Glyphosate: Mechanism in Plants and Toxicokinetics

General Description

Glyphosate, a widely used herbicide with unique physical and chemical properties, acts as a systemic, non-selective herbicide by inhibiting the EPSPS enzyme in the shikimate pathway. This inhibition blocks essential amino acid synthesis and disrupts secondary metabolite production, leading to impaired plant growth. In terms of toxicokinetics, glyphosate is rapidly but incompletely absorbed, with minimal accumulation in the body. It is primarily excreted unchanged, with a small portion metabolizing into AMPA. Glyphosate is mainly eliminated through feces and urine, with a half-life of 6 to 12 hours. Overall, glyphosate's mechanism in plants and toxicokinetic characteristics highlight its effectiveness as a herbicide and provide insights into its fate in the body.

Figure 1. Glyphosate

Overview

Glyphosate, a widely used herbicide, possesses distinct physical and chemical properties. It belongs to the organophosphorus compounds family and was initially synthesized in 1950, but its herbicidal properties were only discovered later and patented several decades afterwards. Chemically, glyphosate is a zwiterrion, featuring phosphonate, carboxylate, and amine functions. This unique structure enables glyphosate to chelate with trivalent and quadrivalent metals. The presence of a covalent bond between carbon and phosphorus atoms, characteristic of organophosphates, gives glyphosate specific chemical and physical characteristics. These include high adsorption, water solubility, and compatibility with other chemicals. Glyphosate's molecular polarity contributes to its high solubility in water but insolubility in organic solvents. Notably, glyphosate lacks a chromophore or fluorophore group, leading to no absorption in the ultraviolet region. It exhibits low ionization, low volatility, and high hydrophilicity. Due to these specific properties, complex analytical methodologies are required for the accurate detection and quantification of glyphosate. Its absence of UV absorption, low ionization, and other factors necessitate sensitive and precise techniques for analysis. In conclusion, glyphosate's physical and chemical properties, such as its zwiterrionic structure, high water solubility, and complex detection requirements, contribute to its effectiveness as a herbicide and the need for sophisticated analysis methods. ¹

Mechanism in Plants

Glyphosate, as a herbicide, has a distinct action mechanism in plants. It is a systemic herbicide, meaning that it is absorbed by the plant and then translocated throughout its system. Additionally, glyphosate is non-selective and post-emerging, indicating that it can target both weeds and germinated grass. The primary mode of action for glyphosate is the inhibition of the shikimate pathway, particularly the enzyme called 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). This enzyme is crucial for the synthesis of essential amino acids like tyrosine, tryptophan, and phenylalanine, which are necessary for plant growth. By inhibiting EPSPS, glyphosate effectively blocks the production of these amino acids, leading to the impairment of plant growth and development. Aside from hindering amino acid synthesis, glyphosate also disrupts the production of secondary metabolites such as lignin. Lignin plays a significant role in providing structural support to plants, and its reduction can further contribute to the overall negative impact of glyphosate on plant health. It is important to note that glyphosate is unique in its ability to inhibit EPSPS compared to other herbicides. This specificity in action contributes to its effectiveness as a herbicide against a broad range of plant species. In summary, glyphosate acts as a systemic, non-selective, and post-emerging herbicide by inhibiting the EPSPS enzyme in the shikimate pathway. This inhibition results in the blockage of essential amino acid synthesis and compromises the production of secondary metabolites, ultimately leading to the suppression of plant growth. ²

Toxicokinetics

Glyphosate, a widely used herbicide, exhibits specific toxicokinetics in the body. Understanding its absorption, distribution, metabolism, and excretion is essential in evaluating its potential impact on human and animal health. When glyphosate is administered orally, studies indicate that it is rapidly absorbed but incompletely, with only 20 to 30% of the dose being absorbed in rats. Higher doses of glyphosate result in lower oral absorption rates. In terms of skin absorption, only about 1 to 3% of glyphosate is absorbed. Once absorbed, glyphosate does not tend to accumulate in the body. After 7 days, only 1% of the absorbed dose remains in the body, indicating minimal accumulation. The highest concentrations of glyphosate are found in the small intestine, liver, kidneys, and bones. Glyphosate is poorly metabolized in both plants and animals. The majority of glyphosate is excreted unchanged, while approximately 1% undergoes hydrolysis, leading to the formation of aminomethylphosphonic acid (AMPA), the primary metabolite of glyphosate. Elimination of glyphosate primarily occurs through feces, with approximately 60 to 70% of the administered dose being excreted via this route in rats. Around 20 to 30% is rapidly eliminated through urine. Bile and lung excretion play minor roles in the elimination process. The half-life of glyphosate is estimated to be between 6 and 12 hours. Within 48 hours, the majority of glyphosate and its metabolites are excreted, and within 7 days, almost all of them have been eliminated from the body. In conclusion, glyphosate demonstrates a rapid but incomplete absorption, minimal accumulation, poor metabolism, and elimination primarily through feces and urine. These toxicokinetic characteristics provide important insights into the fate of glyphosate in the body. ²

Reference

1. Glyphosate. National Center for Biotechnology Information. 2024; PubChem Compound Summary for CID 3496.

2. Soares D, Silva L, Duarte S, Pena A, Pereira A. Glyphosate Use, Toxicity and Occurrence in Food. Foods. 2021; 10(11): 2785.