Irreversible binding of ethynyl-estradiol metabolites to protein and nucleic acids as catalyzed by rat liver microsomes and mushroom tyrosinase

Proliferative liver lesions and sex steroids in rats

In standard two year tumorigenicity studies some gestagens, alone or in combination with an estrogen have been shown to produce proliferative liver lesions (PLL) in rats. Estrogens in general have not produced PLLs; for ethinyl estradiol the situation is equivocal.

The steroids which increase the incidence of PLLs have not been shown to have the characteristics usually associated with classical hepatocarcinogens such as diethylnitrosamine (e.g. negative mutagenicity and cytoxicity, failure to bind covalently to DNA). Therefore, another mechanism must be assumed. A possible explanation is given by the ability demonstrated for a few steroids (mestranol alone or in combination with norethindrone, ethinylestradiol) to promote preneoplastic islands induced by diethylnitrosamine, resulting in an increase in tumor incidence over rats given only the carcinogen. Spontaneous preneoplastic islands in rats occur in various incidence in different strains and we have recently shown that their reaction to steroids—enhanced proliferation—is the same as the effect on preneoplastic islands produced by classical carcinogens. Sex steroids also enhance the growth or normal hepatocytes. It, therefore, seems reasonable to assume that sex steroids produce PLLs by acting on spontaneous preneoplastic lesions and not by the mechanisms usually associated with classical carcinogens.

Extrapolating the incidence of PLLs in rats to a risk in humans may be highly misleading because of different pharmacokinetic patterns. Even in rats treated orally for two years with sex steroids dosed as multiples of the human contraceptive dose, resorption and biovailability may vary by a factor of 10 or more. It is, therefore, inappropriate to state that one sex steroid is more active in the production of PLLs than another, unless the data from pharmacokinetic studies are taken into account.

Available evidence shows that sex steroids act as tumor promotors and enhance the growth of normal hepatocytes. It is extremely doubtful whether a sex steroid producing PLLs in the rat after two years of oral treatment can be classified as an initiating tumorigen for the rat. Substantial risk for humans cannot be inferred, because of the high incidence of spontaneous preneoplastic lesions in rats, and because of the very high sensitivity of the rat liver to a variety of chemically unrelated compounds including sex steroids.

Molecular mechanism of 17α-ethinylestradiol cytotoxicity in isolated rat hepatocytes

17α-Ethinylestradiol (17-EE) is used in formulations of contraceptives and hormone replacement therapy because it is an estradiol derivative. However, it has been associated with an increase in the risk of liver cancers and injury. The carcinogenic properties of 17-EE are similar to that of other estrogens, but the molecular mechanism of liver injury is still unclear. It is important to identify any secondary toxic mechanisms that can be used to prevent or treat the toxicity. The LC50 of 17-EE toward isolated rat hepatocytes was determined to be 150 ± 8 μmol/L. Accelerated cytotoxicity mechanism screening (ACMS) techniques using isolated rat hepatocytes showed that CYP1A inhibitors decreased cytotoxicity, whereas tyrosinase increased toxicity; this suggests that the toxic mechanism involved is the oxidation of 17-EE. A hepatocyte inflammation model also increased 17-EE-induced mitochondrial toxicity, as well as the formation of ROS and H2O2. Cytotoxicity was increased when inhibitors of quinone reduction, catechol-O-methylation, glucuronidation, glutathione conjugation, and sulfation were co-incubated with 17-EE. The hepatocytes could be rescued with antioxidants and quinone trapping agents, thereby suggesting a role for quinoid moiety induced oxidative stress in 17-EE induced cytotoxicity. These mechanisms for 17-EE hepatotoxicity could provide a new perspective for the treating 17-EE-induced liver injury.

S-adenosylmethionine-dependent protein methylation in mammalian cytosol via tyrphostin modification by catechol-O-methyltransferase

It has previously been shown that incubation of mammalian cell cytosolic extracts with the protein kinase inhibitor tyrphostin A25 results in enhanced transfer of methyl groups from S-adenosyl-[methyl-3H]methionine to proteins. These findings were interpreted as demonstrating tyrphostin stimulation of a novel type of protein carboxyl methyltransferase. We find here, however, that tyrphostin A25 addition to mouse heart cytosol incubated with S-adenosyl-[methyl-3H]methionine or S-adenosyl-[methyl-14C]methionine stimulates the labeling of small molecules in addition to proteins. Base treatment of both protein and small molecule fractions releases volatile radioactivity, suggesting labile ester-like linkages of the labeled methyl group. Production of both the base-volatile product and labeled protein occurs with tyrphostins A25, A47, and A51, but not with thirteen other tyrphostin family members. These active tyrphostins all contain a catechol moiety and are good substrates for recombinant and endogenous catechol-O-methyltransferase. Inhibition of catechol-O-methyltransferase activity with tyrphostin AG1288 prevents both base-volatile product formation and protein labeling from methyl-labeled S-adenosylmethionine in heart, kidney, and liver, but not in testes or brain extracts. These results suggest that the incorporation of methyl groups into protein follows a complex pathway initiated by the methylation of select tyrphostins by endogenous catechol-O-methyltransferase. We suggest that the methylated tyrphostins are further modified in the cell extract and covalently attached to cellular proteins. The presence of endogenous catechols in cells suggests that similar reactions can also occur in vivo.

Cytochrome P450 interactions in human cancers: new aspects considering CYP1B1

Molecular epidemiological studies are now a powerful tool to determine differential genetic susceptibilities to cancer-causing agents, and to obtain information on potential mechanisms. Cytochrome P450 (CYP) allelic variants are considered biomarkers of susceptibility to cancer. Such variants have an influence on the bioactivation and thereby on the potency of chemical carcinogens. This is very much straight forward for tobacco smoke-related human cancers. A new aspect is the implication of CYP1B1 in tobacco smoke-related cancers at several organ sites. On this basis, the present review is focused on lung, breast, urinary bladder and head and neck cancer. The CYP profile of the human lung includes CYP1A1, -1B1, -2A6, -2A13, -2B6, -2C18, -2E1, -2F1, -3A5 and -4B1. Polycyclic aromatic hydrocarbons (PAHs) and nitrosamines, as active components of tobacco smoke, appear as primary chemical factors for lung malignancies. For human mammary cancer, the use of hormone replacement therapy (HRT) has been shown to be associated with an increase of breast cancer risk, and there seems to be a link between risks caused by HRT use and modifying polymorphisms of drug/xenobiotic enzymes. Specifically, an association of the CYP1B1*3/*3 genotype with increased breast cancer risks has been postulated. Cigarette smoking is a major cause of human urinary bladder cancer. Arylamines, PAHs and nitrosamines are locally activated within the urothelium. Important CYPs in the bladder epithelium of experimental animals and man are CYP1B1 and -4B1. Alcohol consumption and tobacco smoking are known as the major causes of head and neck cancers. Recently, it appears that a polymorphic variant CYP1B1*3/*3 relates significantly to the individual susceptibility of smokers to head and neck cancer, supporting the view that PAH are metabolically activated through CYP1B1. It appears that CYP1B1 plays a key role for the activation of carcinogens at several organ targets, with a likelihood of complex gene-environment interactions implying Phase II enzymes.

Markers of genetic susceptibility in human environmental hygiene and toxicology: the role of selected CYP, NAT and GST genes

Inherited genetic traits co-determine the susceptibility of an individual to a toxic chemical. Special emphasis has been put on individual responses to environmental and industrial carcinogens, but other chronic diseases are of increasing interest. Polymorphisms of relevant xenobiotic metabolising enzymes may be used as toxicological susceptibility markers. A growing number of genes encoding enzymes involved in biotransformation of toxicants and in cellular defence against toxicant-induced damage to the cells has been identified and cloned, leading to increased knowledge of allelic variants of genes and genetic defects that may result in a differential susceptibility toward environmental toxicants. "Low penetrating" polymorphisms in metabolism genes tend to be much more common in the population than allelic variants of "high penetrating" cancer genes, and are therefore of considerable importance from a public health point of view. Positive associations between cancer and CYP1A1 alleles, in particular the *2C I462V allele, were found for tissues following the aerodigestive tract. Again, in most cases, the effect of the variant CYP1A1 allele becomes apparent or clearer in connection with the GSTM1 null allele. The CYP1B1 codon 432 polymorphism (CYP1B1*3) has been identified as a susceptibility factor in smoking-related head-and-neck squameous cell cancer. The impact of this polymorphic variant of CYP1B1 on cancer risk was also reflected by an association with the frequency of somatic mutations of the p53 gene. Combined genotype analysis of CYP1B1 and the glutathione transferases GSTM1 or GSTT1 has also pointed to interactive effects. Of particular interest for the industrial and environmental field is the isozyme CYP2E1. Several genotypes of this isozyme have been characterised which seem to be associated with different levels of expression of enzyme activity. The acetylator status for NAT2 can be determined by genotyping or by phenotyping. In the pathogenesis of human bladder cancer due to occupational exposure to "classical" aromatic amines (benzidine, 4-aminodiphenyl, 1-naphthylamine) acetylation by NAT2 is regarded as a detoxication step. Interestingly, the underlying European findings of a higher susceptibility of slow acetylators towards aromatic amines are in contrast to findings in Chinese workers occupationally exposed to aromatic amines which points to different mechanisms of susceptibility between European and Chinese populations. Regarding human bladder cancer, the hypothesis has been put forward that genetic polymorphism of GSTM1 might be linked with the occurrence of this tumour type. This supports the hypothesis that exposure to PAH might causally be involved in urothelial cancers. The human polymorphic GST catalysing conjugation of halomethanes, dihalomethanes, ethylene oxide and a number of other industrial compounds could be characterised as a class theta enzyme (GSTT1) by means of molecular biology. "Conjugator" and "non-conjugator" phenotypes are coincident with the presence and absence of the GSTT1 gene. There are wide variations in the frequencies of GSTT1 deletion (GSTT1*0/0) among different ethnicities. Human phenotyping is facilitated by the GST activity towards methyl bromide or ethylene oxide in erythrocytes which is representative of the metabolic GSTT1 competence of the entire organism. Inter-individual variations in xenobiotic metabolism capacities may be due to polymorphisms of the genes coding for the enzymes themselves or of the genes coding for the receptors or transcription factors which regulate the expression of the enzymes. Also, polymorphisms in several regions of genes may cause altered ligand affinity, transactivation activity or expression levels of the receptor subsequently influencing the expression of the downstream target genes. Studies of individual susceptibility to toxicants and gene-environment interaction are now emerging as an important component of molecular epidemiology.

Metabolic activation of 3-hydroxyanisole by isolated rat hepatocytes

A tyrosinase-directed therapeutic approach for malignant melanoma therapy uses the depigmenting phenolic agents such as 4-hydroxyanisole (4-HA) to form cytotoxic o-quinones. However, renal and hepatic toxicity was reported as side effects in a recent 4-HA clinical trial. In search of novel therapeutics, the cytotoxicity of the isomers 4-HA, 3-HA and 2-HA were investigated. In the following, the order of the HAs induced hepatotoxicity in mice, as measured by increased in vivo plasma transaminase activity, or in isolated rat hepatocytes, as measured by trypan blue exclusion, was 3-HA > 2-HA > 4-HA. Hepatocyte GSH depletion preceded HA induced cytotoxicity and a 4-MC-SG conjugate was identified by LC/MS/MS mass spectrometry analysis when 3-HA was incubated with NADPH/microsomes/GSH. 3-HA induced hepatocyte GSH depletion or GSH depletion when 3-HA was incubated with NADPH/microsomes was prevented by CYP 2E1 inhibitors. Dicumarol (an NAD(P)H: quinone oxidoreductase inhibitor) potentiated 3-HA- or 4-methoxycatechol (4-MC) induced toxicity whereas sorbitol (an NADH generating nutrient) greatly prevented cytotoxicity indicating a quinone-mediated cytotoxic mechanism. Ethylendiamine (an o-quinone trap) largely prevented 3-HA and 4-MC-induced cytotoxicity indicating that o-quinone was involved in cytotoxicity. Dithiothreitol (DTT) greatly reduced 3-HA and 4-MC induced toxicity. The ferric chelator deferoxamine slightly decreased 3-HA and 4-MC induced cytotoxicity whereas the antioxidants pyrogallol or TEMPOL greatly prevented the toxicity suggesting that oxidative stress contributed to 3-HA induced cytotoxicity. In summary, ring hydroxylation but not O-demethylation/epoxidation seems to be the bioactivation pathway for 3-HA in rat liver. The cytotoxic mechanism for 3-HA and its metabolite 4-MC likely consists cellular protein alkylation and oxidative stress. These results suggest that 3-HA is not suitable for treatment of melanoma.

Chemical properties of catechols and their molecular modes of toxic action in cells, from microorganisms to mammals

Catechols can undergo a variety of chemical reactions. In this review, we particularly focus on complex formations and the redox chemistry of catechols, which play an inportant role in the toxicity of catechols. In the presence of heavy metals, such as iron or copper, stable complexes can be formed. In the presence of oxidizing agents, catechols can be oxidized to semiquinone radicals and in a next step to o-benzoquinones. Heavy metals may catalyse redox reactions in which catechols are involved. Further chemical properties like the acidity constant and the lipophilicity of different catechols are shortly described as well. As a consequence of the chemical properties and the chemical reactions of catechols, many different reactions can occur with biomolecules such as DNA, proteins and membranes, ultimately leading to non-repairable damage. Reactions with nucleic acids such as adduct formation and strand breaks are discussed among others. Interactions with proteins causing protein and enzyme inactivation are described. The membrane-catechol interactions discussed here are lipid peroxidation and uncoupling. The deleterious effect of the interactions between catechols and the different biomolecules is discussed in the context of the observed toxicities, caused by catechols.

Estrogen affinity crosslinking to tyrosinase-like immunoreactive proteins of rat uterine nuclear extracts

We have previously described tyrosinase-like proteins of rat uterine nuclear extracts with type II estrogen binding characteristics. In this paper we have been able to affinity label these polypeptides with radio-iodinated estradiol. The major label at approximately 33-38 kDa comigrates with a approximately 36 kDa tyrosinase immunoreactive band assessed by autoradiograms and Western blots following electrophoresis. A minor label was also detected at approximately 45 kDa. The label is attenuated by excess quercetin hence these proteins are believed to represent putative type II estrogen binding sites that bind this bioflavonoid. These estrogen binding proteins are distinct from the estrogen receptor as judged by immunoblotting. The affinity crosslinking will be a useful approach in the purification of tyrosinase like proteins.

Intrahepatic cholestasis: a review of biochemical-pathological mechanisms

Intrahepatic cholestasis involves impaired excretion of bile via the hepatobiliary system as a consequence of one or more lesions within the liver. In humans, intrahepatic cholestasis most often results as a side-effect of drug therapy and the clinical manifestation of this condition, jaundice, has been estimated to account for hospitalization in 2 to 5% of the cases for the general population and approaches as much as 20% in the elderly. With the aging of the population and the common occurrence of poly-drug therapy in geriatric patients, it is to be expected that jaundice due to drug-induced intrahepatic cholestasis will become even more prevalent, and accordingly the need to understand the basic mechanisms of this disease condition will become more urgent. The list of culprit agents implicated in the induction of intrahepatic cholestasis in humans is continually expanding. These include various steroid hormones, bile acids, drugs and other chemicals. Experimentally, a wide spectrum of agents has been shown to precipitate intrahepatic cholestasis. Over the years, a number of hypotheses on the biochemical and pathological mechanisms of intrahepatic cholestasis has emerged, including the following: impaired sinusoidal membrane function; interference with the distribution and binding of cytoplasmic endogenous carrier proteins; interference with mitochondrial energy supply; defects in the canalicular membrane including altered Na+/K+ -ATP-ase activity; impairment of microfilament and microtubule functions; interference with bile secretion involving bile acid dependent and independent fractions, and altered bile acid metabolism due to "hypoactive hypertrophic smooth endoplasmic reticulum". In partial agreement with the latter hypothesis, our studies indicated that impairment of the endoplasmic reticulum might represent one of the early stages in the development of intrahepatic cholestasis. Various experimental conditions that induce intrahepatic cholestasis to different degrees resulted in an interference of the synthesis of microsomal phospholipids and altered microsomal function. The conditions included the administration of various hepatotoxic compounds or steroids, pregnancy, delayed development of the endoplasmic reticulum in neonates, and dietary methyl donor or choline deficiency. This review reports the biochemical-pathological mechanisms postulated to be involved in the genesis of intrahepatic cholestasis with specific reference to experimental models of drug-induced intrahepatic cholestasis. The important practical implications of cholestasis are also briefly surveyed.

Isolation of estradiol-2,3-quinone and its intermediary role in melanin formation

We have reported previously that 2-hydroxyestradiol can be oxidized in the presence of catechol by mushroom tyrosinase, with a stoichiometric requirement of molecular oxygen (Jacobsohn, G.M. and Jacobsohn, M.K. (1984) Arch. Biochem. Biophys. 232, 189-196). It is then incorporated into melanin (Jacobsohn et al. (1988) J. Steroid Biochem. 31, 377-385). We now report on the isolation and characterization of the o-quinone as a product of the enzyme reaction from 2-hydroxyestradiol. The o-quinone was isolated from incubates and identified by its FTIR spectrum, in particular, by the appearance of a new band at 1652 cm-1, its migration in HPLC systems, its ultraviolet spectrum, its derivatization with phenylenediamine and comparison of these properties with the periodate oxidation product of the same substrate. The enzyme oxidation of the catechol estrogen was performed at 37 degrees C and did not require an activator; dopa at concentrations higher than 5 microM was inhibitory. At concentrations lower than 5 microM, dopa acted catalytically and was not consumed during the course of reaction. Ascorbic acid inhibited the reaction. The quinone exhibited both reversible and irreversible binding to performed melanin and to melanin actively synthesized by the enzyme. Incubation of 18 microM newly synthesized [4-14C]estradiol-2,3- quinone with mushroom tyrosinase for 45 min at 37 degrees C in presence of 400 microM dopa showed incorporation (irreversible binding) of 6.3 +/- 0.3% of label into melanin produced during the course of reaction. Similar incubations for 45 min of pre-formed melanin prepared from 400 microM dopa showed incorporation of 4.4 +/- 0.2% of the label. Reversible binding was 10-times greater than incorporation for both actively synthesized and preformed melanins. In the absence of dopa or catechol, enzyme incubations of either 2-hydroxy-estradiol or its quinone did not yield melanin. Data suggest that estradiol-2,3-quinone is an intermediate in the incorporation of the catechol estrogen into melanin by tyrosinase.

Multiple pathways for the oxidative metabolism of estrogens in Syrian hamster and rabbit kidney

Microsomal preparations from hamster kidney, a target tissue for the carcinogenic action of stilbene-type and steroidal estrogens, catalyze the oxidative metabolism of diethylstilbestrol (DES). The formation of the major metabolite Z,Z-dienestrol and of reactive intermediates capable of protein binding were mediated by enzyme activities requiring nicotinamide-adenine dinucleotide phosphate (reduced form-NADPH), cumene hydroperoxide, or arachidonic acid (ARA). In addition, hydroxylated DES metabolites were detected in NADPH-supplemented incubations. The NADPH-dependent oxidation of DES was inhibited by SKF 525A and metyrapone. Monooxygenase-catalyzed metabolism was apparently responsible for the majority of DES oxidation in microsomes from whole hamster kidneys in vitro and this activity is preferentially localized in the kidney cortex. However, ARA-dependent, i.e., prostaglandin H synthase (PHS) mediated oxidation of DES and of the catechol estrogen 2-hydroxyestrone was demonstrated as well in the medulla of both rabbit and hamster kidney. It is proposed that monooxygenase and PHS activities act in concert in the metabolic activation of carcinogenic estrogens. This appears to apply in particular to steroidal estrogens, since catechol estrogens formed by monooxygenases are further oxidized to reactive intermediates by PHS and other peroxidatic enzymes.

Characterization of free radicals produced during oxidation of etoposide (VP-16) and its catechol and quinone derivatives. An ESR Study

Spectroscopic evidence for the radical-mediated metabolism of VP-16, VP-16 catechol, and VP-16 quinone during enzymatic oxidation and autoxidation has been obtained. Autoxidation of the catechol yields the primary semiquinone together with the primary molecular product VP-16 quinone, which subsequently undergoes hydrolytic oxidation to form secondary quinones and semiquinones. Both primary and secondary phenoxyl radicals were detected during peroxidatic oxidation of VP-16. Neither the primary nor the secondary radicals react with DNA at a detectable rate. Evidence for the production of hydroxyl radical during iron-catalyzed oxidation of VP-16 catechol was obtained. These free radical reactions may have implications for the mechanism of antitumor action of VP-16.

The metabolism of 17 alpha-ethinyloestradiol by human liver microsomes: formation of catechol and chemically reactive metabolites

The metabolism of 17 alpha-ethinyloestradiol (EE2) to catechol and reactive metabolites by human liver microsomes was investigated. 2-Hydroxyethinyloestradiol (2-OHEE2) was either the sole or principal metabolite. Small amounts of 6-hydroxyethinyloestradiol and 16-hydroxyethinyloestradiol were produced by some of the livers. EE2 (10 microM) underwent substantial (5-20% of incubated drug), though highly variable, NADPH-dependent metabolism to material irreversibly bound to microsomal protein. 2-OHEE2 appeared to be the pro-reactive metabolite. The maximum EE2 2-hydroxylase activity was 0.67 nmol min-1 mg-1 microsomal protein, with a Km value of 8.6 microM. Oestradiol, which is mainly hydroxylated to 2-hydroxyoestradiol, was the most potent inhibitor of hydroxylase activity and exhibited competitive inhibition. Progesterone, which undergoes 2-hydroxylation to a minor extent was also a competitive inhibitor, whereas cholesterol and cortisol did not have any appreciable inhibitory effect. Primaquine was the most potent non-steroidal inhibitor but was non-competitive. Other non-steroidal compounds investigated, e.g. antipyrine, did not show any significant effect on EE2 2-hydroxylation. The results of this study suggest that EE2 2-hydroxylation is metabolised by a form(s) of cytochrome P-450 which has affinity for endogenous steroids.

Mitotic inhibition and aneuploidy induction by naturally occurring and synthetic estrogens in Chinese hamster cells in vitro

We used a predominantly diploid Chinese hamster cell line to test a number of naturally occurring and synthetic estrogens for their ability to arrest cells at metaphase, their potential for allowing anaphase recovery, and their capability of inducing aneuploid progeny. The chemicals employed included diethylstilbestrol, dienestrol, hexestrol, beta-estradiol, ethynylestradiol and estriol. We also tested progesterone, estrone and testosterone in this regard. Only estrogens and their synthetic analogs caused mitotic arrest and aneuploidy, while progesterone, estrone and testosterone did not cause mitotic disturbances. Among the estrogens, DES was the most effective arrestant on a comparative molar basis, whereas dienestrol was most potent over a wide range of concentrations. Estriol was the least potent as an arrestant but was an effective inducer of aneuploidy. The addition of a metabolic activator (S9) did not alter the ability of DES to arrest mitosis. Following the removal of the drugs, cells were able to quickly reorganize a spindle apparatus and enter anaphase. Diethylstilbestrol, dienestrol, hexestrol, beta-estradiol, ethynylestradiol and estriol caused significant increase in aneuploidy within a narrow range of high concentrations in recovering cell populations. Aneuploidy was induced in a non-random manner. Immunofluorescence studies with anti-tubulin antibody indicate that estrogens may have a mechanism of mitotic arrest similar to that of colchicine and colcemid, viz inhibiting the polymerization of tubulin to form microtubules. These data suggest that the interaction between estrogens and microtubules may mediate the induction of aneuploidy in somatic cells. Aneuploidy induction by DES and similar compounds may be related to their carcinogenic potential.

Carcinogenicity and metabolic activation of hexestrol

The carcinogenic activity of the synthetic estrogen hexestrol was measured in male Syrian hamsters. Between 90% and 100% of the animals treated with hexestrol or with 3',3",5',5"-tetradeuteriohexestrol, implanted subcutaneously as 25-mg pellets, were found with renal carcinoma after 6-7 months. In vitro hexestrol metabolism, mediated by phenobarbital-induced rat liver microsomes, led to the formation of 3'-hydroxyhexestrol. This metabolite was identified by comparison with authentic reference material synthesized by oxidation of hexestrol with Fremy's salt. Diethylstilbestrol could not be detected as a metabolite. In urine of male Syrian hamsters, 3'-hydroxyhexestrol, 3'-methoxyhexestrol, 1-hydroxyhexestrol, and other hydroxylated and/or methoxylated hexestrol metabolites were identified. Again, diethylstilbestrol was not detectable as a hexestrol metabolite in vivo. The reactivity of 3'-hydroxyhexestrol was then studied to determine if this catechol estrogen played a role in hexestrol carcinogenicity. Horseradish peroxidase catalyzed the oxidation of 3'-hydroxyhexestrol to 3',4'-hexestrol quinone. This oxidation reaction could also be carried out non-enzymatically using silver oxide or silver carbonate on celite as oxidants. The quinone was unstable (t1/2 in methylene chloride: 53 min). It reacted with sulfur-containing compounds such as mercaptoethanol by Michael addition to form 3'-(2-hydroxyethylthio)-5'-hydroxyhexestrol. 3',4'-Hexestrol quinone reacted with simple amines such as ethylamine to form N-ethyl-aminohexestrol. The chemical reactions described above were carried out to test the reactivity of identified or suspected metabolic intermediates of hexestrol. It was concluded that carcinogenicity of hexestrol was not based on its conversion to diethylstilbestrol. Rather, catechol estrogen formation may be necessary for the carcinogenic action of hexestrol in analogy to events observed earlier with estradiol.

Metabolism of 1-naphthol by tyrosinase

1-Naphthol was metabolized by the polyphenol oxidase, tyrosinase, primarily to 1,2-naphthoquinone and to small amounts of 1,4-naphthoquinone as well as to covalently bound products. The inhibition of covalent binding by ethylenediamine, which reacts specifically with 1,2-naphthoquinone but not 1,4-naphthoquinone, suggested that most of the covalent binding was due to 1,2-naphthoquinone or a metabolite of similar structure. The activation by tyrosinase of 1-naphthol to covalently bound products suggested that it may alter the reaction kinetics of the enzyme. This was investigated by studying the effects of 1-naphthol on the tyrosinase-catalysed oxidation of 4-hydroxyanisole. Preincubation of tyrosinase with 1-naphthol increased the lag period of the oxidation of 4-hydroxyanisole, which may be due to a decrease in the amount of active enzyme, as well as to a reaction of 1-naphthol with 3,4-anisylquinone, an oxidation product of 4-hydroxyanisole. The metabolic activation of 1-naphthol by tyrosinase to covalently bound species suggests that 1-naphthol or a structurally related derivative may be of potential therapeutic application in the treatment of cells high in tyrosinase activity, such as certain melanomas.

The role of 4-bromophenol and 4-bromocatechol in bromobenzene covalent binding and toxicity in isolated rat hepatocytes

4-Bromophenol and 4-bromocatechol are formed as metabolites of bromobenzene in vivo and in isolated rat hepatocytes. Both of these metabolites may potentially contribute to the hepatotoxicity of bromobenzene. Bromobenzene metabolism in hepatocytes isolated from phenobarbital-treated rats forms 0.12 to 0.17 mM 4-bromophenol and 4-bromocatechol in 2 hr, with 1 to 3 mM bromobenzene. The role of activated metabolites derived from 4-bromophenol and 4-bromocatechol in bromobenzene covalent binding and toxicity was investigated with isolated hepatocytes in suspension. The covalent binding of the phenol and the catechol was increased four- to eightfold by the addition of unlabeled bromobenzene. Two-hour incubations of 0.25 mM 14C-labeled 4-bromophenol or 4-bromocatechol with hepatocytes isolated from phenobarbital-treated rats resulted, under these conditions, in no significant toxicity, and approximately 4 and 25%, respectively, of the covalent binding associated with bromobenzene itself. Two- and six-hour incubations with higher 4-bromophenol and 4-bromocatechol concentrations demonstrated that 1 to 3 mM substrate concentrations were required for cytotoxicity. These results show that metabolically produced 4-bromophenol and 4-bromocatechol do not play significant roles in the production of bromobenzene cytotoxicity in isolated hepatocytes, and that they contribute only modestly to bromobenzene covalent binding.

Photosensitized irreversible binding of estrone to protein via a hydroperoxide intermediate: an explanation of (photo-) allergic side-effects of estrogens

After irradiation (lambda greater than 425 nm) for 15 min of a solution of [4-14C]-estrone, albumin and the photosensitizer hematoporphyrin in phosphate buffer, more than 30% of the radioactivity could not be extracted. When the protein was added after irradiation, irreversible binding also occurred. Sephadex gel filtration showed that the radiolabel was bound to albumin as well as to the photosensitizer. A 10 beta-hydroperoxide is the reactive intermediate in this binding. Inasmuch as phenolic steroids coupled to proteins have been used for the induction of estrogenic-specific antibodies, the irreversible binding observed between estrone and albumin by photosensitization might be an explanation for (photo)allergic disorders associated with estrogens.

Estrogen metabolism in rat liver microsomal and isolated hepatocyte preparations--II. Inhibition studies

The abilities of various inhibitors and metabolism modifiers to alter the metabolism of estradiol and the irreversible binding of estradiol to proteins were examined in subcellular microsomal incubations and in intact hepatocyte preparations. In studies with rat liver microsomal preparations containing estradiol and an NADPH-generating system, the irreversible binding of radiolabeled steroid metabolite(s) to the microsomal proteins was 77.59 pmol/mg/min (SD 6.1; 7.6% of total steroid). 2-Bromoestradiol and 4-bromoestradiol, inhibitors of estrogen 2-hydroxylase, effectively decreased this irreversible binding of radiolabeled estradiol metabolite(s) to microsomal proteins to 17 pmol mg-1 min-1 (2.1% of total estradiol). These haloestrogens were also effective inhibitors in the intact hepatocyte cells, decreasing the amounts of organic metabolites, aqueous-soluble conjugates, and protein-bound materials. The HPLC radiochromatograms of the organic-extracted fractions from the 2 h hepatocyte incubations demonstrate that the catechol estrogen products, i.e. 2-hydroxyestrogens and 2-methoxyestrogens, were present in lower amounts in the incubations containing the bromoestrogens than in control incubations containing no inhibitor. Ascorbic acid and cysteine, general modifiers of oxidative pathways of metabolism, also affected estradiol metabolism in microsomal and hepatocyte preparations. Both these agents were able to decrease the irreversible binding of estradiol to proteins in the microsomal assays. Ascorbic acid decreased the general metabolism of estradiol in the hepatocyte incubations but did not decrease irreversible binding to proteins. The addition of cysteine to the hepatocyte incubation resulted in an increased metabolism of estradiol and the production of more aqueous-soluble radiolabeled metabolites than the control incubations; however, cysteine did not decrease the amounts of estradiol metabolite(s) irreversibly bound to proteins. Investigations of steroid metabolism in the isolated hepatocytes thus provide an effective in vitro technique for examining the overall oxidative, reductive, and conjugative pathways that are functional in the liver and enables one to investigate the abilities of inhibitors, regulators, and modifiers to affect the metabolic processes. Also, these hepatocyte studies demonstrate that the inhibitors of estrogen 2-hydroxylase, 2-bromoestradiol and 4-bromoestradiol, can enter and act in the intact cells. Consequently, these agents may be useful pharmacological probes for examining the functions of catechol estrogens in other tissues.

Estrogen metabolism in rat liver microsomal and isolated hepatocyte preparations--I. Metabolite formation and irreversible binding to cellular macromolecules

The metabolism of endogenous estrogens, estradiol and estrone, and the irreversible binding of estrogens to cellular macromolecules have been examined and compared in subcellular microsomal and in intact hepatocyte preparations. In studies with rat liver microsomal preparations containing estradiol, an NADPH-generating system, and denatured DNA, the irreversible binding of radiolabeled steroid metabolite(s) to the microsomal proteins was 3.26 nmoles/mg protein in 1 hr (S.D. 0.39; 7.9% of total steroid) while binding to DNA was found to be 0.288 nmole/mg DNA/mg protein (S.D. 0.025; 0.39% of total steroid). No significant difference was observed between microsomal preparations from untreated, phenobarbital-treated or 3-methylcholanthrene-treated rats. Irreversible binding to proteins was also demonstrated in the intact hepatocyte cell incubations. After 2-hr incubations of estradiol with hepatocytes, 5.9% (S.D. 1.4%) of the steroid(s) was irreversibly associated with cellular proteins (approximately 1.43 pmoles/mg/min). Analysis of the organic-soluble metabolites demonstrated the presence of the catechol estrogens and their metabolites, 2-hydroxyestradiol, 2-hydroxyestrone, 2-methoxyestradiol, and 2-methoxyestrone. Estrone and estriol were also identified. The aqueous-soluble materials isolated from hepatocyte incubations contained glucuronide, sulfate, and apparent thioether conjugates, as determined by liberation from estrogen metabolites by treatment with beta-glucuronidase, sulfatase, and Raney nickel. Thus, extensive primary and secondary metabolism of estrogens occurs in intact hepatocyte incubations. Furthermore, irreversible binding of estrogens to cellular proteins occurs in these intact cells having demonstrated conjugative pathways of metabolism.

Synthesis of 3,4-estrogen-o-quinone

Activated manganese dioxide was found to be an effective reagent for synthesis of estrogen-o-quinones from the corresponding catechols. The isolation and characterization of 1,5(10)-estradiene-3,4,17-trione is described.

Metabolic activation of 1-naphthol by rat liver microsomes to 1,4-naphthoquinone and covalent binding species

1-Naphthol was metabolized by rat liver microsomes, in the presence of an NADPH-generating system, both to methanol-soluble metabolites including 1,4-naphthoquinone and an uncharacterized product(s) (X) and also to covalently bound products. NADH was much less effective as an electron donor than NADPH. Metyrapone, SKF 525-A and carbon monoxide all inhibited the metabolism of 1-naphthol to 1,4-naphthoquinone and to covalently bound products suggesting the involvement of cytochrome P-450 in at least one step in the metabolic activation of 1-naphthol to reactive products. Ethylene diamine, which reacts selectively with 1,2-naphthoquinone but not 1,4-naphthoquinone, did not affect the covalent binding whereas glutathione, which reacts with both naphthoquinones, caused an almost total inhibition of covalent binding. These and other results suggested that 1,4-naphthoquinone, or a metabolite derived from it, was responsible for most of the covalent binding observed and that little if any of the binding was due to 1,2-naphthoquinone.

Interaction of the photosensitization products of oestrone with DNA: comparison with the horseradish peroxidase catalyzed reaction

The interaction with DNA of [4-14C]oestrone upon photosensitization with hematoporphyrin (HP) as a photosensitizer has been investigated. By means of Sephadex LH-20 gel filtration and extraction with dichloromethane it was found that, after irradiation (lambda greater than 425 nm) of a solution of HP, DNA and [4-14C]oestrone 21% of the radiolabel was associated with DNA. If DNA was added after irradiation 23% was bound to DNA, whereas 25% of the oestrone remained after photoreaction under the conditions applied. The binding occurs via the reactive 10 beta-hydroperoxy-1,4-estradien-3,17-dione, which is the only product after photosensitization of oestrone. The hydroperoxide has a strong interaction with DNA compared with that of other steroids. By repeated precipitation with 5 M NaCl and ethanol the association can be broken. It is reported, that binding of oestrone to protein induced by both photosensitization and horseradish peroxidase (HRPO)/H2O2 is irreversible, but that the amount of binding to DNA is dependent on the method of determination. However, neither the hydroperoxide nor its reduced product, a p-quinol, is intermediate or product in the HRPO catalyzed reaction of oestrogens. The tight association of the hydroperoxide product of oestrone with DNA, which may proceed via hydrogen bonding between the -OOH group and oxygen atoms of the backbone phosphate groups or of the furanose ring, might be a cause of chemical modification of DNA and of mutagenic effects.

Metabolism of stilbene estrogens and steroidal estrogens in relation to carcinogenicity

The oxidative metabolism of diethylstilbestrol (DES) and 17 alpha-ethynyl estradiol, as examples of stilbene- and steroid-type estrogens, is discussed with respect to the formation of reactive intermediates. For DES, a genotoxic potential is implied by metabolic studies and positive effects in short-term tests for genetic damage. A particularly important pathway for DES carcinogenicity appears to be peroxidase-mediated oxidation. Although data for steroidal estrogens are more ambiguous, the available evidence suggests that metabolic activation by peroxidatic oxidation may also be of importance for this class of estrogens.

Interaction of estrone and estradiol with DNA and protein of liver and kidney in rat and hamster in vivo and in vitro

[6,7-3H] Estrone (E) and [6,7-3H]estradiol-17 beta (E2) have been synthesized by reduction of 6-dehydroestrone and 6-dehydroestradiol with tritium gas. Tritiated E and E2 were administered by oral gavage to female rats and to male and female hamsters on a dose level of about 300 micrograms/kg (54 mCi/kg). After 8 h, the liver was excised from the rats; liver and kidneys were taken from the hamsters. DNA was purified either directly from an organ homogenate or via chromatin. The radioactivity in the DNA was expressed in the units of the Covalent Binding Index, CBI = (mumol chemical bound per mol DNA-P)/(mmol chemical administered per kg b.w.). Rat liver DNA isolated via chromatin exhibited the very low values of 0.08 and 0.09 for E and E2, respectively. The respective figures in hamster liver were 0.08 and 0.11 in females and 0.21 and 0.18 in the males. DNA isolated from the kidney revealed a detectable radioactivity only in the female, with values of 0.03 and 0.05 for E and E2, respectively. The values for male hamster kidney were less than 0.01 for both hormones. The minute radioactivity detectable in the DNA samples does not represent covalent binding to DNA, however, as indicated by two sets of control experiments. (A) Analysis by HPLC of the nucleosides prepared by enzyme digest of liver DNA isolated directly or via chromatin did not reveal any consistent peak which could have been attributed to a nucleoside-steroid adduct. (B) All DNA radioactivity could be due to protein contaminations, because the specific activity of chromatin protein was determined to be more than 3,000 times higher than of DNA. The high affinity of the hormone to protein was also demonstrated by in vitro incubations, where it could be shown that the specific activity of DNA and protein was essentially proportional to the concentration of radiolabelled hormone in the organ homogenate, regardless of whether the animal was treated or whether the hormone was added in vitro to the homogenate.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigations on the occurrence of non-extractable residues of trienbolone acetate in cattle tissues in respect to their bioavailability and immunological reactivity

3H-trienbolone acetate (TBA) was injected/implanted in cattle and the distribution of radioactivity in liver and muscular tissue was determined, applying rigorously standardized organic or aqueous extraction procedures, either directly or following enzymatic hydrolysis and proteolytic procedures. These steps yielded almost 100% recovery of the radioactivity and indicated that only between 5% and 15% of the total residues present was extractable with organic solvents. The remaining radioactivity was either soluble in aqueous media or stayed bound to tissue structures. Similarly processed liver tissue from a calf treated with 3500 mg TBA 68 days prior to slaughter was examined by applying radioimmunoassay for the determination of TBA/trienbolone (TBOH). Indications of the presence of trienic-steroid type residues were only obtained for fractions containing residues extractable with organic solvents.

Drug-protein conjugates--V. Sex-linked differences in the metabolism and irreversible binding of 17 alpha-ethinylestradiol in the rat

Sex-linked differences in the disposition, biotransformation, excretion and irreversible binding of [6, 7-3H]17 alpha-ethinylestradiol [( 3H]EE2) in Wistar rats have been observed. Three hours after i.v. administration of [3H]EE2 (5 micrograms/kg) the livers of males contained twice as much 3H-labelled material as those of females. The biliary metabolites were largely glucuronides in both sexes, but males also excreted arylsulphates. The principal metabolites liberated from biliary conjugates by enzymes were 2-hydroxyEE2 and 2-methoxyEE2 in females and males, respectively. Biliary elimination of 3H over 3 hr was slightly greater in males (P less than 0.05). Radiolabelled material was irreversibly bound to hepatic microsomal and soluble protein. The material bound to microsomes represented 0.24 +/- 0.07% (mean +/- S.D.) of the dose in males and 0.56 +/- 0.10% in females (P less than 0.001). Oxygenation of the steroid D-ring was not indicated, and 2-hydroxyEE2 appears to be the precursor of the reactive metabolite. The metabolic basis of the sex-linked difference in irreversible binding is discussed.

Drug protein conjugates--III. Inhibition of the irreversible binding of ethinylestradiol to rat liver microsomal protein by mixed-function oxidase inhibitors, ascorbic acid and thiols

The metabolism of [6,7-3H]ethinylestradiol [( 3H]EE2) by rat liver microsomes was studied in vitro. After incubation of [3H]EE2 with rat liver microsomes for 20 min, 90% of the substrate was metabolised and 18% of the 3H-labelled material irreversibly bound to microsomal protein. Ascorbic acid (1 mM) decreased irreversible binding of 3H and produced an accumulation of 2-hydroxyethinylestradiol (2OH-EE2), while mixed-function oxidase inhibitors (0.5 mM) decreased binding of 3H to protein by inhibiting EE2 2-hydroxylation. Addition of thiols gave water-soluble metabolites which were characterised as 1(4)-thioether derivatives of 2OH-EE2 by co-chromatography with synthetic products. The results are consistent with the hypothesis that the chemically reactive metabolite of EE2 formed in vitro is either a quinone or o-semiquinone derived from 2OH-EE2 [1].

Estrogen receptor in malignant melanoma: fact or artefact?

A model system is presented to explain how tyrosinase, an enzyme unique to pigmented cells such as normal and malignant melanocytes can oxidize [3H]-estradiol to radiolabeled products which closely resemble the tight binding of [3H]-estradiol to estrogen receptor. In the model system studied, tyrosinase oxidized [2,4,6,7-3H]-estradiol to [3H]-water and [3H]-estradiol metabolites, the latter of which formed ring-substituted conjugates with nucleophiles like monothioglycerol and BSA. Radiolabeled estradiol without tritium in the C-2 position (i.e. [6,7-3H]-estradiol) failed to liberate [3H]-water when exposed to tyrosinase but, nevertheless, did form ring-substituted [3H]-estradiol adducts with nucleophiles. The [3H]-water and the ring-substituted radiolabeled products possessed several characteristics of a genuine estrogen receptor protein in that they were resistant to dextran-coated charcoal (DCC) adsorption and their enzymatic formation was inhibited with non-steroidal estrogens like diethylstilbestrol. Other natural (estradiol) and synthetic (hydroxytamoxifen) estrogens which contain the phenol grouping also inhibited the enzymatic oxidation of [3H]-estradiol. Although it was difficult to differentiate estrogen receptor from tyrosinase using the conventional DCC assay system, several differences in these two proteins permitted a distinction to be made between them. First, tyrosinase oxidation of [3H]-estradiol was markedly inhibited by sulfhydryl reducing agents (monothioglycerol) that stabilize [3H]-estradiol binding to estrogen receptor. Second, estrogen receptor adsorbed by hydroxylapatite whereas tyrosinase did not, thus permitting the separation of these two proteins prior to incubation with [3H]-estradiol. We conclude that the [3H]-estradiol binding components in melanoma previously reported to be estrogen receptor probably represent instead the radiolabeled products of the tyrosinase-catalyzed oxidation of [3H]-estradiol.

Protection of catechol oestrogen from oxidation during enzymic hydrolysis of biliary conjugates

2-Hydroxyethynyloestradiol (2-OHEE2), a major biliary metabolite of 17 alpha-ethynyloestradiol in female rats, is conjugated largely with glucuronic acid. Accurate quantitation of [3H]2-OHEE2 deconjugated by enzymic hydrolysis depends upon co-incubation with ascorbate (5-10 mM). In the absence of ascorbate, the proportion of [3H]2-OHEE2 declines by 30 +/- 7% (x +/- SD, n = 4) during a 3 h incubation of bile with arylsulphohydrolase and beta-glucuronidase. Over 16 h, decomposition of the catechol leads to a decrease in ether-extractable 3H labelled components.

The biotransformation of 17 alpha-ethynyl[3H]estradiol in the rat: irreversible binding and biliary metabolites

1. The irreversible binding of metabolites of 17 alpha-ethynyl[6,7-3H]estradiol ([3H]EE2) to intracellular proteins, and the biliary metabolites of [3H]EE2, were studied in male rats. 2. Very low levels of irreversible binding to hepatic microsomal and soluble proteins were observed. 3. Approx. 75% of the radiolabelled material excreted in bile was present as beta-glucuronides and arylsulphate esters. 4. The compounds liberated from the biliary conjugates by enzymic hydrolysis consisted of EE2, 2-hydroxy-EE2, 16-hydroxy-EE2, 2-methoxy-EE2, 2-hydroxymestranol and at least three additional metabolites not fully identified. 2-Methoxy-EE2 was the principal metabolite. 5. EE2 and all its identified metabolites were excreted as both beta-glucuronides and arylsulphate esters. The glucuronide fraction contained a greater proportion of EE2, a lower proportion of 2-methoxy-EE2 and a lower ratio of 2-methoxy-EE2 to 2-hydroxymestranol than the arylsulphate fraction.

Covalent binding of drug metabolites to DNA--a tool of predictive value?

The presently available data suggest at least some correlation between covalent binding of drug metabolites to DNA and carcinogenicity of that drug. More data, however, are needed to establish the predictability of covalent DNA binding assays for extrahepatic cancer. A covalent binding assay requires administration of radioactively labelled compound to the experimental animals; the availability of labelled compound and requirements as to radiochemical purity, chemical and biochemical stability are limiting the applicability of this procedure. Many technical pitfalls accompany covalent DNA binding assays. It is concluded that at the present time DNA binding assays do not represent routine procedures within a standard test battery for carcinogenicity, but are invaluable for more in-depth research which probably follows routine testing.

Formation of 3H2O from [2-3H]- and [4-3H] estradiol by rat uteri in vitro: possible role of peroxidase

The mitochondrial fraction of diethylstilbestrol-treated rat uteri, known to contain an estrogen-induced peroxidase, was able to catalyze the release of 3H2O from either [2-3H]- or [4-3H]estradiol. Hydrogen peroxide added to this system increased the yield of 3H2O but had no effect on mitochondrial preparations from ovariectomized rat uteri having only very low peroxidase activity. The reaction was inhibited by catalase and also occurred with lactoperoxidase in the presence of H2O2 but 2-hydroxyestradiol was not detected in any of these experiments. Under similar conditions, tyrosinase catalyzed the formation of the catechol estrogen with loss of 3H from [2-3H]- or [2,4,6,7-3H]- but not [4-3H]- or [6,7-3H]estradiol. It is proposed that the formation of 3H2O from 3H-labeled estradiol in the estrogen-treated rat uterus may occur by a peroxidative mechanism which does not necessarily result in hydroxylation of the steroid.

Binding kinetics of vinyl chloride and vinyl bromide at very low doses

Vinyl chloride and vinyl bromide are metabolically activated by liver microsomal enzymes to intermediates that covalently bind to proteins and nucleic acids. Several lines of evidence suggest the involvement of the epoxides, i.e., chloroethylene oxide or bromoethylene oxide. Proven targets for alkylation are adenine, cytosine and guanine moieties in nucleic acids, and sulfhydryl groups of proteins. For all the halogenated ethylenes studied so far, including vinyl chloride and vinyl bromide, metabolism in vivo is a dose-dependent, saturable process. The metabolic capacity of rats is saturated at atmospheric concentrations of 250 ppm vinyl chloride and 55 ppm vinyl bromide. As recent reports describe a diminishment of hepatocellular glutathione in rats after exposure to vinyl chloride concentrations of 50 ppm and more, we carried out a series of experiments measuring covalent binding of vinyl chloride metabolites after exposure to different concentrations of 14C-vinyl chloride. In all of these experiments, including one of an exposure to only 2 ppm vinyl chloride, hepatic covalent protein binding was related to the dose of vinyl chloride which was actually metabolized, and the ratio between bound and metabolized material was constant. This strongly suggests that hepatic glutathione levels must have only a very limited impact on covalent protein binding of vinyl chloride metabolites, an assumption which is supported by a lacking effect of a pretreatment with diethylmaleate. A scheme of hepatocellular compartimentation of metabolic steps is proposed which serves to explain these findings.

Partial hepatectomy reduces both metabolism and toxicity of benzene

Removal of 70--80% of the liver reduced both the metabolism and the toxicity of benzene in rats. Metabolism was evaluated by measuring the levels of urinary metabolites in both sham-operated and partially hepatectomized rats given 2200 mg/kg [3H]benzene sc. Toxicity was evaluated by measuring the incorporation of 59Fe into circulating erythrocytes according to the method of Lee et al. The observation that partial hepatectomy decreases benzene metabolism and protects against benzene toxicity indicates that the liver may play a primary role in the development of benzene-induced bone marrow toxicity. The fact that benzene administration also reduces the ability of the liver to regenerate after partial hepatectomy suggests that the regenerating liver may serve as a model system in lieu of the bone marrow for studying the mechanism by which benzene inhibits cell proliferation.

Covalent binding of ethinylestradiol and estrone to rat liver DNA in vivo

The covalent binding of [6,7-3H] ethinylestradiol (EE) and [6,7-3H] estrone (E) to liver DNA of 200 g female rats was measured 8 h after the administration of 80 microgram (9.2 mCi) estrogen by gavage. The binding is 1.5 for EE and 1.1 for E, expressed as binding to DNA/dose, in units of mumol hormone/mol DNA phosphate/mmole hormone/kg body wt. It is the same order of magnitude as for benzene and about 10 000 times below the binding of typical liver carcinogens, such as aflatoxin B1 or N,N-dimethylnitrosamine.

Metabolic activation of acetylenic substituents to derivatives in the rat causing the loss of hepatic cytochrome P-450 and haem

1. A number of acetylenic-substituted steroidal and non-steroidal compounds, including 2,2-dipropargylacetamide, pregna-2,4-dien-20-yno[2,3-d]isoxazol-17-ol (Danazol) and acetylene gas, when administered to rats in vivo brought about a decrease in the concentrations of hepatic microsomal cytochrome P-450 and haem. Abnormal haem-breakdown products, ;green pigments', and porphyrins accumulated in the livers of these animals. 2. For loss of microsomal cytochrome P-450 to occur in vitro, metabolic activation of the acetylenic substituent was necessary. The enzyme system responsible required NADPH and air, and was induced by pretreatment of rats with phenobarbitone; these are characteristics typical of the microsomal mixed-function oxidases. 3. When rats were dosed with 17alpha-ethynyl-17beta-hydroxyandrost-4-en-3-one (ethynyltestosterone, 1mmol/kg) the pattern of green pigments extracted from the liver 4h after dosing and separated by t.l.c. was quite different from that in rats given 17beta-hydroxy-17alpha-vinylandrost-4-en-3-one (vinyltestosterone), suggesting that reduction of the unsaturated triple bond to a double bond is not normally part of the metabolic activation pathway of the acetylenic substituent. 4. The green pigments extracted from the livers of rats 4h after the administration of the acetylenic-substituted compounds (1mmol/kg) when separated by silica-gel t.l.c. had variable R(F) values. The number and distribution of green pigments was characteristic for each compound examined. There was little correlation between the total loss of hepatic microsomal haem and the apparent intensity of the green pigments seen on the thin-layer chromatograms. 5. After incubation of [(14)C]acetylene in vitro with microsomal preparations from phenobarbitone-pretreated rats and a NADPH-generating system, no significant covalent binding to microsomal protein was detected over a 30min incubation period, although under similar conditions there was a significant loss of cytochrome P-450.