ABSTRACT
The basis of life is the interaction between organism and chemicals, primarily nutrients and air (oxygen). These and other chemicals should be internalised, utilised and excluded. In order to accomplish all these processes, physical and chemical changes of the exposed chemicals are required. Because the interaction does not solely occur between the organism and the nonliving entity, but also between different live species for the purpose of providing nutrition to each other, every organism in this food chain has developed various protective mechanisms against each other. These mechanisms can be summarised as a surrounding of critical biological structures with special membranes, evolving specialised proteins to react with “foreign” entities and mechanisms to readily exclude them from the organism. All these phases have evolved over several million years from monocellular to multicellular organisms, and are now called “metabolism” as of the last century. Drugs are simply a group of intentionally ingested chemicals with special purposes, and they are not apart from the fate of all other chemicals entering the organism: they are absorbed, distributed, chemically changed (i.e. metabolised and excreted. In this chapter, the fundamental principles and mechanisms of these processes are summarised, and examples of the enzyme-catalysed, chemical modification of drugs (so-called biotransformation) are presented.
