Organic Compounds - Contaminant Geochemistry - page 83

Fig. 4.5 General chemical structures of paraquat and diquat

produce a superoxide anion. This oxygen radical may directly or indirectly cause
cell death. Diquat, 1,1 0 -ethylene-2,2 0 -dipyridylium, is a charged quaternary
ammonium compound often found as the dibromide salt. The structure of diquat
dibromide and that of the closely related herbicide paraquat is shown in Fig. 4.5 .
Paraquat and diquat are nonselective contact herbicides that are relatively
widely used and highly toxic. Paraquat has life-threatening effects on the gastro-
intestinal (GI) tract, kidney, liver, heart, and other organs. The LD50 (lethal dose
to 50 % of the population) in humans is approximately 3-5 mg/kg. The lung is the
primary target organ of paraquat, and pulmonary effects represent the most lethal
and least treatable manifestation of toxicity (Pond 1990 ; Giulivi et al. 1995 ).
However, toxicity from inhalation is rare. Both types I and II pneumatocytes
appear to selectively accumulate paraquat. Biotransformation of paraquat in these
cells results in free-radical production with resulting lipid peroxidation and cell
injury (Pond 1990 ; Giulivi et al. 1995 ; Honore et al. 1994 ). There is a progressive
decline in arterial oxygen tension and CO 2 diffusion capacity. Such a severe
impairment of gas exchange causes progressive proliferation of fibrous connective
tissue in the alveoli and eventual death from asphyxia and tissue anoxia (Harsany
et al. 1987 ). Local skin damage includes contact dermatitis. Prolonged contact
produces erythema, blistering, abrasion and ulceration, and fingernail changes
(Tungsanga et al. 1983 ; Vale et al. 1987 ).
Following ingestion of the substance, the GI tract is the site of initial or phase I
toxicity to the mucosal surfaces. This toxicity is manifested by swelling, edema,
and painful ulceration of the mouth, pharynx, esophagus, stomach, and intestine.
With higher levels, other GI toxicity includes centrizonal hepatocellular injury,
which can cause elevated bilirubin, and hepatocellular enzyme levels such as AST,
ALT, and LDH.
Diquat poisoning is much less common than paraquat poisoning, so that human
reports and animal experimental data for diquat poisoning are less extensive than
for paraquat. However, diquat has severe toxic effects on the central nervous
system (CNS) that are not typical of paraquat poisoning (Vanholder et al. 1981 ;
Olson 1994 ).