The following points highlight the four factors that modify the toxicity of xenobiotic chemicals. The factors are: 1. Factors Pertaining to Xenobiotic Chemicals 2. Factors Pertaining to Exposure 3. Factors Pertaining to Surrounding Medium 4. Factors Pertaining to Organisms.
1. Factors Pertaining to Xenobiotic Chemicals:
(i) Physico-Chemical Properties:
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Actually the physico-chemical properties of a xenobiotic chemical such as solubility, vapour pressure, ionization, valency state, functional groups etc. affect the pharmacological as well as toxicological properties of any toxicant. For example, the toxicity of arsenic in trivalent (+3) state is greater than pentavalent (+5) state.
This is because the former i.e., +3 compounds, bind strongly with the sulph-hydryl (-SH-) groups of proteins resulting in enzyme inhibition, whereas the latter, i.e., +5 compounds, do neither bind to sulph-hydryl groups nor inhibit enzyme systems. However, +5 state compounds (arsenites) inhibit ATP synthesis by oxidative uncoupling of certain reactions. Also the +3 state compounds (arsenates) are easily soluble in water and, hence, are very toxic to animals.
The functional group present in xenobiotic chemical compound appears to be responsible to exert the degree of toxic action. The polar (hydrophilic) chemicals are not easily soluble in lipids. Therefore, these are incapable of crossing the membranous barriers and ultimately cannot easily reach the target sites for appropriate action.
However, non-polar or lipophilic substances are easily soluble in lipid and other organic solvents. These can readily penetrate the lipoprotein layers of the membranes, hence, these readily exert their potential toxic effects. It may thus be inferred that the toxicity of a xenobiotic compound is largely dependent on the chemical structure, valency state, type of functional groups, ionic characteristics and lipid solubility.
(ii) Dose (Concentration) of Xenobiotic Chemical:
Most commonly, the term dose is used to specify the amount of chemical/drug administered, usually expressed per unit body weight. In principle form, the toxic effects exerted by the xenobiotic chemical is directly related to its concentration at the target site. The lower doses of chemical cause less effects whereas high doses may cause pronounced effects. It may, therefore, be inferred that toxicity of xenobiotic chemical is directly proportional to its dose.
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(iii) Translocation of Xenobiotic Chemical:
Effective translocation is actually a key factory to exert toxic effects, especially for those toxicants which produce systemic effects. During the course of translocation, some of the xenobiotic chemicals interact with macromolecules and may be stored in certain inactive tissues i.e., storage depots. The inefficient translocation and storage of toxicants in an inactive form actually diminish their toxic potential, while their effective translocation to the target tissues/organs in an active form enhances their toxicity.
(iv) Biotransformation and Bio-Activation of Toxicants:
Biochemical modifications of the xenobiotic chemical molecules within the living cells may be termed as biotransformation. It actually reduces the toxic potential of the toxicants. In biotransformation, the parent toxic compound, after converting into the metabolites, form conjugate and are finally excreted from the body. This ultimately results in the decrease of toxic action of the toxicants. In some instances the metabolites prove more toxic than the parent toxic compound, consequently causing more toxicity and this process is referred to as bio-activation.
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If the biotransformation leads to conversion of lipophilic forms to hydrophilic forms, then the exertion of xenobiotic is enhanced, consequently toxicant may not proceed at the target sites to exert adverse or deleterious effects. Such processes ultimately reduce the toxicity of xenobiotics.
(v) Chemical Interaction:
The toxicity of xenobiotic in an animal may be increased or decreased by a simultaneous or consecutive exposure to another xenobiotic. If the combined effect is equal to the sum of the effect of each substance given alone, the interaction is considered to be additive, e.g., combinations of most organo-phosphorus insecticides on cholinesterase activity.
If the combined effect is greater than the sum, the interaction is considered to be synergistic and the phenomenon is denoted by synergism. In simple form, synergism is a phenomenon in which the combined effect of two compounds is more than the sum of the individual effects.
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For example, carbon tetrachloride and ethanol on the liver. The term potentiation is used to describe the situation in which the toxicity of a substance on an organ is markedly increased by another substance that alone has no toxic effect on that organ. For example, Isopropanol has no effect on the liver, but it can increase considerably the hepatoxicity of carbon tetrachloride.
The exposure of an organism to a xenobiotic may reduce the toxicity of another. Chemical antagonism denotes the situation wherein a chemical reaction takes place and produces a less toxic product, e.g., chelation of heavy metals by dimercaprol. Functional antagonism exists when two chemicals produce opposite effects on the same physiologic parameters, e.g., the counteraction between CNS stimulants and depressants.
Competitive antagonism exists when the agonist and antagonist act on the same receptor, e.g., the blockade of the effects of nicotine on ganglia by ganglionic blocking agents. Non-competitive antagonism exists when the toxic effect of a chemical is blocked by another with a non-specific action, namely, not occupying the same receptor, e.g., the reduction of the toxicity of cholinesterase inhibitors by atropine.
Mechanism of Chemical Interaction:
Chemical interactions are achieved through a variety of mechanisms. For example, nitrites and certain amines can react in the stomach to form nitrosamines, the majority of which are potent carcinogens, and thus greatly increase the toxicity. On the other hand, the action of many antidotes (an agent that neutralizes the poison or counteracts its effect) is based on their interaction with the toxicants; for example, thiosulphate is used in cases of cyanide poisoning.
Furthermore, a chemical may displace another from its binding site on plasma protein and thereby increases its effective concentration. A chemical may modify the renal excretion of weak acids and weak bases by altering the pH of urine. Competition for the same renal transport system by one chemical can hinder the excretion of another.
An important type of interaction involves the binding of chemicals with their specific receptors. An antagonist (a chemical which opposes or resists the action of other) blocks the action of an agonist (a chemical that initiates a pharmacological response after binding with a receptor) such as a neurotransmitter or a hormone, by preventing the binding of the agonist to the receptor.
The concepts that have been proposed to explain their effects include:
i. There are accessory receptor sites for the competitive antagonists, and an allosteric interaction exists between agonist and antagonist, each binding to its own receptor site on the receptor molecule. The binding of an agonist excludes the binding of an antagonist to the same receptor, and vice versa.
ii. The receptor occurs in two forms, the activated and non-activated, which are in dynamic equilibrium. The agonists stabilize the receptor in the activated form, whereas the antagonists stabilize the receptors in non-activated form.
Another important type of interaction results from alterations of the biotransformation of a chemical by another. Some chemicals are inducers of microsomal and non-microsomal enzymes. The common inducers include phenobarbital, 3-methyl- cholanthrene (3-MC), PCB, DDT, etc. The inducers may lower the toxicity of other chemicals by accelerating their detoxication.
For example, pretreatment with phenobarbital shortens the sleeping time induced by hexa-barbital and the paralysis induced by zoxazolamine. Such pretreatment also reduces the plasma level of aflatoxins. In addition, 3-MC pretreatment greatly reduces the liver necrosis produced by bromobenzene, probably by increasing the activity of the epoxide hydrase.
2. Factors Pertaining to Exposure:
Toxicity of xenobiotics is greatly affected by various factors pertaining to their exposures. The contact reaction between the organism and the xenobiotic is called exposure.
Salient factors associated to exposures are:
(i) Exposure routes
(ii) Exposure duration, and
(iii) Exposure systems.
The route of exposure may affect kinetic factors such as absorption, distribution, biotransformation, and excretion; and may ultimately determine the toxicity of-a xenobiotic. The xenobiotics may enter the body by dermal, oral, inhalation or by intraperitoneal, intramuscular, subcutaneous and intravenous injections. Actually the route of exposure determines the extent of toxicity of xenobiotics.
For example, any xenobiotic produces more rapid and maximum effect when incorporated by intravenous route. It is because through this route the xenobiotic directly reaches to the target sites in active form and thus exerts greatest effects.
The rate of penetration of xenobiotics through injection follows in given order:
Intravenous injection > Intramuscular injection > intracutaneous injection
Overall the exposure routes in descending order (except intravenous which stands first) of toxicity are:
Inhalation > Intraperitoneal > Subcutaneous > Intramuscular > Oral > Topical.
Further, the rate of penetration of xenobiotics may be summarised as:
a. State of Xenobiotic:
Gaseous > Liquid > Solid
b. Xenobiotic Particle Size:
0.2 nm or less > 0.3 nm > more than 0.3 nm
c. Polarity:
Polar > Apolar
d. Solubility:
Hydrophilic > Lipophilic
e. Surface Area and Vascularity:
Large surface area and vascularity > Less surface area and vascularity
With carrier solvent > without carrier solvent
The frequency of exposure may also affect toxicity. Toxic effects can be produced by acute (short-term) or chronic (long-term) exposure to xenobiotics. There are also exposures of intermediate duration, known as subchronic exposures. The acute toxic values in terms of LC50 or LD50 of xenobiotics decrease with increase in duration of exposure. In other words, the exposure for short-term has less effect in comparison to long-terms exposures.
It very simply denotes that the toxicity of xenobiotics increases with increase in exposure duration. Threshold concentration of a xenobiotic is important in toxicity. Xenobiotics elicit an adverse effect only if the organism is exposed to a high enough concentration. At a low concentration, a minimal effect or no effect may result from exposure.
Types of exposure systems affect the toxicity of xenobiotics to certain extent. A variety of exposure systems may be selected for the exposure of toxicants.
For instance, in aquatic medium, various exposure systems are used for xenobiotics, such as:
(a) Renewal System:
Where the toxicant solution is renewed after certain interval.
(b) Static System:
Where toxicant is mixed in the water and the organisms are exposed to the toxicant in still water.
(c) Recirculatory System:
Where the toxicant solution is recirculated through certain pumps.
(d) Flow Through System:
Where the toxicant solution flows into and out of test chamber intermittently or continuously.
3. Factors Pertaining to Surrounding Medium (Environmental Factors):
Any factor of the external environment that is involved in the bioavailability of the xenobiotic affects its toxicity in the organism. For instance, DO, pH temperature salinity hardness of water, and dissolved solids in water would affect toxicity of a xenobiotic.
Salient environmental factors that affect toxicity are:
Temperature of the environment can alter the toxic effect. For example, the toxicity of strychnine, nicotine, atropine, malathion, and sarin is increased in animals exposed to cold temperature. However, the toxicity of parathion and another organo-phosphorous insecticides is reduced by hypothermia. Further, colchicine and digitalis are more toxic to the rat than to the frog. But their toxicity to the frog can be increased by raising the environmental temperature. The duration of the response, however, is shorter when the temperature is higher.
The effect of environmental temperature on the magnitude and duration of the response is apparently related to the temperature-dependent biochemical reactions responsible for the effect and for the biotransformation of the xenobiotics.
In aquatic medium, the temperature is expected to affect greatly the toxicity of xenobiotics. The increased water temperature increases the solubility of many xenobiotic chemicals. Several pesticides have been observed to be more toxic at increased temperatures while some others present stronger lethal action at low temperatures.
Sanders and Cope (1966) reported that DDT was three times more toxic against cladocerans at 10°C than at 27°C.
Interest in the effect of barometric pressure on the toxicity of chemicals stems from human exposure to them in space and is saturation diving vehicles. At high altitudes, the toxicity of digitalis and strychnine decreases whereas that of amphetamine is increased. The influence of changes in barometric pressure on the toxicity of chemicals seems attributable mainly, if not entirely, to the altered oxygen tension rather than to a direct pressure effect.
Whole body irradiation increases the toxicity of CNS stimulants but decreases that of CNS depressants. However, it has no effect on analgesics such as morphine.
The effects of toxicants often show a diurnal pattern that is mainly related to the light cycle. In the rat and the mouse, the activities of cytochrome P-450 are the greatest at the beginning of the dark phase.
Caging can affect the LD50 of a chemical in several ways. For example, the LD50 of isoproterenol was less than 50 mg/kg in rats caged individually, whereas it was about 800 mg/kg in rats caged in groups of ten. However, the LD50 values of most chemicals are only slightly affected, if at all, by this factor. The type of cage (mesh versus solid) and the type of litter material can also affect the reaction of the animals of toxicants.
A higher relative humidity may increase the acute toxicity, resulting in a lower LD50 dose.
Oxygen is essentially required for respiration to all biota. Increase in temperature, as an example, decreases the dissolved oxygen content of the water. Reduction in DO content of water imposes stress on the aquatic organisms, which may greatly enhance the toxicity of a xenobiotic.
Toxicity of certain xenobiotics is greatly affected by pH of the water. For example, cyanide toxicity. Appreciable proportion of ionic cyanide i.e., CN–, occurs at pH 8.5 whereas the molecular form i.e., HCN, predominates at 5.0 – 60 pH. The un-dissociated form of cyanide i.e., HCN is twice as toxic as the ionic form. This clearly indicates that changes in pH of water may greatly affect the toxicity of cyanide in particular and certain other xenobiotics viz., NH3 etc. in general.
In general, salinity does not significantly affect the toxicity of xenobiotics. However, a particular species may be more affected with change in water salinity. For example, toxicity of Zn and NH4Cl to rainbow trout and Atlantic salmon decreases with an increase in salinity.
The total hardness of water has little effect on the toxicity of most of the xenobiotics, however, metal toxicity is greatly affected. Many heavy metals are more toxic to fishes and other aquatic organisms in soft water in contrast to the hard water.
4. Factors Pertaining to Organisms:
Actually the factors associated to the organisms are of great significance in the study of toxicity of xenobiotics.
The salient toxicity modifying factors pertaining to organisms are:
Differences of toxic effect from one species to another have long been recognized. Knowledge in this field has been used to develop, e.g., pesticides, which are more toxic to pests than to humans and other mammals. Among various species of mammals, most effects of toxicants are similar.
This fact forms the basis of predicting the toxicity to humans from results obtained in toxicologic studies conducted in other mammals, such as the rat, mouse, dog, rabbit and monkey. There are, however, notable differences in detoxification mechanisms.
Even in different individuals of same species, the toxicity of chemical varies because of variation in susceptibility owing to certain genetic factors. It can, therefore, be concluded that certain individuals of a species may be susceptible to a chemical whereas the others may be resistant to the same chemical.
As an example, among various fish species, the toxicity of a xenobiotic may vary. Spear and Pierre (1979) observed that salmonids and minnows are approximately 15 times more susceptible to copper than that of sunfish. Pickering et al (1962) found 6 to 90-fold variations in the sensitivity to organophosphorus pesticides between sensitive (bluegills and guppies) and tolerant species (fathead minnows and gold fish).
(ii) Sex, Hormonal Status and Pregnancy:
The toxicity of xenobiotics differ with respect to sexes because the male and females differ in their responses due to hormonal and metabolic differences. Actually male and female individuals of the same strain and species usually react to toxicants’ similarly. There are, however, notable quantitative differences in their susceptibility, especially in the rat. For example many barbiturates induce more prolonged sleep in female rats than in males.
The shorter duration of action of hexobarbital in male rats is related to the higher activity of the liver microsomal enzymes to hydroxylate this chemical. This higher activity may be reduced by castration or pretreatment with estrogen. Similarly, in male rats, demethylate aminopyrine and acetylate sulfanilamide metabolize faster than females, and the males are thus less susceptible.
Female rats are also more susceptible than the males to such organophosphorus insecticides as azinphosmethyl and parathion. Castration and hormone treatments may reverse this difference. However, unlike hexobarbital, parathion is metabolized more rapidly in the female rat than in the male. This faster metabolism of parathion results in a higher concentration of its metabolite, paraoxon, which is more toxic than the parent compound.
This higher toxicity resulting from greater bioactivation in female rats, compared to males, is also true with aldrin and heptachlor, which undergo epoxidation. The female rat is also more susceptible to warfarin and strychnine. On the other hand, male rats are more susceptible than females to ergot and lead.
Differences in susceptibility between the sexes are also seen with other xenobiotics. For example, chloroform is acutely nephrotoxic in the male mouse but not in the females. Castration or the administration of estrogens reduces this effect in the males, and treatment with androgens enhances susceptibility to chloroform in the females.
The greater susceptibility of male mice was explained on the basis of a much higher concentration of cytochrome P-450. Nicotine is also more toxic to the male mouse, and digoxin is more toxic to the male dog. However, the female cat is more susceptible to dinitrophenol and the female rabbit is more to benzene.
Imbalances of non-sex hormones may also alter the susceptibility of animals -to toxicants. Hyperthyroidism, hyperinsulinism, adrenalectomy, and stimulation of the pituitary-adrenal axis have all been shown to be capable of modifying the effects of certain toxicants.
There is some evidence that the pregnant rat is more susceptible to the carcinogenic activity of ethylnitrosourea. Highly malignant tumours, apparently of trophoblastic origin, developed in these animals and were rapidly fatal.
It has been considered that neonates and very young animals, in general, are more susceptible to toxicants such as morphine. For a great majority of toxicants, the young are 1.5 -10 times more susceptible than adults.
The available information reveals that the greater susceptibility of the juvenile or young animals too many toxicants may be attributed to deficiencies of various detoxication enzyme systems. Both phase I and phase II reactions of biotransformation may be responsible. For example, hexobarbital at a dose of 10 mg/kg induced a sleeping time of longer than 360 min in 1-day-old mice compared to 27 min in the 21- day-old.
The proportion of hexobarbital metabolized by oxidation in 3 hours in these animals was 0% and 21-33%, respectively. On the other hand, chloramphenicol is excreted mainly as a glucuronide conjugate. When a dose of 50 mg/ kg was given to 1-or 2-day-old infants, the blood levels were 15 μg/ml or higher over a period of 48 hours. In contrast, children aged 1-11 years maintained such blood levels for only 12 hours.
However, not all xenobiotics are more toxic to the young. Certain chemicals, notably CNS stimulants, are much less toxic to neonates. Lu et al (1965) reported that the LD50 of DDT was more than 20 times greater in newborn rats than in adults, in sharp contrast to the effect of age on Malathion.
This insensitivity to the toxicity of DDT may be significant in assessing the potential risk of this pesticide, because of one very much larger intake in young babies via breast feeding and cow’s milk, especially on the unit body weight basis.
The effect of age on the Susceptibility to other CNS stimulants, including other organochlorine insecticides, appears less marked (generally in the range of 2-10 times). Most organophosphorus insecticides are more toxic to the young; Schradan (octamethyl pyrophosphoramide) and phenyl- thiourea are notable exceptions. Pandey and Shukla (1982) reported that fries and fingerlings of fishes are most sensitive stages for toxicants.
Old animals as well as humans are also more susceptible to certain xenobiotics. This problem has not been studied as extensively as in the young. However, the available evidence reveals that the aged patients are generally more sensitive to many drugs. The possible mechanisms include reduced biotransformations and an impaired renal excretion. In addition, the distribution of xenobiotics in the body may also be altered because of increased body fat and reduced body water.
The toxicity of chemicals to organisms is affected by health and nutritional status. The biotransformation of toxicants is catalyzed by the microsomal mixed-function oxidase system (MMFO). A deficiency of essential fatty acids generally depresses MMFO activities.
This is also true with protein deficiency. The decreased MMFO has different effects on the toxicity of xenobiotic chemicals. For example, hexobarbital and aminopyrine are detoxicated by these enzymes and are thus more toxic to male rats and mice with these nutrient deficiencies.
On the other hand, the toxicities of Alfa-toxin, carbon tetrachloride, and heptachlor are lower in such animals because of depressed bioactivation of these toxicants. Rats fed low-protein diets were 2-26 times more sensitive to a variety of pesticides. MMFO activities are decreased in animals fed with high levels of sugar.
Many carcinogenesis studies have revealed that restriction of food intake decreases tumour yield. Deficiency of protein generally lowers tumourigenicity of carcinogens, such as aflatoxin B1 and dimethylnitrosamine. The importance of diet on carcinogenesis is further demonstrated by the fact that rats and mice fed diets rich in fats have higher tumour incidences compared to those that are given a restricted diet.
Vitamin A deficiency depressed the MMFO. In general, this is also true with deficiencies of ascorbic acid i.e., vitamin C and also E. But thiamine deficiency has the opposite effect. Vitamin A deficiency, in addition, boosts the susceptibility of the respiratory tract to carcinogens.
Some foods contain appreciable amounts of xenobiotic chemicals that are strong inducers of the MMFO, e.g., safrole, flavones, xanthines, and indoles. . In addition, potent inducers such as DDT and polychlorinated biphenyls (PCBs) are present as contaminants in various foods.
The liver is the principal organ wherein biotransformation of xenobiotic chemicals takes place. Such diseases as acute and chronic hepatitis, cirrhosis of the liver, and hepatic necrosis often decrease the biotransformation of xenobiotics. The microsomal and non-microsomal enzyme systems as well as the phase II reactions may be involved.
Renal diseases may also affect the toxic manifestations of xenobiotics. This effect stems from disturbances of the excretory and metabolic functions of the kidney. Heart diseases, when severe, can increase the toxicity of chemicals by impairing the hepatic and renal circulation, thus affecting the metabolic and excretory functions of these organs.
The toxicity of xenobiotics is also affected by the size of the animal. Often larger size animals are more resistant to toxicants and this has been found true m case of certain tropical fresh water fishes.
The animals acclimated to sublethal levels of toxicants may become more tolerant or more weakened, depending upon the mode of action of toxicants and potential of the bio-transformational mechanisms of the animals.