Toxicity of Pesticides and the Ecosystem

Common Pesticide

Introduction

Chlorpyrifos is a pesticide that has been widely used in agriculture for many years. However, recent studies have shown that it can be harmful to human health, particularly for children. The pesticide works by attacking the nervous system of insects, causing them to become paralyzed and die. However, it can also have a similar effect on humans, causing neurological damage. Studies have found that children who are exposed to chlorpyrifos are more likely to experience developmental delays and problems with their IQ. As a result, the use of this pesticide is now being restricted in many countries (Gander, 2021; US EPA, 2022). While it is still used in some agricultural applications, it is important to be aware of the potential risks associated with chlorpyrifos exposure. A common pesticide used in the US is Eraser, which contains the chemical chlorpyrifos, the main active ingredient that blocks an enzyme and prevents messages from moving from one nerve cell to another in the targeted pest (Gander, 2021).

Four Cornerstones

The four cornerstones of xenobiotic pharmacokinetics are absorption, distribution, metabolism, and excretion. Each of these processes is affected in different ways by the pesticide chlorpyrifos.

Chlorpyrifos is absorbed through the skin and gastrointestinal tract. Once absorbed, it rapidly distributes to all tissues, with highest concentrations in the liver, kidney, and blood. Chlorpyrifos is then metabolized in the liver by oxidation and hydrolysis to form active and inactive metabolites (CDC, 2017). The active metabolites bind to cholinesterase enzymes, resulting in inhibition of acetylcholinesterase activity. This leads to accumulation of acetylcholine at nerve endings, which causes overstimulation of the nervous system and ultimately death of the targeted organism. Excretion of chlorpyrifos occurs primarily through the urine.

Metabolites

When chlorpyrifos is applied to crops, it is ingested by the pests that feed on the plants. Once inside the pest\'s body, the chlorpyrifos is metabolized into several different chemicals, including 3,5,6-trichloro-2-pyridinol (TCPy) and 2,5-dichloro-4-nitrophenol (DCNP). These metabolites are more toxic than the parent compound and can persist in the environment for long periods of time (National Pesticide Information Center, 2010). Studies have shown that DCNP is particularly harmful to aquatic life, and can reduce the lifespan of fish and amphibians (Gander, 2021). TCPy has been shown to be toxic to birds, and has been linked to reduced reproduction rates in certain populations (CCOHS, 2017). Given the toxicity of these metabolites, it is clear that care must be taken when using chlorpyrifos to ensure that it does not enter the environment where it can cause harm.

There are two cornerstone processes associated with CPF metabolism: oxidation and hydrolysis. Oxidation is catalyzed by a series of enzymes known as cytochrome P450s, which are found in liver cells. This process converts CPF into a more polar and less toxic compound called 3,5,6-trichloro-2-pyridinol (TCP). TCP is then excreted in urine. Hydrolysis is also catalyzed by enzymes, specifically carboxylesterases. This process breaks down CPF into two less toxic compounds, dichlorovinyl and pyridinol. Dichlorovinyl is then further metabolized into carbon dioxide and chlorine. Pyridinol is excreted in urine.

While both oxidation and hydrolysis play important roles in detoxifying CPF, they are not equally effective at all stages of life. Adults typically have higher levels of cytochrome P450s than children, leading to more rapid oxidation of CPF. In contrast, carboxylesterases are more active in children, resulting in higher rates of hydrolysis. This difference may partly explain why children are more susceptible than adults to CPF toxicity.

Thoughts on Toxicity