Introduction to Xenobiotics

Study Notes

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Xenobiotics or toxicants or drugs or pollutants are substances foreign to the body that can cause changes in structure and functions of an organism
They get into the body from external sources (not produced within the body) with a few endogenous ones

Exogenous: foreign molecules not normally ingested or utilized by the organism, entering through dietary foodstuffs or in the form of certain medicines/drugs used for therapeutic cause or inhaled through environment
Endogenous: synthesized in the body or produced as metabolites of various processes in the body, having effects similar to exogenous xenobiotics

Allergy: an altered or abnormal tissue reaction, an exaggerated response by certain tissues to some foreign substances, occurring only in hypersensitive people
Allergic reaction results from contact between a foreign protein called allergen and body tissues that are sensitive to it
Allergy reactions to substances in food are sometimes spontaneous, peculiar to certain humans, and rarely demonstrable in lower animals

The gut is a major immunological organ containing about 10^10 lymphocytes
Substances surviving the effects of digestive juices and the protective effect of mucous secretion, including the effects of immunoglobulin antibodies, may still be able to penetrate the epithelial barrier of the gut

Biochemical reactions involved in the conversion of foreign, toxic, or water-insoluble molecules to non-toxic, water-soluble, and excretable forms
These reactions convert lipophilic to hydrophilic, detoxify, and induce metabolic activation, changing the solubility characteristics of compounds
The liver is the main organ involved in biotransformation or detoxification reactions due to the presence of various enzymes involved in detoxification processes

A situation where biotransformation reactions of some xenobiotics conversely increase the toxicity of these compounds
Example: methanol toxicity, where methanol is metabolized in the liver to formaldehyde and then formic acid, causing metabolic acidosis and damage to the optic nerve

Phase 1 reactions: oxidation, reduction, and hydrolysis reactions, introducing or exposing a functional group that serves as the active center or point of conjugation for sequential conjugation in a phase II reaction
Phase 2 reactions: conjugation reactions, making the products of phase 1 reactions more water-soluble and excretable, including glucuronidation, sulphuric acid ester synthesis, hippuric acid synthesis, glutamine conjugation, glutathione conjugation, acetylation, methylation, and thiocyanate synthesis

Pesticides: substances or mixtures of substances intended for preventing, destroying, repelling, or lessening the damage of any pest
Herbicides: substances or mixtures of substances intended for preventing, destroying, repelling, or lessening the damage of weeds

Classification based on mode of entry: systemic, contact, stomach poisons, fumigants, and repellents
Classification based on pesticide function and pest organism they kill: insecticides, herbicides, fungicides, rodenticides, and other less well-known pesticides### Classification of Pesticides
Pesticides can be classified based on their function, such as:
- Growth regulators (stimulate or retard pest growth)
- Defoliants (cause plants to drop leaves)
- Desiccants (speed up plant drying or kill insects)
- Repellents (repel pests)
- Attractants (attract pests, usually to a trap)
- Chemosterilants (sterilize pests)
Some pesticides can control multiple classes of pests and be classified into more than one category
Examples of multi-class pesticides include:
- Aldicarb (acaricide, insecticide, and nematicide)
- 2,4-D (herbicide and plant growth regulator)

Chemical composition is a useful method for classifying pesticides
Pesticides are classified into four main groups based on their chemical composition:
- Organochlorines
- Organophosphorus
- Carbamates
- Pyrethrin and pyrethroids
Modern pesticides are primarily organic chemicals, including synthetic and plant-based compounds
Some inorganic compounds are also used as pesticides