HEPATIC METABOLISM AND BIOACTIVATION OF MYCOTOXINS …€¦ · metabolizes foreign compounds (xenobiotics) into more polar, more excretable and less toxic metabolites. During metabolism

HEPATIC METABOLISM AND BIOACTIVATION OF MYCOTOXINS AND PLANT TOXINS 1

R. A. Swick

Monsanto Company 2 , St. Louis, MO 63167

Summary

The intoxication of livestock from ingested feed contaminated with naturally occurring toxicants is of great economic importance. Certain toxicants require enzymatic bioactivation before harmful effects are exhibited. In the liver, the cytochrome P-450 dependent mixed function oxidase (MFO) system normally metabolizes foreign compounds (xenobiotics) into more polar, more excretable and less toxic metabolites. During metabolism of aflatoxins and pyrrolizidine alkaloids (PAL oxidized metabolites are highly reactive with cellular components and are more toxic than the parent compounds. Hepatic bioactivation is of considerable importance because active metabolites are formed directly within the tissues of the animal. In addition, the MFO system has only limited specificity and is dependent on many factors including diet, age, sex, species and environmental influences. The interrelationship of these factors on aflatoxicosis and pyrrolizidine alkaloidosis are often responsible for the chronic nature and difficult etiology of these diseases. A better under- standing of the factors affecting metabolism and bioactivation of these toxicants may lead to the development of techniques for protecting animals that are subjected to contaminated feeds. (Key Words: Livestock, Mycotoxins, Pyrroli- zidine Alkaloids, Bioactivation, Metabolism, Mixed Function Oxidase.)

Invitational paper presented at the symposium on "Natural Plant Toxins and Their Effects on Live- stock Production," Joint Annu. Meet. of Can.-Amer. Soc. of Anita. Sci., Univ. of Guelph, Guelph, Ontario, Canada; August 9, 1982.

: Nutrition Chemicals Division. Received November 16, 1982. Accepted September 9, 1983.

t n t rodu~ion

Naturally occurring toxicants are responsible for great economic loss to the livestock industry with contamination of feed sometimes resulting in large outbreaks of disease. In England, outbreaks of Turkey "X" disease in 1960 caused the death of more than 100,000 turkey poults (Blount, 1961). The disease was later traced to contamination of Brazilian peanut meal with aflatoxins produced from the mold Aspergillus flavus growing in the meal (Sargeant et al., 1961). In the U.S. Pacific Northwest, heavy losses ($1.2 million estimated in 1972) to livestock producers have been attributed to pyrrolizidine alkaloid (PA) toxicity caused by the poisonous plant Senecio jacobaea (Snyder, 1972). More often, however, the economic loss from these toxicants is difficult to quantitate and may go unnoticed by animal producers for long periods. The subtle effects are insidious and can be the cause of long-term economic disaster in the form of reduced productivity.

Low level consumption of aflatoxin and(or) other mycotoxins, for example, have been documented to cause poor growth and feed efficiency (Smith and Hamilton, 1970), real- absorption syndrome and decreased bone strength (Osborne et al., 1982), decreased dry matter and amino acid digestibility (Nelson et al., 1982), decreased resistance to disease and vaccine failure (Pier et al., 1977), leg problems and increased carcass condemnations (Jones et al., 1982). Long-term ingestion of PA have also been documented as causing chronic disease in animals. Signs and symptoms include poor growth, cumulative liver degeneration characterized by megalocytosis and centrilobular necrosis, reproductive failure and a general slow wasting away (Bull et al., 1968; McLean, 1970).

In addition to their economic ramifications, aflatoxin and PA have certain features in

1017 JOURNAL OF ANIMAL SCIENCE, Vol. 58, No. 4, 1984

1018 SWICK

common that have stimulated much research and investigative effort in recent years. Although chemically unrelated, both are products of secondary metabolism in plant tissue and metabolized similarly in animals. Hepatic oxidation of both aflatoxin (Clifford and Rees, 1967) and PA (Mattocks, 1968) result in metabolites that are more active or toxic than the parent compounds. This bioactivation, as it is called, can be altered by dietary and nutritional factors, hormone changes in the body, administered drugs and pesticides, age and species of animals. Thus expression of toxicity for these bioactivated toxicants can be extremely variable under field conditions adding to the insidious nature of the problem. The objective of this paper is to explore the importance of hepatic metabolism and bioactivation within the realm of animal production with specific emphasis on aflatoxins and PA.

Hepatic Metabolism and Bioactivation

The intensity of effect that foreign compounds or xenobiotics wiU have on animals is determined in large part by their rate of metab- otism. Although xenobiotic metabolizing capa- bility exists in all tissues, the liver is the most active site. The essential effect of this metabolism is detoxification and elimination, involving the conversion of lipophilic compounds into more polar and water soluble metabolites that are more easily excreted via bile or urine (La D u e t al., 1971). While oxidations account for most of these transformations, reductions, hydrolyses and conjugations also occur. Several metabolic products may be formed from one parent compound as a result of multiple enzymes acting in sequence. Oxidations are catalyzed in the microsomal fraction of liver tissue by the cytochrome P-450 dependent mixed function oxidases (MFO). Generally, the oxidation of a lipophilic compound increases the electrophilicity of the molecule rendering it reactive with nucleophiles in the cell. Reaction of the electrophile with soluble nucleophiles in the ceU such as glucuronides, sulfates, amines, metcapturates or amino acids results in conjugation, leaving the original compound detoxified and readily excretable (Testa and Jenner, 1976). Unfortunately, certain compounds including the PA and aflatoxins, are oxidized into extremely electrophilic and reactive intermediates that are capable of irreversible covalent binding to critical biopolymers such as nucleic acids and(or) functional proteins within cells.

The expression of toxicity from this type of intoxication is difficult to predict because the MFO and conjugation reactions are dependent on a myriad of factors.

Factors Affecting M FO Activity

The activity of the MFO can be markedly altered when animals are treated with drogs, pesticides and other xenobiotics. Induction of MFO activity appears to represent an increase in concentration of enzyme protein and level of cytochrome P-450. Compounds known to stimulate enzyme induction are extremely diverse and there is no apparent relationship between structure and inducing ability except that most are lipid at physiological pH (Conney, 1967). Several inducing agents are of practical interest to the livestock producers and may be responsible for variations in response to toxicants and drugs. The use of red cedar chips as bedding for animals was first shown to be related to changes in drug metabolism by Ferguson (1966). It was suggested that volatile components in cedar were the active MFO inducers. The organochlorine insecticides DDT, chlordane, dieldrin and others have been shown to be active and potent MFO inducers (Hart et al., 1963; Hart and Fours, 1963), while organo- phosphate insecticides inhibit rather than stimulate MFO activity when given in chronic doses (Welch et al., 1967). The commonly used barbiturates phenobarbital, pentobarbital and hexobarbital (Conney et al., 1960; Seawright et al., 1972) and phenothiazines (Wattenberg and Leong, 1965) are MFO inducers. Steriod hormones also alter MFO activity and may account for differences in xenobiotic metabolism between males and females. Testosterone administration to castrated male rats increased MFO activity while estradiol decreased MFO activity (Kramer et al., 1979). When hypo- physectomized rats were used, however, the hormones did not affect MFO activity. The effect of growth promoting anabolic agents used in livestock production have not been fully examined for their effects on xenobiotic metabolism. Research in this area appears warranted.

Nutrients in the feed also have an important influence on xenobiotic metabolism. A reduc- tion in dietary protein quantity or quality, for example, causes a depression in MFO activity and will generally decrease the metabolism of xenobiotics (Kato et al., 1968; Campbell and Hayes, 1974). Alterations in dietary lipids

METABOLISM O F MYCOTOXINS AND PLANT TOXINS 1019

also affect MFO activity. Fat free diets have been shown to cause a depression in cytochrome P-450 and reduce MFO activity (Campbell and Hayes, 1974). In addition, MFO inducing agents such as phenobarbital have been shown to be more effective in animals fed diets containing fish oils or linoleic acid as opposed to coconut oil or beef fat (Marshall and McLean, 1971; Century, 1973; Gefferth and Blaskovits, 1977).

The interrelationships of diet and drugs on MFO activity and xenobiotic metabolsim have been thoroughly studied. For more information the reader is directed to excellent reviews by Conney (1967), Campbell and Hayes (1974), Testa and Jenner (1976), La Du et al., (1971) and Giri (1982).

A f l a t o x i n s

The chemical and biological characteristics of secondary fungal metabolites are highly diverse making it difficult to assign these compounds into toxic or nontoxic catagories. Over 200 compounds have been reported as mycotoxins, although the toxicological features have been elucidated in only a limited number of cases. Of the known mycotoxins, none has been more thoroughly studied than the ariatoxins that are produced by several strains of the fungi Aspergillus riavus and A. parasiticus. Excellent reviews on historical perspective, chemistry, toxicity, carcinogenicity, natural occurrence, biosynthesis and contamination of foods and feeds have been presented by Goldblatt (1969), Wogan and Busby (1980), Bryden (1982) and Applebaum et al. (1982).

Ariatoxins are highly substituted coumarins containing a fused dihydrofurofuran moiety. Four major metabolites are produced from ariatoxigenic fungi (figure 1'). Ariatoxins Bx and 132 (AFB1 and AFB 2) are so designated because of their strong blue fluorescence under ultraviolet light; ariatoxins G1 and G 2 (AFG1 and AFG2) fluoresce greenish yellow (Wogan and Busby, 1980).

Large species variation exists with regard to ariatoxin susceptibility. Ducks, swine and trout are considered most susceptible while sheep, rats and chickens are more resistant. Of the metabolites produced, AFB1 is the most toxic. The oral LDso (mg/kg) body weight values for ducklings receiving the various ariatoxin metabolites are as follows: Bx, .36; B2, 1.7; G1, .8; G2, 2.5. In contrast, the LDs0 values for the

aflatoxins in rats range from 5.5 to 17.9 mg/kg (Applebaum et al., 1982).

Various metabolites of AFB1 have been shown to be generated in vitro and in vivo as a result of MFO activity and other reactions in the liver. The relative abundance of each metabolite formed is related to the metabolic state of the animal at the time of intoxication in terms of species, age, diet and prior exposure to chemical treatment. The pathways of formation of the major known metabolites are shown in figure 2.

Alfatoxin M1 (AFM1) was the first metabolite to have been identified and was so named because of its presence in milk where it occurs principally in the protein fraction (Allcroft and Carnaghan, 1963). Microsomal enzymes, speci- fically the MFO, have been suggested as responsible for AFM1 production via hydroxylation of AFB 1 (Portman et al., 1968). Other ariatoxins of the M series found in milk include M2, GM1, GM2, M2a and GM2a. They are hydroxylated derivatives of AFB2, AFG1, AFG2, AFB2a and AFG2, respectively (Apple- baum et al., 1982). Although AFM1 has received the most attention in the literature presumably because of public health concerns, its formation generally represents less than 3% of an ingested dose of AFB 1 (Polan et al., 1974-). In addition, several reports suggest that AFMa is less toxic (Garner et al., 1972) and carcinogenic (Sinnhuber et al., 1970) than AFB1. The transformation of AFM1 to conjugated metabolites, rates of AFMa excretion and other tissue sites for AFMx production may represent factors in addition to the primary hepatic metabolism of AFB1 to AFM1 that are

0 0 0 0

Aflatoxin B 1 Aflatoxin B 2

0 0 0 0

Aflatoxin G~ Aflatoxin G 2

Figure 1. Toxins produced by Aspergillus flavus and A. parasiticus.

1020 SWICK

o

AFLATOXIN B1-2,3-OXIDE

HIGHLY REACTIVE, ~<~ . . ~ ELECTROPHILIC,

.O ~'~ INTERMEDIATE

2 MFO

AFLATOXIN B 1

~CYTOPLASMIC MFO AND \ \REOUCTASE NON-MFO \ \

BEHYDROGENASE ~

0 ON

AFLATOXICOL (PROPOSED AS A RESERVOIR OF TOXICITY)

COVALENT BIND4NG TO NUCLEIC ACIDS OR PROTEINS (N7 GUANINE, METHIONINE OR HISTIDINE)

-- or --

CONJUGATION WITH SOLUBLE NUCLEOPHILES (GLUTATHIONEI GLUCURONIDES, ETC.)

O O

No - j AFLATOXIN B2a

O % q ' ~ O O

AFLATOXIN Q1

O O 0

"O ~ "0" ~ ~OCH3 --

AFLATOXIN M I 'MILK' METABOLITE

Figure 2. Metabolism of aflatoxin B 1 (adapted from Wogan and Busby, 1980).

~TOXICITY CARCINOGENICITY

EXCRETION "~w'DETOXIFICA'flO N

CONJUGATION AND

EXCRETION

operative in the tissue desposition of this metabolite.

Aflatoxin B2 a, the hemiacetal metabolite of AFB1 may be formed under the acidic conditions found in the stomach (Purchase and Steyn, 1969) or by liver microsomal preparations (Schabort and Steyn 1969; Patterson and Roberts, 1970). Under physiological conditions AFB2a was found to be unstable and easily bound to protein or hepatic microsomes (Gurtoo and Dahms, 1974). Toxicity studies have shown AFB2a to be essentially nontoxic (Wogan, 1973), however, Schabort and Pitout (1971) have shown the metabolite to inhibit pancreatic DNase.

Aflatoxin P1, the O-demethylated metabolite of AFBz, is produced as a result of MFO metabolism (Dalezios et al., 1971; Roebuck and Wogan, 1977). The overall role of AFP 1 in

aflatoxin toxicity has not been thoroughly elucidated. Because the molecule is hydroxylated, it readily undergoes conjugation reactions rendering it excretable as a glucuronide or sulfate (Dalezios et al., 1972). Wong and Hsieh (1980) have found that species relatively resistant to the carcinogenic effect of AFBt were more active in the in vivo conversion of the AFBz to A F P 1 and water soluble conjugates. Although formation of this metabolite may represent a detoxication pathway for AFBz, the molecule still retains both the 2,3-unsaturated terminal furan and lactone portion of coumarin generally assumed to be required for aflatoxin toxicity.

Aflatoxin Qz is another metabolite generated by the action of MFO on AFB1. It is hydroxylated at carbon atom ~ to the carbonyl function of the cyclopentenone ring. Aflatoxin O.1

METABOLISM OF MYCOTOXINS AND PLANT TOXINS 1021

represents one-third to one-half of the metabolites produced from AFB1 by human (Biichi et al., 1974) or monkey liver (Masri et al., 1974b) microsomes but only a small fraction of the AFB1 metabolites produced by chicken or rat liver microsomes (Masri et al., 1974a). Aflatoxin Q1 appears to be much less toxic than AFBt (Hsieh et al., 1974) suggesting its formation to be a mechanism of detoxification in primates.

Afiatoxicol (AFL), another metabolite of AFB1, is carcinogenic and is produced by nonMFO enzymes in the soluble fraction of liver homogenates. Rabbit and bird liver homogenates were observed to be active AFL producers whereas rodent and sheep preparations were inactive (Patterson and Roberts, 1972). Although the toxicity of AFL was determined to be much less than that of AFB1 the reaction was found to be reversible and has been suggested as providing a reservior of toxicity for AFB1. The interconversion of AFB1 to AFL in target hepatocytes presumably can prolong cellular exposure to AFB1 and thereby enhance its carcinogenic effect (Wong and Hsieh, 1978). In rats, AFL has been identified in plasma as a major metabolite of AFB1 but was not detected in the plasma of similarly dosed monkeys and mice, which are resistant to the carcinogenic effect of AFBI (Wong and Hsieh, 1980). Recently, the conversion of AFB1 to AFL was found to occur in red blood cell suspensions with resistant species having the highest rate of interconversion (Kumagai et al., 1983).

Bioactivation of AFB1 by MFO to a highly reactive and labile intermediate AFB1 2,3- epoxide has been proposed as the ultimate mechanism by which toxicity is manifested (Garner et al., 1972). These researchers were unable to characterize the metabolite and suggested that because of its extreme reactivity it became rapidly bound to various nucleophiles, making isolation difficult or impossible. In later research, Swenson et al. (1974) isolated 2,3-dihydro-2,3-dihydroxyaflatoxin B1 from the livers of rats treated with AFBI thus supporting the hypothesis that AFB1 2,3- epoxide is the ultimate toxicant. The proposed AFB1 2,3-epxoide intermediate has been suggested to react with various nucleophiles in the cell and bind covalently with important biopolymers such as DNA, RNA and protein. Binding of the intermediate to the N-7 position on guanylic acid in DNA molecules has been

demonstrated in vivo and in vitro (Essigmann et al., 1977; Croy et al., 1978). Protein synthesis was inhibited following binding to RNA and reactions with free amino groups in functional protein results in acute toxicity (Patterson, 1973). These reactions are no doubt responsible for the mutagenesis and carcinogenesis observed during aflatoxicosis.

The overall toxicity of aflatoxin in an animal appears to be determined by the rate of formation of the reactive intermediate, its binding to target macromolecules and the rate of detoxification and other competing reactions. Because many factors can alter the relative rates of formation of the various metabolites, it is not surprising that there is considerable variation in animal response to afiatoxins.

The metabolism and thus expression of aflatoxicosis can be altered by a variety of dietary and environmental factors in any given animal species. Conversely, the nutritional status of an animal may be affected by exposure to aflatoxin. Diets high in lipid have a mortality- sparing effect against aflatoxicosis, and a growth sparing effect if high in unsaturated fatty acids (Hamilton, 1977). Consumption of alfatoxin by chickens at concentrations not inhibitory to growth has been shown to produce a malabsorption syndrome characterized by steatorrhea, hypocarotenoidemia and decreased concentrations of bile salts and pancreatic lipase, trypsin, amylase and RNase (Osborne et al., 1982).

An interaction between vitamin A and aflatoxicosis has been suggested in several studies. Liver concentrations of vitamin A were reduced in cattle, swine and chickens by aflatoxin suggesting a modification of vitamin A metabolism and induced vitamin A deficiency (Hamilton, 1977). However, Bryden et al. (1979) found vitamin A deficiency to have a protective effect against afiatoxicosis in broiler chickens. Diets deficient in vitamin D or riboflavin have been shown to increase the toxicity of aflatoxin, while thiamine deficiency appeared protective in chickens (Hamilton et al., 1974).

The effects of dietary protein and amino acids have also been investigated. Protein deficiency, was found to reduce the binding of AFB1 to DNA in rats (Preston et al., 1976), whereas in other research (Smith et aL, 1971) acute afiatoxicosis caused an increase in the requirement for protein. This apparent disparity might be explained in terms of aflatoxin metab-

i022 SWlCK

olism. It is probable that protein deficiency reduces MFO activity and thus decreases bioactivation. In normal diets, however, the need for amino acids to function as nucleophilic substituents during conjugation reactions may cause an increase in the protein requirement. Injection of cysteine or methionine was found to be protective against acute aflatoxicosis in goats (Hatch et al., 1979). It was suggested that administration of these compounds might be protective by causing an increased availability of liver glutathione required for conjugation reactions.

Other agents of potential concern to animal producers have been studied for their effect on aflatoxin metabolism. Ethynylestradiol, a syn- thetic therapeutic estrogenic agent induced hepatic lesions in rats dosed with AFBI and caused decreased levels of UDP-glucuronosyl- transferase and cytochrome P-450 but increased epoxide hydrolase activity (Kamden et al., 1983). The insecticide dieldrin was shown to exert a slight cocarcinogenic effect on AFBx in trout (Hendricks et al., 1979) while cycloheximide, a fungicide used to control turf and orchard diseases, had a protective effect against acute AFB1 toxicity in rats (Chi'ih et al., 1980).

The results indicate that aflatoxin metabolism and toxicity can be influenced by many factors both protective and antagonistic. The information ~uggests that actual field cases of aflatoxicosis in animals are highly variable making it difficult to ascribe the condition to cases of decreased productivity or subacute disease.

Other Mycotoxins

The metabolism of several other mycotoxins has been studied in an attempt to elucidate mechanisms of toxicity. Penicillic acid, an a, unsaturated lactone produced by both Aspergil- lus ochraceus and Penicillium puberulum, has been recently suggested to undergo metabolic activation (Chan et al., 1980a,b; Chan et al., 1982). Administration of penicillic acid caused a depletion of liver GSH, with increased toxicity after pretreatment with phenobarbital. In research involving distribution and excretion pathways, Chan and Hayes (1981) found water soluble sulfhydryl and glucuronide conjugates of penicillic acid in urine. This evidence supports the hypothesis that an active epoxide metabolite may be involved in penicillic acid toxicosis. The proposed metabolic pathway is presented in figure 3.

Metabolism of ochratoxin A (Stormer et al., 1981; Syversten and Stormer, 1983), verruculogen (Perera et al., 1982) and T-2 toxin (Yoshizawa et al., 1982) have also been recently examined. Results suggest these mycotoxins are metabolized by the MFO, but further studies will be necessary to determine if bioacti- ration is requisite for intoxication.

Pyrrolizidine Alkaloids

Pyrrolizidine alkaloids are constituents of several species of plants in the genera Senecio, Heliotropium, Amsinckia, Echium, Crotalaria, Symphytum and others. These plants pose health related problems for animals when feedstuffs or pastures become contaminated (Bull et al., 1968). Human exposure has also been reported as a result of contamination of honey (Dienzer et al., 1977), milk (Dickinson et al., 1976) herbal teas (Huxtable et al., 1977) and other foodstuffs (Mohabbat et al., 1976) by the alkaloids or their metabolites. In addition to chronic and acute toxicity several PA have been shown to be carcinogenic (Schoental, 1968; Harris and Chert, 1970).

All of the toxic and carcinogenic PA are ester derivatives of 1-hydroxymethyl-l,2-dehydropyrrolizidine with esterification being possible at positions 1 and 7 (see figure 4). The esterfied portion may form a closed cyclic structure as in retrorsine, jacobine or monocrotaline or may be open as in lasiocarpine or heliotrine. Several structural requirements for toxicity have been identified and summarized by McLean (1970). Toxic PA must contain a double bond between carbon one and two in the pyrrolizidine ring system, and carry esterified hydroxyl groups with at least one containing a branched carbon chain.

Although the toxic PA are chemical stable compounds, they are known to undergo several kinds of metabolic transformations in animal tissues including N-oxidation, ester hydrolysis and conjugation reactions (Bull et al., 1968). These reactions usually result in detoxification. Dehydrogenation by MFO was proposed by Mattocks (1968) as the pathway that renders PA toxic within animals. Evidence has been presented in many studies to support the hypothesis that PA undergo bioactivation in the liver resulting in the formation of highly reactive pyrrole derivatives (dihydropyrrolizi- dines).

The metabolism of PA is depicted in figure 5. Pyrroles were first observed in liver tissue


H 3 C O ~ H

o II

HsC~H

PENICILLIC ACID

MFO DEMETHYLATION,

EPOXIDATION

r

H 3 C O ~ H

c~/C ~ ~'0 ~ " 0

-OR-

o II CH2

GLUTATHIONE CONJUGATION, EXCRETION

COVALENT BINDING TO MACROMOLECULES,

TOXICITY

ACTIVE METABOLITE

Figure 3. Proposed metabolism of penicillic acid (adapted from Chan et al., 1980b).

and urine of rats dosed with PA (Mattocks, 1972; White et al., 1973) and were later found to be formed by rat liver microsomes (Jago et al., 1970). Pyrroles in liver tissue were found to be high in susceptible species and low in resistant species after dosing with PA (White et al., 1973). With the exception of rabbits, Shull et al. (1976) found in vitro formation of pyrroles low in resistant species and high in susceptible species. Recent research with an in vitro mammalian cell transformation test lends support to the hypothesis that a simple alkylating pyrrole is the biologically active chemical agent derived from the toxic PA (Styles et al., 1980).

Pyrroles formed from PA in liver tissue are highly electrophilic intermediates that can react with nucleophiles in the cell. If the reaction involves soluble constituents such as thiols, amines or water, conjugation and excretion may occur. Pyrroles may also react with macromolecules such as nucleic acids or protein and become covalently bound causing chemical lesions. Nucleophilic substitution by cysteine and glutathione occurs preferentially at (2-7 of the pyrrole dehydroretronecine (Wogan and

Busby, 1980) suggesting this as the initial site on pyrroles for covalent binding. Pyrrole derivatives have also been shown to act as bi- functional alkylating agents that may cross-link DNA (McLean, 1970; White and Mattocks, 1972). Presumably, the binding occurs at positions C-7 and C-9 on the pyrrole molecule. The kinetics of pyrrole alkylation have been studied by Karchesy and Deinzer (1981) with rate data indicating pyrroles may fit two reaction patterns; those exhibiting first order kinetics and others whose rate data fit a biexpo- nential expression. Electronic differences in structure between the various pyrrole derivatives appears to be reahed to the rate of alkylation and thus to toxicity.

Much like the aflatoxins, PA metabolism in the liver can be altered by dietary and nutritional factors and ingestion of foreign chemicals. Conversion of the PA retrorsine to pyrrole in vitro was lower in liver microsomal fractions taken from rats that were starved or fed a protein free diet (Mattocks and White, 1971). Cheeke and Garman (1974), however, found that high dietary protein (fish meal) had a

1024 SWICK

H3C/CH3 O CH II / ,,OCH3

/C ---. C ~ C H HO CH2.,.O I I OH CH 3

.3c~,. o . H /CH 2 - C.~. ~.N...-CH2OH

,, . /C~c C---O n3t., ~ I

o / /C\o /O CH2

HELIOTRINE RETRORSINE

H CH 3 HO CH 3 \ / O \ / C=C |1 C'--CH,,

./ ~ ...C ~ . . / ~ H3C C--O CH2"" O /~' ~'~H "H

o* ">--~ .o ~- ' - ~ ? oc.,

LASIOCARPINE

H3C HO H . C / ~ - - IC/CH3

" c ~ O H3C/I OH I

o,~Ck /O O CH2

MONOCROTALINE

HO CH2OH

5 4 3

1-HYDROXYMETHYL-1,2- DEHYDROPYRROLIZIDINE

Figure 4. Pyrrolizidine alkaloids.

H CH3 N /

c~_,,o c . ~ c o. \C/--CH3

C = O I

0

JACOBINE

protective effect against Senecio toxicity. It is possible that low protein diets were protective by lowering the activity of MFO and thus pyrrole formation, while high levels of high quality protein enhance conjugation pathways and result in detoxification. Cysteine and other thiol compounds such as mercaptoethylamine

(MEA) were shown to be protective against PA intoxication. Dosing with MEA or cysteine prolonged survival and improved the growth of rats that received the PA monocrotaline (Hayashi and Lalich, 1968). Protective effects of MEA against lasiocarpine toxicity have also been observed (Rogers and Newberne, 1971).


O II

RO CH2'-O-C--R

PARENT ALKALOID

- - LIVER TISSUE - - COVALENT BINDING TO /

NUCLEIC ACIDS AND PROTEIN /

ALKYLATION TO MACROMOLECULES

TOXICITY, CARCINOGENIClTY

Figure 5. Metabolism of pyrrolizidine alkaloids.

MFO OR OTHER OXIDATION

MFO ~I~ O

RO C H 2 - O - C - R

PYRROLE DERIVATIVE HIGHLY REACTIVE

\ CONJUGATION

AND EXCRETION

O II

RO CH2 - O - C -R

0 N-OXIDE

/ EXCRETION

j DETOXIFICATION

Buckmaster et al. (1976) found improvement in survival time of rats consuming Senecio by the addition of cysteine to the diet. Cysteine was presumed to enhance pyrrole conjugation in these experiments. This is likely because liver sulfhydryls are reduced during PA intoxication (Shull et al., 1976) and dietary cysteine en- hances liver glutathione levels (White, 1976). Dietary carbohydrates may also affect PA toxicity. Rats fed pure sucrose diets showed reduced liver pyrroles in vitro and were pro- tected against PA intoxication in vivo (Mattocks, 1972). Detoxification of pyrroles by conjugation with glucuronides may be implicated. A study conducted by Thorpe and Ford (1968) seems to support this. These investigators found depleted liver glycogen levels in calves fed Senecio suggesting the monomer sugars were being used for glucuronide formation.

Exposure to xenobiotics has also been shown to cause changes in the pattern of PA intoxication. Susceptibility of rats to acute lethal and hepatotoxic effects of injected lasiocarpine was prevented by oral pretreatment with pregnenolone-16-a-carbonitrile, spirono- lactone, ethylestrenol, phenobarbital and di- phenylhydantoin. In contrast, similar pretreatment prior to monocrotaline or heliotrine administration resulted in marked aggravation of intoxication (Tuchweber et al., 1974). Because MFO are responsible for pyrrole

formation, it follows that induction should increase toxicity. The contrasting results obtained with the different alkaloids tested suggests that MFO induction under certain instances may enhance detoxification possibly through formation of water soluble N-oxides. Pretreatment of rats with DDT enhanced pyrrole formation in vitro in one study (Mat- tocks and White, 1971) but prolonged survival and performance in rats in another study (Cheeke and Garman, 1974). Phenobarbital pretreatment enhanced pyrrole formation from monocrotaline in vitro in rats (Chesney et al., 1974) but failed to enhance toxicity or greatly alter MFO activity in sheep fed Senecio (Swick et al., 1983). These results illustrate that the metabolic behaviors of individual PA are quite different from one another and that species differences, diet and other factors are important when considering the toxicity of these compounds.

Conclusion

Naturally occuring toxicants are econom- ically important to animal production. Con- tamination of feeds with aflatoxins or PA are of particular concern because animal response is highly variable and often subtle. Because these substances require metabolic activation by MFO within the animal, toxic effects are

1026 SW1CK

d e p e n d e n t o n m a n y fac to rs inc luding die t , sex, species, e n v i r o n m e n t a l cond i t ions , exposu re to o the r xenob io t i c s and hea l th of the animal . Exposure o f an imals to c o m m o n l y used mater ia l s in an imal p r o d u c t i o n such as o r ganoch l o r i ne insect icides, b a r b i t u r a t e s or cedar chips have been n o t e d for the i r abi l i ty to i nduce the act ivi ty of M F O and ( o r ) increase t he t ox ic i ty of b ioac t iva ted tox ican t s . N u t r i t i ona l supp lemen- t a t i on has b e e n f o u n d to a t t e n u a t e t ox i c i ty u n d e r cer ta in c i rcumstances . The in t e r r e l a t i on - ship of these fac to rs makes i t d i f f icu l t to f o r m u l a t e genera l i za t ions conce rn ing in tox ica- t ions par t i cu la r ly u n d e r f ield cond i t ions . These in f luenc ing fac to rs are none the l e s s i m p o r t a n t to cons ider w h e n a t t e m p t i n g to cor re la te a f l a tox in or PA in take wi th decreased p r o d u c t i v i t y or increased inc idence o f disease.

Literature Cited

Allcroft, R. and R.B.A. Carnaghan. 1963. Groundnut toxicity: An examination for toxin in human food products from animals fed toxic groundnut meal. Vet. Rec. 75:259.

Applebaum, R. S., R. E. Brackett, D. W. Wiseman and E. H. Marth. 1982. Aflatoxin: Toxicity to dairy cattle and occurrence in milk and milk products - A review. J. Food Protect. 45:752.

Blount, W. P. 1961. Turkey "X" disease. Turkeys 9:52.

Bryden, W. L. 1982. Aflatoxins and animal production: An Australian perspective. Food. Technol. Austral. 34:216.

Bryden, W. L., R. B. Cumming and D. Belnave. 1979. The influence of vitamin A status on the response of chickens to aflatoxin B, and changes in liver lipid metabolism associated with aflatoxicosis. Brit. J. Nutr. 41:529.

Biichi, G. H., P. D. Mueller, B. D. Roebuck and G. N. Wogan. 1974. Aflatoxin Q, : A major metabolite of aflatoxin B,. produced by human liver. Res. Comm. Chem. Pathol. Pharmacol. 8: 585.

8uckmaster, G. W., P. R. Cheeke and L. R. Sbull. 1976. Pyrrolizidine alkaloid poisoning in rats: Protective effects of dietary cysteine. J. Anita. Sci. 43:464.

Bull, L. B., C.C.J. Culvenor and A. T. Dick. 1968, The Pyrrolizidine Alkaloids. John Wiley and Sons, Inc., New York.

Campbell, T. C. and J, R. Hayes. 1974. Role of nutrition in the drug-metabolizing enzyme system. Pharmacol. Rev. 26:171.

Century, B. 1973. A role of the dietary lipid in the ability of phenobarbital to stimulate drug detoxification. J. Pharmacol. Exp. Therap. 185:185.

Chan, P. K. and A. W. Hayes. 1981. Hepatotoxicity of the mycotoxin peniciUic acid: A pharmacokinetics consideration. J. Anim. Oil Chem. Soc. 58:1017.

Chan, P. K., A. W. Hayes, E. F. Meydrech and A. Ciegler. 1980a. The protective role of glutathione in penicillic acid-induced hepatotoxicity in male mice and possible involvement of an active metabolite. Toxicol. Appl. Pharmacol. 55:291.

Chan, P. K., A. W. Hayes and M. Y. Siraj. 1982. Excretion of conjugated metabolites of the mycotoxin penicillic acid in male mice. Toxicol. Appl. Pharmacol. 66:259.

Chan, P. K., C. S. Reddy and A. W. Hayes. 1980b. Acute toxicity of penicillic acid and its interaction with pentobarbita/and other compounds. Toxicol. Appl. Pharmacol. 52:1.

Cheeke, P. R. and. G. R. Garman. 1974. Influence of dietary protein and sulfur amino acid levels on the toxici W o f Senecio jacobaea (tansy ragwort) to rats. Nutr. Rep. Int. 9:197.

Cbesney, C. F., I. C. Hsu and J. R. Allen. 1974. Modifications of the in vitro metabolism of the hepatoto~c pyrrolizidine alkaloid, monocrotaline. Res. Comm, Chem. Pathol. Pharmacol. 8:567.

Chi'ih, J. J., S. Wang and H. Feng, 1980. Protective effect of cycloheximide on acute aflatoxin B I intoxication. Proc. Soc. Exp. Biol. Med. 164:35.

Clifford, J. I. and K. R. Rees. 1967. The action of aflatoxin B, on rat liver. Biochem. J. 102:65.

Conney, A. H. 1967. Pharmacological implications of microsomal enzyme induction. Pharmacol. Rev. 19:317.

Conney, A. H,, C. Davison, R. Gastel and J. J. Burns. 1960. Adaptive increases in drug-metabolizing enzymes induced by phenobarbital and other drugs. J. Pharmacol. Exp. Therap. 130:1.

Croy, R. G., J. M. Essigmann, V. N. Reinhold and G. N. Wogan. 1978. Identification of the principal aflatoxin B,-DNA adduct formed in vivo in rat liver. Proc. Natl. Acad. Sci. USA 75:1745.

Dalezios, J. I., R. I. Kreiger and D.P.H. Hsieh. 1972. The in vitro metabolism of aflatoxin B: in rhesus monkey liver. Fed. Proc. 31:733 (Abstr.).

Dalezios, J. I., G. N. Wogan and S. M. Weinbeb. 1971. Aflatoxin P~ : A new aflatoxin metabolite in monkeys. Science 171 : 584.

Dickinson, J. O., M. P. Cooke, R. R. King and P. A. Mohamed.-1976. Milk transfer of pyrrolizidine alkaloids in cattle. J. Amer. Vet. Med. Assoc. 169:1192.

Dienzer, M. L., P. A. Thomson, D. M. Burgetr and D. L. Isaacson. 1977. Pyrrolizidine alkaloids: Their occurrence in honey from tansy ragwort (Senecio jacobaea L.). Science 195:497.

Essigman, J. M., R. G. Croy, A. M. Nadzan, W. F. Busby, V. N. Reinhold, G. Bi~chi and G. N. Wogan. 1977. Structural identifieation of the major DNA adduct formed by aflatoxin BI in vitro. Proc. Natl. Acad. Sci. USA 74:1870.

Ferguson, H. C. 1966. Effect of red cedar chip bedding on hexobarbital and pentobarbital sleep time. J. Pharmaceut. Sci. 55:1142.

Garner, R. C., E. C. Miller and J. A. Miller. 1972. Liver microsomal metabolism of aflatoxin B, to a reactive derivative toxic to Salmonella typhi- murium TA 1530. Cancer Res. 32:2058.

Gefferth, G. and A. Blaskovits. 1977. The effect of different oil contents of the diet on the mixed function oxidases in the liver of rats. Nutr. Metab. 21 (Suppl. 1):246.

Girl, S. N. 1982. Drug metabolism and nutrition. In: J. C. Woodward and M. Bruss (Ed.) Comparative Aspects of Nutritional and Metabolic Diseases. pp 25--40. CRC, Boca Raton, FL.

METABOLISM OF MYCOTOXINS AND PLANT TOXINS 102 7

Goldblatt, L. A. 1969. Aflatoxin: Scientific Back- ground, Control, and Implications. Academic Press, Inc., New York.

Gurtoo, H. L. and. R. Dahms, 1974. On the nature of the binding of aflatoxin B~a to rat hepatic microsomes. Res. Comm. Chem. Pathol. Pharma- col. 9:107.

Hamilton, P. B. 1977. Interrelationships ofmycotoxins with nutrition. Fed. Proc. 36:1899.

Hamilton, P. B., H. T. Tung, R. D. Wyatt and W. E. Donaldson. 1974. Interaction of dietary aflatoxin with some vitamin deficiencies. Poul. Sci. 53: 871.

Harris, P. N. and K. K. Chert. 1970. Development of hepatic tumors in rats following ingestion of Senecio longilobus. Cancer Res. 30: 2881.

Hart, L. G. and J. R. Fours. 1963. Effects of acute and chronic DDT administration on hepatic microsomal drug metabolism in the rat. Proc. Soc. Exp. Biol. Med. 114:388.

Hart, L. G., R. W. Shultice and J. R. Fouts. 1963. Stimu|atory effects of chlordane on hepatic microsomal drug metabolism in the rat. Toxicol. App1. Pharmacol. 5: 371.

Hatch, R. C., J. D. Clark, A. V. Jain and E. A. Mahaffey. 1979. Experimentally induced aflatoxicosis in goats treated with ethyl maleate, glutathione precusors or thiosolfate. Amer. J. Vet. Res. 40: 505.

Hayashi, Y. and J. J. Lalich. 1968. Protective effect of mercaptoethylamine and cysteine against monocrotaline intoxication in rats. Toxicol. Appl. Pharmacol. 12:36.

Hendricks, J. D., T. P. Putnam and R. O. Sinnhuber. 1979. Effect of dietary dieldrin on aflatoxin B z carcinogenesis in rainbow trout (Salmo gairdneri). J. Environ. Pathol. Toxicol. 2: 719.

Hsieh, D.P.H., A. S. Salhab, J. J. Wong and S. L. Yang. 1974. Toxicity of aflatoxin QI as evaluated with the chicken embryo and bacterial auxotrophs. Toxicol. Appl. Pharmacol. 30:237.

Huxtable, R., A. Stillman and D. Ciaramitaro. 1977. Characterization of alkaloids involved in human Senecio (pyrrolizidine) poisoning. Proc. West. Pharmacol. Soc. 20:455.

Jago, M. V., J. A. Edgar, L. W. Smith and C.C.J. Culvenor. 1970. Metabolic conversion of heliotridine-based pyrrolizidine alkaloids to dehydro- heliotridine. Mol. PharmacoL 6:402.

Jones, F. T., W. G. Hagler and P. B. Hamilton. 1982. Association of low levels of aflatoxin in feed with productivity losses in commercial broiler opera- tions. Poul. Sci. 61:861.

Kamden, L., J. Magdalou, G. Siest, M. Ban and D. Zissu. 1983. Induced hepatotoxicity in female rats by aflatoxin B 1 and ethynylestradiol interaction. Toxicol. Appl. Pharmacol. 67:26.

Karchesy, J. J. and M. L. Deinzer. 1981. Kinetics of alkylation reactions of pyrrolizidine alkaloid derivatives. Heterocycles 16:631.

Kato, R., T. Oshima and S. Tomizawa. 1968. Toxicity and metabolism of drugs in relation to dietary protein. Jpn. J. Pharmacol. 18:356.

Kramer, R. E., J. W. Greiner, R. C. Rumbaugh, T. D. Sweeny and H. D. Colby. 1979. Requirement of the pituitary gland for gonadal hormone effects on heptic drug metabolism in rats. J. Pharmacol. Exp. Therap. 208:19.

Kumagai, S., N. Nakano and K. Aibara. 1983. Inter- actions of aflatoxin Bt and blood components of various species in vitro: Interconversion of aflatoxin B~ and afiatoxicol in the blood. Toxicol. Appl. Pharmacol. 67: 292.

La Du, B. N., H. G. Mandel and E. L. Way. 1971. Fundamentals of Drug .Metabolism and Drug Disposition. Williams and Wilkins Co., Baltimore.

Marshall, W. J. and. A.E.M. McLean. 1971. A requirement for dietary lipids for induction of cytochrome P-450 by phenobarbitone in rat liver microsomal fraction. Biochem. J. 122:569.

Masri, M., N. Booth and P. H. Hsieh. 1974a. Compara- tive metabolic conversion of aflatoxin B 1 to M t and Qt by monkey, rat and chicken liver. Life Sci. 15:203.

Masri, M. S., W. F. Haddon, R. E. Lundin and D.P.H. Hseih. 1974b. Aflatoxin QI: A newly identified major metabolite of aflatoxin Bt in monkey liver. J. Agr. Food Chem. 22:512.

Mattocks, A. R. 1968. Toxicity of pyrrolizidine alkaloids. Nature 217:723.

Mattocks, A. R. 1972. Acute hepatotoxicity and pyrrolic metabolites in rats dosed with pyrrolizidine alkaloids. Chem. Biol. Interact. 5:227.

Mattocks, A. R. and. I.N.H. White. 1971, The conversion of pyrrolizidine alkaloids to N-oxides and to dihydropyrrolizidine derivatives by rat liver microsomes in vitro. Chem.-Biol. Interact. 3: 383.

McLean, E. K. 1970. The toxic actions of pyrrolizidine (Senecio) alkaloids. Pharmacol. Rev. 22:429.

Mohabbat, O., M. S. Younos, A. A. Merrao, R. N. Srivastave, S. G. Ghaos and C. N. Aram. 1976. An outbreak of hepatic veno-occlusive disease in Northwestern Afghanistan. Lancet 2:269.

Nelson, T. S., Z. B. Johnson, L. K. Kirby and J. N. Bcasley. 1982. Digestion of dry matter and amino acids and energy utilization by chicks fed molded corn containing mycotoxins. Poul. SCI. 61:584.

Osborne, D. J., W. E. Huff, P. B. Hamilton and H. R. Burmeister. 1982. Comparison of ochratoxin, afiatoxin, and T-2 toxin for their effects on selected parameters related to digestion and evidence for specific metabolism of carotenoids in chickens. Poul. Sci. 61:1646.

Patterson, D. S. 1973. Metabolism as a factor in deter- mining toxic action of afiatoxins in different animal species. Food Cosmet. Toxicol. 11:287.

Patterson, D.S.P. and B. A. Roberts. 1970. The formation of aflatoxins B z a and G z a and their degradation products during the in vitro detoxification of aflatoxin by livers of certain avian and mammalian species. Food Cosmet. Toxicol. 8:527.

Patterson, D.S.P. and B. A. Roberts. 1972. Steroid sex hormones as inhibitors of aflatoxin metabolism in liver homogenates. Experientia 28:929.

Perera, K.P.W.C., J. B. Day, P. G. Mantle and L. Rodrigues. 1982. Metabolisan of verruculogen in rats. Appl. Environ. Microbiol. 43-503.

Pier, A. C., S. J. Cysewski, J. L. Richard and J. R. Thurston. 1977. Mycotoxins as a veterinary problem. In: J. V. Rodricks, C. W. Hesseltine and M. A. Mehiman (Ed.) Mycotoxins in Human and Animal Health. pp 745--750. Pathotox. Pub., Park Forest South, IL.

1028 SWICK

Polan, C. E., J. R. Hayes and T. C. Campbell. 1974. Consumption and fate of aflatoxin B, by lactating cows. J. Agr. Food Chem. 22:635.

Portman, R. S., K. M. Plowman and T. C. Campbell. 1968. Aflatoxin metaoblism by liver microsomal preparations of two different species. Biochem. Biophys. Res. Comm. 33:711.

Preston, R. S., J. R. Hayes and T. C. Campbell. 1976. The effect of protein deficiency on the in vivo binding of aflatoxin Bt to rat liver marcomole- cules. Life Sci. 19:1191.

Purchase, LF.N., and M. Steyn. 1969. The metabolism of aflatoxin Bt in rats. Brit. J. Cancer 23:800.

Roebuck, B. D. and G. N. Wogan. 1977. Species comparison of in vitro metabolism of aflatoxin B 1 . Cancer Res. 37:1649.

Rogers, A. E. and P. M. Newberne. 1971. Lasiocarpine: Factors influencing its toxicity and effects on liver cell division. Toxicol. Appl. Pharmacol. 18:356.

Sargeant, K., A. Sheridan, J. O'KeUy and R.B.A. Carnag- ban. 1961. Toxicity associated with certain samples of groundnuts. Nature 192:1096.

Sehabort, J. C. and. M. J. Pitout. 1971. The relationship between the chemical structure of aflatoxins and their effect on bovine pancreas deoxyribonuclease. Enzymologia 41:201.

Schabort, J. C. and M. Steyn. 1969. Substrate and phenobarbital inducible aflatoxin-4-hydroxylation and aflatoxin metabolism by rat liver microsomes. Biochem. Pharmacol. 18:2241.

Schoental, R. 1968. Toxicology and carcinogenic action of pyrrolizidine alkaloids. Cancer Res. 28:2237.

Seawright, A. A., D. P. Steele, A. W. Mudie and R. Bishop. 1972. The effect of diet and drugs on hepatic microsomal aminopyrine N-demethylase activity in vitro and susceptibility to carbon tetrachloride in sheep. Res. Vet. Sci. 13: 245.

Shull, L. R., G. W. Buckmaster and P. R. Cheeke. 1976. Factors influencing pyrrolizidine (Senecio) alkaloid metabolism: Species, liver sulfhydryls, and rumen fermentation. J. Anita. Sci. 43:1247.

Sinnhuber, R. O., D. J. Lee, J. H. Wales, M. K. Landers and A. C. Keyl. 1970. Aflatoxin Mt, a potent liver carcinogen for rainbow trout. Fed. Proc. 29:568 (Abstr.).

Smith, J. W. and P. B. Hamilton. 1970. Aflatoxicosis in the broiler chicken. Poul. Sci. 49:207.

Smith, J. w., C. H. Hill and P. B. Hamilton. 1971. The effect of dietary modifications on aflatoxicosis in the broiler chicken. Poul. Sci. 50:768.

Snyder, S. P. 1972. Livestock losses due to tansy ragwort poisoning. Oregon Agr.-Record, No. 255. pp 2--4.

Stormer, F. C., C. E. Hansen, J. I. Pedersen, G. Hvistendahl and A. Aasen. 1981. Formation of (4R)- and (4S)-4-hydroxyochratoxin A from ochratoxin A by liver microsomes from various species. Appl. Environ. Microbiol. 42:1051.

Styles, J., J. Ashby and A. R. Mattocks. 1980. Evalua- tion in vitro of several pyrrolizidine alkaloid

carcinogens: Observations on the essential pyrrolic nucleus. Carcinogenesis 1:161.

Swenson, D. H., E. C. Miller and J. A. Miller. 1974. Aflatoxin B1-2,3-oxide: Evidence for its formation in rat liver in vivo and by human liver microsomes in vitro. Biochem. Biophys. Res. Comm. 60:1036.

Swick, R. A., C. L. Miranda, P. R. Cheeke and D. R. Buhler. 1983. Effect of phenobarbital on toxicity of pyrrolizidine (Senecio jacobaea) alkaloids in sheep. J. Anita. Sci. 56:887.

Syversten, C. and F. C. Stormer. 1983. Oxidation of two hydroxylated ochratoxin A metabolites by alcohol dehydrogenase. Appl. Environ. Microbiol. 45:1701.

Testa, B. and P. Jenner. 1976. Drug Metabolism: Chemical and Biochemical Aspects~ Marcel Dekker, New York.

Thorpe, E. and E.J.H. Ford. 1968. Development of hepatic lesions in calves fed with ragwort (Senecio jacobaea). J. Comp. Pathol. 78:195.

Tuchweber, B., K. Kovacs, M. V. Jago and T. Beaulieu. 1974. Effects of steriodal and nonsteroidal microsomal enzyme inducers on the hepatotoxicity of pyrrolizidine alkaloids in rats. Res. Comm. Chem. Pathol. Pharmacol. 7:459.

Wattenberg, L. W. and J. L. Leong. 1965. Effects of phenothiazines on protective systems against polycyclic hydrocarbons. Cancer Res. 25: 365.

Welch, R. M., W. Levin and A. H. Conney. 1967. Insecticide inhibition and stimulation of steriod hydroxylases in rat liver. J. Pharmacol. Exp. Therap. 155:167.

White, I.N.H. 1976. The role of liver glutathione in the acute toxicity of retrorsine to rats. Chem.-Biol. Interact. 13 : 333.

White, I.N.H. and A. R. Mattocks. 1972. Reaction of dehydropyrrolizidines with deoxyribonucleic acids in vitro. Biochem. J. 128: 291.

White, I.N.H., A. R. Mattocks and W. H. Butler. 1973. The conversion of the pyrrolizidine alkaloid retrorsine to pyrrolic derivatives in vivo and in vitro and its acute toxicity to various animal species. Chem.-Biol. Interact. 6: 207.

Wogan, G. N. 1973. Aflatoxin carcinogenesis. In: H. Busch (Ed.) Methods in Cancer Research. pp 309-344. Academic Press, New York.

Wogan, G. N. and. W. F. Busby, Jr. 1980. Naturally occurring carcinogens. In: I. E. Liener (Ed.) Toxic Constituents of Plant Foodstuffs. Academic Press, Inc., New York.

Wong, Z. A. and D.P.H. Hsieh. 1978. Aflatoxicol: Major aflatoxin Bt metabolite in rat plasma. Science 200:325.

Wong, Z. A. and D,P.H. Hsieh. 1980. The comparative metabolism and toxicokinetics of aflatoxin B t in the monkey, rat, and mouse. Toxicol. AppI. Pharmacol. 55:115.

Yoshizawa, T., T. Sakamoto, Y. Ayano and C. J. Microcha. 1982. 3'-hydroxy T-2 and 3'-hydroxy HT-2 toxins: New metabolites of T-2 toxin, a tricothecene mycotoxin, in animals. Agr. Biol. Chem. 46:2613.

HEPATIC METABOLISM AND BIOACTIVATION OF MYCOTOXINS …€¦ · metabolizes foreign compounds (xenobiotics) into more polar, more excretable and less toxic metabolites. During metabolism

Documents