Feature image: Movement of xenobiotics in our body
First, let’s learn a new term — xenobiotic. A xenobiotic is a compound that is foreign to an organism, i.e., it does not play a role in the organism’s normal functioning. A compound that is native to one organism can be foreign to another. For example, plants make chemicals that are xenobiotic and toxic to animals.
Pollutants are often xenobiotic to all organisms. So what happens when we (or any other organism) gets exposed to a xenobiotic?
Uptake: Xenobiotics mainly enter us either through the body surface (absorbed), mouth (eating) or through nose (breathing). These routes involve crossing membranes (example: skin, gut and lungs). A xenobiotic must have some affinity to both fats and water for effective uptake. This is because membranes contain fat, while water is found within them. Smaller, uncharged compounds cross membranes more easily. Warm temperatures also increase uptake as they make the fat more fluid-like.
Distribution: If a xenobiotic has greater affinity for water, it will be transported by blood or lymph. If it has greater affinity for fats, it will get transported by binding to a molecule or to membranes of blood cells. As the brain has a membrane barrier, fat-loving xenobiotics enter it more easily.
Storage: Xenobiotics are mostly stored in fat deposits (if fat-soluble) and bones (if minerals), though some are also stored by binding to proteins. While storing toxic xenobiotics is protective in the short term (as they are not free to move and act), they can be released later and cause toxicity. This usually happens when an animal breaks down fats for greater energy needs, i.e., during illness, starvation, egg-laying or migration.
Metabolism: Our bodies can alter a compound — this is called metabolism. We typically make xenobiotics more water soluble so that they can be excreted out of our body. This also often reduces the xenobiotic’s toxicity. However, in some cases, this can activate the xenobiotic and make it more toxic. The major enzymes responsible for metabolizing xenobiotics are oxidoreductases (catalyze oxidation-reduction reactions), hydrolases (catalyze hydrolysis reactions), and transferases (catalyze transfer of chemical groups).
Elimination: The final step is the excretion of the xenobiotic. While very water-soluble and fat-soluble xenobiotics can be directly excreted, most are excreted after they are made water-soluble (see metabolism). Excretion can happen through urine, feces, saliva, breast milk, etc. Many aquatic organisms remove xenobiotics through their gills or skin.
The above steps (uptake, distribution, storage, metabolism and elimination) describe how pollutants move through us. Let’s next see how they harm us.
Principles of Ecotoxicology (4th Edition) by C.H. Walker, R.M. Sibly, S.P. Hopkin and D.B. Peakall