Mechanism of Action of DDT and DDT Analogues as Pesticides

Insects sprayed with DDT exhibit hyperactivity and convulsions consistent with the disruption of the nervous system by DDT. Many theories have been suggested for the toxic effect of DDT, and the exact mechanism is not known. A theory that appears to be plausible and has some experimental basis suggests that the DDT molecules are of the correct size to be trapped in the pores of the nerve membranes, which are thus distorted, allowing sodium ions to leak through and depolarize the nerve cell so that it can no longer transmit impulses. This theory states that the toxicity of DDT is not due to its chemical reactivity but rather to its size and geometry, which allow for the blockage of the pores of the nerve membranes. This theory is supported by the observation that a variety of quite different compounds that are stereochemically similar to DDT exhibit DDT-like activity.

This "special fit'' hypothesis is supported by the observation that the biological activity of methoxychlor analogues decreases rapidly when R in the following formula contains five carbon atoms or more.

Methoxychlor analogue; R is substituted for CH3

Methoxychlor analogue; R is substituted for CH3

Presumably the longer chain methoxychlor analogues are not able to fit into the nerve pores.

Insects are especially susceptible to DDT because it is readily absorbed through the insect cuticle. DDT is not appreciably absorbed through the protein skin of mammals and does not exhibit acute toxicity. This is why it could be safely applied directly to humans to kill head lice. If the polarity of the DDT analogue is increased by introducing —N02, —C02H, —C02CH3, and —OH substituents into the aromatic ring, the toxicity to insects is lost. This loss is probably due in part to decreased absorption through the exoskeleton and in part to decreased adsorption on the nonpolar nerve membrane.

Continue reading here: Chronic Toxicity of DDT and Related Compounds

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