Definition
Table 1 Phase I and Phase II pathways in drug metabolism.
Metabolism (sometimes referred to as biotransformation) is the conversion of a substance from one form to another by the actions of enzymes or organisms. Its primary function is to turn the drug into a chemical form that can be removed from the body. Most drugs have to be lipid-soluble in order to be able to cross membranes, move around the body and reach their site of action. However, they cannot leave the body in this form because they would be easily reabsorbed from the urinary tract or gut and would remain in the body indefinitely. Metabolism reduces lipid solubility and prepares drug molecules for excretion.
Metabolism also changes the pharmacological nature of the drug, usually leading to a reduction or complete loss of activity, but sometimes increasing its activity or changing its nature.
Metabolism occurs predominantly in the liver and is normally divided into two distinct phases:
Table 1 lists the different types of Phase I and II reactions and describes the processes involved.
