Biotransformation, Detoxification, and Biodegradation

Some chemicals that are stored may remain in the body for years without exhibiting appreciable effects. One such chemical is DDT. Accumulation or buildup of free chemicals may be prevented until the storage sites are saturated. Selective storage limits the amount of foreign chemicals to be excreted, however. Since bound or stored toxicants are in equilibrium with their free forms, a chemical will be released from the storage site as it is metabolized or excreted. On the other hand, accumulation may result in illnesses which develop slowly, as exemplified by fluorosis and lead and cadmium poisoning. 

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As applied to hazardous wastes in the biosphere, distinguish among biodegradation, biotransformation, detoxification, and mineralization. 

The examples provided above give only a brief overview of the variety of enzymatic functions that alter, biotransform, and biodegrade xenobiotics. In many instances numerous enzymes are known, as in the case of the mixed function oxidases. In order to provide a concrete example of a system of detoxification enzymes that is widely distributed, we have chosen the organophosphate acid anhydrolases — enzymes that may aid in the understanding of organophosphate intoxication and may also provide a means for the detoxification and bioremediation of these materials. 

It has become clear over the past ten years that the 1980s demonstration of anaerobic microbial dechlorination of PCBs is probably the most important discovery in the field of PCB biotransformations since Ahmed and Focht first demonstrated in 1973 that PCBs were biodegradable. Many new anaerobic microbial activities have been enriched and characterized from anaerobic fresh water and marine environments and heavily polluted industrial sediments. These anaerobic cultures are capable ofdechlorinating PCBs, thereby transforming highly chlorinated Aroclors to lower-chlorinated mixtures. This natural attenuation process is an important contributor to PCB degradation and detoxification in the environment and can form the basis for intrinsic remediation of many PCB-contaminated sites. 

In this chapter we discuss some of our recent studies of several different groups of anaerobes and their biodegradative capabilities. The approach we take helps to distinguish between biodegradation and biotransformation of a compound. This distinction is important because biodegradation indicates complete mineralization and, hence, detoxification of a toxic compound. In contrast, biotransformation is only a partial metabolism of a compound in which one or a few biochemical reactions have taken place with the major structure of the original compound still intact. In some cases, biotransformation can produce metabolites with similar or even greater toxicity. 

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