). Likewise, the mean differences between the baseline and ethanol-treated (0.8 g/kg) CYP2E1 activities were also significantly different (two-tailed p value = 0.0005, n = 7). The reduction in in vivo CYP2E1 activity by DAS is consistent with reported inhibition observed in vitro. The marked reduction in CYP2E1 activity following acute ingestion of ethanol is consistent with a competitive inhibition mechanism of CZX metabolism. The inhibitory effect of DAS maybe additive with daily consumption of Allium vegetables in particular. This may explain the lower 6-OHCZX/CZX metabolic ratios measured in various European and Mexican cohorts and is consistent with the lower incidence of stomach, liver and colon cancers observed in southern Europeans.

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.

A historical perspective on garlic and cancer

Epidemiological and laboratory studies provide insight into the anticarcinogenic potential of garlic and its constituent compounds. Both water- and lipid-soluble allyl sulfur compounds are effective in blocking a myriad of chemically induced tumors. Part of the protection from these compounds probably relates to a block in nitrosamine formation and metabolism. However, blockage in the initiation and promotion phases of the carcinogenicity of various compounds, including polycyclic hydrocarbons, provide evidence that garlic and its constituents can alter several phase I and II enzymes. Their ability to block experimentally induced tumors in a variety of sites including skin, mammary and colon, suggests a general mechanism of action. Changes in DNA repair and in immunocompetence may also account for some of this protection. Some, but not all, allyl sulfur compounds can also effectively retard tumor proliferation and induce apoptosis. Changes in cellular thiol and phosphorylation stains may account for some of these antitumorigenic properties. The anticarcinogenic potential of garlic can be influenced by several dietary components including specific fatty acids, selenium, and vitamin A. Since garlic and its constituents can suppress carcinogen formation, carcinogen bioactivation, and tumor proliferation it is imperative that biomarkers be established to identify which individuals might benefit most and what intakes can occur with ill consequences.

Mechanisms of inhibition of chemical toxicity and carcinogenesis by diallyl sulfide (DAS) and related compounds from garlic

Diallyl sulfide (DAS) is a flavor compound derived from garlic and is sequentially converted to diallyl sulfoxide (DASO) and diallyl sulfone (DASO(2)) by cytochrome P(450) 2E1 (CYP2E1). These compounds have been shown to reduce the incidence of a multitude of chemically induced tumors in animal models. The impediment of phase I activation of these carcinogens is hypothesized to be accountable for the reduction in tumor incidence. Indeed, DAS, DASO and DASO(2) are competitive inhibitors of CYP2E1. DASO(2), in addition, is a suicide inhibitor of CYP2E1. These compounds have been shown to reduce carbon tetrachloride-, N-nitrosodimethylamine- and acetaminophen-induced toxicity in rodents. All three chemicals are substrates for CYP2E1. The protective effect was observed when the organosulfur compounds were given before, during or soon after chemical treatment. DAS and DASO(2) inhibited the bioactivation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and related lung tumorigenesis in A/J mice. Because CYP2E1 does not play a key role in NNK activation, the inhibition of other CYP enzymes active in NNK metabolism is likely. DAS also has been shown to induce other CYP and phase II enzymes as well as decrease hepatic catalase activity. All of these effects are observed at concentrations much higher than what is normally ingested by humans. The biological activities of garlic and its related compounds at lower concentrations that mimic human consumption remain to be studied further.

Decrease of hepatic catalase level by treatment with diallyl sulfide and garlic homogenates in rats and mice

Diallyl sulfide (DAS) is a flavor compound derived from garlic and is active in the inhibition of chemically induced cytotoxicity and carcinogenicity in animal models. This study was conducted to examine the effects of the treatment of DAS and garlic homogenates on the activities of catalase, glutathione peroxidase, and superoxide dismutase. Male Sprague-Dawley rats were treated with DAS i.g. at daily doses of 50 or 200 mg/kg for 8 days, causing the hepatic catalase activity to decrease by 55 and 95%, respectively. Such a decrease in hepatic catalase activity was also observed when the DAS treatment was extended to 29 days. Western blot analysis showed that the DAS treatments resulted in corresponding decreases in the liver catalase protein level. No significant change in the catalase activity in the kidney, lung, and brain was observed with the treatments, but a slight decrease in heart catalase activity was observed. These treatments did not cause significant changes in superoxide dismutase and glutathione peroxidase activities in these tissues. Treatment with DAS at a daily dose of 200 mg/kg for 1-7 days resulted in a gradual decrease in the liver catalase activity to 5% of the control level, but it did not decrease the erythrocyte catalase activity. Treatment of rats with fresh garlic homogenates (2 or 4 g/kg, i.g., daily for 7 days) caused a 35% decrease in liver catalase activity. A/J mice treated with DAS and garlic homogenates also showed a decrease in the liver catalase activity. Diallyl sulfone (DASO2), a DAS metabolite, however, did not effectively decrease catalase activity in mice. The catalase activity was not inhibited by either DAS or DASO2 in vitro. The present results demonstrate that treatment with DAS and garlic homogenates decrease the hepatic catalase level in rats and mice.

Acetone catabolism by cytochrome P450 2E1: studies with CYP2E1-null mice

Previous experiments in vitro have suggested that cytochrome P450 2E1 (CYP2E1) is involved in acetone catabolism by converting acetone to acetol and then to methylglyoxal, both intermediates in the gluconeogenic pathway. In the present study, CYP2E1-null mice were used to demonstrate the role of CYP2E1 in acetone catabolism in vivo. The blood acetone level in male CYP2E1-null mice was 3.3 +/- 0.9 microg/mL, which was similar to levels of their sex- and age-matched parental lineage strains C57BL/6N (2.3 +/- 0.2 microg/mL) and 129/Sv (3.5 +/- 0.3 microg/mL) mice (both are CYP2E1 wild-type). After fasting for 48 hr, the blood acetone levels in the CYP2E1 wild-type mice were increased by 2.5- to 4.4-fold, but that in the CYP2E1-null mice increased 28-fold. These results clearly demonstrate that CYP2E1 plays a vital role in the catabolism of acetone under fasting conditions.

Presystemic intestinal metabolism of N-nitrosodimethylamine in mouse intestine

N-Nitrosodimethylamine (NDMA), a common food contaminant, is a potent liver carcinogen in rodents. A high presystemic intestinal metabolism has been shown for several nitrosamines including environmentally important compounds. We determined the metabolism of 1 micron [14C]-NDMA in isolated perfused mouse intestinal segments. We found NDMA to be equally distributed between the absorbed fluid and the perfusate. During a 2-h perfusion period, 0.13% of the radioactivity was converted to CO2. The formation of CO2 was decreased by pretreatment with diallylsulfide or addition of SKF 525A, and slightly increased by phenobarbital. Hydrophilic metabolites were found in the absorbate (0.9%) and perfusate (3.8%) of untreated mice. The amount of metabolites in the absorbate was increased by treatment with acetone or phenobarbital (8-fold), but not after starvation, with formaldehyde being present only in phenobarbital-treated animals. Treatment with diallylsulfide or addition of SKF 525A reduced the amount of metabolites in acetone-treated animals to control values. In conclusion, intestinal turnover does not significantly reduce the body burden of orally ingested NDMA and thus is not a first-line defense against this carcinogenic nitrosamine. NDMA metabolism has been attributed to the presence of cytochrome P450IIE1, which has not been detected in the intestine of untreated animals. The low turnover of NDMA, the induction by acetone and phenobarbital treatment, and the inhibition by diallylsulfide suggest the presence of low amounts of this or related cytochrome P450 isozyme(s) in mouse intestine.

Mechanism-based cancer prevention approaches: targets, examples, and the use of transgenic mice

Humans are exposed to a wide variety of carcinogenic insults, including endogenous and man-made chemicals, radiation, physical agents, and viruses. The ultimate goal of carcinogenesis research is to elucidate the processes involved in the induction of human cancer so that interventions may be developed to prevent the disease, either in the general population or in susceptible subpopulations. Progress to date in the carcinogenesis field, particularly regarding the mechanisms of chemically induced cancer, has revealed several points along the carcinogenesis pathway that may be amenable to mechanism-based prevention strategies. The purpose of this review is to examine the basic mechanisms and stages of chemical carcinogenesis, with an emphasis on ways in which preventive interventions can modify those processes. Possible ways of interfering with tumor initiation events include the following: i) modifying carcinogen activation by inhibiting enzymes responsible for that activation or by direct scavenging of DNA-reactive electrophiles and free radicals; ii) enhancing carcinogen detoxification processes by altering the activity of the detoxifying enzymes; and iii) modulating certain DNA repair processes. Possible ways of blocking the processes involved in the promotion and progression stages of carcinogenesis include the following: i) scavenging of reactive oxygen species; ii) altering the expression of genes involved in cell signaling, particularly those regulating cell proliferation, apoptosis, and differentiation; and iii) decreasing inflammation. In addition, the utility for mechanism-based cancer prevention research of new animal models that are based on the overexpression or inactivation of specific cancer-related genes is examined.

Experimental evidence for cancer preventive elements in foods

The last decade has witnessed an incredible advance in our understanding of how fruits and vegetables work to prevent cancer. Epidemiological studies have suggested that a diet rich in fruits and vegetables is associated with reduced risk for a number of common cancers. Food chemists and natural product scientists have identified hundreds of 'phytochemicals' that are being evaluated for the prevention of cancer. Food components can modify carcinogenesis in one of five different ways. They may: (1) modify carcinogen activation by inhibiting Phase 1 enzymes; (2) modify how carcinogens are detoxified through Phase 2 pathways; (3) scavenge DNA reactive agents; (4) suppress the abnormal proliferation of early, preneoplastic lesions; and (5) inhibit certain properties of the cancer cell.

Effects of cytochrome P450 2E1 modulators on the pharmacokinetics of chlorzoxazone and 6-hydroxychlorzoxazone in rats

A previously observed correlation between the rate of 6-hydroxylation of chlorzoxazone (CZX), a potent skeletal muscle relaxant, and cytochrome P450 2E1 activity in vitro led to the postulation that this drug may be used as a non-invasive probe for P450 2E1 activity in vivo. In this study, comparative pharmacokinetics of CZX and 6-hydroxychlorzoxazone (OH-CZX) were conducted in rats pretreated with an inhibitor or inducer of P450 2E1. After administration of CZX (150 mumol/kg, i.v.) to rats, blood samples were taken at different time points and the plasma concentrations of CZX and OH-CZX were determined by HPLC. The concentrations for CZX and OH-CZX over time were simultaneously fitted to a model of first-order elimination of CZX and first-order formation and elimination of OH-CZX using the computer program PCNONLIN to give pharmacokinetic parameters. Diallyl sulfide, a P450 2E1 inhibitor, at an oral dose of 50 or 200 mg/kg 12 hr prior to the CZX dose markedly inhibited the hydroxylation of CZX. Pretreatment with ethanol (15% in the drinking water for six days), a condition known to induce P450 2E1, slightly enhanced the formation of OH-CZX. To observe possible involvement of enzymes other than P450 2E1 in CZX metabolism, dexamethasone and phenobarbital were also used. Pretreatment with dexamethasone (50 mg/kg, i.p. daily for four days) did not cause changes in CZX and OH-CZX pharmacokinetics. Pretreatment with phenobarbital (75 mg/kg, i.p. daily for three days) enhanced CZX metabolism slightly. Our results suggest that P450 2E1 plays a major role in CZX hydroxylation in rats, but other factors may also be involved in the metabolism in vivo.

Organosulfur compounds and cancer

There is evidence that organosulfur compounds can inhibit the induction and growth of cancer. Several organosulfur compounds are dietary constituents and Allium species are a rich source of such molecules. Some but not all epidemiological studies have suggested that consumption of garlic can decrease cancer incidence. There is substantial evidence that constituents of garlic including diallyl sulfides can inhibit the induction of cancer in experimental animals. Effects on both tumor initiation and promotion have been documented. Some effects may be mediated by modulation of carcinogen metabolism involving altered ratios of phase I and phase II drug metabolizing enzymes. 59,60 Inhibitory actions on the growth of tumor cells have been documented and, for some tumor cells, differentiating effects of diallyl sulfides can occur. A definitive mechanism of action has not been established and evidence exists for effects at several sites in carcinogen metabolism and regulation of tumor growth. It is not always clear that laboratory studies can be extrapolated to reasonable levels of consumption by humans of garlic or other Allium species.