DNA damage and cell death induced by 1,2-dibromo-3-chloropropane (DBCP) and structural analogs in monolayer culture of rat hepatocytes: 3-aminobenzamide inhibits the toxicity of DBCP

1,2-Dibromo-3-chloropropane (DBCP) and a number of halogenated propane analogs induced DNA damage in rat hepatocytes in vitro measured by an automated alkaline elution method. Short-term (2 hrs) cytotoxic effects of DBCP were not observed until the DBCP concentration exceeded 1 mM. The short-term cytotoxicity of all the DBCP analogs occurred in the same concentration range. Significant membrane damage, measured as cell detachment, was observed after extended exposure to lower concentrations of DBCP (100 microM) for 20 hrs. The relative, delayed cytotoxic effect of DBCP and analogs correlated with their ability to cause DNA damage. In general, the halogenated propanes with more bromines relative to chlorines were the more potent compounds. Propane analogs lacking the third halogen had little cytotoxic activity. The addition of the proposed specific poly(ADP-ribosyl)transferase inhibitor 3-aminobenzamide (3-ABA) protected against DBCP-induced cytotoxic effects and NAD+ depletion. However, 3-ABA also reduced DBCP-induced DNA damage, DBCP metabolic loss, and the formation of water soluble and covalently bound DBCP metabolites. Thus, 3-ABA may block DBCP-induced cell death by decreasing the formation of reactive DBCP-metabolites.

Partial characterization of biliary metabolites of pulegone by tandem mass spectrometry. Detection of glucuronide, glutathione, and glutathionyl glucuronide conjugates

The hepatotoxic monoterpene pulegone is a major constituent of the herbal abortifacient pennyroyal oil. An approximately equimolar mixture of 2H3- and 14C-labeled pulegone was administered to rats to study its phase II metabolism. Radioactive conjugates that were excreted into the bile were isolated by selective derivatization and HPLC separation, and subsequently characterized from the daughter ion mass spectra of protio- and deutero-analogs of each metabolite. The biliary metabolites characterized were glucuronide and glutathione (GSH) conjugates, accounting for approximately 3% of the radioactivity excreted in bile. The glucuronides, which were 2-fold more abundant than GSH conjugates, were mainly of hydroxylated pulegone and hydroxylated, reduced pulegone. The three GSH conjugates contained xenobiotic moieties that varied in their oxidation state; one of these was tentatively identified as the GSH conjugate of the proximate oxygenated metabolite, menthofuran. The two other GSH conjugates apparently underwent subsequent glucuronidation since novel glutathionyl glucuronide conjugates were identified that contained nonhydroxylated xenobiotic moieties. The results indicate that pulegone is bioactivated via at least three distinct pathways, each marked by a different GSH conjugate. Characterization of these conjugates represents a first step in the identification of the reactive metabolites from which they are derived.

Effect of bromine and chlorine positioning in the induction of renal and testicular toxicity by halogenated propanes

A series of halogenated propanes were studied for renal and testicular necrogenic effects in the rat and correlated to their ability to induce in vivo renal and testicular DNA damage and in vitro testicular DNA damage. 1,2-Dibromo-3-chloropropane (DBCP) and 1,2,3-tribromopropane were most potent in causing organ damage in both kidney and testes. Extensive necrosis was evident at 85 mumol/kg in kidney and at 170 mumol/kg in testis. The dibromomonochlorinated analogue 1,3-dibromo-2-chloropropane was less organ toxic than DBCP and 1,2,3-tribromopropane, but induced more organ damage than the dichloromonobrominated analogues 1-bromo-2,3-dichloropropane and 1,3-dichloro-2-bromopropane. Dihalogenated propanes were even less necrogenic. These observed differences in toxic potency between the halogenated propanes could not be explained by relative differences in tissue concentrations. The ability of the halogenated propanes to induce DNA damage in vivo correlated well with their ability to induce organ damage. However, DNA damage occurred at lower doses and at a shorter period of exposure than organ necrosis. This indicates that DNA damage might be an initial event in the development of organ necrosis by halogenated propanes in general. Further, testicular DNA damage induced by the halogenated propanes in vivo correlated well with the DNA damage observed in isolated testicular cells in vitro, showing that toxicity was due to in situ activation. The numbers, positions, and the types of halogen substituents appear to be important determinants in causing DNA damage and necrogenic effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of acetaminophen-reactive metabolite formation by methylxanthines and known cytochrome P-450 activators

Previous in vitro studies suggested that caffeine enhanced acetaminophen (APAP) oxidation to N-acetyl-p-benzoquinone imine (NAPQI) by selectively activating the male-specific constitutive cytochrome P-450IIIA2. Monomethylxanthine and dimethylxanthine analogs of caffeine (also metabolites) were studied for their potential effect to accelerate NAPQI formation in various preparations of rat liver microsomes. In contrast to caffeine, none of the mono- and dimethylxanthines (2.5 mM) activated P-450. Rather, the analogs either inhibited NAPQI formation or had no effect; 1-methylxanthine (2.5 mM) was the only compound which consistently inhibited (25-70%) APAP oxidation in all microsomal preparations. Thus, all three methyl groups appear to be required for P-450 activation by methylxanthines. Because of the highly selective activation effect of caffeine, it was of particular interest to determine whether other known P-450 activators could enhance APAP oxidation. Both acetone (400 mM) and flavone (50 microM) accelerated NAPQI formation in all microsomal preparations, whereas metyrapone caused only inhibition. Flavone (50 microM) caused a pattern of activation similar to that observed with 5 mM caffeine (maximal activation of 125-300%), except that NAPQI formation was increased approximately 40% by flavone in microsomes prepared from adult females, whereas no activation was caused by caffeine. Acetone yielded a pattern of P450 activation very different from that of either caffeine or flavone; the maximal degree of activation (3 times control) was observed in microsomes prepared from adult females. In contrast to caffeine and flavone, the degree of activation by acetone in microsomes prepared from juvenile animals was considerably lower (50%) than that observed in adult microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative cytotoxic effects of acetaminophen (N-acetyl-p-aminophenol), a non-hepatotoxic regioisomer acetyl-m-aminophenol and their postulated reactive hydroquinone and quinone metabolites in monolayer cultures of mouse hepatocytes

Toxic effects of acetaminophen (paracetamol, N-acetyl-p-aminophenol, APAP) in monolayer cultures of mouse hepatocytes developed over a period of 18 hr. N-Acetyl-m-aminophenol (AMAP) was approximately 10-fold less toxic than APAP, despite the fact that it bound covalently to a greater extent to hepatocyte macromolecules. AMAP did not deplete glutathione to as great an extent as APAP, indicating that their reactive metabolites may bind to different proteins or that oxidative damage in addition to arylation of proteins may be involved in the development of cell death. The toxicity of 3-methoxy-acetyl-p-aminophenol was similar to that of APAP, whereas the other hydroquinone and quinone metabolites were 8-10 times more cytotoxic than APAP. The potencies of these analogs were in the order: acetyl-m-aminophenol-p-benzoquinoneimine greater than or equal to 2,5-dihydroxyacetanilide greater than or equal to 3-methoxy-p-benzoquinone greater than or equal to N-acetyl-p-benzoquinone imine (NAPQI) greater than or equal to acetyl-m-aminophenol-o-benzoquinone greater than or equal to 3-hydroxy-acetyl-p-aminophenol. The relative toxic potencies of the hydroquinone and quinone metabolites of AMAP were comparable to that of NAPQI, and do not readily explain the marked difference between the cytotoxic effects of AMAP and APAP.

Parasympathetic withdrawal is an integral component of autonomic imbalance in congestive heart failure: demonstration in human subjects and verification in a paced canine model of ventricular failure

Although enhanced sympathetic tone is a well recognized component of the autonomic profile characteristic of congestive heart failure, the contribution of parasympathetic withdrawal to this autonomic imbalance is less well described. The technique of spectral analysis of heart rate variability provides a dynamic map of sympathetic and parasympathetic tone and was thus used to define the nature of sympathetic-parasympathetic interactions in humans with idiopathic dilated cardiomyopathy and in a paced canine model of congestive heart failure. Humans with cardiomyopathy were found to have an augmentation of the sympathetically mediated low frequency area of the power density spectrum. Parasympathetic withdrawal was demonstrated by significant reductions in the parasympathetically mediated high frequency area (p less than 0.05) and the ratio of high to low frequency areas (p less than 0.01). Administration of atropine to normal subjects resulted in a significant reduction in the high frequency area (p less than 0.05) and the high/low frequency area ratio, both of which decreased within the range noted in patients with congestive heart failure. Administration of isoproterenol in normal subjects led to an augmentation of the low frequency area but to only a small decrease in the high/low frequency area ratio. Induction of congestive heart failure in a paced canine model resulted in alterations in the autonomic profile that resembled those seen in humans with ventricular failure. The prominent high frequency region of the spectrum at baseline, indicating a predominance of parasympathetic tone, was absent after the evolution of congestive heart failure, and there was a marked augmentation of the low frequency region of the spectrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Necrotizing enterocolitis in a neonatal piglet model

The aim of this study was to develop an animal model for necrotizing enterocolitis (NEC). Twenty-five neonatal Hanford minipigs had carotid artery and external jugular vein catheters and rectal Clinical Tonomitors placed under anesthesia. Experimental animals were subjected to a hypoxic insult (50% reduction in baseline PaO2 for 30 minutes) and hypothermic stress (core temperature reduced to 35 degrees C for 30 minutes). Regular oral diet was resumed and the survivors were euthanized 3 to 4 days later. All animals underwent necropsy with gross and histopathological evaluation of the entire bowel. Of 22 experimental animals, 14 survived (64%) and 8 (36%) died of pulmonary hemorrhage. Of the 14 survivors, 8 (57%) had gross and microscopic evidence of NEC. Six of the total 25 animals (24%) sustained rectal perforations from the tonometer. Of 3 control animals, one died of pulmonary hemorrhage and the two survivors had normal intestine. This model successfully produced gross and histological evidence of NEC. The tonometer shows promise as a predictor of NEC provided technical modifications can reduce the complication rate.

Inhibition and activation of acetaminophen reactive metabolite formation by caffeine. Roles of cytochromes P-450IA1 and IIIA2

Caffeine has previously been shown to diminish or potentiate acetaminophen (APAP) hepatotoxicity in rats, depending on induction state. To elucidate the P-450 forms involved in these divergent effects, rat liver microsomes, prepared after pretreatment with various inducers, were used to examine the effect of caffeine on N-acetyl-p-benzoquinone imine (NAPQI) formation. The addition of caffeine to incubations with 3-methylcholanthrene (MC)-induced microsomes resulted in a biphasic effect on the formation of NAPQI. A 43% decrease in NAPQI formation was observed as caffeine concentration was increased from 0 to 0.5 mM; however, NAPQI formation was accelerated as caffeine concentration increased, exceeding the control (no caffeine) value, at caffeine concentrations greater than 2.5 mM. Incubations with purified P-450IA1 showed that as caffeine concentration increased from 0 to 5 mM, a 50% inhibition was observed with no evidence of acceleration. In contrast to MC microsomes, the addition of caffeine to incubations with uninduced and phenobarbital-induced adult rat microsomes resulted in a marked (3- to 4-fold) acceleration of NAPQI formation with no evidence of inhibition. Caffeine (5 mM) also accelerated NAPQI formation in microsomes isolated from diabetic rats, but to a substantially lesser extent (120%), suggesting a modest (if any) effect on P-450IIE1, a form previously shown to form NAPQI from APAP. Interestingly, caffeine caused a 3- to 4-fold increase in NAPQI formation by juvenile male and female rat microsomes, but no activation was observed with adult female rat microsomes. These results suggested that caffeine activated a member of the cytochrome P-450IIIA subfamily.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatotoxicity after 3'-hydroxyacetanilide administration to buthionine sulfoximine pretreated mice

The administration of 3'-hydroxyacetanilide, a regioisomer of acetaminophen, to mice failed to produce hepatotoxicity even after the administration of diethyl maleate. In contrast, hepatotoxicity did occur when 3'-hydroxyacetanilide was administered to buthionine sulfoximine pretreated mice. Although the administration of 3'-hydroxyacetanilide in conjunction with either diethyl maleate or buthionine sulfoximine depleted total hepatic glutathione, only the combined buthionine sulfoximine-3'-hydroxyacetanilide treatment decreased hepatic mitochondrial glutathione concentrations to below 20% of control values. In addition, pretreatment with buthionine sulfoximine increased the amount of 3'-hydroxyacetanilide bound to mitochondrial proteins. These results, in conjunction without previous results on the involvement of mitochondrial damage in the pathogenesis of hepatotoxicity caused by acetaminophen, suggest a probable relationship between mitochondrial damage caused by the buthionine sulfoximine-3'-hydroxyacetanilide treatment and hepatotoxicity caused by this treatment.

Inhibition of the metabolism of paracetamol by isoniazid

1. The effect of isoniazid given daily for 7 days on paracetamol (acetaminophen) kinetics and metabolism was studied in 10 healthy volunteers. Paracetamol, 500 mg, was given before isoniazid, on day 7 of isoniazid administration, and 2 days after the last dose of isoniazid. 2. On day 7, isoniazid markedly inhibited the formation clearance of the glutathione and catechol metabolites by 69.7% and 62.2%, respectively. Total paracetamol clearance was lowered by 15.2%. There was no effect of isoniazid on the non-oxidative pathways of paracetamol elimination. 3. Two days after isoniazid was discontinued, paracetamol metabolism had returned to pre-isoniazid values.

Co-culture systems for assessing the stability and genotoxicity of reactive 1,2-dibromo-3-chloropropane (DBCP) metabolites

1,2-Dibromo-3-chloropropane (DBCP) induced DNA damage, measured by alkaline elution, at low concentrations (5-10 microM) in suspensions of hepatocytes and testicular cells isolated from rats. At higher concentrations (greater than or equal to 100 microM) DBCP caused DNA damage and increased the frequency of sister chromatid exchanges in Chinese hamster V79 cells. When DBCP (2.5-10 microM) was tested for its ability to cause unscheduled DNA synthesis (UDS) in monolayers of liver cells isolated from untreated rats, a clear positive response was obtained. No increase in UDS was detectable when liver cells isolated from PCB-pretreated rats were used. In contrast, DBCP (greater than or equal to 50 microM) was metabolized in hepatocytes from PCB-pretreated rats to products mutagenic to Salmonella typhimurium TA100 in co-culture with the hepatocytes, whereas control hepatocytes were substantially (40-fold) less active. No bacterial mutagenicity could be detected when the TA100 strain was co-incubated with isolated rat testicular cells and DBCP. In co-cultures of hepatocytes and V79 cells DBCP-induced DNA damage in V79 cells occurred at low concentrations (10 microM DBCP) compared to the concentration (100 microM) needed to induce DNA damage in the V79 cells incubated without hepatocytes. Testicular cells were not able to enhance DBCP-induced DNA damage in the V79 cells. The data indicate that the putative reactive DBCP episulphonium ion metabolite formed in the testicular cells presumably responsible for testicular cell DNA damage, is not capable of escaping the cell where it is formed. Other reactive DBCP metabolites generated in the liver cells seem to be able to interact with the DNA of neighbouring cells.

Differential effects of arylating and oxidizing analogs of N-acetyl-p-benzoquinoneimine on red blood cell membrane proteins

Incubation of human red blood cell membranes (white ghosts) with N-acetyl-p-benzoquinone imine (NAPQI), a toxic metabolite of acetaminophen, or with either an arylating or an oxidizing analog of NAPQI, resulted in the inhibition of membrane ion transporting systems and the modification of cytoskeletal proteins. NAPQI and 2,6-dimethyl-NAPQI, which primarily arylates protein thiols, inhibited the calmodulin-activated Ca pump ATPase activity, the basal (calmodulin-independent) Ca pump ATPase activity and the Na,K pump ATPase activity. In contrast, 3,5-dimethyl-NAPQI, which primarily oxidizes protein thiols, caused selective inhibition of the calmodulin-activated Ca pump ATPase activity. Sodium dodecyl sulfate gel electrophoresis of red blood cell (RBC) membrane proteins revealed that NAPQI and 2,6-dimethyl-NAPQI, but not 3,5-dimethyl-NAPQI, decreased the intensity of band 3 corresponding to the anion transporter, whereas NAPQI as well as 2,6-dimethyl-NAPQI, and to a lesser extent 3,5-dimethyl-NAPQI, caused a decrease of cytoskeletal protein bands, including spectrin, actin, and bands 4.1 and 4.2. These modifications were associated with increased formation of high molecular weight protein aggregates that did not enter the gel. Treatment of 3,5-dimethyl-NAPQI-exposed ghosts with the reducing agent dithiothreitol (DTT), resulted in the recovery of the affected cytoskeletal protein bands. Conversely, the modifications caused by NAPQI and 2,6-dimethyl-NAPQI were only partially reversed by DTT treatment. Taken together our results suggest that NAPQI and its two analogs modified ion transporting systems and cytoskeletal proteins by reacting with protein thiols. Both oxidation and arylation of protein thiols can alter the functional properties of important RBC membrane proteins. Of the two reactions, arylation appeared to be the less specific and more damaging event.

Hepatic protein arylation, glutathione depletion, and metabolite profiles of acetaminophen and a non-hepatotoxic regioisomer, 3'-hydroxyacetanilide, in the mouse

The metabolism and disposition of acetaminophen (APAP) and a non-hepatotoxic regioisomer, 3'-hydroxyacetanilide (AMAP), were investigated in the mouse using 14C-labeled analogues. Covalent binding of metabolites of both compounds was observed on the order of 1 nmol/mg tissue protein. AMAP binding was much higher than that of APAP at 1 hr, but by 24 hr, AMAP binding was significantly lower than that of APAP. APAP binding peaked at 3 hr and did not decrease significantly thereafter. Despite the high early levels of covalent binding, AMAP was not as effective in causing glutathione depletion as was APAP. This was reflected in the urinary metabolite profiles of the two compounds. Approximately twice as much APAP was cleared through thioether conjugation compared to AMAP, based on an analysis of urinary metabolites. These results and results of other studies suggest that electrophilic metabolites of AMAP are more reactive than those of APAP, and do not diffuse as far from their site of formation, which may spare some critical target proteins from damage.

Metabolic activation of 1,2-dibromo-3-chloropropane to mutagenic metabolites: detection and mechanism of formation of (Z)- and (E)-2-chloro-3-(bromomethyl)oxirane

1,2-Dibromo-3-chloropropane (DBCP), a haloalkane nematocide and soil fumigant, is metabolically activated to chemically reactive species that are direct-acting mutagens in a Salmonella typhimurium TA 100 test system. Studies in vitro with rat liver microsomes indicated that oxidation at carbon 3 resulted in the formation of an unstable gem-chlorohydrin that rearranged with elimination of hydrogen bromide to form (Z)-2-chloro-3-(bromomethyl)oxirane [(Z)-CBPO] and (E)-2-chloro-3-(bromomethyl)oxirane [(E)-CBPO]. Gas chromatography-mass spectrometry (GC-MS) with positive ion chemical ionization (CI) was employed to identify (Z)-CBPO and (E)-CBPO by comparison of characteristic fragment ions in their CI mass spectra with those observed for authentic standards. Quantitative GC-MS methodology was exploited to quantitate the rate of formation of (Z)-CBPO and (E)-CBPO from DBCP and analogues of DBCP specifically deuterated at carbon 1 and carbon 3. The rate of formation of Z- and E-isomers of CBPO was 31 and 33 pmol/(min.mg of protein), respectively, from DBCP; substitution with deuterium at carbon 1 increased the rate of epoxide formation by 50%, whereas CBPO formation could not be detected from a substrate labeled with deuterium at carbon 3. Both epoxides were directly acting mutagens to S. typhimurium TA 100. (Z)-CBPO caused approximately twice as many his+ revertants/nmol compared to (E)-CBPO. Oxidation at carbon 2 of DBCP resulted in the formation of a bifunctional alkylating agent, 1-bromo-3-chloroacetone, presumably via the intermediacy of an unstable gem-bromohydrin.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidant-induced changes in the cellular energy homeostasis. A study with 3,5-dimethyl N-acetyl-p-benzoquinone imine and isolated hepatocytes

Exposure of isolated hepatocytes to 400 microM 3,5-dimethyl N-acetyl-p-benzoquinone imine (3,5-diMe NAPQI), rapidly induced the formation of plasma membrane blebs. More than 50% of the viable cells were affected after 1 min incubation with 3,5-diMe NAPQI. Rapid loss of mitochondrial ATP, and sequential increases in ADP and AMP accompanied hepatocyte blebbing. 3,5-diMe NAPQI also induced a pronounced elevation of mitochondrial NADP level, whereas the NAD concentration was unaffected. Similar alterations in the adenine and pyridine nucleotide pools were found to occur in the cytosol, although at slower rates. During the initial phase of ATP loss and NADP production, there was also a concomitant decrease in the oxygen uptake of the hepatocytes. The decreases in energy substrates occurred in parallel to an increased uptake of trypan blue into the cells. Treatment of the hepatocytes with dithiothreitol, following 4 min exposure of the cells to 3,5-diMe NAPQI, reversed the quinone imine-induced changes in nucleotide levels and reduced the cytotoxicity. It is concluded that alteration of mitochondrial function, which results in changes in the cellular energy homeostasis, is an important event in the development of cytotoxicity caused by 3,5-diMe NAPQI.

Neuroactive carbamate adducts of beta-N-methylamino-L-alanine and ethylenediamine. Detection and quantitation under physiological conditions by 13C NMR

beta-N-Methylamino-L-alanine (BMAA) reacts with dissolved carbon dioxide to form two carbamate compounds at physiological pH, temperature and bicarbonate concentration. The reversible reactions of BMAA with dissolved carbon dioxide were monitored by 13C NMR. At 37 degrees C and pH 7.4, the fraction of BMAA existing as the alpha-N-carboxy adduct is 0.22 while the fraction of BMAA existing as the beta-N-carboxy adduct is 0.09. Although both adducts could be implicated in the bicarbonate-dependent neurotoxicity of BMAA (Weiss, J. H., and Choi, D. W. (1988) Science 241, 973-975; Mroz, E. A., Weiss, J. W., and Choi, D. W. (1989) Science 243, 1613), the beta-N-carboxy adduct shares structural characteristics with the appropriate endogenous ligand, glutamic acid. Analogously, the GABA-mimetic properties of ethylenediamine have been attributed to a carbamate adduct, ethylenediamine monocarbamate (Kerr, D. I. B., and Ong, J. (1987) Br. J. Pharmacol. 90, 763-769). Using the same method, we were able to detect directly and quantify the formation of this carbamate under physiological conditions. Information on the carbamate equilibria of these compounds is essential in order to address questions of their neuroactive potency.

Metabolic activation of 1,2-dibromo-3-chloropropane: evidence for the formation of reactive episulfonium ion intermediates

The nematocide and soil fumigant 1,2-dibromo-3-chloropropane (DBCP) is a carcinogen and a mutagen and displays target-organ toxicity to the testes and the kidney. It has been proposed that both cytochrome P-450 mediated activation and glutathione (GSH) conjugation pathways are operative in DNA damage and organotropy induced by DBCP. To determine the chemical mechanisms involved in the bioactivation of DBCP and to assess a role for an episulfonium ion intermediate, the mechanism of formation of GSH conjugate metabolites of DBCP was investigated. Five biliary GSH conjugates of DBCP were isolated from rats and identified by fast atom bombardment tandem mass spectrometry: S-(2,3-dihydroxy-propyl)glutathione (I), S-(2-hydroxypropyl)glutathione (IIA), S-(3-chloro-2-hydroxypropyl)glutathione (III), 1,3-di(S-glutathionyl)propan-2-ol (IV), and 1-(glycyl-S-cysteinyl)-3- (S-glutathionyl)propan-2-ol (V). The mechanisms of conjugate formation were addressed by assessing deuterium retention in conjugates derived from [1,1,2,3,3-2H5] DBCP (D5-DBCP). GSH conjugates I, III, IV, and V displayed quantitative retention of deuterium, an observation consistent with the formation of an episulfonium ion intermediate. GSH conjugate IIA, however, retained three atoms of deuterium, thus invoking a P-450 mechanism in its genesis. The involvement of glutathione transferase (GST) and sequential episulfonium ion intermediates in the formation of metabolites I, III, and IV was demonstrated in vitro. Upon incubation of DBCP with GST, metabolites I, III, and IV were identified by tandem mass spectrometry and were found to arise with quantitative retention of deuterium when D5-DBCP was employed as a substrate. An additional GSH conjugate, 1,2,3-tri(S-glutathionyl)propane (VI), was observed as the major metabolite in incubations of GST with DBCP. When the incubations of DBCP with GST were performed in H2(18)O, metabolite I incorporated two atoms of 18O, and metabolites III and IV incorporated one atom of 18O. The ability of GST to catalyze the formation of the four GSH conjugates observed in vivo, with quantitative retention of deuterium and incorporation of 18O from H2(18)O, may be rationalized by a mechanism invoking the initial formation of S-(2-bromo-3-chloropropyl)glutathione. Rearrangement of this unstable conjugate via several reactive episulfonium ions, with either hydrolysis by water or alkylation of GSH at various stages, would account for the pattern of metabolites and their status of isotopic enrichment observed under various incubation conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Menthofuran-dependent and independent aspects of pulegone hepatotoxicity: roles of glutathione

Pulegone, a monoterpene that protects source plants against predators, is a hepatotoxic constituent of the folklore abortifacient pennyroyal oil. In the rat, pulegone extensively depleted glutathione measured in both liver tissue and plasma, and its toxicity was markedly enhanced in animals treated with buthionine sulfoximine. The glutathione-depleting effect of pulegone was compromised following inhibition of cytochrome P-450 by piperonyl butoxide. In addition, we found no evidence for conjugation of glutathione to unchanged pulegone in vitro. Administration of menthofuran, a known oxidative and hepatotoxic metabolite of pulegone, only marginally affected glutathione levels in plasma and liver, and toxicity was not augmented by buthionine sulfoximine. These results provide indirect evidence for cytochrome P-450-catalyzed bioactivation of pulegone via at least two independent pathways: 1) the formation and subsequent activation of menthofuran from pulegone; and 2) the formation of reactive intermediate(s) from pulegone, but not menthofuran, which can be detoxified through a mechanism requiring reduced glutathione.

Species differences in kidney necrosis and DNA damage, distribution and glutathione-dependent metabolism of 1,2-dibromo-3-chloropropane (DBCP)

Species differences and mechanisms of 1,2-dibromo-3-chloropropane (DBCP) nephrotoxicity were investigated by studying DBCP renal necrosis and DNA damage, distribution and glutathione-dependent metabolism in rats, mice, hamsters and guinea pigs. Extensive renal tubular necrosis was observed in rats 48 hr after a single intraperitoneal administration (21-170 mumol/kg) of DBCP. Significantly less necrosis was found in mice and guinea pigs, whereas no renal damage was evident (less than 680 mumol/kg) in hamsters. The activation of DBCP to DNA damaging intermediates in vivo, as measured by alkaline elution of DNA isolated from kidney nuclei 60 min. after intraperitoneal injection of DBCP, was compared in all four species. Distinct DNA damage was detected in rats, mice and hamsters as early as 10 min. after administration of DBCP and within 30 min. in guinea pigs. Rats and guinea pigs showed similar sensitivity towards DBCP-induced DNA damage (extensive DNA damage greater than 21 mumol/kg DBCP), whereas in mice and hamsters a 10-50 times higher DBCP dose was needed to cause a similar degree of DNA damage. Renal DBCP concentrations at various time-points (20 min., 1, 3 and 8 hr) after intraperitoneal administration (85 mumol/kg) revealed that the initial (20 min.) DBCP concentration was substantially higher in rats and guinea pigs compared to the other two species. Furthermore, kidney elimination of DBCP occurred at a significantly lower rate in rats than in mice, hamsters and guinea pigs.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetaminophen-induced oxidation of protein thiols. Contribution of impaired thiol-metabolizing enzymes and the breakdown of adenine nucleotides

The administration of a hepatotoxic dose of acetaminophen (250 mg/kg) to mice induced the loss of protein thiols in mouse liver. Our data suggest that a significant portion of this loss was due to protein thiol oxidation. The administration of the nonhepatotoxic regioisomer, 3'-hydroxyacetanilide (600 mg/kg) did not produce a similar decrease in liver protein thiols despite producing similar levels of covalent binding. Mice treated with acetaminophen exhibited decreased glutathione peroxidase activity, decreased thioltransferase activity, and decreased adenine nucleotide concentrations in the liver. The increase in urinary allantoin after the administration of acetaminophen suggests that the decrease in adenine nucleotides was due to their degradation in the liver. Acetaminophen also promoted the conversion of the enzyme xanthine dehydrogenase to the oxidase form, and pretreatment of mice with allopurinol, an inhibitor of xanthine oxidase, significantly decreased acetaminophen-mediated hepatotoxicity. The conversion of xanthine dehydrogenase to the oxidase form may lead to a transient increase in the production of activated oxygen species. The increase in activated oxygen species coupled with decreases in glutathione peroxidase and thioltransferase activity may be responsible in part for the increased levels of oxidized protein thiols observed following acetaminophen administration.