Detoxification
When many toxic substances are introduced into the environment, they do not remain in their original form, but are transformed to other products by a variety of biological and non-biological processes. The chemicals and their transformation products are degraded, converted progressively to smaller molecules, and eventually utilized in various natural cycles, such as the carbon cycle. Toxic metals are not degraded but are interconverted between available and nonavailable forms. Some organic compounds, such as polychlorinated biphenyl (PCBs), are degraded over a period of many years to less toxic compounds, while compounds such as the organophosphate insecticides may break down in only a few hours.
The chemical transformations that occur may either increase (referred to as intoxication, or activation if the parent compound was nontoxic) or decrease (referred to as detoxification) the toxicity of the original compound. For example, elemental mercury, which has low toxicity, can be converted to methylmercury, a very hazardous chemical, through methylation. Parathion is a fairly nontoxic insecticide until it is converted in a living system or by photochemical reactions to paraoxon, an extremely toxic chemical. However, parathion can also be degraded to less toxic products by the process of hydrolysis.
In microbial degradation, the ultimate fate of the toxic chemicals may be mineralization (that is, the conversion of an organic compound to inorganic products), which results characteristically in detoxification; however, intermediates in the degradation sequence, which may be toxic or have unknown toxicity, may persist for a period of time, or even indefinitely. Likewise, since degradation pathways may contain many steps, detoxification may occur early in the degradation pathway, before mineralization has occurred. Detoxification may also be accomplished biologically through cometabolism, the metabolism by microorganisms of a compound that cannot be used as a nutrient; cometabolism does not result in mineralization, and organic transformation products remain. Studies have also shown that the structure and toxicity of many organic compounds can be altered by plants.
In modifying a toxic chemical, detoxifying processes destroy its actual or potential harmful influence on one or more susceptible animal, plant, or microbial species. Detoxification may be measured with the use of bioassays. A bioassay involves the determination of the relative toxicity of a substance by comparing its effect on a test organism with the conditions of a control. The scale or degree of response may include the rate of growth or decrease of a population, colony, or individual; a behavioral, physiological, or reproductive response; or a response measuring mortality. Bioassays can be used for environmental samples through time to determine detoxification of chemicals.
Both acute and chronic bioassays are used to assess detoxification. In an acute bioassay, a severe and rapid response to the toxic chemical is observed within a short period of time (for example, within four days for fish and other aquatic organisms and within 24 hours to two weeks for mammalian species). Detoxification of a chemical may be detected if there is a decrease in the observed toxicity of a test solution over the time of the acute test, indicating removal of the toxic chemical by degradation or other processes. Similarly, an increase in toxicity could indicate the formation of a more toxic transformation product.
Chronic bioassays are more likely to provide information on the rates of degradation, transformation, and detoxification of toxic compounds. Partial or complete life cycle bioassays may be used, with measurements of growth, reproduction, maturation, spawning, hatching, survival, behavior, and bioaccumulation.
Detoxification of chemicals should also be measured by toxicity testing that involves changes in different organisms and interactions among organisms, especially if the chemicals are persistent and stable and may accumulate and magnify in the food chain/web. Model ecosystems can be used to simulate processes and assess detoxification in a terrestrial-aquatic ecosystem. A typical ecosystem could include soil organisms, lake-bottom fauna, a plant, an insect, a snail, an alga, a crustacean, and a fish species maintained under controlled conditions for a period of time.
Most major types of reactions that result in transformation and detoxification of toxic chemicals can be accomplished either by biological (enzymatic) or by non-biological (nonenzymatic) mechanisms. Although significant changes in structure and properties of organic compounds may result from non-biological processes, the biological mechanism is the major and often the only mechanism by which organic compounds are converted to inorganic products. Microorganisms are capable of degrading and detoxifying a wide variety of organic compounds; presumably, every organic molecule can be destroyed by one or more types of microorganisms (referred to as the "principle of microbial infallibility.") However, since some organic compounds do accumulate in the environment, there must be factors such as unfavorable environmental conditions that prevent the complete degradation and detoxification of these persistent compounds. There are many examples where certain microorganisms have been identified as capable of detoxifying specific organic compounds. In some cases, these microorganisms can be isolated, cultured, and inoculated into contaminated environments in order to detoxify the compounds of concern.
The major types of transformation reactions include: oxidation, ring scission, photodecomposition, combustion, reduction, dehydrohalogenation, hydrolysis, hydration, conjugation, and chelation. Conjugation is the only reaction mediated by enzymes alone, while chelation is strictly nonenzymatic. Primary changes in organic compounds are usually accomplished by oxidative, hydrolytic, or reductive reactions.
Oxidation reactions are reactions in which energy is used in the incorporation of molecular oxygen into the toxic molecule. In most mammalian systems, a monooxygenase system is involved. One atom of molecular oxygen is added to the toxic chemical, which usually results in a decrease in toxicity and an increase in water solubility, as well as provides a reaction group that can be used in further transformation processes such as conjugation. Microorganisms use a dioxygenase system, in which oxidation is accomplished by adding both atoms of molecular oxygen to the double bond present in various aromatic (containing benzene-like rings) hydrocarbons.
Ring scission, or opening, of aromatic ring compounds also can occur through oxidation. Though aromatic ring compounds are usually stable in the environment, some microorganisms are able to open aromatic rings by oxidation. After the aromatic rings are opened, the compounds may be further degraded by other organisms or processes. The number, type, and position of substituted molecules on the aromatic ring may protect the ring from enzymatic attack and may retard scission.
Photodecomposition can also result in the detoxification of toxic chemicals in the atmosphere, in water, and on the surface of solid materials such as plant leaves and soil particles. The reaction is usually enhanced in the presence of water; photodecomposition is also important in the detoxification of evaporated compounds. The ultraviolet radiation in sunlight is responsible for most photodecomposition processes. In photooxidation, for example, photons of light provide the necessary energy to mediate the reactions with oxygen to accomplish oxidation.
Combustion of toxic chemicals involves the oxidation of compounds accompanied by a release of energy. Often combustion does not completely result in the degradation of chemicals, and may result in the production of very toxic combustion products. However, if operating conditions are properly controlled, combustion can result in the detoxification of toxic chemicals.
Under anaerobic conditions, toxic compounds may be detoxified enzymatically by reduction. An example of a reductive detoxifying process is the removal of halogens from halogenated compounds. Dehydrohalogenation is another anaerobic process that also results in the removal of halogens from compounds.
Hydrolysis is an important detoxification mechanism in which water is added to the molecular structure of the compound. The reaction can occur either enzymatically or nonenzymatically. Hydration of toxic compounds occurs when water is added enzymatically to the molecular structure of the compound.
Conjugation reactions involve the combination of foreign toxic compounds with endogenous, or internal, compounds to form conjugates that are water soluble and can be eliminated from the biological organism. However, the toxic compound may still be available for uptake by other organisms in the environment. Endogenous compounds used in the conjugation process include sugars, amino acid residues, phosphates, and sulfur compounds.
Many metals can be detoxified by forming complexes with organic compounds by sharing electrons through the process of chelation. These complexes may be insoluble or nonavailable in the environment; thus the toxicant can not affect the organism. Sorption of toxic compounds to solids in the environment, such as soil particles, as well as incorporation into humus, may also result in detoxification of the compounds.
Generally, the complete detoxification of a toxic compound is dependent on a number of different chemical reactions, both biological and non-biological, proceeding simultaneously, and involving the original compound as well as the transformation products formed.
Biomagnification; Chemical Bond; Environmental Stress; Incineration; Oxidation Reduction Reaction; Persistent Compound; Water Hyacinth
Resources
Books
Burnside, O. C. "Prevention and Detoxification of Pesticide Residues in Soils." In Pesticides in Soil and Water, edited by W. D. Guenzi. Madison, WI: Soil Science Society of America, 1974.
Dauterman, W. C., and E. Hodgson. "Chemical Transformations and Interactions." In Introduction to Environmental Toxicology, edited by F. E. Guthrie, and J. J. Perry. New York: Elsevier, 1980.
Rand, G. M. "Detection: Bioassay." In Introduction to Environmental Toxicology, edited by F. E. Guthrie, and J. J. Perry. New York: Elsevier, 1980.
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