Inhibition of hepatic mixed function oxidase activity by propyl gallate

2 Final Report on the Safety Assessment of Propyl Gallate

Propyl Gallate acid is used as an antioxidant in cosmetic products at concentrations normally less than 0.1 percent. Propyl Gallate is absorbed when ingested, methylated, conjugated, and excreted in the urine. Acute animal toxicity studies indicate that Propyl Gallate is slightly toxic when ingested and practically nontoxic when applied to the skin. Numerous chronic oral toxicity studies indicate that Propyl Gallate at concentrations up to 5 percent is practically nontoxic to rats, mice, dogs, and guinea pigs.

Propyl Gallate is nonirritating to human skin at concentrations up to 10 percent; however, it is sensitizing at this and higher concentrations. Propyl Gallate was nonphototoxic. It is concluded that Propyl Gallate is safe as a cosmetic ingredient at concentrations not exceeding 1 percent.

Carcinogenesis Bioassay of Propyl Gallate in F344 Rats and B6C3FJ Mice

Chronic toxicity studies were conducted by maintaining groups of 50 F344 rats and 50 B6C3F1 mice of each sex on nutritionally complete diets containing 0%, 0.6%, or 1.2% propyl gallate for 103 weeks. Survival of rats and mice of both sexes was not significantly affected by the administration of this compound. Dosed rats and mice showed growth retardation and reduced feed utilization efficiency. Increased incidence of hepatic cytoplasmic vacuolization and suppurative inflammation of the prostate gland were observed in dosed male rats and were considered to be related to propyl gallate administration. Tumors of the preputial gland, islet ceil tumors of the pancreas, and pheochromocytoma of the adrenal gland were observed with significantly (p < 0.05) higher incidence in the low-dose male rats; however, there was little evidence of a dose response or of an effect in the high-dose group. Rare tumors (an astrocytoma and a glioma) were found in the brains of two low-dose female rats but none was found in the high-dose group. Malignant lymphoma occurred with a significant (p < 0.05) positive trend in male mice and the incidence in the high-dose group was significantly (p < 0.05) higher than that of the concurrent controls. However, the high-dose incidence was not significantly different from the historical control rate for the laboratory that conducted the bioassay. Under the conditions of the bioassay, propyl gallate was not considered to be clearly carcinogenic for F344 rats, although the increased incidence of preputial gland tumors, islet-cell tumors of the pancreas, and pheochromocytoma of the adrenal glands in low-dose male rats may have been related to compound administration. Thus, the evidence for carcinogenicity in male rats is regarded as being equivocal, while there was no indication of a carcinogenic response in female rats. Propyl gallate was not considered to be carcinogenic for B6C3F1 mice, although the increased incidence of malignant lymphoma in dosed male mice may have been related to administration of the test compound.

Final report on the amended safety assessment of Propyl Gallate

Propyl Gallate is the n-propyl ester of gallic acid (3,4,5-trihydroxybenzoic acid). It is soluble in ethanol, ethyl ether, oil, lard, and aqueous solutions of polyethylene glycol (PEG) ethers of cetyl alcohol, but only slightly soluble in water. Propyl Gallate currently is used as an antioxidant in a reported 167 cosmetic products at maximum concentrations of 0.1%. Propyl Gallate is a generally recognized as safe (GRAS) antioxidant to protect fats, oils, and fat-containing food from rancidity that results from the formation of peroxides. Data on dermal absorption are not available, but Propyl Gallate is absorbed when ingested, then methylated, conjugated, and excreted in the urine. The biological activity of Propyl Gallate is consistent with its free-radical scavenging ability, with effects that include antimicrobial activity, enzyme inhibition, inhibition of biosynthetic processes, inhibition of the formation of nitrosamines, anesthesia, inhibition of neuromuscular response to chemicals, ionizing/ultraviolet (UV) radiation protection, chemoprotection, antimutagenesis, anticarcinogenesis and antitumorigenesis, antiteratogenesis, and anticariogenesis. Animal toxicity studies indicate that Propyl Gallate was slightly toxic when ingested, but no systemic effects were noted with dermal application. Propyl Gallate is a strong sensitizer when tested intradermally, less sensitizing when tested topically, and nonsensitizing topically at 0.1% in one study. In a second study, Propyl Gallate (15 mg dissolved in 8 ml vehicle) was sensitizing to guinea pigs. Acute eye irritation tests conducted on nine cosmetic formulations, each containing less than 1% Propyl Gallate, were negative. A phototoxicity study conducted on a cosmetic formulation containing 0.003% Propyl Gallate determined that the product was not phototoxic to guinea pigs. In one study, female rats fed 0.5 g Propyl Gallate had substantially increased fetal resorption rates when compared to controls, but in four other studies, Propyl Gallate at doses up to 2.04 g/kg was nonteratogenic in rats, rabbits, mice, and hamsters. In clinical cumulative irritancy tests, Propyl Gallate was nonirritating at concentrations up to 10%. Patch tests at concentrations less than 1% yielded positive elicitation responses. Repeat-insult patch tests using cosmetic formulations with 0.003% Propyl Gallate produced no irritation or sensitization. Propyl Gallate at a concentration of 10% in alcohol was nonphototoxic in 25 subjects. Cosmetic formulations, each containing 0.003% Propyl Gallate, produced no signs of photosensitization or phototoxicity in a total of 371 subjects. Although Propyl Gallate is not a skin irritant in clinical tests, the available data demonstrate that it is a skin sensitizer and that it may be a sensitizer at lower concentrations than originally thought, i.e., at concentrations less than 1%. In actual practice, cosmetic formulations contain Propyl Gallate at concentrations up to 0.1% and usage has increased over the past 20 years. In spite of the increased exposure associated with increased use, it is the clinical experience of the Panel that the use of Propyl Gallate in cosmetics has not resulted in sensitization reactions. Therefore, the Panel believes that a concentration limitation of 0.1% in cosmetics is necessary (given the evidence of sensitization at concentrations less than 1%) and sufficient (given that current products are not producing adverse reactions).

Effects of various plant polyphenols on bladder carcinogen benzidine-induced mutagenicity

Benzidine (Bz), a human bladder carcinogen, was strongly mutagenic to Salmonella TA102 tester strain in the Ames Salmonella microsome/mutagenicity assay in the presence of rat liver S9 mix. Various non-mutagenic plant polyphenols were included in the assay to test their inhibitory effects on the Bz-induced mutations. Coumestrol, ellagic acid (EA), (-)-epicatechin (EC), (-)-epichatechingallate (ECG), gallic acid (GA), (-)-gallocatechin (GC), plumbagin, propyl gallate (PG), taxifolin, and 2,2',4'-trihydroxychalcone were found to have a strong inhibitory effect on Bz-induced mutations. (-)-Epigallo-catechingallate (EGCG), fisetin, (-)-gallocatechingallate (GCG), and piceatannol were moderately inhibitory to the mutations; whereas, (-)-catechin, (-)-catechingallate (CG), and reseveratrol were weakly inhibitory to the mutations. (-)-Epigallocatechin (EGC) and 7,3',4'-trihydroxy isoflavon were not inhibitory to the Bz-induced mutations. Isoliquirtigenin, quercetin dihydrate, and rhein were found to be mutagenic in tester strain TA102. Benzidine mediated lipid peroxidation was conducted employing the thiobarbituric acid reactive substances (TBARS) assay using linoleic acid as a substrate. In the presence of rat liver S9 mix, Bz could cause lipid peroxidation as an outcome of production of oxygen free radicals. Incorporation of the above mentioned non-mutagenic plant polyphenols significantly inhibited benzidine mediated lipid peroxidation in a time dependent manner. These polyphenols also effectively reduced the iron mediated lipid peroxidation. Thus, it is concluded that the inhibition of oxidative mutagenicity of Bz by plant polyphenols could be due to an inhibitory effect of plant polyphenols on the bioactivating enzymes such as cytochrome P-450 and peroxidase and the chelation of iron present in the cytochrome P-450 in the S9 mix. Thus, these plant polyphenols play a significant inhibitory role on Bz-induced mutagenicity.

The effect of temperature on monoxygenase reactions in the microsomal membrane

The effect of temperature on the rates of monoxygenase reactions was studied with microsomes prepared from phenobarbital pretreated rats. The rates of the N-demethylation of ethylmorphine, benzphethamine, aminopyrine, and p-nitroanisole were studied. Breaks at temperatures around 24 degrees C were observed in the Arrhenius plots of all these reactions. The energy of activation of these reactions has values of 10-12 and 19-21 kcal per mol at temperature ranges above and below the break temperature, respectively. The break, however, was not observed if 30% glycerol was added to the microsomes. The Arrhenius plot of the microsomal NADPH-cytochrome c reductase activity also did not show any break. The implications of these observations in relationship to the fluidity of the membrane, the translational mobility of membrane enzymes, and the rate of monoxygenase reactions are discussed.

The application of in vitro data in the derivation of the acceptable daily intake of food additives

The acceptable daily intake (ADI) for food additives is commonly derived from the NOAEL (no-observed-adverse-effect level) in long-term animal in vivo studies. To derive an ADI a safety or uncertainty factor (commonly 100) is applied to the NOAEL in the most sensitive test species. The 100-fold safety factor is considered to be the product of both species and inter-individual differences in toxicokinetics and toxicodynamics. Although in vitro data have previously been considered during the risk assessment of food additives, they have generally had no direct influence on the calculation of ADI values. In this review 18 food additives are evaluated for the availability of in vitro toxicity data which might be used for the derivation of a specific data-derived uncertainty factor. For the majority of the food additives reviewed, additional in vitro tests have been conducted which supplement and support the short- and long-term in vivo toxicity studies. However, it was recognized that these in vitro studies could not be used in isolation to derive an ADI; only when sufficient in vivo mechanistic data are available can such information be used in a regulatory context. Additional short-term studies are proposed for the food additives which, if conducted, would provide data that could then be used for the calculation of data-derived uncertainty factors.

Inhibition of murine hepatic cytochrome P450 activities by natural and synthetic phenolic compounds

1. The effect of the phenolic compounds protocatechuic acid, chlorogenic acid, tannic acid, gallates and silybin on ethoxyresorufin O-dealkylase (CYP1A1), methoxyresorufin O-dealkylase (CYP1A2) and pentoxy-O-dealkylase (CYP2B) was examined in mouse liver microsomes from induced animals. 2. All compounds tested could inhibit cytochrome P450-mediated enzyme activities, but to different extents. Tannic acid was the most potent inhibitor, especially toward EROD activity with an IC50=2.6 microM. Synthetic dodecyl gallate was also relatively selective toward this enzyme activity with an IC50=120 microM. 3. Protocatechuic acid, chlorogenic and silybin were more selective towards PROD and MROD activities. Their relative inhibitory potency for PROD activity was as follows: chlorogenic acid > protocatechuic acid > silybin > dodecyl gallate > propyl gallate. Protocatechuic acid was a more effective inhibitor of MROD activity than chlorogenic acid, and propyl gallate more effective than dodecyl gallate. Thus, no clear structure-activity or selectivity relationship was observed. 4. Analysis of the kinetics of inhibition revealed that the inhibition in most cases was non-competitive in nature.

Relationship between mitochondrial dysfunction and toxicity of propyl gallate in isolated rat hepatocytes

The relationship between cytotoxicity and mitochondrial dysfunction caused by propyl gallate (PG) has been studied in hepatocytes freshly prepared from fasted rats. Hepatocytes isolated from fasted (18 h) rats were significantly more susceptible to the toxicity of PG than hepatocytes from fed rats. The addition of fructose (15 mM), an alternative carbohydrate source, to hepatocyte suspensions resulted in the prevention of PG (1 mM)-induced cell killing accompanied by decrease in intracellular ATP loss during a 3 h-incubation period. Despite this, fructose did not completely prevent an abrupt loss of intracellular glutathione caused by PG, but effectively inhibited the loss of protein thiol levels. Fructose elicited a concentration (0.5-20mM)-dependent protection against the cytotoxicity of 1.5 mM PG. The incubation of hepatocytes with sodium azide (4 mM), an inhibitor of oxidative phosphorylation, enhanced the toxicity induced by PG (1 mM), but coincubation with fructose delayed the onset of toxicity. Neither azide alone nor fructose plus azide did affect the cell viability during the incubation period. Furthermore, the addition of 2 mM salicylamide, nontoxic to hepatocytes during the incubation period, enhanced PG (1 mM)-induced cytotoxicity and decreased the loss of free PG. These results indicate that the onset of cytotoxicity caused by PG may depend on the intracellular energy status and that mitochondria are critical target for the compound. In addition, the toxicity caused by the inhibition of mitochondrial ATP synthesis is related to the concentration of PG remaining in cell suspensions.

Cytotoxicity of propyl gallate and related compounds in rat hepatocytes

The cytotoxic effects of propyl gallate (PG), its related gallates and gallic acid have been studied in freshly isolated rat hepatocytes. Addition of PG (0.5-2.0 mM) to hepatocyte suspension elicited concentration-dependent cell death accompanied by losses of intracellular ATP, adenine nucleotide pools, glutathione (GSH) and protein thiols. The rapid loss of intracellular ATP preceded the onset of cell death caused by PG. In the comparative toxic effects of PG and related gallates at concentration of 1 mM, octyl gallate (OG), dodecyl gallate (DG) and butyl gallate (BG) elicited an abrupt depletion of ATP, followed by an acute cell death. These gallates were more toxic than PG; the toxic effects of PG were similar to those of methyl gallate (MG) and ethyl gallate (EG). In mitochondria isolated from rat liver, PG caused a concentration-dependent increase in the rate of state 4 oxygen consumption, indicating an uncoupling effect. The rate of state 3 oxygen consumption was inhibited by OG and DG. According to the respiratory control index, the order of impairment potency to mitochondria was OG > BG, DG > PG > EG, MG > gallic acid. These results indicate that PG and related gallates are toxic to hepatocytes and that the acute cytotoxicity may be due to mitochondrial dysfunction.

Inhibition of microsomal lipid peroxidation and monooxygenase activities by eugenol

Previously we reported that eugenol (4-allyl-2-methoxyphenol) inhibits non-enzymatic peroxidation in liver mitochondria (E. Nagababu and N. Lakshmaiah, 1992, Biochemical Pharmacology. 43, 2393-2400). In the present study, we examined the effect of eugenol on microsomal mixed function oxidase mediated peroxidation using Fe+3-ADP-NADPH, carbon tetrachloride (CCL4)-NADPH and cumene hydroperoxide (CumOOH) systems. In the presence of eugenol the formation of thiobarbituric acid reactive substances (TBARS) was decreased in all the systems (IC50 values: 14 microM for Fe+3-ADP-NADPH, 4.0 microM for CCl4-NADPH and 15 microM for CumOOH). Oxygen uptake was also inhibited to a similar extent with Fe+3-ADP-NADPH and CumOOH systems. A comparative evaluation with other antioxidants showed that in Fe+3-ADP-NADPH and CumOOH systems, the antioxidant efficacy was in the order: butylated hydroxytoluene (BHT) > eugenol > alpha-tocopherol, while in CCl4-NADPH system the order was alpha-tocopherol > BHT > eugenol. Time course of inhibition by eugenol indicated interference in initiation as well as propagation of peroxidation. Eugenol did not inhibit cytochrome P-450 reductase activity but it inhibited P-450 - linked monooxygenase activities such as aminopyrine-N-demethylase, N-nitrosodimethylamine demethylase, benzo(a)pyrene hydroxylase and ethoxyresorufin-O-deethylase to different extents. However, CumOOH supported monooxygenases (aminopyrine-N-demethylase and benzo(a)pyrene hydroxylase) required much higher concentrations of eugenol for inhibition. The concentration of eugenol required to inhibit monooxygenase activities was more than that required to inhibit peroxidation in all the systems. Eugenol elicited type 1 changes in the spectrum of microsomal cytochrome P-450. These results suggest that the inhibitory effect of eugenol on lipid peroxidation is predominantly due to its free radical quenching ability. Eugenol significantly protected against the degradation of cytochrome P-450 during lipid peroxidation with all the systems tested. These findings suggest that eugenol has the potential to be used as a therapeutic antioxidant. Further evaluation may throw more light on this aspect.

Boldo and boldine: an emerging case of natural drug development

Boldo (Peumus boldus Mol.), a Chilean tree traditionally employed in folk medicine and recognized as a herbal remedy in a number of pharmacopoeias, mainly for the treatment of liver ailments, has recently been the subject of increasing attention. Boldine, in particular, the major and most characteristic alkaloidal constituent of this plant species, now emerges as its most interesting active principle from the pharmacological viewpoint. The recent demonstration that boldine is an effective antioxidant in both biological and non-biological systems has opened up the perspective of a broad range of uses in medicine and industry. Given the toxicological data on this alkaloid, its antioxidative properties situate it as a potentially useful substance in many disease states featuring free-radical related oxidative injury. This review attempts to cover and discuss the studies conducted over the last four decades on the chemical and pharmacological properties of boldo and its main constituent.

Inhibition of rat liver microsomal lipid peroxidation by boldine

The alkaloid boldine, found in the leaves and bark of boldo, was an effective inhibitor of rat liver microsomal lipid peroxidation under a variety of conditions. The following systems all displayed a similar sensitivity to boldine: non-enzymatic peroxidation initiated by ferrous ammonium sulfate; iron-dependent peroxidation produced by ferric-ATP with either NADPH or NADH as cofactor; organic hydroperoxide-catalyzed peroxidation; and carbon tetrachloride plus NADPH-dependent peroxidation. Boldine inhibited the excess oxygen uptake associated with microsomal lipid peroxidation. Thus, boldine was effective in inhibiting iron-dependent and iron-independent microsomal lipid peroxidation, with 50% inhibition occurring at a concentration of about 0.015 mM. Boldine did not appear to react efficiently with superoxide radical or hydrogen peroxide, but was effective in competing for hydroxyl radicals with chemical scavengers. Concentrations of boldine which produced nearly total inhibition of lipid peroxidation had no effect on microsomal mixed-function oxidase activity nor did boldine appear to direct electrons from NADPH-cytochrome P450 reductase away from cytochrome P450. Boldine completely protected microsomal mixed-function oxidase activity against inactivation produced by lipid peroxidation. The effectiveness of boldine as an anti-oxidant under various conditions, and its low toxicity, suggest that this alkaloid may be an attractive agent for further evaluation as a clinically useful anti-oxidant.

Antimutagenicity profiles for some model compounds

The concept of activity profile listings and plots, already applied successfully to the display of mutagenicity data, has been modified for application to antimutagenicity data. The activity profiles are bar graphs that have been organized in two general ways: for antimutagens that have been tested in combination with a given mutagen and for mutagens that have been tested in combination with a given antimutagen. Doses from both the mutagen and the antimutagen are displayed and plotted together with results on enhancement or inhibition of mutagenic activity. The short-term tests that have been used extensively to identify mutagens and potential carcinogens are increasingly being used to identify antimutagens and potential anticarcinogens. Three model mutagens, N-methyl-N'-nitro-N-nitrosoguanidine, aflatoxin B1 and benzo[a]pyrene, and 4 model antimutagens, butylated hydroxyanisole, butylated hydroxytoluene, glutathione and disulfiram, were selected from the data surveyed in the published literature. It is not clear at the present time whether the inhibition of carcinogen-induced mutation is a good indicator of anticarcinogenic properties, and further research is needed. Nevertheless, the activity profiles are useful for the assessment of the available antimutagenesis data by providing rapid visualization of considerable dose information and experimental results.

Inhibition of bleomycin-induced DNA strand breaks in V 79 Chinese hamster cells by the antioxidant propylgallate

Bleomycin induced DNA single-strand breaks in Chinese hamster V 79 cells which were detected by the alkaline filter elution assay. In the presence of propylgallate, an antioxidant, the amount of DNA single-strand breaks was significantly reduced. The production of DNA strand breaks by methylnitronitrosoguanidine used as a positive control was not influenced by propylgallate. It is suggested that propylgallate inhibits the generation of DNA single-strand breaks by trapping reactive oxygen species produced by the bleomycin-iron(II) complex.

Distribution of Microcystis aeruginosa peptide toxin and interactions with hepatic microsomes in mice

Purified 14C-labelled peptide toxin from the cyanobacterium Microcystis aeruginosa was administered intraperitoneally to mice and the distribution of label determined between the major organs. Seventy per cent of the label was localized in the liver after 1 min.; this value increasing to almost 90 per cent after 3 hours. Label associated with the lungs and other individual organs varied between 10 and 1 per cent of the 14C recovered throughout. Three microsomal enzyme inducers, beta-naphthoflavone, 3-methylcholanthrene and phenobarbital, afforded protection against liver damage and extended survival if given to mice before the administration of an LD50 dose of toxin. Toxin-dependent changes in liver cytochrome levels were also reduced by the enzyme inducers.

Immobilization of pig liver microsomes. Stability of cytochrome P-450-dependent monooxygenase activities

Microsomes from pig liver were covalently coupled to Sepharose activated by CNBr and to Sephadex activated by 1,1'-carbonyldiimidazole. Microsomes were also entrapped inside Ca-alginate and kappa-carrageenan gels. The concentration of immobilized cytochrome P-450 was determined by CO-difference spectra. The activity of the monooxygenase system was demonstrated by the N-demethylation of aminopyrine, the O-demethylation of p-nitroanisole, and the hydroxylation of perhexiline maleate. Upon immobilization, a 30-40% and a 60-70% decrease in Vappmax for the O- and N-demethylations were respectively observed. The Vappmax values for the hydroxylation of perhexiline maleate were essentially the same for the different immobilized forms and for the freely suspended microsomal cytochrome P-450. Under storage at 4 degrees C, microsomes entrapped inside kappa-carrageenan and Ca-alginate were less stable than the free microsomes, whereas immobilization on CNBr-activated Sepharose improved the stability of the hepatic microsomal monooxygenase system at the same temperature. These types of immobilized microsomes have the advantage of being easily recovered and reused for other assays. Finally, microsomes entrapped inside kappa-carrageenan or Ca-alginate can be used to follow up the continuous metabolization of p-nitroanisole for several hours in a stirred-batch reactor.

Synthetic antioxidants: biochemical actions and interference with radiation, toxic compounds, chemical mutagens and chemical carcinogens

Biological actions of 4 commonly used synthetic antioxidants--butylated hydroxyanisole, butylated hydroxytoluene, ethoxyquin and propyl gallate--on the molecular, cellular and organ level are complied. Such actions may be divided into modulation of growth, macromolecule synthesis and differentiation, modulation of immune response, interference with oxygen activation and miscellaneous. Moreover, an overview of beneficial and adverse interactions of these antioxidants with exogenous noxae is given. Beneficial interactions include radioprotection, protection against acute toxicity of chemicals, antimutagenic activity and antitumorigenic action. Possible mechanisms of the antitumorigenic action of antioxidants are discussed. This discussion is centered around antioxidant properties which may contribute to a modulation of initiation-related events, especially their ability to interfere with carcinogen metabolism. The beneficial interactions of antioxidants with physical and chemical noxae are contrasted to those leading to unfavorable effects. These include radiosensitization, increased toxicity of other chemicals, increased mutagen activity and increased tumor yield from chemical carcinogens. At present, the latter one can most adequately be characterized as tumor promotion at least in the case of butylated hydroxytoluene. It is concluded that current information is insufficient to promote expectations as to the use of antioxidants in the prevention of human cancer.

Interaction of Clostridium difficile toxins and mouse hepatic microsomes

Intraperitoneal administration of toxins of Clostridium difficile to mice resulted in loss of hepatic cytochrome P450 and peroxidation of microsomal lipids. Pretreatment with the microsomal enzyme inducer beta-naphthoflavone partially alleviated these effects and increased survival time of intoxicated animals.

Influence of gallic acid esters on drug-metabolizing enzymes of rat liver

The effect of three antioxidants, propyl, octyl and dodecyl gallate, on hepatic drug metabolism in male rats was studied in vivo and in vitro. When fed at a dietary concentration of 1% for 14 days, only dodecyl gallate increased relative liver weight. Cytochrome P-450 content was not influenced, but a slight increase in cytochrome b5 content was observed after the feeding of propyl gallate. Monooxygenase activity (benzo[a]pyrene-hydroxylase and ethoxycoumarin-deethylase activities) was not affected by propyl or octyl gallate, but a significant decrease in benzo[a]pyrene-hydroxylase activity was apparent in rats fed dodecyl gallate. Study of benzo[a]pyrene-metabolite formation in liver microsome preparations from control and propyl gallate-treated rats showed an overall decrease in metabolite production following gallate treatment, the decrease being statistically significant for the formation of the 9,10-dihydrodiol. Epoxide-hydratase activity was enhanced by a factor of 1.5 in rats fed propyl gallate; glutathione-transferase activity was unaffected. In vitro, the gallates proved to be potent inhibitors of ethoxycoumarin deethylation in liver microsomes from untreated and phenobarbital-treated rats; however, when cytochrome P-448 had been induced by pretreatment with 3-methylcholanthrene, ethoxycoumarin deethylase was less sensitive to the inhibitory action of the gallates.

Screening of antioxidants and other compounds for antimutagenic properties towards benzo[a]pyrene-induced mutagenicity in strain TA98 of Salmonella typhimurium

Antioxidants and several other compounds, some of which are known to inhibit carcinogenicity, have been screened for their effectiveness as inhibitors of benzo[a]pyrene (BP) mutagenicity towards Salmonella typhimurium strain TA98 in the Ames test. A total of 32 compounds were tested. In the assay, metabolic activation of BP (8.2 nmoles/plate) was mediated by the S9 fraction from beta-naphthoflavone-induced rat livers. Among compounds which are known to inhibit carcinogenicity, retinol, phenothiazine, disulfiram, phenethylisothiocyanate and phenylisothiocyanate were the most effective inhibitors of BP mutagenicity, being effective at equimolar concentrations. Several other compounds showed inhibition at higher concentrations of antioxidant and the remainder showed little or no inhibition. Dose-response curves have been obtained for the 17 most active compounds. No general pattern of inhibition is obvious from our studies, inhibitors are not drawn ;from any single class of compounds, nor does a particular compound necessarily appear to inhibit more than one mutagen.

Effect of diphenylhydantoin and its main hydroxylated metabolite on the pharmacokinetics and the urinary and biliary excretion of phenobarbital and its p-hydroxy metabolite

When rats which had been pretreated with a high dose of diphenylhydantoin (80 mg/kg) for 5 days were given a single intravenous dose of phenobarbital (30 mg/kg): (a) There was no increase in the rate at which phenobarbital (PB) disappeared from the plasma or the tissues of pretreated rats. (b) The percentages of phenobarbital and p-hydroxyphenobarbital (free and conjugated) excreted in the urine were similar in both treated and control animals. However, the percentage of conjugated p-hydroxyphenobarbital excreted, was almost twice that of the control group. (c) Pretreatment with diphenylhydantoin (DPH) markedly increased bile flow rates. Therefore these rats excreted more PB than their controls. The biliary excretion of hydroxylated metabolites of PB (free and conjugated), was similar to that found in urine. Hydroxylation was not increased although, there was a significant elevation in the percentage of conjugated metabolite excreted. In a study to establish whether the main metabolites of diphenylhydantoin interfered with the metabolism of phenobarbital the following results were obtained: (a) Intravenous administration of DPH together with PB caused a two-fold increase in the half-life of phenobarbital elimination. (b) Intravenous administration of PB, to bile duct cannulated rats which had been pretreated for 5 days with DPH, caused a significant reduction in the excretion of hydroxylated phenobarbital in comparison with their control group. However, all the excreted DPH was present in the conjugated form. (c) The DPH pretreated rats had significantly lower cytochrome P-450 and mono-oxygenase activities in their hepatic microsomes than the pretreated controls, and higher UDP-glucuronyltransferase activity with DPH itself as the substrate.

Effect of aflatoxin B1 on hepatic drug-metabolizing enzymes in female rats. Interaction with a contraceptive agent

1. The effect of a contraceptive on aflatoxin B1 toxicity has been studied in female rats treated for 15 consecutive days with repeated doses of aflatoxin (0.40 mg/kg/day), norethindrone (0.60 mg/kg/day) and ethynylestradiol (0.012 mg/kg/day). 2. Increases occurred in hepatic microsomal cytochrome P-450 content (31%), and in the activities of epoxide hydrase (77%), UDP-glucuronyltransferase (67%) and gamma-glutamyltransferase (78%). 3. Aflatoxin alone caused a 22% increase in GSH S-epoxide transferase activity, whereas the contraceptive given alone or combined with aflatoxin increased the hepatic reduced glutathione. 4. The effect of aflatoxin plus contraceptive was not additive. 5. The effects caused by aflatoxin and the contraceptive were similar, and the contraceptive (depending on its progestogen/estrogen ratio), may modify aflatoxin toxicity by increasing the drug-metabolizing enzyme activities and the concentration of hepatic glutathione.

Interaction between NADPH-cytochrome P-450 reductase and hepatic microsomes

Solubilized NADPH-cytochrome P-450 reductase has been purified from liver microsomes of phenobarbital-treated rats. When added to microsomes, the reductase enhances the monoxygenase, such as aryl hydrocarbon hydroxylase, ethoxycoumarin O-dealkylase, and benzphetamine N-demethylase, activities. The enhancement can be observed with microsomes prepared from phenobarbital- or 3-methylcholanthrene-treated, or non-treated rats. The added reductase is believed to be incorporated into the microsomal membrane, and the rate of the incorporation can be assayed by measuring the enhancement in ethoxycoumarin dealkylase activity. It requires a 30 min incubation at 37 degrees C for maximal incorporation and the process is much slower at lower temperatures. The temperature affects the rate but not the extent of the incorporation. After the incorporation, the enriched microsomes can be separated from the unbound reductase by gel filtration with a Sepharose 4B column. The relationship among the reductase added, reductase bound and the enhancement in hydroxylase activity has been examined. The relationship between the reductase level and the aryl hydrocarbon hydroxylase activity has also been studied with trypsin-treated microsomes. The trypsin treatment removes the reductase from the microsomes, and the decrease in reductase activity is accompanied by a parallel decrease in aryl hydrocarbon hydroxylase activity. When purified reductase is added, the treated microsomes are able to gain aryl hydrocarbon hydroxylase activity to a level comparable to that which can be obtained with normal microsomes. The present study demonstrates that purified NADPH-cytochrome P-450 reductase can be incorporated into the microsomal membrane and the incorporated reductase can interact with the cytochrome P-450 molecules in the membrane, possibly in the same mode as the endogenous reductase molecules. The result is consistent with a non-rigid model for the organization of cytochrome P-450 and NADPH-cytochrome P-450 reductase in the microsomal membrane.

Disturbance of microsomal detoxication mechanisms in liver by chlorophenol pesticides

The pesticide pentachlorophenol known as an uncoupler of mitochondrial oxidative phosphorylation was shown to disturb liver microsomal detoxication functions by a selective inhibition of the terminal oxygenation enzyme P-450. At lower concentrations the flavin moiety of this enzyme chain is not inhibited but rather is stimulated, whereby a qualitative shift in detoxication of aromatic amines from C-oxygenation to N-oxygenation is obtained. The effects were due to the pentachlorophenol itself and not to a metabolite. Similar effects of varying strength were also obtained with other chlorophenol pesticides; 2,4,di-, 2,4,6,-tri and 2,3,4,6-tetrachlorophenol, di- and hexachlorophen, tri- and nonachloro-2-hydroxydiphenyl ethers. The relevance of these findings to the possible synergistic influence of chlorophenols on the carcinogenic effects of polyaromatic amines and hydrocarbons is discussed.