The Organochlorine Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 105

A major, albeit slow, route of detoxication in animals is conversion to the water-
soluble acid p,p ′-DDA, which is excreted unchanged, or as a conjugate. In one study,
the major urinary metabolites of p,p ′-DDT in two rodent species were p,p ′-DDA-
glycine, p,p ′-DDA-alanine, and p,p ′-DDA-glucuronic acid (Gingell 1976). The route
by which p,p ′-DDA is formed remains uncertain. Early studies suggested that con-
version might be via p,p ′-DDD, but the later observation that this is a postmortem
process has cast some doubt on these findings. Some or all of the p,p ′-DDD found in
livers in these studies would have been generated postmortem because analysis was
carried out after a period of storage. Another possibility is that this process, similar to
dehydrochlorination, takes place via glutathione conjugation. After conjugation and
consequent loss of HCl, the DDE moiety, which remains bound to glutathione, may
undergo hydrolysis, leading eventually to deconjugation and formation of p,p ′-DDA.
A mechanism of this type has been proposed for the conversion of dichloromethane
to HCHO (Schwarzenbach et al. 1993, p. 514; Chapter 2, Figure 2.15 of this topic).
One other biotransformation deserving mention is the oxidation of p,p ′-DDT to
kelthane, a molecule that has been used as an acaricide. This biotransformation occurs in
certain DDT-resistant arthropods, but does not appear to be important in vertebrates.
Unchanged p,p ′-DDT tends to be lost only very slowly by land vertebrates. There
can, however, be a certain amount of excretion by females into milk or across the pla-
centa into the developing embryo (mammals) or into eggs (birds, reptiles, and insects).

5.2.3 e n v i r of n m e n T a l f a T e of f ddT

In discussing the environmental fate of technical DDT, the main issue is the per-
sistence of p,p ′-DDT and its stable metabolites, although it should be born in mind
that certain other compounds—notably, o,p ′-DDT and p,p ′-DDD—also occur in the
technical material and are released into the environment when it is used. The o,p ′
isomer of DDT is neither very persistent nor very acutely toxic; it does, however,
have estrogenic properties (see Section 5.2.4). A factor favoring more rapid metabo-
lism of the o,p ′ isomer compared to the p,p ′ isomer is the presence, on one of the
benzene rings, of an unchlorinated para position, which is available for oxidative
attack. p,p ′-DDD, the other major impurity of technical DDT, is the main component
of technical DDD, which has been used as an insecticide in its own right (rhothane).
p,p ′-DDD is also generated in the environment as a metabolite of p,p ′-DDT. In prac-
tice, the most abundant and widespread residues of DDT found in the environment
have been p,p ′-DDE, p,p ′-DDT, and p,p ′-DDD.
When DDT was widely used, it was released into the environment in a number of
different ways. The spraying of crops, and the spraying of water surfaces and land to
control insect vectors of diseases, were major sources of environmental contamina-
tion. Waterways were sometimes contaminated with effluents from factories where
DDT was used. Sheep-dips containing DDT were discharged into water courses.
Thus, it is not surprising that DDT residues became so widespread in the years after
the war. It should also be remembered that, because of their stability, DDT residues
can be circulated by air masses and ocean currents to reach remote parts of the
globe. Very low levels have been detected even in Antarctic snow!