IB 486: Pesticide Toxicology
CARBAMATE INSECTICIDES
Physostigmine is a naturally occurring inhibitor of AChE, known to western medicine at least
since the 19th century. It was isolated from the calabar bean, Physostigma venenosum, in 1864,
and its structure was known by 1923. Physostigmine was the first drug against glaucoma and
myasthenia gravis; although it has mostly been superseded by synthetic derivatives,
physostigmine is still used to diagnose myasthenia.
Figure 1: Physostigmine
Figure 2: Generalized carbamate structure
The quaternary nitrogen compounds, including physostigmine and acetylcholine itself, have no
insecticidal activity, apparently because they cannot penetrate the lipid sheath of the insect nerve
(due to the formally charged quaternary nitrogen). RL Metcalf, R Fukuto, and their colleagues at
Riverside began with the structure of physostigmine, and tested related uncharged structures
(notably the isomeric dimethylaminophenyl N-methylcarbamates) to find potential insecticides.
Their structure-activity experiments also used information about the structure of AChE that
indicated that, in insects, approximately 5 A separate the anionic from the esteratic site. This
work was subsequently used by Bayer AG to develop the first commercial carbamate
insecticides. Thus the carbamate insecticides were the first class of insecticides derived from
deliberate considerations of structure-activity relationships.
Like the OPs, carbamates inhibit AChE by forming an enzyme/inhibitor complex at the esteratic
site of the enzyme. Like the OPs, the insecticidal carbamates have a "leaving group" that is
hydrolyzed during formation of the insecticide-enzyme complex. However, whereas the
OP/AChE complex "ages" by removal of a second portion of the OP molecule, AChE is
regenerated from the AChE/carbamate complex. As a result of this regeneration of native
enzyme, and concomitant destruction of a molecule of insecticide, recovery of AChE following
exposure to carbamates is far more rapid than following exposure to OPs. In fact, recovery of
the enzyme is so rapid that it is difficult to document AChE inhibition by carbamates with the
standard Ellman assay1. For the same reason, atropine - which prevents ACh from reaching its
post-synaptic receptor - is a far more effective antidote in most cases of carbamate exposure than
are oximes (like 2-PAM) which cleave the enzyme/insecticide complex. In fact, in the case of
carbaryl, the inherent toxicity of oximes outweighs their beneficial effects, and they are
contraindicated.
1
Since dilution is particularly conducive to regeneration of the enzyme, modifications to the standard
assay are necessary to prevent evidence of AChE inhibition from disappearing.
1
Procarbamates are a newer class of carbamates that contain an (often complex) substituent on the
carbamyl nitrogen instead of the H or CH3 present in simpler carbamates. The cleaving of the
complex group releases the active carbamate. In some cases, the cleaved moiety may itself be
pesticidal. Thus formamidine-carbamates combine the formamidine demethylchlordimeform,
with its strong acaricidal activity, with a classical carbamate (e.g., the phenolic moiety of
carbofuran). These compounds are sometimes more effective against both mites and insects
because of superior penetration of the arthropod cuticle. Moreover, procarbamates often have
lower acute mammalian toxicity than do their parent carbamates, primarily because they are
only slowly converted to the active carbamate, allowing for alternative metabolism in mammals.
Figure 5: Carbaryl or SevinTM
Figure 4: a procarbamate, carbosulfan
The carbamates as a class vary considerably in their acute toxicity, from the highly toxic aldicarb
to the quite benign carbaryl. Environmentally, they are similarly varied. Although far less
bioaccumulative than the older organochlorines, a few persist in water to an undesirable extent
(e.g., aldicarb, q.v.) Carbamates are also generally devoid of chronic or delayed health risks.
Other than mutagenesis/carcinogenicity, the exceptions are peculiar to specific compounds.
Carbaryl causes a transient form of delayed neuropathy (see below). Aldicarb has been suggested
to depress immune function, but the data are controversial and not well confirmed.
A generalized suspicion of carcinogenicity of this class of insecticides lies in their potential
interaction with nitrates and nitrites. Nitroso-derivatives of N-methyl carbamates can be formed
in acid environments (such as the mammalian stomach) in the presence of nitrates. Nitrates are
present in the dietary, both from foods (e.g., as preservatives in sausage) and in many drinking
water supplies in agricultural regions2. There has been concern that nitrosocarbamates formed in
situ could be carcinogens. The evidence for such risk is indirect, but substantial. First, the
nitrosocarbamates are mutagenic in bacteria and cause chromosome aberrations in rodent cells in
vitro. Second, they are formed at pH 3, the pH of the stomach. Injected subcutaneously,
nitrosocarbamates cause sarcomas in rats; applied to skin, they cause skin tumors in mice.
Perhaps most important, they cause stomach tumors in rats when administered by gavage. On
the other hand, in experiments in which the components were delivered by gavage (sodium
nitrite plus carbaryl), no increase in cancer was seen in treated rats or their offspring; oral
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For example, during the spring planting season, Danville often issues warnings that nitrates in the water
are high, and families with infants should use bottled water.
2
administration of nitrosocarbamates (nitrosocarbaryl, nitrosopropoxur) did not cause
chromosomal aberrations in rats or mice; and feeding of nitrate with parent carbamates did not
cause chromosomal aberrations in the bone marrow of rats. These results suggest that processes
of nitrosocarbamate carcinogenesis may be species-specific or depend on the exact site of
delivery (or generation) of the active molecules.3
Aldicarb
LD50: po, rats, 0.9 mg/kg (EPA); 0.46-1.23 (American Cyanamid); dermal, 5 mg/kg; 3.2 - >10
(American Cyanamid). po, birds: 3.4 mg/kg (domestic duck)
LC50 (not found)
Long-term, chronic or delayed effects: none identified with certainty. A decrease in immune
system function at low (but not higher) doses was found in one study and not confirmed
in a second; carcinogenesis assays were negative. There is no evidence of delayed
neuropathy or teratogenesis.
Persistence in water: soluble to 6,000 ppm; stable in acidic media but decomposed in alkaline
media. Persistent in groundwater.
Degradation at insecticidal levels: Under "summer" conditions (high light, heat, humidity) of the
Metcalf microcosm, aldicarb was still present in both water and fish at end of study.
Degradation at high levels: not explicitly examined?
Problems encountered in presumably proper and legal uses: persistence in groundwater led to
banning of aldicarb in Wisconsin, New York (Long Island) and Florida. Use in NY and
Wisconsin was in sandy soil, allowing easy percolation to groundwater; in Florida,
applications against nematodes required extremely high concentrations, and allowed
penetration to groundwater.
Aldicarb is transported into and through plants. When California farmers used
aldicarb (illegally) on land planted in watermelons, enough aldicarb was present in the
melons to make over 1000 people ill. Since the use was de facto illegal (farmers
maintained that aldicarb was carried over from applications to the previous - nonfood crop), accurate information on application rate, timing, etc are disputed.
A woman who ate mint leaves 24 days after treating soil around a nearby rose
plant, showed signs of AChE inhibition severe enough to require atropine.
3
In a situation like this, it is advisable for interested individuals to go back to the original literature and
review all the data critically, not trusting reviewers. One must assume that every paper published in this
area reflects the bias of the investigator and that every investigator has a bias (or underlying philosophy)
that affects data interpretation.
3
Carbaryl
LD50: po, rats, 250-350 mg/kg; dermal, 2,000 mg/kg po, birds: ??
LC50:
Long-term, chronic or delayed effects: A single study in dogs suggests that carbaryl causes
problems in delivery of the litter, and may also cause developmental toxicity. These
problems occurred at maternally toxic levels, and are not seen in hamster, rats or mice.
However, anecdotal evidence suggests that dogs may be susceptible to carbaryl-induced
problems of parturition after flea treatment4. Therefore, although carbaryl is unlikely to
be a developmental toxicant in humans, dog owners should be careful with its use on
pregnant bitches.
Carbaryl induces a syndrome similar to OPIDN after heavy or chronic exposure. The
syndrome can be induced in both humans and chickens. Unlike OPIDN, the weakness
and parasthesias do regress; recovery seems to be complete.
Persistence in water: brief.
Degradation at insecticidal levels: rapid
Degradation at high levels: not explicitly examined?
Problems encountered in presumably proper and legal uses: Carbaryl is extremely toxic to
honeybees and must not be applied when bees are visiting the crop. In the case of
orchards, spraying should occur in late afternoon or early evening to prevent loss of
hives.
4
Anecdotal in the extreme: conversation with owner of St Bernard, who reported several bitches had
problems giving birth after carbaryl treatment during a particularly flea-ridden summer.
4
Carbofuran
LD50: po, rats, 5.3-8 mg/kg; [human, 11 mg/kg]; dermal, rat, 120 mg/kg; 2,400; human, 10,000
mg/kg po, birds: 6 mg/kg, chicken; 0.415 mg/kg, duck; 0.1-0.4, wild bird (species not
identified); dermal wild bird, 100 mg/kg.
LC50:
Long-term, chronic or delayed effects: none identified.
Persistence in water: Under the "summer" conditions of the Metcalf microcosm, only traces of
carbofuran were identified in water or the organisms after 33 days.
Degradation at insecticidal levels: increases as bacterial populations in soil adapt to using
carbofuran as carbon source, to the point that its effectiveness against corn rootworms is
lost.
Problems encountered in presumably proper and legal uses: Lack of effectiveness after relatively
few years of use, due to increasing degradation by soil microbes. Because of its extreme
toxicity, accidents are likely to be serious.
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