The relationship between the concentration of the pyrrolizidine alkaloid monocrotaline and the pattern of metabolites released from the isolated liver

Metabolism-mediated cytotoxicity and genotoxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) and PA N-oxides are common phytotoxins produced by over 6000 plant species. Humans are frequently exposed to PAs via ingestion of PA-containing herbal products or PA-contaminated foods. PAs require metabolic activation to form pyrrole-protein adducts and pyrrole-DNA adducts which lead to cytotoxicity and genotoxicity. Individual PAs differ in their metabolic activation patterns, which may cause significant difference in toxic potency of different PAs. This review discusses the current knowledge and recent advances of metabolic pathways of different PAs, especially the metabolic activation and metabolism-mediated cytotoxicity and genotoxicity, and the risk evaluation methods of PA exposure. In addition, this review provides perspectives of precision toxicity assessment strategies and biomarker development for the risk control and translational investigations of human intoxication by PAs.

Monocrotaline-induced liver toxicity in rat predicted by a combined in vitro physiologically based kinetic modeling approach

The aim of the present study was to use an in vitro–in silico approach to predict the in vivo acute liver toxicity of monocrotaline and to characterize the influence of its metabolism on its relative toxic potency compared to lasiocarpine and riddelliine. In the absence of data on acute liver toxicity of monocrotaline upon oral exposure, the predicted dose–response curve for acute liver toxicity in rats and the resulting benchmark dose lower and upper confidence limits for 10% effect (BMDL₁₀ and BMDU₁₀) were compared to data obtained in studies with intraperitoneal or subcutaneous dosing regimens. This indicated the predicted BMDL₁₀ value to be in line with the no-observed-adverse-effect levels (NOAELs) derived from availabe in vivo studies. The predicted BMDL₁₀–BMDU₁₀ of 1.1–4.9 mg/kg bw/day also matched the oral dose range of 1–3 mg PA/kg bw/day at which adverse effects in human are reported. A comparison to the oral toxicity of the related pyrrolizidine alkaloids (PAs) lasiocarpine and riddelliine revealed that, although in the rat hepatocytes monocrotaline was less toxic than lasiocarpine and riddelliine, due to its relatively inefficient clearance, its in vivo acute liver toxicity was predicted to be comparable. It is concluded that the combined in vitro-PBK modeling approach can provide insight in monocrotaline-induced acute liver toxicity in rats, thereby filling existing gaps in the database on PA toxicity. Furthermore, the results reveal that the kinetic and metabolic properties of PAs can vary substantially and should be taken into account when considering differences in relative potency between different PAs.

A systematic review and quality assessment of case reports of adverse events for borage (Borago officinalis), coltsfoot (Tussilago farfara) and comfrey (Symphytum officinale)

Symphytum officinale (comfrey), Tussilago farfara (coltsfoot) and Borago officinalis (borage) have long histories of therapeutic use, but their safety has been questioned due to the presence of unsaturated pyrrolizidine alkaloids (PAs). The evidence base underlying these concerns relies in part on case reports. This systematic review assesses these case reports for their reliability to inform this debate.

METHOD

Study selection was restricted to case reports describing possible pyrrolizidine alkaloid related harm and ingestion of comfrey, coltsfoot or borage. An extensive search of academic databases was conducted. Papers meeting the criteria were critically appraised.

RESULTS

The search resulted in 11 appropriate case reports, none of which involved borage. Nine reports were assessed for causality and indicated some degree of association between the material ingested and the adverse event. Lack of unequivocal identification of the species ingested compromised attribution and was a significant source of uncertainty. Three levels of identity confusions were found; misidentification or substitution at the level of the whole herb; omission of appropriate botanical identification and attribution of a specific PA to either comfrey or coltsfoot when it is a constituent found in other plants of established toxicity.

CONCLUSION

These cases are an unreliable body of evidence on which to draw conclusions about the safety of the oral consumption of Symphytum officinale and Tussilago farfara. Toxicological studies based on oral ingestion of phytochemically-complex preparations of these herbs may be the most accurate methodology for assessing clinical risk.

Oxidative hepatotoxicity effects of monocrotaline and its amelioration by lipoic acid, S-adenosyl methionine and vitamin E

Liver is the major site for several xenobiotics metabolism, and formation of toxic metabolites that may be hepatotoxic, therefore the burden of metabolism and exposure to dangerous chemicals make liver vulnerable to a variety of disorders. Our work aimed to investigate the effects of some antioxidants such as lipoic acid (LA), S-adenosyl methionine (SAM) and vitamin E in a trail to investigate the possibility of using these substances to relieve and protect liver from exposure to monocrotaline (MCT). Twenty-five mature adult rats were classified into five groups (five rats in each group), control group, MCT-induced hepatic damage, LA+MCT, SAM+MCT and vitamin E+MCT group. Homogenates of liver samples were used for measuring the oxidative biomarkers and hepatic antioxidant status. The results showed that administration of vitamin E, SAM and LA caused a significant increase in liver glutathione contents, glutathione reductase, glutathione peroxidase and glutathione-S-transferase activities and a significant decrease in hepatic catalase and superoxide dismutase. We could conclude that administration of natural LA, SAM and vitamin E before and after MCT injection modulate the hepatic oxidative stresses induced by MCT in various extents.

Organic cation transporter 1 mediates the uptake of monocrotaline and plays an important role in its hepatotoxicity

Monocrotaline (MCT) is a kind of toxic retronecine-type pyrrolizidine alkaloids (PAs) from plants of Crotalaria, which can be bio-activated by cytochrome P450 (CYP) enzymes in liver and then induce hepatotoxicity. Since CYPs are localized in the endoplasmic reticulum, the influx of MCT to the liver is the key step for its hepatotoxicity. The objective of the present study was to investigate the role of organic cation transporter 1 (OCT1), a transporter mainly expressed in liver, in the uptake of MCT and in hepatotoxicity induced by MCT. The results revealed that MCT markedly inhibited the uptake of 1-methyl-4-phenylpyridinium (MPP(+)), an OCT1 substrate, in Madin-Darby canine kidney (MDCK) cells stably expressing human OCT1 (MDCK-hOCT1) with the IC50 of 5.52±0.56μM. The uptake of MCT was significantly higher in MDCK-hOCT1 cells than in MDCK-mock cells, and MCT uptake in MDCK-hOCT1 cells followed Michaelis-Menten kinetics with the Km and Vmax values of 25.0±6.7μM and 266±64pmol/mg protein/min, respectively. Moreover, the OCT1 inhibitors, such as quinidine, d-tetrahydropalmatine (d-THP), obviously inhibited the uptake of MCT in MDCK-hOCT1 cells and isolated rat primary hepatocytes, and attenuated the viability reduction and LDH release of the primary cultured rat hepatocytes caused by MCT. In conclusion, OCT1 mediates the hepatic uptake of MCT and may play an important role in MCT induced-hepatotoxicity.

Sorafenib attenuates monocrotaline-induced sinusoidal obstruction syndrome in rats through suppression of JNK and MMP-9

BACKGROUND & AIMS

Sinusoidal obstruction syndrome (SOS) is a drug-induced liver injury that occurs with oxaliplatin treatment and is associated with postoperative morbidity after hepatectomy. The aim of this study was to investigate the effects of sorafenib in a monocrotaline (MCT)-induced model of SOS in rats.

METHODS

Rats were divided into groups treated with sorafenib (2mg/kg) or vehicle, 36 h and 12h before MCT (90 mg/kg) administration by gavage. Liver tissues and blood were sampled 48 h after MCT administration to evaluate SOS. Survival after hepatectomy was examined and immunohistochemistry and electron microscopy were performed to assess sinusoidal injury.

RESULTS

In the vehicle group, liver histology showed sinusoidal dilatation, coagulative necrosis of hepatocytes, endothelial damage of the central vein, and sinusoidal hemorrhage. In the sorafenib group, these changes were significantly suppressed, total SOS scores were significantly decreased, and the elevation of serum transaminase levels observed in the vehicle group was significantly reduced. Survival after hepatectomy was significantly higher in the sorafenib group compared to the vehicle group (45% vs. 20%, p=0.0137). Immunohistochemistry and electron microscopy revealed a protective effect of sorafenib on sinusoidal endothelial cells at 6h after MCT treatment. Sorafenib also attenuated the activity of metallopeptidase-9 (MMP-9) and phosphorylation of c-Jun N-terminal kinase (JNK).

CONCLUSIONS

Sorafenib reduced the severity of MCT-induced SOS in rats through suppression of MMP-9 and JNK activity, resulting in improvement of survival after hepatectomy.

The protective effects of cerium oxide nanoparticles against hepatic oxidative damage induced by monocrotaline

Objective

The objective of the present study was to determine the ability of cerium oxide (CeO₂) nanoparticles to protect against monocrotaline (MCT)-induced hepatotoxicity in a rat model.

Method

Twenty male Sprague Dawley rats were arbitrarily assigned to four groups: control (received saline), CeO₂ (given 0.0001 nmol/kg intraperitoneally [IP]), MCT (given 10 mg/kg body weight IP as a single dose), and MCT + CeO₂ (received CeO₂ both before and after MCT). Electron microscopic imaging of the rat livers was carried out, and hepatic total glutathione (GSH), glutathione reductase (GR), glutathione peroxidase (GPX), glutathione S-transferase (GST), superoxide dismutase (SOD), and catalase (CAT) enzymatic activities were quantified.

Results

Results showed a significant MCT-induced decrease in total hepatic GSH, GPX, GR, and GST normalized to control values with concurrent CeO₂ administration. In addition, MCT produced significant increases in hepatic CAT and SOD activities, which also ameliorated with CeO₂.

Conclusions

These results indicate that CeO₂ acts as a putative novel and effective hepatoprotective agent against MCT-induced hepatotoxicity.

Dehydromonocrotaline induces cyclosporine A-insensitive mitochondrial permeability transition/cytochrome c release

Monocrotaline (MCT) is a pyrrolizidine alkaloid present in plants of the genus Crotalaria that causes cytotoxicity and genotoxicity in animals and humans. It is well established that the toxicity of MCT results from its hepatic bioactivation to dehydromonocrotaline (DHM), an alkylating agent, but the exact mechanism of action remains unknown. In a previous study, we demonstrated DHM's inhibition of mitochondrial NADH-dehydrogenase activity at micromolar concentrations, which is an effect associated with a significant reduction in ATP synthesis. As a follow-up study, we have evaluated the ability of DHM to induce mitochondrial permeability transition (MPT) and its associated processes in isolated rat liver mitochondria. In the presence of 10 microM Ca(2+), DHM (50-250 microM) elicited MPT in a concentration-dependent, but cyclosporine A-independent manner, as assessed by mitochondrial swelling, which is associated with mitochondrial Ca(2+) efflux and cytochrome c release. DHM (50-250 microM) did not cause hydrogen peroxide accumulation but did deplete endogenous glutathione and NAD(P)H, while oxidizing protein thiol groups. These results potentially indicate the involvement of mitochondria, via apoptosis, in the well-documented cytotoxicity of monocrotaline.

Molecular characterization of sheep ruminal enrichments that detoxify pyrrolizidine alkaloids by denaturing gradient gel electrophoresis and cloning

An enrichment of strictly anaerobic bacteria from ovine rumen fluid, which has previously been named L4M2, is known to detoxify animal hepatotoxins from the pyrrolizidine alkaloid family. These toxins are present in the tansy ragwort plant (Senecio jacobaea). These plants have been described in livestock animals' range forages in regions of the world such as the Northwest United States and South Africa. The bacterial enrichment was characterized by molecular cloning techniques and by the molecular fingerprinting technique of denaturing gradient gel electrophoresis (DGGE). Phylogenetic analysis of the enrichment revealed that the consortium is composed of no more than five putative bacterial species which associated to the Anaerovibrio, Desulfovibrio, Megasphaera, Prevotella, and Synergistes generas. These are all known to exist in the upper gastrointestinal tract of ruminant animals. This work improved upon previous attempts to characterize the consortium by obtaining nearly full-length ribosomal 16S rDNA sequences through cloning. The DGGE results were directly compared to the cloning data by polymerase chain reaction (PCR) amplifying eight phylogenetically representative clones and analyzing them by DGGE. Direct DGGE analysis of the enrichment displayed greater 16S diversity than the clone library used in this study, suggesting that at least one of the organisms present in the enrichment comprises less than 1% of the total cell population. These data will be used to further refine the enrichment in hopes of future use as a probiotic, which could be administered to animals challenged by the presence of tansy ragwort in their forage.

Molecular analysis of a consortium of ruminal microbes that detoxify pyrrolizidine alkaloids

Members of a consortium of bacteria, isolated from the rumen of sheep, that degrades pyrrolizidine alkaloids (PAs) found in tansy ragwort (Senecio jacobaea) were characterized. An enrichment of ruminal bacteria was isolated from a sample of ruminal fluid using standard anaerobic techniques. The PA degradative capacity of the enrichment was tested by spiking purified PA extract from tansy ragwort. Length heterogeneity analysis by PCR (LH-PCR) and restriction fragment length polymorphism (RFLP) analysis was used to identify members of the consortium. Phylogenetic analysis of the 16S rDNA gene revealed differing results based on the molecular method used. LH-PCR identified 7 different organisms in 3 groups while RFLP identified 6 organisms with differing banding patterns in 5 groups. After the phylogenetic analyses of both methods were combined, the combined isolates represented 6 groups. The majority of the members of this consortium are <97.0% homologous with known bacteria, indicating this consortium may contain novel organisms able to detoxify PAs found in tansy ragwort. Further understanding of the metabolic pathways used by this consortium to degrade PAs could lead to the use of the consortium as a probiotic therapy for livestock and horses afflicted with tansy ragwort toxicosis.Key words: pyrrolizidine alkaloids, ruminal bacteria, tansy ragwort, RFLP, LH-PCR.

Pyrrolizidine Alkaloids—Genotoxicity, Metabolism Enzymes, Metabolic Activation, and Mechanisms

Pyrrolizidine alkaloid-containing plants are widely distributed in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Because of their abundance and potent toxicities, the mechanisms by which pyrrolizidine alkaloids induce genotoxicities, particularly carcinogenicity, were extensively studied for several decades but not exclusively elucidated until recently. To date, the pyrrolizidine alkaloid-induced genotoxicities were revealed to be elicited by the hepatic metabolism of these naturally occurring toxins. In this review, we present updated information on the metabolism, metabolizing enzymes, and the mechanisms by which pyrrolizidine alkaloids exert genotoxicity and tumorigenicity.

Augmented Pulmonary Vascular and Venous Constrictions to NG-Nitro-L-Arginine Methyl Ester in Rats with Monocrotaline-Induced Pulmonary Hypertension

The hemodynamic effects of N(G)-nitro-L-arginine methyl ester (L-NAME, inhibitor of nitric oxide (NO) synthase) were examined in thiobutabarbital-anesthetized control-rats and rats with monocrotaline-induced pulmonary hypertension. L-NAME (1-16 mg/kg i.v.) increased mean arterial pressure, systemic vascular resistance, venous resistance and pulmonary vascular resistance, and decreased cardiac output in both the control and pulmonary hypertensive rats. Relative to the controls, L-NAME (16 mg/kg) caused a smaller increase (approximately 50% of control) in mean arterial pressure in the pulmonary hypertensive rats, but greater increases in venous (approximately 200%) as well as pulmonary vascular (approximately 400%) resistances and a greater decrease in cardiac output (approximately 140%). The results show that NO is an important dilator within the arterial, venous and pulmonary circulation; its pulmonary and venous dilator roles are augmented in pulmonary hypertension.

Pyrrolizidine poisoning: a neglected area in human toxicology

Pyrrolizidine poisoning in humans is regarded by most clinical toxicologists as of little relevance. However, a number of individual case studies in the West and some severe cases of mass poisoning by contaminated grains have led to increased interest in these alkaloids. The increasing use of herbal remedies, some of which contain toxic pyrrolizidines, suggests that the incidence of pyrrolizidine poisoning is likely to increase. In this review the authors describe the chemistry and metabolism of pyrrolizidine alkaloids, the salient features of pyrrolizidine poisoning, and the methods available for detection of these compounds in human fluids.

Lack of protective action of cysteine against the fetotoxic effect of monocrotaline

Monocrotaline (MCT), a pyrrolizidine alkaloid present in Crotalaria species, has hepatotoxic, nephrotoxic, pneumotoxic and fetotoxic effects. However, the toxic effects of exposure to MCT in adult rats can be prevented by cysteine. Thus, the present study was conducted to evaluate the possible prevention by cysteine of the toxic effects of MCT on pregnant rats. Thirty-six pregnant rats were used. The females in the experimental groups were fed ration containing 0.02% MCT, 0.02% MCT + 1% cysteine, or 1% cysteine from day 6 to day 21 of pregnancy; the control group was fed only common ration for the same period of time. All rats were killed on day 21 of pregnancy and their blood was collected for determination of liver and kidney function. General toxicity to pregnant dams was assessed. Fetuses were removed by caesarian section and embryofetotoxic parameters were examined. Results showed impaired body weight gain in rats fed MCT, with or without cysteine supplementation. Plasma levels of AST, ALT, LDH, GGT, urea and creatinine were increased in MCT animals compared to controls. The pathology study revealed lesions only in dams from the MCT group. The weights of the placentas and fetuses of the MCT and MCT + cysteine groups were significantly lower than those of the control group. Thus, the present data suggests some protective action of 1% of cysteine in ration against the toxic effects of MCT on the dams but not on the litter.

Chronic pulmonary hypertension--the monocrotaline model and involvement of the hemostatic system

Monocrotaline (MCT) is a toxic pyrrolizidine alkaloid of plant origin. Administration of small doses of MCT or its active metabolite, monocrotaline pyrrole (MCTP), to rats causes delayed and progressive lung injury characterized by pulmonary vascular remodeling, pulmonary hypertension, and compensatory right heart hypertrophy. The lesions induced by MCT(P) administration in rats are similar to those observed in certain chronic pulmonary vascular diseases of people. This review begins with a synopsis of the hemostatic system, emphasizing the role of endothelium since endothelial cell dysfunction likely underlies the pathogenesis of MCT(P)-induced pneumotoxicity. MCT toxicology is discussed, focusing on morphologic, pulmonary mechanical, hemodynamic, and biochemical and molecular alterations that occur after toxicant exposure. Fibrin and platelet thrombosis of the pulmonary microvasculature occurs after administration of MCT(P) to rats, and several investigators have hypothesized that thrombi contribute to the lung injury and pulmonary hypertension. The evidence for involvement of the various components of the hemostatic system in MCT(P)-induced vascular injury and remodeling is reviewed. Current evidence is consistent with involvement of platelets and an altered fibrinolytic system, yet much remains to be learned about specific events and signals in the vascular pathogenesis.

Effects of monocrotaline, a pyrrolizidine alkaloid, on glutathione metabolism in the rat

Monocrotaline (MONO), a pyrrolizidine alkaloid, causes veno-occlusive disease of the liver, pulmonary arterial hypertension, and right ventricular hypertrophy. Toxicity is due to the hepatic formation of a pyrolic metabolite that can be detoxified by conjugation with glutathione (GSH). We have shown that the GSH content of the liver affects the quantity of the pyrrolic metabolite that is released from the liver. We have now examined whether MONO, in turn, affects GSH metabolism. Twenty-four hours after administration of MONO to rats (65 mg/kg, i.p.), the highest concentration of bound pyrrolic metabolites was found in the liver, followed by the lung and kidney. Heart and brain contained lower concentrations of these metabolites. Significantly higher levels of GSH were found in liver and lungs of MONO-treated rats than in saline-injected control animals. In the liver, activities of the following enzymes were elevated: gamma-glutamylcysteine synthetase, GSH synthetase, gamma-glutamyl transpeptidase, dipeptidase, and microsomal GSH transferase. The same changes were seen in the lung. In the heart, gamma-glutamyl transpeptidase activity was decreased markedly, and cytosolic GSH transferase activity was elevated. In the kidney, the activities of GSH synthetase, gamma-glutamyl transpeptidase, and cytosolic GSH transferase were increased. Our results establish a mutual interaction of MONO and sulfur metabolism. It appears that an early metabolic action of MONO is to modify sulfur amino acid metabolism, diverting cysteine metabolism from oxidation to taurine towards synthesis of GSH.

Physicochemical and metabolic basis for the differing neurotoxicity of the pyrrolizidine alkaloids, trichodesmine and monocrotaline

Monocrotaline and trichodesmine are structurally closely related pyrrolizidine alkaloids (PAs) exhibiting different extrahepatic toxicities, trichodesmine being neurotoxic (LD(50) 57 mu mol/kg) and monocrotaline pneumotoxic (LD(50) 335 mu mol/kg). We have compared certain physicochemical properties and metabolic activities of these two PAs in order to understand the quantitative and qualitative differences in toxicity. Both PAs were metabolized in the isolated, perfused rat liver to highly reactive pyrrolic dehydroalkaloids that appear to be responsible for the toxicity of PAs. More dehydrotrichodesmine (468 nmol/g liver) than dehydromonocrotaline (116 nmol/g liver) was released from liver into perfusate on perfusion for 1 hr with 0.5 mM of the parent PA. Dehydrotrichodesmine had a significantly longer aqueous half-life (5.4 sec) than that of dehydromonocrotaline (3.4 sec). In vivo, significantly higher levels of bound pyrroles were found in the brain 18 hr after injection of trichodesmine (25 mg/kg; i.p.) than were seen following either an equal dose (25 mg/kg; i.p.) or an equitoxic dose (90 mg/kg; i.p.) of monocrotaline. Trichodesmine had a higher partition coefficient than monocrotaline for both chloroform and heptane, indicating its greater lipophilicity. The pK(a) of trichodesmine (7.07) was only slightly higher than that of monocrotaline (pK(a¿ 6.83), suggesting that a difference in degree of ionization was not a major factor affecting the relative ability of the dehydroalkaloids to cross the blood-brain barrier. We conclude that the greater lethality and neurotoxicity of trichodesmine compared to monocrotaline is due to two structural characteristics: (i) steric hindrance at position 14 of dehydrotrichodesmine results in greater resistance to hydrolysis, allowing more to be released from the liver and to be delivered to the brain; (ii) the larger isopropyl substituent at position 14 of dehydrotrichodesmine renders the molecule more lipophilic, leading to greater penetration of the brain.

Effect of the pyrrolizidine alkaloid, monocrotaline, on bile composition of the isolated, perfused rat liver

Monocrotaline is a hepatotoxic pyrrolizidine alkaloid, releasing high levels of metabolites into bile of isolated, perfused liver. Although perfusion of rat liver with 0.5 mM monocrotaline does not affect bile flow over a 1 hr study period, it markedly affects bile composition. Biliary release of conjugated and free GSH increases 30-fold. Marked increases are also observed in the biliary concentration of the related sulfur-containing substances, cysteine and cysteinylglycine. However, biliary release of the sulfur amino acids, taurine and methionine, is unaffected. Only two amino acids show mildly increased releases, 23% for glycine and 46% for aspartate. Release of bile acids, cholesterol and phospholipids also decrease, both in terms of mM concentration in bile and in terms of nmol secreted per g liver. Thus, exposure to monocrotaline causes disturbances in sulfur metabolism in the liver and in the composition of bile. The consequences of the digestive properties of bile and gastrointestinal toxicity remain to be established. As sulfhydryl compounds are involved in detoxification of monocrotaline metabolites, these findings indicate a mutual interaction of pyrrolizidine toxicity and sulfur metabolism. This suggests that dietary sulfur amino acid intake may influence susceptibility to pyrrolizidine poisoning.

Micronuclei in mice treated with monocrotaline with and without phenobarbital pretreatment

Monocrotaline is a very potent toxin, producing significant effects of pneumotoxicity, hepatotoxicity, and teratogenicity, as well as carcinogenicity. In addition, the compound has been clearly shown to be mutagenic after metabolic activation. The goal of the experiments reported here was to confirm the reported clastogenesis induced by this agent in vivo and to evaluate the impact of modulation of metabolic activity by phenobarbital, a potent P-450 inducer (both Phase I and Phase II enzymes). The method used in addressing this problem relied on a new technique for monitoring clastogenesis in vivo, i.e., the acridine orange micronucleus assay method originally exploited by Hayashi et al. [1990]. The result of our experiments confirmed monocrotaline to be an effective clastogen in vivo, using the acridine orange method of assessment. The peak in induction of micronuclei occurred on the second day following intraperitoneal administration of the drug. Administration of phenobarbital prior to monocrotaline did appear to modulate the micronucleus induction. At 30 mg/kg bw monocrotaline, the pretreatment with phenobarbital appears to increase the intensity of monocrotaline clastogenesis, while the effect at higher doses (60 and 125 mg/kg bw) is a reduction in potency, presumably reflecting increased importance of Phase II metabolism for monocrotaline at these doses. Thus the study reported here confirms the potent in vivo clastogenesis of monocrotaline, and provides evidence for a dose-related shift in mechanism for the phenomenon.