Pesticide Toxicology Week 2: Organophosphate Insecticides a. Acute toxicity

History World War II nerve gases –Britain –Germany Gerhard Schrader TEPP as first commercial OP insecticide Post-WWII transfer of IG Farben chemicals to US –Parathion

Basics Esters of phosphoric acid Inhibit acetylcholinesterase (AChE) –Prevent dissociation of acetyl choline (ACh) into acetate and choline –Consequences: facilitation of transmission of nerve impulses across cholinergic synapses –All insecticidal action of OPs is due to inhibition of AChE Estimated 250,000 insecticidal structures

A brief pause to discuss transmission of neural signals across synapses

plasma membrane of axon ending of presynaptic cell synaptic vesicle membrane receptor for neurotransmitter plasma membrane of receiving cell

Structure of Acetylcholine

AChE dimer

Michaelis Complex Between AChE and ACh

Interaction of OP and AChE

General Structure of AChE-inhibiting OPs X - leaving group –X may be F –X may also be alkyl, aryl, O-alkyl, O-aryl, S-alkyl, or S-aryl R1 and R2 R1 and R2 may be alkyl, aryl, O-alkyl, O- aryl, N-alkyl, N-aryl 250,000 active OPs possible

TEPP: Tetraethyl pyrophosphate

Parathion-ethyl

Role of =S vs =O =S does not interact with AChE Conversion of =S to =O is quite rapid in mammals and insects, but not instantaneous =S provides a safety factor, especially in field exposures

Other Structural Variables Affecting Toxicity Electronegativity of leaving group –All else being equal, the more the leaving group pulls electrons from P, the more toxic the OP will be to mammals Length of alkyl chain for R1 and R2 –All else being equal, methyl (OCH 3 ) is less toxic to mammals than ethyl (OC 2 CH 5 ) –Isopropyl is even less toxic than methyl, but insecticidal activity begins to suffer

Other Structural Variables Affecting Toxicity

Clinical Toxicity of OPs Acute toxicity depends on % AChE inhibited Symptoms –Inhibition > 50% Bronchoconstriction Tremors increasing to convulsions Light sensitivity Salivation, nausea, vomiting, diarrhea –Inhibition > 70-80% is lethal Suffocation due to mucus production Exhaustion CNS effects?

Determinants of Inhibition Species –Fit of OP on AChE Insect and mammalian AChE differ in dimensions of active site Distance between esteratic, anionic sites Age and sex may influence toxicity Within species: –Dose –Route of administration Slower absorption through skin may reduce peak inhibition –Relative affinity of OP for AChE vs other ChEs chlorpyrifos

AChE Inhibition Depends on specific OP Results from interaction between –Affinity for AChE, ChE –Regeneration O-methyl >O-ethyl > O-isopropyl Regeneration may occur either in plasma and brain –Dichlorvos Degradation vs activation –Malathion is rapidly degraded in mammals, but not in insects –Aging isopropyl > O-methyl >/= O-methyl

Antidotes Atropine –Competes for binding sites on postsynaptic receptor Toxic in its own right Paralyzes muscles –Acts at any time –Large doses necessary 2-PAM –Regenerates free enzyme from OP-AChE complex Acts only before complex ages