In this article we will discuss about:- 1. Classification of Organophosphate Insecticides 2. Mechanism of Toxic Action of OPIs 3. Absorption and Excretion 4. Metabolism 5. Reactions of Different Enzymes.
Classification of Organophosphate Insecticides:
OPIs are classified into two broad groups on the following basis:
1. On the basis of their activity
2. According to the manner in which OPIs exert their insecticidal action
1. On the Basis of Their Activity:
On this basis OPIs are grouped into two:
(i) Direct Action OPIs:
These compounds act by directly inhibiting the cholinesterase enzyme, e.g., Fenchlorvos, Dichlorvos, Disopropyl fluorophosphate (DFP), Tabun, Serin, Soman, Tetraethylpyrophosphate (TEPP), Mevinphos, Disulfoton, Chlorothion, Triazophos, Chlorpyriphos, Bidrin and Anilophos.
(ii) Indirectly Acting OPIs:
These insecticides undergo microbial conversion into metabolites which inhibit cholinesterase enzyme e.g.:
2. According to the Manner in which OPIs Exert their Insecticidal Action:
On this Basis OPIs are grouped into two:
(i) Contact Poisons:
These organophosphorus insecticides exert their insecticidal action like the chlorinated hydrocarbon insecticides and are called contact poisons e.g., paraxon, parathion, Malathion, EPN, methyl parathion.
(ii) Selective Systemic Insecticides:
Such compounds are absorbed into the sap of plants, remaining active and soluble for a reasonable period and are toxic to the plant pests but not to their predators e.g., mipafox, dimethoate, demethon schradan.
Mechanism of Toxic Action of OPIs:
The OPIs, in general, are less persistent than the organochlorines, a property due to which they are preferred to the organochlorines. In principle, OP compounds react with the active site of AChE (a serine hydroxyl group). In other words, all OP compounds are anticholinesterase agents i.e., they inhibit the enzyme cholinesterase (ChE) at the neuromuscular junction.
During normal muscle contraction, the cholinergic nerve fibers of autonomic system liberate acetylcholine (ACh) at the neuromuscular junction. The ACh excites muscle fibers and, consequently, muscle contractions take place. Very soon ACh is splitted by ChE present at the neuromuscular junction into choline and acetyl CoA.
Actually, splitting of ACh by ChE prevents its accumulation at the neuromuscular junction and thus assists in muscle relaxation after each contraction.
In its usual course of action, the ChE first gets acylated by losing one H+ and then fast recovers by using one molecule of metabolic HOH.
Actually, the OP compounds are structurally similar to ACh. At the enzyme site, they compete with ACh and bind with ChE, thus inhibiting its normal function. The OP compounds phosphorylate the ChE and form a stable complex. Ultimately the enzyme fails to recover quickly.
These events bring about an accumulation and, consequently, increase of ACh at the neuromuscular junction, which, in turn, respectively, excites muscle fibers and produces repetitive muscle contraction that causes tremors in the body muscles of the insect — and the insect finally dies of exhaustion.
Absorption and Excretion of OPIs:
Mostly, OPIs are highly lipid-soluble except few like ecothiopate, and are rapidly absorbed practically by all routes including GIT, skin, mucus membrane and lungs. Dermal absorption is highly influenced by the solvent used. The extent of absorption is more if these are applied in organic solvents.
Usually, the OPIs are excreted in milk, urine and bile. Some OPIs and their metabolites may cross the placenta and inhibit the fetal AChE enzyme, producing toxicity in the fetus.
Metabolism of OPIs:
OPIs basically undergo two types of metabolism — either activative or degradative. Activative metabolism converts a poor anticholinesterase to a stronger one, whereas degradative metabolism is the reverse.
It involves P = S to P = O conversion called desulfuration, hydroxylation (found only in case of phosphoramides), thioether oxidation, cyclization and microsomal activation.
It occurs primarily by hydrolytic routes, which leave an anionic group attached to the phosphorus, thereby reducing its positivity. It involves a number of enzymes such as phosphatases, carboxylesterases and amidases. The most common hydrolysis is by phosphatases which convert insecticides to their degradation products e.g.-
Sumithion → Ethyl sumithion
Dichlorvos → Dimethyl phosphate
Mevinphos → Dimethyl phosphate
Malathion → Demethyl Malathion.
Organophosphate containing carboxamide group are cleaved by amidases,
e.g.-
Dimethoate → dimethoate acid
Imidan → Phthalamic acid
In compounds containing carboxyester group cleavage occurs by carboxylesterase
e.g.- Malathion → malathion acid
Some other metabolic processes may also play a role in degradation of OPIs.
These reactions result in severe lessening of the positivity of phosphorus. The commonest mechanisms are hydrolytic which introduce an anion.
N-Dealkylation and N-Hydroxylation:
The N-derivatives of amino or amidic organophosphates differ considerably in their anticholinesterase activity. The removal or modification of N-substituents leads to activation, degradation, or neither. N-dealkylation is catalysed by hepatic microsomes and preceded by hydroxylation of the alkyl group.
Organophosphates undergo certain non- enzymatic reactions which alter the biological properties of these compounds in important ways. The most crucial among these reactions are hydrolysis and isomerization.
All the organophosphates can be hydrolysed at a rate directly related to the alkalinity and this reaction determines the half-life of an OPI.
For example-
Certain amino acids, hydroxamic acids, chlorine, inorganic phosphates, copper and molybdenum ions promote the hydrolysis of organophosphates.
Isomerization:
The properties of the isomers of certain organophosphates differ in salient ways from those of the parent compound.
e.g., S-ethyl parathion, an isomer of parathion and
S-alkyl Malathion, an isomer of Malathion.
These isomers are responsible for the direct antichlolinesterae activity, increased susceptibility to hydrolysis and increased potency against cholinesterase.
Reactions of Different Enzymes with OPIs:
i. Acetylcholinesterase:
Most organic phosphorus pesticides bind with the esteratic site on the AChE molecule. The reacting group of the esteratic site is the hydroxyl group of serine. The bond between phosphorus atom of OPI and the esteratic site of AChE is more stable than the bond between carboxyl carbon of ACh and the same enzyme site. Thus the phosphorylated enzyme is inhibited because its active site is occupied and, therefore, is incapable of hydrolysing ACh.
ii. Carboxylesterase:
Carboxylesterase present in plasma hydrolyse organophosphate compounds to products that are entirely inactive as cholinesterase inhibitors, thus reducing their toxicity. Thus OPIs serve as substrates for carboxylesterases and are detoxified by them.
iii. Inhibition of Other Enzymes:
Organophosphates phosphorylate several other enzymes including acid phosphatases, aliesterases, lipases, trypsin, chymotrypsin, succinoidase, ascorbic acid oxidase, dehydrogenase and sulphydryl enzymes. The reaction with these enzymes is generally slower than that with cholinesterases.
The potentiation of certain OPIs by others is partly due to inhibition of aliesterases.
Parathion decreases the activity of β-glucu-ronidase in liver and choline acetyl transferase in CNS which is responsible for the synthesis of ACh.
iv. Microsomal Enzymes:
OPIs inhibit a wide range of microsomal enzymes in contrast to CHIs which are microsomal enzyme inducers.
v. Transferases:
Organic phosphorus molecules may split not only by carboxyesterase and mixed function oxygenases but also by certain transferases including glutathion-s-alkytransferase and glutathion-s-aryl transferase.