The human body is a very delicate structure that is composed of different chemicals. Without our immune system we would not be able to survive due to the hazards we are exposed to from chemicals and microbes.
There is enormous number of synthetic and natural chemicals that range in function from beneficiary chemicals such as amino acids and different types of medication that we consume to hazardous and toxic chemicals. Chemicals have so diverse functions due to the different structure each molecule has, and due to the unique way each molecule reacts with the tissue of our body.
Our body is composed mainly of proteins and carbohydrates and lipids. The synthesis of these macromolecules is very organized and specific for each compound in our body. This specificity is due to the presence of special types of proteins that are called enzymes.
These enzymes catalyze the different synthesis of macromolecules and its degradation process.
Many medications that we consume target these enzymes that catalyze the synthesis of essential proteins in microbes and their cellular wall. Other medications target DNA synthesis catalyzed by other enzymes. These chemicals are called inhibitors of the enzymes responsible for catalyzing these reactions such as DNA polymerase.
One feature that differs enzymes from small chemicals used in medicine is that enzymes are very specific whereas chemicals or medications are not specific to the same extent as enzymes. Therefore there is accompanied with the administration of medications side effects due to there non specificity.
Due to the delicate structure of our body tissues enzymes can be targeted to good action such as the case with the administration of medications and to bad actions such as toxic chemicals.
Many chemicals can have dual effect, both beneficiary and harmful. An example is an alkylating agent such as nitrogen mustard which can be used to treat cancer as well as can be used in chemical warfare. Sulfur mustard another alkylating agent which has harmful effect of the DNA of the cells due to its anchimeric assistance that helps the binding of the DNA bases to it.
Another class of chemicals that have dual effect on enzymes is acetyl-choline esterese inhibitors. In this case also the inhibitor molecules can have dual function. It can function as a medication such as in alzheimer disease and in myasthenia gravis in which there is an increase in the concentration of acetylcholine in the tissues. The othe bad function of acetylcholine esterase inhibitors is as a nerve gas in chemical warfare in which acetylcholine accumulates in the tissue leading to death.
Atropine is a chemical which functions as an anticholinergic compound that is used as a treatment for acetylcholine toxicity.
In general the harmful effect of chemicals exceeds the beneficiary effect and this is due to the non specificity toward human tissues.