Biotransformation (Metabolism) of Pesticides - Pesticide Biotransformation and Disposition - page 73

metabolism , nucleic acid metabolism , and the like. Such aspects, as they deal with the
processing of the normal endogenous constituents of the body and the effect of pesti-
cides on them, are dealt with in several chapters of the third edition of the Handbook
of Pesticide Toxicology ( Krieger, 2010 ). The word metabolism may also be used to desig-
nate the effect of an organism, through its enzymes, on the chemical structure of foreign
compounds now more often referred to as xenobiotics. These effects, also called bio-
transformation, are the subject of this chapter as they apply to pesticides. The enzymes
involved in these biotransformations are frequently referred to as xenobiotic-metabolizing
enzymes (XMEs).
Given the enormous literature on pesticide metabolism, it is no longer possible to
provide an exhaustive review of the subject. The more recent reviews and book chap-
ters referred to throughout are recommended as sources of more detailed and recent
information.
It should also be noted that pesticides not only are substrates for XMEs, but may
also act as inhibitors or inducers, in either case often with selectivity for specific iso-
forms. Inhibition and/or induction and interactions consequent to them are consid-
ered in Chapter 7.
EXTERNAL TRANSFORMATION
The finding of a derivative of a compound in the tissues or excreta of an animal is not
necessarily proof that the compound is the result of biotransformation in that organ-
ism. Compounds, especially in thin films, may undergo chemical change when exposed
to light or heat. As early as 1961, Mitchell (see Matsumura, 1975 ) reported the effects of
ultraviolet light on 141 pesticides, and Matsumura (1975, 1985) summarized the effects of
light and other physical factors on pesticides and their movement in the environment.
The rate and extent of the photochemical degradation of pesticides depend upon
the chemical nature of the pesticide, the wavelength of the light, and the presence of
other chemicals. These last may act as photosensitizers, forming reactive light-energized
intermediates that react with pesticides, or they may react with photoenergized pesti-
cides. The four best known types of photochemical reactions of aromatic pesticides are
ring substitution, hydrolysis, oxidation, and polymerization. Examples summarized by
Matsumura (1975) include the following: substitution of a ring chlorine in 2,4-D by
a hydroxyl group, hydrolysis of carbaryl, oxidation of parathion, and polymerization of
pentachlorophenol. A more recent review ( Stangroom et al., 2000 ) discusses the photo-
chemical and thermochemical transformation in water and soil of a number of pesticides,
including carbamate, organophosphorus (OP), and pyrethroid insecticides, as well as urea,
chlorophenoxy, and triazine herbicides. The reactions involved are often pH-dependent
and some may be catalyzed by metal and other ions. A more narrowly focused review
( Pehkonen and Zhang, 2002 ) concerns the degradation of OPs in natural waters.