Polymorphism of human cytochrome P-450

Roles of cytochrome p450 in development

Cytochrome P450 (CYP) forms are ubiquitous in nature, appearing in almost all phyla, with many forms appearing in any organism. About 50 different forms have been identified in man, and some of these are found in the embryo, some showing temporal dependence. Many of the forms of cytochrome P450 present in one species have homologues in other species. For example, CYP1A2 is present in many species, including man, rabbits, rodents, fish and fowl. The amino acid sequence identity of these homologues is often in excess of 70%. CYP26, too, has more than 61% identity in amino acid sequence between fish, fowl and mammals. In view of the high degree of conservation of sequence as well as of enzymatic activities, it is only reasonable to assume that such strong conservation of sequence also reflects a conservation of function. Since the 'xenobiotic metabolizing' enzymes predate the production of the many xenobiotics they are known to metabolize, perhaps it is reasonable to consider endobiotics as natural substrates for their metabolism. Of the identified forms of cytochrome P450 that are present in embryonic tissue, we consider the possibility that they serve the organism in support of morphogenesis of the embryonic tissue. These forms may either function to generate morphogenic molecules or to keep regions free of them, thereby creating temporal and spatial regions of morphogen action and supporting region-specific changes in cells. One known morphogen, retinoic acid, has the enzymes retinal dehydrogenase (RALDH) and CYP26 maintaining its actions, the former responsible for its generation and the latter for its elimination. Another form of cytochrome P450, CYP1B1 appears also to be involved in differentiation of tissue, with its absence resulting in primary congenital glaucoma. However, the nature of the morphogen it may maintain still remains to be elucidated.

Soft drugs. 12. Design, synthesis, and evaluation of soft bufuralol analogues

In the search for more potent but still short-acting beta-blockers (BB), the methyl, ethyl, isopropyl, tert-butyl, cyclohexyl, 2-(1-adamantyl)ethyl, and methylthiomethyl esters of the acidic inactive metabolite of bufuralol were synthesized based on the "inactive metabolite" approach. The cleavage of the ester bond by blood and tissue esterases rapidly deactivates these compounds, resulting in an ultrashort duration of action. The beta-antagonist potencies and time courses of actions of the new "soft" BBs were characterized by recording ECG and intra-arterial blood pressure (BP) in rats. In the isoproterenol-induced tachycardia model, while bufuralol at an iv dose of 1 mg/kg (3.8 micromol/kg) diminished heart rate (HR) for at least 2 h, the effects of the soft drugs lasted for only 10-30 min at equimolar dose. The inactive metabolite did not decrease HR significantly. The first four members of this series of compounds showed the highest beta-blocking potencies, ranging between 25% and 50% of that of bufuralol. Next, the effects of these most active compounds on resting HR and BP were evaluated in comparison to esmolol. Infused for 10 min at a rate of 20 micromol/kg/min, esmolol decreased HR and mean arterial pressure (MAP) by 40% and 60%, respectively. The soft drugs at doses ranging only between 2 and 4 micromol/kg/min resulted in a 20-40% decrease in HR and a 30-50% reduction in MAP. However, the time courses of both the bradycardic and hypotensive effects of the soft drugs were superimposable to that of esmolol, diminishing within 60 min after the discontinuation of the infusions.

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.

Recent advances in the metabolism of cannabinoids

This review describes recent advances in the metabolism of cannabinoids. Cannabidiol was metabolized to cannabielsoin, 6 beta-hydroxymethyl-delta 9-tetrahydrocannabinol and an oxepine derivative through epoxide intermediates by hepatic microsomal enzymes containing cytochrome P450 of animals. Cannabidiol inactivated cytochrome P450 UT-2 (CYP2C11) not equal to in male rats and a member of 3A subfamily in mouse liver. These inactivations may be very important because serious drug-drug interactions will occur in the case that cannabidiol is co-administered with drugs which are metabolized mainly by the enzyme system containing these P450 isozymes. A member of cytochrome P450 belonging to 2C subfamily was the major isozymes responsible for the cannabinoid metabolism in many experimental animals and that of 3A subfamily made some contribution to the metabolism of cannabinoids by human hepatic microsomes. Microsomal aldehyde oxygenase, a particular isozyme of cytochrome P450 catalyzing the oxidation of 11-oxo-tetrahydrocannabinol to tetrahydrocannabinol-11-oic acid, was found for the first time by the authors. Cytochrome P450 MUT-2 (CYP2C29) is the major isozyme responsible for the microsomal aldehyde oxygenase activity in mouse hepatic microsomes.

Characterization of cannabidiol-mediated cytochrome P450 inactivation

Cannibidiol (CBD) has been shown to impair hepatic drug metabolism in several animal species and to markedly inhibit mouse hepatic microsomal delta 1-tetrahydrocannabinol (THC) metabolism by inactivating specific cytochrome P450s (P450) belonging to the 2C and 3A subfamilies. Elucidation of the mechanism of CBD-mediated P450 inhibition would be clinically very important for predicting its effect on metabolism of THC and the many other clinically important drugs known to be metabolized by P450s 2C and 3A. CBD-mediated inactivation of mouse hepatic microsomal P450s did not decrease hepatic microsomal heme content. However, [14C]CBD was found covalently bound to microsomal protein in an approximately equimolar ratio to P450 loss. Immunoprecipitation of microsomal protein with antibodies raised against either P450 2C or 3A revealed that approximately equal amounts of [14C]-CBD were bound to each of these P450s after CBD-mediated inactivation. Furthermore, this specific P450 binding was equivalent to P450 loss and accounted for nearly all of the microsomal [14C]CBD irreversible binding. Although > 80% of the enzyme activities attributed to P450s 2C and 3A were inactivated by CBD at the anticonvulsant dose of 120 mg/kg, P450 2C was approximately 3-fold more sensitive than P450 3A at the lower CBD doses tested. CBD analogs were synthesized in order to elucidate the chemical pathways for CBD-mediated P450 inactivation in vivo. Oxidations at allylic carbon positions or saturation of either the exocyclic double bond or both double bonds of the terpene moiety did not markedly affect the inhibitory properties of the analogs. Methylation of both phenolic groups of the resorcinol moiety completely blocked the P450-inhibitory properties of this analog, revealing the involvement of a free hydroxyl group in the inactivation process. Rotation of the resorcinol moiety in abnormal-CBD did not impair the inhibitory properties of the analog, suggesting that the position of the hydroxyl group relative to the terpene ring is unimportant. Further studies are required to fully understand the chemical basis of CBD-mediated P450 inactivation.

Effect of alpha-tocopherol on hepatic mixed function oxidases in hepatic ischemia/reperfusion

This study was done to determine the relationship between microsomal lipid peroxidation during hepatic ischemia/reperfusion and alteration in cytochrome P-450-dependent drug metabolism. Rats were pretreated with alpha-tocopherol to inhibit lipid peroxidation or with vehicle (soybean oil) and then subjected to 60 min no-flow hepatic ischemia in vivo. Control animals were time-matched sham-ischemic animals. After 1, 5 or 24 hr of reperfusion, liver microsomes were isolated and cytochrome P-450 and mixed function oxidases were studied. In vehicle-treated ischemic rats, serum ALT levels peaked at 5 hr (5,242 +/- 682 U/L) and were significantly reduced by alpha-tocopherol pretreatment (1,854 +/- 229 U/L, p less than 0.01). Similarly, microsomal lipid peroxidation was elevated in the vehicle-treated ischemic group, but this elevation was prevented by alpha-tocopherol pretreatment. Microsomal cytochrome P-450 content and aminopyrine-N-demethylase activity were both decreased in vehicle-treated ischemic rats to 60% and 70% of sham-ischemic control levels, respectively. Although alpha-tocopherol restored cytochrome P-450 content to the level of sham-ischemic control rats, aminopyrine-N-demethylase activity remained at 76% of control with alpha-tocopherol treatment (p less than 0.01 compared with sham-ischemic control). In contrast to what was seen with cytochrome P-450 and aminopyrine-N-demethylase, aniline p-hydroxylase activity was elevated in the vehicle-treated ischemic rats compared with sham-ischemic control rats. These increases were prevented by alpha-tocopherol pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential inhibition of individual human liver cytochromes P-450 by cimetidine

Cimetidine binds to cytochrome P-450 and inhibits hepatic metabolism of various drugs in humans. However, cytochrome P-450 is a family of enzymes rather than a single protein, and effects of cimetidine on individual human liver cytochromes P-450 have not been previously characterized. Metabolism of selected substrates and cimetidine-binding assays have been performed using human liver microsomes, purified human liver cytochromes P-450, and cytochrome P-450 complementary DNA-expressed yeast proteins to probe interaction of cimetidine with these individual enzymes. Cimetidine (3.0 mmol/L) in incubations reduced bufuralol hydroxylase activity by 80% and strongly inhibited microsomal nifedipine oxidation (23% +/- 13% of control activity). The same concentration of cimetidine produced intermediate inhibition of cytochrome enzymes responsible for ethoxyresorufin deethylation and aniline hydroxylation (77% +/- 6% and 68% +/- 17% of activity in control microsomal incubations, respectively), but little effect on tolbutamide hydroxylation was observed. Concordantly, the calculated binding constant for the binding of cimetidine to a purified cytochrome P-450 with high tolbutamide hydroxylase activity was 4.4 mmol/L, whereas the calculated binding concentration constant for a purified cytochrome P-450-metabolizing nifedipine was 0.7 mmol/L. These studies show a high variability in the effect of cimetidine on drug metabolism by individual human liver cytochromes P-450. In vitro studies using human liver microsomes and genetically engineered human cytochromes P-450 can be very useful in exploring important clinical questions of hepatic drug metabolism.

Ethnic differences in the pharmacokinetics of oral nifedipine

1. The pharmacokinetics of a single oral dose of 20 mg nifedipine, given as capsules, has been compared in five South Asian volunteers with data for 27 Caucasian volunteers. 2. The area under the plasma concentration-time curve (AUC) of nifedipine was three fold higher in South Asians (989 +/- 166 ng ml-1 h) than in Caucasians (323 +/- 116 ng ml-1 h). 3. The ratio of the AUC of nifedipine to that of the nitropyridine analogue, which is formed largely as a first pass metabolite, was significantly higher in South Asians (4.6 +/- 1.9) than in Caucasians (2.3 +/- 1.1) indicating a lower first pass metabolism in South Asians. 4. The terminal half-lives of nifedipine and the nitropyridine metabolite were significantly greater in South Asians than in Caucasians. 5. Consumption of a spicy curry diet for 3 days by Caucasians did not significantly affect the pharmacokinetics of a single oral dose of nifedipine. 6. The treatment of patients of South Asian origin with nifedipine should be initiated with lower doses than would be given to Caucasians.

Metabolism of debrisoquine and susceptibility to breast cancer

There may exist an association between the genetically determined oxidation status of the antihypertensive agent debrisoquine (DEB) and the propensity to develop tumours. The metabolism of DEB is extensive in 90% of healthy subjects (metabolic ratio = MR = 0-12.6; MR = % DEB excreted divided by % 4-hydroxy-DEB excreted) and poor in 10% (MR greater than 12.6). In patients with cancer of the lung, urinary bladder, and gastrointestinum, the percentage of high metabolizers is increased to greater than 98%. The poor metabolizer mode is almost devoid of cancer patients. It was investigated whether breast cancer patients show a similar association with respect to the oxidative status of DEB. 108 breast cancer patients and 123 women with benign gynecologic disorders received 1 tablet of 10 mg DEB orally in the evening. Urine was collected for the subsequent 8 hrs and analysed for its content of DEB and its main urinary metabolite 4-OH-DEB by means of HPLC. No decreased amount of poor metabolizers was seen in the cancer group.

Expression of cytochrome P-450 enzymes in cultured human hepatocytes

Hepatocytes from adult and newborn humans were put into primary culture and exposed to phenobarbital, 3-methylcholanthrene, or rifampicin, three well-known inducers of cytochrome P-450 in animals. The expression of four cytochrome P-450 enzymes (or groups of enzymes, namely P-450 IIIA, P-450 IIC8/9/10, P-450 IIE1, and P-450 IA2) was investigated. These enzymes were found to remain expressed during the period of culture studied. Treatment with the inducers for three days resulted in different responses, depending upon the inducer and the enzyme. Phenobarbital and rifampicin increased P-450 IIC8/9/10 mRNA transcripts and the corresponding protein, while 3-methylcholanthrene was ineffective. Both P-450 IIIA mRNA and protein were strongly induced by rifampicin. All of the hepatocytes were found to synthesize P-450 IIIA in response to rifampicin, as shown by immunoperoxidase staining. P-450 IIIA expression was not affected by phenobarbital and was decreased by 3-methylcholanthrene. P-450s IA2 and IIE1 decreased to 25-50% of the initial level during these cultures. P-450 IA2 and ethoxyresorufin O-deethylase activity (which is a monooxygenase activity related to P-450 IA family) were increased only by 3-methylcholanthrene and did not respond to the other inducers. P-450 IIE1 was not induced by any of these compounds. P-450 IIC8/9/10 and P-450 IIIA mRNA levels were also measured in human hepatocytes from one newborn. P-450 IIC8/9/10 was barely expressed in freshly isolated cells but increased dramatically with time in culture. P-450 IIIA transcripts were abundant in both freshly isolated and cultured cells derived from a newborn. These results clearly demonstrate that human hepatocytes continue to express cytochrome P-450 enzymes and respond to inducers in culture. This model system provides a useful approach for investigating the effects of drugs on maturation and expression of drug-metabolizing enzymes in human liver.

New concepts affecting the use of antiarrhythmic agents

Three underappreciated concepts having an important impact on the use of antiarrhythmic agents having Class I activity are discussed. These are stereochemical influences on antiarrhythmic action, the modulated receptor theory, and pharmacogenetics. The stereoisomers of some antiarrhythmic agents behave differently in terms of their potency, disposition, and antiarrhythmic action. For example, the enantiomers of both tocainide and mexiletine are cleared at different rates, and those of disopyramide have opposite effects on repolarization. The modulated receptor theory suggests that the affinity of antiarrhythmic drugs to bind to a specific receptor on or near the sodium channel depends on whether the sodium channel is open, resting, or inactivated. Study of the interaction between the state of the sodium channel and the differing actions of the antiarrhythmic agents have provided evidence for synergistic drug combinations. Pharmacogenetics relates to the differences observed in drug metabolism among individuals, which can result in variations of two- to fourfold in clearance and plasma concentration in some cases. There is still much to learn about Class I antiarrhythmic agents. These concepts should lead to a better understanding of their actions and increase their utility.

In vivo oxidative cleavage of a pyridine-carboxylic acid ester metabolite of nifedipine

The pharmacokinetics of the primary pyridine metabolite of nifedipine (2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinecarboxylic acid dimethylester) (M-0) and its [2H6]dimethylester analog ([2H6]M-0) were studied in male rats. A large, 5.8-fold deuterium isotope effect for the formation clearance of the monomethylester (M-1) was observed, which is strongly indicative for an oxidative reaction mechanism involving the abstraction of a hydrogen atom, presumably by cytochrome P-450. M-0 exhibited a high systemic blood clearance (104 +/- 27 ml/min/kg) (mean +/- SD) which was not significantly influenced by deuterium substitution (125 +/- 13 ml/min/kg). Its systemic clearance is presumably flow limited, and extrahepatic metabolism can be anticipated. The major metabolic pathway for M-0 in male rats seems to be a direct oxidation at the 2-methyl position and subsequently a rapid conversion of the unstable 2-hydroxymethyl-dimethylester to the lactone of the monomethylester (M-2), as has been shown by others in vitro. Non-oxidative ester cleavage of M-0 in our rats was negligible. Deuterium substitution of M-0 at the ester methyl groups induced "metabolic switching" in favor of the direct oxidation of M-0 to M-2.

Testing for bimodality in frequency distributions of data suggesting polymorphisms of drug metabolism--histograms and probit plots

1. The shape of histograms used to illustrate density distributions of indices of polymorphic drug metabolism was shown to be sensitive to the position of the cell divisions. 2. Non-linearity of the probit plot was shown not to indicate bimodality of the original density distribution. Computer simulation was used to generate examples of unimodal density distributions with curvilinear probit plots. 3. Using the same technique probit plots for bimodal density distributions were constructed. Some were shown to differ less from the probit plots of certain unimodal distributions than did the original density distributions. 4. The position of the antimode was shown not to coincide with inflections seen in the probit plots. 5. A new method for determining the linearity of probit plots is suggested.

Xenobiotic metabolism in human brain--presence of cytochrome P-450 and associated mono-oxygenases

The cytochromes P-450, a family of heme proteins, play an important role in the oxidation of drugs and carcinogens, as well as endogenous substrates. We report the presence of cytochrome P-450 and associated mono-oxygenase activity in human brain regions and their selective enrichment in the brainstem. Immunocytochemical studies on human medulla with antibodies raised to phenobarbital-inducible rat liver cytochrome P-450 indicate that the enzyme is primarily localized in the neuronal cell bodies and to a lesser extent in the axons. These observations indicate that the human brain could be involved in metabolism of xenobiotics and endogenous compounds, mediated through cytochrome P-450.

Induction of liver microsomal cytochrome P-450 isozymes by 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide

1. 1-(2-Chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide (RP 52028), an antagonist of peripheral type benzodiazepine binding sites, a potential anticonvulsant, has been shown to have an inducing effect on drug-metabolizing enzymes. 2. RP 52028 was administered orally at 20-800 mg/kg for 1 week, and enzymic activities were determined using a panel of substrates. Western blot analyses were performed using several specific polyclonal and monoclonal antibodies directed against cytochrome P-450 isozymes. 3. RP 52028 appears to be an inducer of cytochrome P-450 II B1 (P-450b) and related enzymic activities; i.e. benzphetamine, ethylmorphine and aminopyrine demethylation.

Developmental aspects of P450IIIA: prenatal activity and inducibility

We have used antiserum of defined specificity as well as a specific inducers and inhibitors of P450IIIA1(2) to determine the fetal occurrence and inducibility of this enzyme in rats. Apparently absent from uninduced fetal rat liver (or present in extremely low amounts) cytochrome P450IIIA1(2) becomes increasingly inducible as a function of gestational age. In adult rats, it is now apparent that there are at least two inducible members and one male-specific constitutive member of the IIIA subfamily. The ontogenesis of these enzymes from 2 weeks post partum to puberty has also been determined. The male-specific occurrence of P450IIIA2 subject to testosterone imprinting and maintenance has been proposed. Inconsistencies persist, however. Waxman et al. have proposed the perinatal occurrence in male and female rats with subsequent suppression in females, whereas others have not detected P450IIIA1(2) in uninduced perinatal rat liver. These differences remain unresolved and reflect the difficulties in defining the individual enzyme specificities for various substrates and of antiserum reactivity. Approaches recently applied to investigations of the IIB subfamily of cytochromes P-450 should contribute greatly to the elucidation of factors governing the ontogenesis of IIIA in rats and humans. Recently, cDNA probes capable of discriminating P450IIB1 and P450IIB2 (commonly referred to as P450s b and e, respectively) were utilized to discriminate the developmental regulation of these immune cross-reactive enzymes. cDNA probes specific for the constitutive and inducible P450IIIA enzymes should clarify the P450IIIA ontogeny in rats. However, in light of regulatory differences among the human and rat members of P450IIIA, it is apparent that the extrapolation of human biotransforming potential from results of animal models must be approached with great caution.

In vitro inhibition studies of tolbutamide hydroxylase activity of human liver microsomes by azoles, sulphonamides and quinolines

1. A number of compounds have been examined for their ability to inhibit tolbutamide hydroxylase activity in human liver microsomes (control value at a substrate concentration of 150 microM being 0.27 +/- 0.12 nmol min-1 mg-1 protein; mean +/- s.d.; n = 7). 2. IC50 (concentration of inhibitor producing 50% inhibition) values were determined for a range of sulphonamides, imidazoles and aminoquinoline compounds. The most potent inhibition was evident with the 1-substituted imidazole antimycotic drugs ketoconazole, clotrimazole and miconazole and the sulphonamide sulphaphenazole (IC50 values of 16.5, 2.5, 0.85 and 0.5 microM respectively). A number of compounds showed little or no inhibition of tolbutamide hydroxylase as judged by an IC50 of greater than or equal to 500 microM. 3. The Km value for tolbutamide hydroxylase was 125 microM and Vmax, 0.44 nmol min-1 mg-1 protein. All the substituted imidazoles examined in kinetic studies 1v vs 1s, Line-weaver-Burk plots) produced either non-competitive or mixed inhibition. The sulphonamides exhibited competitive inhibition, the Ki for sulphaphenazole being 0.22 microM. Primaquine showed mixed inhibition. Dixon plots confirmed the type of inhibition produced. 4. Although the competitive inhibition between some sulphonamides and tolbutamide is consistent with metabolism by the same isozyme of cytochrome P-450 it does not prove it and further studies with purified enzymes will be necessary to confirm this.

The pharmacokinetics of oral nifedipine--a population study

1. The pharmacokinetics and metabolism of nifedipine have been studied in a population of 59 young male volunteers following administration of a 10 mg capsule. 2. The area under the plasma concentration-time curves (AUC) for nifedipine demonstrated a skewed but not a bimodal distribution (mean 154 ng ml-1 h; range 54-306 ng ml-1h). 3. Calculation of the ratio of the AUC of nifedipine to the AUC of its nitropyridine metabolite did not separate those individuals with high AUC values of nifedipine from the remainder of the population. 4. Similarly there was no evidence for bimodality in the excretion of the major urinary metabolite. Those subjects with high plasma AUC values for nifedipine excreted similar amounts to the remainder of the population. 5. In contrast to a previous study using a 20 mg oral dose, there was no evidence of polymorphism in the pharmacokinetics or metabolism of nifedipine following a single 10 mg oral dose.

Selective inhibition of drug oxidation after simultaneous administration of two probe drugs, antipyrine and tolbutamide

The effects of sulphaphenazole, cimetidine and primaquine on the disposition of antipyrine and tolbutamide in healthy volunteers have been investigated. The model substrates were administered simultaneously in order more clearly to define any selective effects of the potential inhibitors. Sulphaphenazole produced a significant increase in the half-life of tolbutamide (7.10 to 21.50 h) and a corresponding decrease in its clearance (0.260 to 0.084 ml.min-1.kg-1). Clearance to hydroxytolbutamide (OHTOL) and carboxytolbutamide (COOHTOL) was also significantly decreased. In contrast, sulphaphenazole had no effect on the disposition of antipyrine. Administration of cimetidine did not significantly alter the disposition of either model drug. However, a 1.6-times higher dose of cimetidine did increase the half lives both of tolbutamide and antipyrine (6.21 to 9.04 h and 14.2 to 19.2 h, respectively) and decrease their clearance (0.226 to 0.148 and 0.50 to 0.31 ml.min-1 kg-1, respectively). Clearance to OHTOL and hydroxymethylantipyrine (HMA) was reduced. A single dose of primaquine had no demonstrable effect on tolbutamide disposition whereas the half-life of antipyrine was increased (12.1 to 15.0 h) and its clearance decreased (0.63 to 0.38 ml.min-1.kg-1). The partial clearance to HMA, 4-hydroxyantipyrine (OHA) and norantipyrine (NORA) was also significantly reduced. The two main inferences are first, that tolbutamide and antipyrine are metabolised by different forms of cytochrome P-450, and second that a battery of model substrates is needed to investigate the inhibitory effects of a drug in man.

Is the variational scope of individual sensitivity given enough consideration in evaluation of risks caused by toxicological substances in food?

The contemporary concept of evaluation of risks with humans caused by xenobiotics in food is critically reviewed. In this context it is pointed out that epidemiologic studies, in most cases, give little reliable evidence only. Mankind's risk factors are of a multi-dimensional composition, depending on genetic aspects and the differring health and nutrition statuses of the individuals. This leads to an enormously wide scope of variations in the individual sensitivity to xenobiotics. Contemporary knowledge leaves the question open, whether predisposed population groups are always sufficiently protected by the level of the permissible amount of xenobiotics.