Properties and Uses of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 19

reported from metabolism studies conducted in aerobic aquatic systems (Kennard
1996 ; Reeves and Mackie 1993 ). A half-life of 30 d was reported in an aqueous
photolysis study of CPY that was conducted under natural sunlight in sterile pH 7
phosphate buffered solution (Batzer et al. 1990 ). Data on the dissipation of CPY
from aquatic systems are summarized in SI Table A-5.
Field-scale analyses of runoff have demonstrated little potential for CPY to be
transported with runoff water (Racke 1993 ). Chlorpyrifos has been extensively
examined in field studies under varying conditions, including greater and lesser
antecedent soil moisture, incomplete and full canopy development stages, 2 h to 7 d
intervals between application and rainfall, maximum soil erosion conditions, differ-
ent soils properties, and a range of rainfall events up to a 1-in-833 year return fre-
quency (Cryer and Dixon-White 1995 ; McCall et al. 1984 ; Poletika and Robb 1994 ;
Racke 1993 ). Resulting concentrations of CPY in runoff ranged from 0.003 to 4.4%
of the amount applied (McCall et al. 1984 ; Poletika and Robb 1994 ). A field runoff
study conducted in Mississippi indicated that the majority of chemical mass was
transported in the dissolved chemical phase (Poletika and Robb 1994 ), while a
study conducted in Iowa under record high rainfall conditions concluded that the
majority of compound was transported attached to eroded sediment (Cryer and
Dixon-White 1995 ).

3 Toxicity of CPY

The primary mode of action of organophosphorus insecticides, such as CPY, is
well known and has been characterized in mammals (Testai et al. 2010 ) and in
aquatic organisms, particularly fish (Giesy et al. 1999 ). Chlorpyrifos inhibits the
enzyme acetylcholinesterase (AChE) in synaptic junctions of the nervous sys-
tem. As a result of this inhibition, acetylcholine accumulated in the synapse
causes repeated and uncontrolled stimulation of the post-synaptic axon.
Disruption of the nervous system that results is the secondary effect that causes
the death of the animal. The amino acid sequence of acetylcholinesterase is
highly conserved in animals, with the result that CPY is toxic to most groups of
animals, although differences in toxicokinetics (adsorption, distribution, metab-
olism, and excretion—ADME) account for differences in susceptibility among
taxa (Timchalk 2010 ).

3.1 Mechanism of Action

The mechanism of action (toxicodynamics) of CPY involves activation by biotic
transformation to CPYO, followed by covalent binding to the serine-hydroxyl in the
active site of the acetylcholinesterase molecule (Testai et al. 2010 ) (Fig. 4 ). While
this can occur in the environment (Mackay et al. 2014 ), in animals this reaction is