Pesticide Residues in Man - Pesticides: Evaluation of Environmental Pollution - page 510

15.8.2 Metabolic Processes
Microbial metabolism is a process by which a chemical is absorbed into the cell membrane
of the microbe. Enzymes within the microbe break down the chemical into smaller frag-
ments. Pesticides are generally degraded by microbes along with the material excreted
by the roots of plants. Bacteria account for 65% of the total biomass in soil, so they gener-
ally account for most degradation processes. Bacteria dominate the degradation process in
alkaline soils and water (pH > 5.5). Fungi dominate the degradation process in acidic soils.
The surrounding conditions will determine whether aerobic or anaerobic metabolism will
occur in the degradation of a pesticide. In soils, the first layer degrades the chemicals via
aerobic metabolism because air (O 2 ) flows over the surface. Below this layer, anaerobic
metabolism occurs because of the lack of oxygen (Linde 1994).
The toxicity of pesticides, as with that of any toxicants, involves the interaction of the
pesticide or one or more of its metabolites with a target macromolecule. Therefore, toxicity
cannot be thought of as a single defining molecular event. On the contrary, the expression
of a toxic endpoint is the final event in a cascade of events that begins with exposure. This
cascade involves absorption, distribution, metabolism to reactive metabolites, further dis-
tribution, and interaction with target molecules. At the same time, pesticides may be meta-
bolically detoxified or excreted or the potentially toxic lesion may be repaired. Factors that
affect any of these interactions may affect the ultimate expression of toxicity, although, in
general, metabolism and interaction with target molecules play the most important roles.
Thus, any of the factors that affect pesticide metabolism may affect the ultimate toxicity of
the pesticide (Hodgson 2001). On the other hand, the major processes affecting the fate or
persistence of a pesticide in the human body are governed by several factors, which play
an important role in the magnitude of the endpoint effect. For example, absorption of the
pesticide through the skin differs with age (Shah et al. 1987) and thickness of the skin of
the individuals (Feldman and Maibach 1974) as well as the chemical properties of the toxi-
cant and environmental factors. These factors may all affect the dermal absorption and the
rate of penetration of pesticides (Baynes and Rivier 2001).
In this respect, it may be convenient to mention the chemical group known as “persistent
organic pollutants (POPs).” These POPs have key characteristics in common, which cause
their ubiquitous distribution and accumulation in the environment and living organisms
including humans. Although many different forms of POPs may exist, both natural and
anthropogenic, POPs that are noted for their persistence and bioaccumulative characteris-
tics include many of the first-generation OC insecticides such as dieldrin, DDT, toxaphene,
and chlordane and several industrial chemical products or by-products, including PCBs,
dibenzo-p-dioxins (dioxins), and dibenzo-p-furans (furans). Many of these compounds
have been or continue to be used in large quantities and, due to their environmental per-
sistence, have the ability to bioaccumulate and biomagnify (Anonymous 2001). POPs may
have the following properties, generally (UNIDO 2003):
i. POPs are environmentally persistent compounds; they resist breakdown by natu-
ral processes, and in some cases, remain in the environment for decades.
ii. POPs are characterized by low water solubility and high fat solubility, which make
them bioavailable to mammals. They bioaccumulate exponentially up to the top of
the food chain. Some of these compounds, such as PCBs, may persist in the environ-
ment for long periods of time and may bioaccumulate by factors up to 70,000 fold.
iii. POPs are semivolatile compounds, enabling them to move long distances in the
atmosphere before deposition occurs. They are volatilized at warm temperatures