Rat Liver Microsomal System Research Articles

Biological Activity and Metabolism of Phosphoramidate Insecticides

The mode of action of phosphoramidothioates was examined with reference to their anti-AChE activity. N-Isopropylphosphoramidothioate, isofenphos, was the most toxic to the adzuki bean weevil among N-alkyl substituted compounds. LD50 values for the insects were not correlated with the in vitro anti-AChE activity of these oxons. It was demonstrated that phosphoramidothioates and these oxons became potent inhibitors of AChE by the incubation with the rat liver microsomal NADPH system and the chemical treatment with peracid. The inhibition of AChE activity was reduced by the addition of SKF 525-A to the microsomal system. The stereospecificity of isofenphos and its oxon on biological activity was obvious. The (+)-isomers of both the compounds were more toxic to insects than the (-)-isomers. Chiral isomers and racemate of isofenphos-oxon were activated with the microsomal NADPH system and the I50 values became 1.47×10-7M, and 6.19×10-5M for the (+)- and (-)-oxon, respectively. The data suggested that the (+)-isomer was easily bioactivated and the I50 values of chiral isomers after the bioactivation were sufficient to explain their insecticidal activity. 14C-Isofenphos metabolism was different between animals and plants. In mammals and insects, isofenphos was rapidly transformed into the water soluble metabolites, and in plant isofenphos and isofenphos-oxon were very persistent. Isofenphos was metabolized by (1) oxidative desulfuration, (2) desphenylation, (3) desN-alkylation and (4) N-desalkylation, and the formation of water soluble metabolites through isofenphos-oxon was proposed to be the main metabolic pathway. In all experiments, the conversion to water soluble metabolites was dominant in the cleavage of P-O-C (aryl) bond and only desphenyl compounds were found in plants. The effect of chirality was seemingly small in the metabolism of isofenphos based on the examination in the rat liver microsomal system and in the housefly. The amount of isofenphos-oxon metabolized from the (-)-isofenphos was always larger than that from the (+)-isomer, but quantitative differences of other metabolites between both the chiral isofenphos was obscure.

Tannin antioxidants from Osbeckia chinensis

Six hydrolysable tannins, casuarinin ( 1), casuariin ( 2), punicacortein A ( 3), degalloyl-punicacortein A ( 4), 2,3-[( S)-4,4′,5,5′,6,6′-hexahydroxydiphenoyl]- d-glucopyranoside ( 5), and 4,6-[( S)-4,4′,5,5′,6,6′-hexahydroxydiphenoyl]- d-glucopyranoside ( 6), and related compounds, gallic acid, methyl gallate and ellagic acid, were isolated from the methanol extract of Osbeckia chinensis. In order to confirm the structure-activity relationship, the antioxidant activities of these compounds were examined both in the food model and in vitro systems. The antioxidative efficiency, using the thiocyanate and TBA methods, increased in the order of gallic acid <α-tocopherol<methyl gallate<( 4)<( 3)<( 5)=( 6)<( 2)<( 1)=BHA<ellagic acid. The in vitro antioxidant order is α-tocopherol<gallic acid<methyl gallate<( 4)<( 3)=( 5)=( 6)=ellagic acid<( 2)<( 1) in the rabbit erythrocyte membrane ghost system, and ellagic acid <( 4)<gallic acid<( 3)=( 5)=( 6)<methyl gallate<( 2)<( 1) = α-tocopherol in the rat liver microsome system.

Formation of glutathione adducts of carbon tetrachloride metabolites in a rat liver microsomal incubation system

Metabolism of CCl 4 by rat liver microsomes causes damage to the membrane. GSH diminishes that damage. One mechanism of GSH protection has been characterized. It involves formation of diglutathione carbonate from two molecules of GSH and one molecule of phosgene, an oxygenated metabolite of CCl 4. The present studies were done to seek other GSH adducts of CCl 4 metabolites and to examine the effect of oxygen tension on their formation. Incubations were carried out in sealed flasks under oxygen concentrations ranging from 0.14 to 21% at 37°. The CCl 4 concentration was 72 μM. 14CCl 4 and 35S-GSH were used to label metabolites. High pressure liquid chromatographic analysis of the aqueous phase demonstrated two GSH adducts of CCl 4 metabolites. One adduct was oxygendependent and was identified as diglutathione carbonate by its co-elution with a diglutathione carbonate standard. Its formation showed no evidence of saturation when GSH concentrations as high as 10 mM were used, indicating that the overall process was nonenzymatic. Formation of the other adduct was greatest under the lowest oxygen concentration studied and none occurred at oxygen tensions of 5% or greater. Based on experiments with radiolabeled CCl 4 and GSH, this metabolite appeared to be a product of one molecule each of CCl 4 and GSH. Formation of this adduct had enzymatic characteristics. It was saturable with respect to GSH with an apparent K m of 70 μM, and other thiol compounds that were tested could not substitute for GSH. The adduct was unstable during isolation attempts and was not characterized further. Formation of these two GSH adducts could account for some of the protection by GSH against CCl 4 injury.

Genotoxic and mutagenic activation of aflatoxin B 1 by constitutive forms of cytochrome P-450 in rat liver microsomes

Cytochrome P-450-mediated activation of aflatoxin B 1 (AFB 1) to genotoxic and mutagenic products which subsequently cause induction of an umu gene expression in Salmonella typhimurium TA1535/pSK1002 has been studied in a rat liver microsomal or reconstituted monooxygenase system. Liver microsomes from male Sprague-Dawley rats had a 1.5-fold higher activity to catalyze AFB 1 than did those from female rats. In addition, the activation was not increased in liver microsomes from rats pretreated with phenobarbital, 3-methylcholanthrene, a polychlorinated biphenyl mixture, or dexamethasone, suggesting that the constitutive forms of cytochrome P-450 have important roles for the activation of AFB 1 in rat liver microsomes. Using 15 forms of cytochrome P-450 purified from liver microsomes of untreated and phenobarbital- and 3-methylcholanthrene-treated rats, three isozymes from untreated male rats and one isozyme from untreated female rats were found to have high reactivities in metabolizing AFB 1 to genotoxic products. Cytochrome P-450 forms isolated from inducer-treated rats were relatively less active. The close correlation between induction of umu gene expression and mutagenicity with Ames/ S. typhimurium TA98 system by activated metabolites of AFB 1 in the reconstituted monooxygenase system suggested that the constitutive forms of cytochrome P-450 had major roles for genotoxic and mutagenic activation of AFB 1 in rat liver microsomes.

The effect of 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide on the rat liver microsomal glucose-6-phosphatase system.

The effect of 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide (CMC) on the reactions catalyzed by the glucose-6-phosphatase system of rat liver microsomes was studied. Modification of the intact microsomes by CMC leads to the inhibition of the glucose-6-phosphatase, pyrophosphate:glucose and carbamoyl-phosphate : glucose phosphotransferase activities of the system. The activities are restored by the disruption of the microsomal permeability barrier. The mannose-6-phosphate, pyrophosphate, and carbamoyl-phosphate phosphohydrolase activities of the intact as well as the disrupted microsomes were not affected by CMC. It follows from the results obtained that CMC inactivates the microsomal glucose-6-phosphate translocase, the inactivation is a result of the modification of a single sulfhydryl or amino group of the translocase; pyrophosphate, carbamoyl phosphate and inorganic phosphate are transported across the microsomal membrane without participation of the glucose-6-phosphate translocase; pyrophosphate and carbamoyl phosphate may act as the phosphate donors in the glucose phosphorylation reactions in vivo.

Site of inhibition of rat liver microsomal fatty acid chain elongation system by dec-2-ynoyl coenzyme A. Possible mechanism of inhibition.

The present study examines the effect of the acetylenic thioester dec-2-ynoyl-CoA (delta 2 10 identical to 1-CoA) on the microsomal fatty acid chain elongation pathway in rat liver. When the individual reactions of the elongation system were measured in the presence of delta 2 10 identical to 1-CoA, the trans-2-enoyl-CoA reductase activity was markedly inhibited (Ki = 2.5 microM), whereas the activities of the condensing enzyme, the beta-ketoacyl-CoA reductase, and the beta-hydroxyacyl-CoA dehydrase were not affected. The absence of inhibition of total microsomal fatty acid elongation was attributed to the significant accumulation of the intermediates, beta-hydroxyacyl-CoA and trans-2-enoyl-CoA, without formation of the saturated elongated product, indicating that the trans-2-enoyl-CoA reductase-catalyzed reaction was the only site affected by the inhibitor. The nature of the inhibition was noncompetitive. In contrast to the delta 2 10 identical to 1-CoA, delta 3 10 identical to 1-CoA did not inhibit trans-2-enoyl-CoA reductase activity, suggesting that the mode of inhibition was not via formation of the 2,3-allene derivative. Based on the observation (a) that p-chloromercuribenzoate markedly inhibits reductase activity, (b) that dithiothreitol protects the enzyme against inactivation by delta 2 10 identical to 1-CoA, (c) of the spectral manifestation of the interaction between thiol reagents and delta 2 10 identical to 1-CoA depicting an absorbance peak similar to that of the beta-ketoacyl thioester-Mg2+ enolate complex, (d) of a similar absorbance spectrum formed by the interaction between delta 2 10 identical to 1-CoA and liver microsomes, and (e) of the absence of formation of a similar spectrum by delta 3 10 identical to 1-CoA, trans-2-10:1-CoA, or delta 2 10 identical to 1 free acid with liver microsomes, we propose that delta 2 10 identical to 1-CoA inactivates trans-2-enoyl-CoA reductase by covalently binding to a critical sulfhydryl group at or in close proximity to the active site of the enzyme.

The Activation of the Cytochrome P-450 Dependent Monooxygenase System by Light

The cytochrome P-450 dependent monooxygenase system of rat liver microsomes is investigated by light dosimetry (action spectroscopy). The scoparone-O-demethylation activity is enhanced by light and depends on the wavelength of the irradiating light. The relative increase of the activity (about 15%) by the irradiating light (approximately 0.5 mW/cm2) is maximal at a wavelength of 400 nm. The light induced enhancement of the 7-ethoxycoumarin-O-deethylase activity was measured in reconstituted systems, consisting of one of two P-450 enzymes (P-450 beta NF-B and P-450 PB-B) with the NADPH-cytochrome P-450 reductase. The action spectrum of the reconstituted P-450 beta NF-B:NADPH-P-450 reductase complex shows a maximum between 420 and 440 nm. The relative increase of the activity induced by light of 420 nm was 7.3% and 9% for the reconstituted systems of P-450 beta NF-B and P-450 PB-B, respectively. The results are discussed in analogy to the classic experiments of Warburg with its blocked CO-enzyme-complex. The results can best be explained by the assumption that the light induced enhancement of the enzyme activity is due to an excitement of those intermediate states of the P-450 catalytic cycle (ferric and ferrous state of the heme iron) which are rate limiting.

Factors responsible for the formation of different N-alkylated porphyrins in rat liver microsomal systems exposed to norethindrone. The role of 3 alpha-hydroxysteroid dehydrogenase.

Incubation of rat liver microsomes with norethindrone and a NADPH-generating system leads to the formation of one N-alkylated porphyrin (green pigment, GP1). Administration of this steroid to male rats in vivo results in the formation of three more-polar green pigments (GP2, 3 and 4). A cytosolic protein (green-pigment converting protein) has been purified from rat liver that, when added to liver microsomal mixtures containing norethindrone (0.03 mM) and a NADPH-generating system, results in the formation of all four green pigments (GP1, 2, 3 and 4). Field-desorption mass spectrometry of the purified green pigments gave protonated molecules, [M + H]+, at m/z 905 for GP1, m/z 909 for GP2, m/z 925 for GP3 and 4. The Mr of the purified cytosolic protein on SDS/polyacrylamide-gel electrophoresis or gel filtration was 37000. Polyacrylamide-gel isoelectric focusing gave a pI value of 5.9. Antibodies raised in rabbits against this protein, after preincubation with rat liver cytosol, subsequently prevented the formation of the more-polar norethindrone-induced green pigments (GP2, 3 and 4). The purified protein in the presence of either NADH or NADPH catalysed the reduction of delta 4-ring-reduced norethindrone, 5 alpha-oestran-17 alpha-ethynyl-17 beta-ol-3-one and, with the appropriate cofactor, the oxidation and reduction of steroids lacking the ethynyl function, e.g. androsterone or dihydrotestosterone. Indomethacin inhibited the reduction of dihydrotestosterone by this protein with an I50 (concn. causing 50% inhibition) value of 4.9 microM. From its physical and enzymic properties it is concluded that green-pigment converting protein is the same as 3 alpha-hydroxysteroid dehydrogenase (EC 1.1.1.50).

Metabolic activation of olefins: Conversion of 1-octene to a putative reactive intermediate 1-octen-3-one: An alternative pathway to epoxidation

1. The enzymic activation of a model olefin oct-1-ene was studied in rat liver microsomal systems in vitro. 2. An active metabolite was trapped using N-acetylcysteine and identified by means of capillary GLC/ mass spectrometry and 360 MH z 1H NMR as S-3-oxo-octyl- N-acetylcysteine. A two step pathway for the formation of this adduct was proposed involving first the production of oct-1-en-3-ol by NADPH dependent mixed function oxidases and secondly a NADP or NAD linked oxidation, independent of cytochrome P-450, to yield the putative reactive intermediate oct-1-en-3-one. 3. Under physiological conditions, oct-1-en-3-one, prepared chemically, reacted non-enzymically with N-acetylcysteine with a t 1 2 of about 6 sec. 4. Enzymes catalysing the NADP-dependent oxidation of octen-3-ol were present in microsomal preparations from a number of organs apart from the liver, those from adrenal and intestinal epithelia showing the next highest levels of activity. Unlike the activation of octene, rates of hepatic activation of octen-3-ol were not induced by pretreatment of rats with phenobarbitone or 3-methylcholanthrene. 5. Using 1-octene as the substrate, comparisons were made of alternative routes of hepatic metabolism activation. Relative to the rate of formation of the 3-oxo intermediate trapped with N-acetylcysteine, epoxidation of octene and subsequent hydrolysis to octane-1,2-diol was over 40 times more rapid. The rate of formation of a presumptive oxirane precursor trapped with the haem of cytochrome P-450 as N-(2-hydroxyoctyl) protoporphyrin IX was about 17-fold lower.

Regulation of linoleic acid Δ6-desaturation by a cytosolic lipoprotein-like fraction in isolated rat liver microsomes

A peripheral component of the Δ 6-fatty acid-desaturase system of rat liver microsomes has been isolated from the cytosol by ultracentrifugation at a saline density of 1.26 g/ml. It exhibited lipoprotein characteristics with an approximate protein/lipid ratio of 1.22 and free fatty acids and phosphatidylcholine as its main lipid components. Linoleic acid desaturation activity diminished in washed microsomes, since they lost the adsorbed cytosolic fraction. Addition of the factor reactivated the reaction and the recovery was dependent on the concentration of the factor in the medium. Linoleic acid and linoleyl-CoA were bound by the cytosolic fraction. However, the transport of substrate to the desaturase was not apparently a main function of the cytosolic fraction, since transport occurred equally in the absence of the factor. Moreover, the solubilization of linoleyl-CoA was not enhanced and the free monomeric concentration was not altered by the presence of the cytosolic fraction. In addition, the factor did not divert Δ 6-desaturase substrate to or from other metabolic pathways such as esterification to phospholipids. γ-Linolenic acid produced by Δ 6-desaturation of linoleic acid in the microsomes inhibited the desaturase, but it was removed by the factor from the membrane towards the cytosol, preventing the inhibition. The anti-inhibitory effect of the cytosolic factor was blockaded by addition of columbinic acid or γ-Linolenic acid to the factor. Moreover, the inhibitory effect of arachidonic acid was not prevented by addition of the cytosolic fraction. These results suggest that the cytosolic fraction studied would optimize the Δ 6-desaturation of linoleic acid in vitro in rat liver microsomes by removal of the product, γ-linolenic acid, as it is formed.

Metabolism of the bay-region diol-epoxide of chrysene to a triol-epoxide and the enzyme-catalysed conjugation of these epoxides with glutathione.

Metabolic activation of chrysene in mouse skin appears to involve r-1,t-2-dihydroxy-t-3,4-oxy-1,2,3,4-tetrahydrochrysene (anti-chrysene-1,2-diol 3,4-oxide) and 9-hydroxy-r-1,t-2-dihydroxy-t-3,4-oxy-1,2,3,4-tetrahydrochrysene (anti-9-OH-chrysene-1,2-diol 3,4-oxide). The enzyme-catalysed conjugation of these epoxides with [35S]glutathione has been studied in experiments in which the glutathione conjugates were separated by h.p.l.c. and examined by fluorescence spectrophotometry. Both anti-chrysene-1,2-diol 3,4-oxide and anti-9-OH-chrysene-1,2-diol 3,4-oxide formed conjugates nonenzymically and both were shown to be substrates for rat liver glutathione transferases. When anti-chrysene-1,2-diol 3,4-oxide was incubated with [35S]glutathione and a rat liver microsomal metabolizing system, glutathione conjugates with h.p.l.c. and fluorescence spectral characteristics identical to those of conjugates formed from both anti-chrysene-1,2-diol 3,4-oxide and anti-9-OH-chrysene-1,2-diol 3,4-oxide were detected. This finding provides evidence that anti-chrysene-1,2-diol 3,4-oxide can be further metabolized to the triol-epoxide, anti-9-OH-chrysene-1,2-diol 3,4-oxide by rat liver microsomal systems.

Production of hypoprothrombinaemia by cefazolin and 2-methyl-1,3,4-thiadiazole-5-thiol in the rat.

Recent findings have suggested that the 1-methyltetrazole-5-thiol (MTT) group contained in several beta-lactam antibiotics may be responsible for the hypoprothrombinaemia associated with these drugs. In order to determine if the hypoprothrombinaemia associated with cefazolin is due to the presence of the structurally related 2-methyl-1,3,4,-thiadiazole-5-thiol (MTD) group which it possesses, the ability of MTD and cefazolin to produce hypoprothrombinaemia in rats was examined. Female rats maintained on a vitamin K-deficient diet for ten days developed hypoprothrombinaemia after the intravenous administration of cefazolin for two subsequent days. Hypoprothrombinaemia was also produced by the oral administration to vitamin K-deficient rats of either cefazolin or MTD, while the oral administration of cefotaxime, which does not contain a thiol group, had no effect. In a rat liver microsomal system, MTD was found to be a much more potent inhibitor than cefazolin of the vitamin K-dependent step in clotting factor synthesis, the gamma-carboxylation of glutamic acid. These results suggest that the hypoprothrombinaemia associated with cefazolin may be due to the MTD group.

Microsomal oxidation of 2-dimethylamino-3-chloro-1,4-naphthoquinone. The possibility of substrate activation by cytochrome P-450.

2-Dimethylamino-3-chloro-1,4-naphthoquinone (DCNQ) is bound to microsomal cytochrome P-450 as a type I substrate (lambda max = 391 nm, lambda min = 420 nm). The Ks is 40.5 microM. In a rat-liver microsomal system, the N-demethylation of DCNQ produces formaldehyde (rate 225 pmol/min per mg of protein). Induction by phenobarbital increases the rate of formation, while addition of metyrapone and SKF-525A into the system decreases the rate by 52% and 35%, respectively. The microsomal N-demethylation of DCNQ is not inhibited by CO. Under full anaerobiosis, the microsomal oxidation of DCNQ again gives formaldehyde (rate 416 pmol/min per mg of protein). The anaerobic oxidation of DCNQ is inhibited by metyrapone and SKF-525A. The microsomal, chemical and electrochemical reduction of DCNQ to the corresponding semiquinones and hydroquinones have been studied. Non-enzymic DCNQ reduction is insufficient for the formation of formaldehyde. Under anaerobic conditions the microsomal DCNQ oxidation is assumed to occur via the intramolecular oxazole bond which is then hydrolysed, yielding formaldehyde. This may be a new example of substrate activation by cytochrome P-450.

In vitro DNA-binding of microsomally-activated fluoranthene: evidence that the major product is a fluoranthene N2-deoxyguanosine adduct.

Incubation of 3-[3H]fluoranthene with a rat liver microsomal activation system in the presence of calf thymus DNA resulted in radioactivity bound to the DNA. The fluoranthene-modified DNA was enzymatically digested and a DNA adduct elution profile developed using h.p.l.c. The fraction containing the major fluoranthene--DNA adduct was further purified by h.p.l.c. and separated into two subfractions. Treatment of these with perchloric acid liberated guanine in both cases. Evidence that both were N-2 guanine derivatives was based on the pK values of these adducts determined before and after treatment with nitrous acid. The major adduct (70% of total modified deoxyribonucleosides) was further characterized by high resolution, fast atom bombardment mass spectroscopy which yielded a molecular ion consistent with a fluoranthene triol bound to the N-2 position of deoxyguanosine. Synthetic syn and anti 3-[3H]2,3-dihydroxy-1,10b-epoxy-1,2,3-trihydrofluoranthene were reacted directly with DNA and the 8-[3H]-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrofluoranthene was incubated with DNA and microsomes. The DNA from each of these reactions was enzymatically hydrolyzed and h.p.l.c. adduct profiles were developed. The major adduct formed from reaction of the anti 2,3-dihydroxy-1,10b-epoxy-1,2,3-trihydrofluoranthene with DNA and the major N-2 fluoranthene derived adduct had identical elution times on two different h.p.l.c. systems, similar pK values (before and after nitrous acid treatment) and the same u.v. spectra. In addition, derivatization of both adducts with ethyl methanesulfonate yielded identical products, as determined by h.p.l.c. analysis.

In vitro DNA strand scission and inhibition of nucleic acid synthesis in L1210 leukemia cells by a new class of DNA complexers, the anthra[1,9- cd]pyrazol-6(2 H)-ones (anthrapyrazoles)

CI-937 and CI-942 belong to a new class of DNA complexes, the anthra[1,9- cd]pyrazol-6(2 H)-ones (anthrapyrazoles), and are being further developed as antitumor drugs based on their curative properties against murine solid tumour models. The biochemical effects of these agents were studied in L1210 leukemia in relation to other clinically used intercalators. After a 1-hr exposure, CI-937 and CI-942 reduced the cloning efficiency of L1210 cells by 50% at 3.0 × 10 −8 and 1.5 × 10 −7 respectively. Based on an ethidium displacement assay, these drugs bound strongly to DNA, reducing the fluorescence of an ethidium-DNA complex by 50% at concentrations of 23 and 33 nM for CI-937 and CI-942 respectively. This was comparable to mitoxantrone at 15 nM, but much more potent than Amsacrine which required over 1.3 μM. A distinct property of the anthrapyrazoles was a much more potent inhibitory effect on whole cell DNA synthesis than on RNA synthesis. After L1210 cells were exposed to drug for 2 hr the concentration needed to inhibit DNA synthesis by 50% was 0.33 and 0.57 μM for CI-937 and CI-942, respectively, whereas 2.0 and 11.3 μM were required to inhibit RNA synthesis by the same extent. This was in contrast to Adriamycin and mitoxantrone which inhibited both activities equally at similar concentrations. It was apparent that the inhibition of these processes was not due to substrate depletion since intracellular ribonucleoside and deoxyribonucleoside triphosphates either remained constant or were elevated after a 2-hr exposure to 1 or 10 μM drug. A similar discriminatory effect was observed on DNA and RNA polymerase in permeabilized cells, and the inhibition of nucleic acid synthesis in this system could be reversed by exogenously added DNA. Since the high incidence of cardiotoxicity associated with the administration of anthracyclines has been related to the formation of reactive oxygen species, the ability of the anthrapyrazoles to augment superoxide dismutase sensitive oxygen consumption was observed in a rat liver microsomal system. CI-937 and CI-942 induced 5- and 10-fold less oxygen consumption than Adriamycin, producing rates of 12.4, 24.2 and 138.9 nmoles/min/mg microsomal protein, respectively, at a drug concentration of 0.5 mM. The anthrapyrazoles were similar to other known intercalators in that they caused both single- and, to a lesser extent, double-strand DNA breaks that were tightly associated with protein in a concentration-dependent manner. The rates at which these lesions formed, however, were quite different between drugs. CI-937 at a concentration of 1 μM produced maximum strand breakage after 30 min, whereas CI-942 was much slower requiring at least 3 hr. If L1210 cells were treated with the anthrapyrazoles such that substantial strand breaks occurred and the drug removed, the breaks were repaired very slowly over the first 30 min but then additional lesions occurred thereafter for at least 2 hr. Additional DNA damage after drug was removed was also observed with Adriamycin and mitoxantrone but not Amsacrine with which 90% of the strand breaks disappeared within 30 min.

Stereoselective hydroxylation at the aliphatic carbons of 7,8- and 9,10-dihydrobenzo[a]pyrenes by rat liver microsomes

Optically active 7-hydroxy-7,8-dihydrobenzo[a]pyrene and 8-hydroxy-7,8-dihydrobenzo[a]pyrene were identified as two of the major metabolites formed by incubation of 7,8-dihydrobenzo[a]pyrene with rat liver microsomes. Optically active 9-hydroxy-9,10-dihydrobenzo[a]pyrene and 10-hydroxy-9,10-dihydrobenzo[a]pyrene were similarly identified as two of the minor metabolites of 9,10-dihydrobenzo[a]pyrene. The formation of these metabolites was abolished either by prior treatment of liver microsomes with carbon monoxide or the absence of NADPH, but was not inhibited by an epoxide hydrolase inhibitor. The results indicate that the aliphatic carbons of dihydro polycyclic aromatic hydrocarbons may undergo stereoselective hydroxylation reactions catalyzed by the cytochrome P-450 system of rat liver microsomes.

Alterations of drug-induced toxicity in the mouse lymphoma assay by a rat hepatic microsomal metabolizing system (S-9).

Large differences in induced cellular toxicity were observed in the presence or absence of a rat liver microsomal metabolizing system (S-9) during drug testing in the mouse lymphoma assay. After studying the fate of three drugs in this test system, several mechanisms were demonstrated whereby S-9 reduced cellular toxicity. For N-(4-hydroxyphenyl)retinamide (HPR), fenoctimine sulfate and methyl palmoxirate, the drug concentrations (EC50) in the presence of S-9 were, respectively, 11.5, 14.3 and 4.1 times the concentrations required to achieve comparable levels of toxicity in the absence of S-9. HPR was metabolized by the S-9 and sequestered in the microsomal membranes. This was associated with a marked reduction in the cellular accumulation of the drug. The reduced toxicity of fenoctimine sulfate in the presence of S-9 was associated with extensive biotransformation to polar metabolites. This was accompanied by a reduction of radioactivity associated with the cells from 5.7% to 0.4% of the administered drug. Methyl palmoxirate was rapidly converted to its acid, palmoxirate, by horse serum enzymes present in the treatment medium. This provides an example of metabolism by a test system component other than the S-9 or lymphoma cells. The reduced toxicity of this drug in the presence of S-9 was attributed to further metabolism of palmoxirate and a reduction of the proportion of total radioactivity associated with the cells from 3.1% to 0.4%. These results emphasize the need for pilot toxicity studies, especially when components of the test system are varied, to assess the effect of drug concentration on the toxic response.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of the inhibition of the gamma-carboxylation of glutamic acid by N-methylthiotetrazole-containing antibiotics.

Antibiotics that contain a 1-N-methyl-5-thiotetrazole (MTT) side group have been associated with hypoprothrombinemia. In a detergent-treated rat liver microsomal system, MTT inhibited the carboxylation of the gamma carbon of glutamic acid, a necessary reaction in the synthesis of four of the clotting factors. In the present work, the inhibition by MTT was found to be slow in onset, with a lag time of 15 min before significant inhibition occurred. A preincubation of MTT with the microsomes decreased the lag time and increased the extent of inhibition. Glutathione at 1 mM was found to markedly decrease the ability of MTT to inhibit this reaction. The disulfide dimer of MTT was a more potent inhibitor of the system than was MTT, with inhibition detected as low as 1 microM dimer. Disulfiram also inhibited the carboxylation system. These results indicate that the sulfhydryl group of MTT is important for the inhibitory effect of MTT and suggest that a slowly formed metabolite of MTT may be directly responsible for the observed inhibition. The inhibitory mechanism of MTT may be analogous to that of disulfiram, which would explain some pharmacologic effects in common with disulfiram. In addition, the in vitro observations presented here and a closer examination of the clinical evidence raise the possibility that MTT-containing antibiotic-induced hypoprothrombinemia may not be a vitamin K reversible phenomenon.

O-53) - Effect of propylthiouracil on the electron transfer system of rat liver microsomes

O-53) - Effect of propylthiouracil on the electron transfer system of rat liver microsomes

Induction of sister chromatid exchanges by the peroxisome proliferators 2,4-D, MCPA, and clofibrate in vivo and in vitro.

Phenoxy acid herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA) have been found to induce proliferation of peroxisomes in the liver cells of rodents in the same manner as the hypolipidemic drug clofibrate. Both phenoxy acid herbicides and clofibrate (ethyl-alpha-p- chlorophenoxyisobutyrate ) are suspected carcinogens. The present study reports the effect of these agents on the induction of sister chromatid exchange (SCE) in the blood lymphocytes of exposed rats (100 mg/kg with 2,4-D and MCPA, 200 mg/kg with clofibrate for 2 weeks in one intragastric dose/day), in the bone marrow cells of exposed Chinese hamsters (100 mg/kg, treatments as above), and in Chinese hamster ovary (CHO) cells in vitro (10(-5), 10(-4), and 10(-3) M, for 1 h). In the experiments in vitro, the effects of purified 2,4-D and MCPA phenoxy acids were studied, in addition to those of the commercial herbicide formulations and clofibrate. No increase of SCE frequency was observed in the blood lymphocytes of the exposed rats in comparison with the controls. In the bone marrow cells of the exposed Chinese hamsters, a slight increase of SCE was found in the group treated with MCPA but not in the groups treated with 2,4-D or clofibrate. A slight increase in the number of SCEs was characteristic of all the treated CHO cell cultures, both with and without a rat liver microsomal activation system (S9 mix). No clear dose-related effects, however, could be discerned with any of the compounds, and no differences in the SCE induction were observed between the commercial herbicide products and the purified phenoxy acetic acids. The present results support the data which indicate that 2,4-D, MCPA, and clofibrate do not act as direct DNA-damaging agents.