Mutagenic activity of the pyrrolizidine alkaloids seneciphylline and senkirkine in Drosophila and their transfer into rat milk

Chemical Constituents and Biological Properties of Genus Doronicum (Asteraceae)

The genus Doronicum, belonging to tribe Senecioneae (Fam. Asteraceae), is found mainly in the Asia, Europe and North Africa. This genus of plant has always been used in traditional medicinal treatments due to the many biological properties shown such as killing parasitic worms and for relieving constipation, as well as to improve heart health, to alleviate pain and inflammation, to treat insect bites, etc. According to the World Flora the genus Doronicum contains 39 subordinate taxa.^[1-3] The purpose of this article, which covers data published from 1970 to 2021 with more than 110 articles, aims to carry out a complete and critical review of the Doronicum genus, examining traditional uses and reporting the antioxidant, antimicrobial, anti-inflammatory and antitumor activity shown from crude extracts or essential oils, and from single isolated compounds. Furthermore, critical considerations of the published data have been highlighted by comparing them with the results obtained from species of other genus belonging to the Asteraceae family.

Identification and Simultaneous Determination of the Main Toxical Pyrrolizidine Alkaloids in a Compound Prescription of Traditional Chinese Medicine: Qianbai Biyan Tablet

Qianbai biyan tablet (QT) is a compound prescription of traditional Chinese medicine which is used to treat nasal congestion, rhinitis, and nasosinusitis, with Senecio scandens as its main plant material. Several pyrrolizidine alkaloids (PAs) were reported in Senecio scandens and others of Senecio species. Although Senecio scandens is assigned as the legal plant material of QT, whether replaced use of it by other Senecio plants can bring toxicity is unknown because of the lack of quantitative data about toxic PAs between different Senecio species. In the present study, adonifoline, senkirkine, and another PA presumed as emiline have been identified in QT; however, there was no senecionine detected in all tablets. PA contents in QTs varied in different companies and different batches. Adonifoline existed only in Senecio scandens, and senecionine was detected in all eight Senecio plants investigated in the present study. Data showed that replaced use of Senecio scandens with a low level of senecionine by other Senecio plants such as Senecio vulgaris containing a high level of senecionine is advertised to be forbidden. Data of the present study may be used as a reference to make new drug quality regularity and recommendation guideline for the safety of QT.

Prediction of in vivo genotoxicity of lasiocarpine and riddelliine in rat liver using a combined in vitro-physiologically based kinetic modelling-facilitated reverse dosimetry approach

Pyrrolizidine alkaloids (PAs) are naturally occurring genotoxic compounds, and PA-containing plants can pose a risk to humans through contaminated food sources and herbal products. Upon metabolic activation, PAs can form DNA adducts, DNA and protein cross links, chromosomal aberrations, micronuclei, and DNA double-strand breaks. These genotoxic effects may induce gene mutations and play a role in the carcinogenesis of PAs. This study aims to predict in vivo genotoxicity for two well-studied PAs, lasiocarpine and riddelliine, in rat using in vitro genotoxicity data and physiologically based kinetic (PBK) modelling-based reverse dosimetry. The phosphorylation of histone protein H2AX was used as a quantitative surrogate endpoint for in vitro genotoxicity of lasiocarpine and riddelliine in primary rat hepatocytes and human HepaRG cells. The in vitro concentration-response curves obtained from primary rat hepatocytes were subsequently converted to in vivo dose-response curves from which points of departure (PoDs) were derived that were compared to available in vivo genotoxicity data. The results showed that the predicted PoDs for lasiocarpine and riddelliine were comparable to in vivo genotoxicity data. It is concluded that this quantitative in vitro-in silico approach provides a method to predict in vivo genotoxicity for the large number of PAs for which in vivo genotoxicity data are lacking by integrating in vitro genotoxicity assays with PBK modelling-facilitated reverse dosimetry.

New glutathione conjugate of pyrrolizidine alkaloids produced by human cytosolic enzyme-dependent reactions in vitro

RATIONALE

The toxic metabolites of pyrrolizidine alkaloids (PAs) are initially formed by cytochrome P450-mediated oxidation reactions and primarily eliminated as glutathione (GSH) conjugates. Although the reaction between the reactive metabolites and GSH can occur spontaneously, the role of the cytosolic enzymes in the process has not been studied.

METHODS

The toxic metabolites of selected PAs (retrorsine, monocrotaline, senecionine, lasiocarpine, heliotrine or senkirkine) were generated by incubating them in 100 mM phosphate buffer (pH 7.4) containing liver microsomes of human, pig, rat or sheep, NADPH and reduced GSH in the absence or presence of human, pig, rat or sheep liver cytosolic fraction. The supernatants were analyzed using liquid chromatography connected to Finnigan LTQ ion-trap, Agilent QTOF or Thermo Scientific Q Exactive Focus quadrupole-orbitrap mass spectrometers.

RESULTS

Retrorsine, senecionine and lasiocarpine yielded three GSH conjugates producing [M - H]^- ions at m/z 439 (7-GSH-DHP (CHO)), m/z 441 (7-GSH-DHP (OH)) and m/z 730 (7,9-diGSH-DHP) in the presence of human liver cytosolic fraction. 7-GSH-DHP (CHO) was a novel metabolite. Monocrotaline, heliotrine and senkirkine did not produce this novel 7-GSH-DHP (CHO) conjugate. 7-GSH-DHP (CHO) disappeared when incubated with hydroxylamine, and a new oxime derivative was formed. This metabolite was formed only by the human liver cytosolic enzymes but not in the presence of rat or sheep liver cytosolic fractions under otherwise identical reaction conditions.

CONCLUSIONS

7-GSH-DHP (CHO) has not been reported before, and thus it was considered as a novel metabolite of PAs. This may clarify the mechanisms involved in PA detoxification and widely observed but less understood species differences in response to PA exposure.

Oxidant effects and toxicity of Croton campestris in Drosophila melanogaster

CONTEXT

Croton campestris A.St.-Hil. (Euphorbiaceae) is a species native to Northeast Brazil used by traditional communities for the treatment of a variety of health problems. However, potential toxicological effects of this plant are unknown.

OBJECTIVE

The potential toxicity of the hydroalcoholic extract of C. campestris leaves on Drosophila melanogaster insect model, additionally with phytochemical constitution and cellular mechanisms mediating the action of extract were analysed in this study.

MATERIALS AND METHODS

Constituents of the extract were evaluated by HPLC. In vitro antioxidant potential of extract was analysed by DPPH, ABTS and FRAP. Flies injected culture medium mixed with extract (0.1-50 mg/mL) for 72 h. After, ROS production was evaluated by DCF-DA oxidation. Phosphorylation of MAPK signalling pathway was investigated by Western blotting method. Activity of antioxidant enzymes was analysed in homogenates.

RESULTS

Major components of the extract include quercetin (38.11 ± 0.06 mg/g), caffeic acid (20.06 ± 0.17 mg/g) and kaempferol (15.45 ± 0.05 mg/g). Consumption of the extract impaired locomotor performance and induced fly death of flies (LC₅₀ of 26.51 mg/mL). Augmented ROS formation and SOD, CAT and GST activity were observed from 0.1 mg/mL. JNK and p38 kinases phosphorylation was modulated and Paraquat-induced toxicity was augmented by extract.

DISCUSSION AND CONCLUSION

Our data show important toxicological effects of C. campestris leading to increased mortality and impaired locomotor performance accompanied by induction of cell stress markers in flies. The study draws attention to the indiscriminate use of plant extracts.

Interim relative potency factors for the toxicological risk assessment of pyrrolizidine alkaloids in food and herbal medicines

Pyrrolizidine alkaloids (PAs) are among the most potent natural toxins occurring in a broad spectrum of plant species from various families. Recently, findings of considerable contamination of teas/herbal infusions prepared from non-PA plants have been reported. These are obviously due to cross-contamination with minor amounts of PA plants and can affect both food and herbal medicines. Another source of human exposure is honey collected from PA plants. These findings illustrate the requirement for a comprehensive risk assessment of PAs, hampered by the enormous number of different PA congeners occurring in nature. Up to now, risk assessment is based on the carcinogenicity of certain PAs after chronic application to rats using the sum of detected PAs as dose metric. Because of the well-documented large structure-dependent differences between sub-groups of PA congeners with respect to their genotoxicity and (cyto)toxicity, however, this procedure is inadequate. Here we provide an overview of recent attempts to assess the risk of PA exposure and the available literature on the toxic effects and potencies of different congeners. Based on these considerations, we have derived interim Relative Potency (REP) factors for a number of abundant PAs suggesting a factor of 1.0 for cyclic di-esters and open-chain di-esters with 7S configuration, of 0.3 for mono-esters with 7S configuration, of 0.1 for open-chain di-esters with 7R configuration and of 0.01 for mono-esters with 7R configuration. For N-oxides we suggest to apply the REP factor of the corresponding PA. We are confident that the use of these values can provide a more scientific basis for PA risk assessment until a more detailed experimental analysis of the potencies of all relevant congeners can be carried out.

Plants containing pyrrolizidine alkaloids: toxicity and problems

Pyrrolizidine alkaloids (PA) are toxic for human and livestock. They undergo a metabolic toxication process in the liver which is the first target organ for PA poisoning. Worldwide many episodes of human PA intoxications are well reported. In many cases the reason for these intoxications has been PA contamination in food. The main tools for analysing food and fodder on PA content are based on GC and HPLC separation, followed by MS(-MS) detection. Actual incidents with toxic PA are the 'Jacobaea vulgaris (syn. Senecio jacobaea) problem' in Europe and the 'Ageratum conyzoides problem' in Ethiopia.

Genotoxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are common constituents of many plant species around the world. PA-containing plants are probably the most common poisonous plants affecting livestock and wildlife. They can inflict harm to humans through contaminated food sources, herbal medicines and dietary supplements. Half of the identified PAs are genotoxic and many of them are tumorigenic. The mutagenicity of PAs has been extensively studied in different biological systems. Upon metabolic activation, PAs produce DNA adducts, DNA cross-linking, DNA breaks, sister chromatid exchange, micronuclei, chromosomal aberrations, gene mutations and chromosome mutations in vivo and in vitro. PAs induced mutations in the cII gene of rat liver and in the p53 and K-ras genes of mouse liver tumors. It has been suggested that all PAs produce a set of (+/-)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine-derived DNA adducts and similar types of gene mutations. The signature types of mutations are G : C --> T : A transversion and tandem base substitutions. Overall, PAs are mutagenic in vivo and in vitro and their mutagenicity appears to be responsible for the carcinogenesis of PAs.

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.

Honey from plants containing pyrrolizidine alkaloids: a potential threat to health

Following scientific risk assessments, several countries have imposed strict regulations on herbal medicines containing 1,2-dehydro-pyrrolizidine alkaloids. Using published data on the plants used in honey production, pyrrolizidine alkaloid-containing plants are shown in this review to represent a significant source of honey worldwide. This observation, honey consumption data, reported levels of pyrrolizidine alkaloids in honeys, and consideration of tolerable exposure levels determined for pyrrolizidine alkaloids in herbal medicines, leads to the conclusion that some honey is a potential threat to health, especially for infants and fetuses, and further investigation is warranted.

Pyrrolizidine alkaloids in human diet

Pyrrolizidine alkaloids are the leading plant toxins associated with disease in humans and animals. Upon ingestion, metabolic activation in liver converts the parent compounds into highly reactive electrophiles capable of reacting with cellular macromolecules forming adducts which may initiate acute or chronic toxicity. The pyrrolizidine alkaloids present a serious health risk to human populations that may be exposed to them through contamination of foodstuffs or when plants containing them are consumed as medicinal herbs. Some pyrrolizidine alkaloids (PA) adducts are persistent in animal tissue and the metabolites may be re-released and cause damage long after the initial period of ingestion. PAs are also known to act as teratogens and abortifacients. Chronic ingestion of plants containing PAs has also led to cancer in experimental animals and metabolites of several PAs have been shown to be mutagenic in the Salmonella typhimurium/mammalian microsome system. However, no clinical association has yet been found between human cancer and exposure to PAs. Based on the extensive reports on the outcome of human exposure available in the literature, we conclude that while humans face the risk of veno-occlusive disease and childhood cirrhosis PAs are not carcinogenic to humans.

Risks associated with consumption of herbal teas

Plants have been used for medicinal purposes for centuries. Health-oriented individuals are turning to herbal teas as alternatives to caffeinated beverages such as coffee, tea, and cocoa and for low-caloric supplements. The popularity of herbal tea consumption has increased significantly during the past two decades in the U.S. Hundreds of different teas made up of varied mixtures of roots, leaves, seeds, barks, or other parts of shrubs, vines, or trees are sold in health food stores. Although chemists have been characterizing toxic plant constituents for over 100 years, toxicological studies of herbal teas have been limited and, therefore, the safety of many of these products is unknown. Plants synthesize secondary metabolites that are not essential in the production of energy and whose role may be in the defense mechanisms as plant toxins to their interactions with other plants, herbivores, and parasites. Pyrrolizidine alkaloids (PAs) were among the first naturally occurring carcinogens identified in plant products, and their presence in herbal teas is a matter of public health significance. Some herbal tea mixtures and single-ingredient herbal teas have been analyzed for toxic/mutagenic potential by bioassay and chromatographic techniques. Numerous human and animal intoxications have been associated with naturally occurring components, including pyrrolizidine alkaloids, tannins, and safrole. Thus, the prevention of human exposure to carcinogens or mutagens present in herbal tea mixture extracts is crucial. Preparation of infusion drinks prepared from plants appears to concentrate biologically active compounds and is a major source of PA poisoning. The quantity and consumption over a long period of time is of major concern. It is recommended that widespread consumption of herbal infusions should be minimized until data on the levels and varieties of carcinogens, mutagens, and toxicants are made available.

Toxicity and carcinogenicity of riddelliine following 13 weeks of treatment to rats and mice

Toxicity studies of riddelliine, a member of a class of pyrrolizidine alkaloids, were conducted because riddelliine has been found to contaminate human food sources. Groups of male and female Fischer rats were administered riddelliine by gavage in phosphate buffer at doses up to 10 mg/kg, and B6C3F1 mice at doses up to 25 mg/kg, five times a week. The animals were necropsied after 13 weeks of treatment or after a 7 or 14 week recovery period. Body weight gains were inversely related to dose in both rats and mice. Body weight of the 1.0 and 3.3 mg/kg female rats and 10.0 and 25.0 mg/kg mice remained depressed during the 14 week recovery period. At 13 weeks, significant findings included dose-related hepatopathy and intravascular macrophage accumulation in rats and hepatocytomegaly in mice. During the 14 week recovery period these lesions persisted and hepatic foci of cellular alteration in male rats and bile duct proliferation in female rats and male and female mice increased in severity. In the 10 mg/kg group of female rats adenomas of the liver occurred in two of ten at 13 weeks and in one of five at the 14 week recovery period. In separate studies, the frequency of micronucleated erythrocytes in peripheral blood was increased in male mice administered a single dose (150 mg/kg) of riddelliine. Increases in unscheduled DNA and S-phase syntheses were detected in primary hepatocytes from rats and mice treated with riddelliine at doses up to 25.0 mg/kg for 5 or 30 days. In mating trials in rats and mice, pup weights from treated dams at birth and during suckling were lower than controls. Thus, riddelliine is genotoxic and carcinogenic and may cross the placenta and/or be found in milk, causing developmental toxicity in rodents.

Chapter 1 Allelochemical Properties or the Raison D'être of Alkaloids

This chapter provides evidence that alkaloids are not waste products or functionless molecules as formerly assumed, but rather defense compounds employed by plants for survival against herbivores and against microorganisms and competing plants. These molecules were developed during evolution through natural selection in that they fit many important molecular targets, often receptors, of cells, which are seen in molecules that mimic endogenous neurotransmitters. The chapter discusses that microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. Many herbivores have potent mechanisms to detoxify xenobiotics, which allow the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. It needs more experimental data to understand fully the intricate interconnections between plants, their alkaloids, and herbivores, microorganisms, and other plants.

Structure/activity relationships of the genotoxic potencies of sixteen pyrrolizidine alkaloids assayed for the induction of somatic mutation and recombination in wing cells of Drosophila melanogaster

Sixteen pyrrolizidine alkaloids (PAs) were examined for their genotoxic potency in the wing spot test of Drosophila melanogaster following oral application. This in vivo assay tests for the induction of somatic mutation and mitotic recombination in cells of the developing wing primordia. All PAs tested except the C9-monoester supinine were clearly genotoxic. Depending on their chemical structure, however, genotoxicity of the PAs varied widely in a range encompassing about three orders of magnitude. In general, macrocyclic diester-type PAs were the most and 7-hydroxy C9-monoester types the least genotoxic representatives studied, while open diesters were intermediate in this respect. Stereoisomeric PAs mostly showed similar, but sometimes also clearly unequal genotoxicity. An increasing number of hydroxy groups in the PA molecule seemed to reduce its genotoxic potency. With respect to the structure/activity relationships, there appears to be a good correlation between hepatotoxicity of PAs in experimental rodents and genotoxicity in the wing spot test of Drosophila. This suggests that PAs are bioactivated along similar pathways in the mammalian liver and in the somatic cells of Drosophila. The genotoxic potential of PAs in the Drosophila wing spot test and their carcinogenic potential in mammals also seem correlated, although the information in the literature on carcinogenicity of the non-macrocyclic PAs with moderate to low genotoxic potency is concededly limited. Comparisons with other genotoxicity tests suggest that the wing spot test is particularly suitable for genotoxins like PAs, on the one hand because of the versatile metabolic bioactivation system of Drosophila and on the other hand also because of its excellent sensitivity to the crosslinking agents among the genotoxins.

Lipid peroxidation and cellular damage caused by the pyrrolizidine alkaloid senecionine, the alkenal trans-4-hydroxy-2-hexenal, and related alkenals

Lipid peroxidation was examined as a possible mechanism for cell injury by trans-4-OH-2-hexenal, the macrocyclic pyrrolizidine alkaloid senecionine and related alkenals in isolated rat hepatocytes. Each compound elicited a positive dose response for peroxidation of cellular lipids as measured by the formation of thiobarbituric acid-reactive products. The addition of the anti-oxidant N,N'-diphenyl-p-phenylenediamine to the hepatocyte suspensions inhibited the production of thiobarbituric acid-reactants. However, the presence of the anti-oxidant had no protective effects on the cell membrane integrity as evidenced by the leakage of lactate dehydrogenase from the cells into the surrounding media. These results suggest that lipid peroxidation which occurs in the presence of senecionine, trans-4-OH-2-hexenal or related alkenals is not entirely responsible for the cellular damage in isolated rat hepatocytes.

Genotoxicity and cytotoxicity of selected pyrrolizidine alkaloids, a possible alkenal metabolite of the alkaloids, and related alkenals

Recently our laboratory isolated trans-4-OH-2-hexenal from the hepatic microsomal metabolism of the macrocyclic pyrrolizidine alkaloid (PA) senecionine and demonstrated in vivo that hepatic necrosis occurred following injection into the hepatic portal vein. To demonstrate similarities in the toxic effects of these compounds, as well as additional macrocyclic PAs and alkenals, genotoxicity and cytotoxicity were examined in primary cultures of rat hepatocytes. A positive cytotoxic response was exhibited by senecionine, retrorsine, seneciphylline, 19-OH-senecionine, trans-4-OH-2-hexenal, trans-4-OH-2-nonenal, and nonenal as measured by the release of LDH. A weaker response was elicited by hexenal. Dosages used of each of these compounds ranged from 30 to 600 nmol/10(6) cells, with each compound exhibiting a linear dose response within this range. All eight compounds exhibited a positive, dose-related genotoxic response as measured by autoradiographic detection of unscheduled DNA synthesis. These results would predict a carcinogenic role for both the PAs and the alkenals. This would suggest similarities in the mechanisms of action of the PAs and alkenals, lending support to the proposed role of trans-4-OH-2-hexenal as an important toxic metabolite of the PAs.

Metabolism of the pyrrolizidine alkaloid metabolite trans-4-hydroxy-2-hexenal by mouse liver aldehyde dehydrogenases

The metabolism of trans-4-hydroxy-2-hexenal (t-4HH) was compared with that of propionaldehyde (PAL) and trans-2-hexenal (t-2H). Kinetic experiments with hepatic cytosol as a source for aldehyde dehydrogenase(s) (ALDH) (E.C. 1.2.1.3) resulted in linear Lineweaver-Burk plots for both t-4HH and t-2H, while PAL produced biphasic plots. Values of 43 and 3 microM were obtained for the apparent Km values for t-4HH and t-2H, respectively, with the two apparent Kms for PAL being 5 microM and 0.8 mM. DEAE-cellulose chromatography of cytosol isolated two peaks capable of oxidizing PAL, but only one peak was able to metabolize t-4HH. Unlike cytosol, mitochondria produced biphasic Lineweaver-Burk plots for t-4HH with two estimated Kms of 52 microM and 0.2 mM. The majority of activity for the oxidation of t-4HH, like PAL, was localized in the cytosolic fraction. This study indicates that ALDH(s) may play an important role in the detoxification of the pyrrolizidine alkaloid metabolite t-4HH, as well as other 4-hydroxyalkenals that arise from membrane lipid peroxidation.

In vivo covalent binding of retronecine-labelled [3H]seneciphylline and [3H]senecionine to DNA of rat liver, lung and kidney

Retronecine-labelled [3H]seneciphylline ([3H]SPH) and [3H]senecionine ([3H]SON) of high specific radioactivity (22 and 49 mCi/mmol, respectively) were prepared biosynthetically with seedlings of Senecio vulgaris L. using [2,3-3H]putrescine as precursor. [2,3-3H]Putrescine was synthesized by Gabriel synthesis of 1,4-diamino-2-butene from 1,4-dibromo-2-butene and catalytic hydrogenation of the product with tritium gas. Rats of both sexes were treated with the labelled pyrrolizidine alkaloids (PAs) (75-215 microCi SPH or 40-485 microCi SON/kg body wt.) and killed after 6 h or 4-5 days. SON-treated females excreted 83.4 +/- 0.2% of applied radioactivity in faeces and urine within 4 days whereas equally treated males excreted 90.9 +/- 3.2% in the same time. Excretion of 3H-activity from SPH-treated females was completed within 5 days (104.7 +/- 6.4%). Corresponding with these results, tissue levels were highest in SON-treated females. DNA and proteins were isolated from liver, lungs and kidneys and covalent binding of the alkaloids to DNA was determined. A Covalent Binding Index (CBI, mumol alkaloid bound per mol nucleotides/mmol alkaloid administered per kg body wt.) of 210 +/- 12 was found for the liver from SON-treated females whereas binding to liver DNA of males was lower by a factor of 4. The DNA damage determined six hours after treatment persisted during the following 4 days. Administration of [3H]SPH to female and male rats resulted in a CBI of 69 +/- 7 and 73/92, respectively, for the liver DNA. Furthermore we found binding of both alkaloids to DNA of lungs and kidneys in male and female rats. The in vivo formation of [3H]SON derived DNA adducts could be proved by HPLC analysis of hydrolyzed DNA.

Induction of SCEs by some pyrrolizidine alkaloids in V79 Chinese hamster cells co-cultured with chick embryo hepatocytes

Genotoxicity of heliotrine, monocrotaline, seneciphylline and senkirkine was studied with the sister-chromatid exchange assay in V79 Chinese hamster cells. Exposure to these pyrrolizidine alkaloids, in the presence of co-cultured primary chick embryo hepatocytes, resulted in a high induction of SCEs. It was also shown for heliotrine that co-cultivation with chick embryo hepatocytes resulted in a much higher response than addition of activated rat-liver homogenate to the test medium.