Drugs and their metabolites are also removed from the body via metabolism. The liver is the most important site as it contains the greatest number of metabolising enzymes. However, it is important to realize that the liver isn’t the only place metabolism takes place and that metabolism doesn’t always mean the inactivation of a drug. Some people have begun using the term biotransformation instead of metabolism to reflect the body’s ability to not only breakdown a drug, but to transform a pro-drug into an active drug.
In addition, there are several different types of enzymes responsible for metabolism. Hepatic microsomal enzymes are responsible for the bulk of the body’s breakdown of lipophilic drugs via oxidation and conjugation. A smaller percentage of drugs are metabolised by hepatic non-microsomal enzymes which include acetylation, sulphation, GSH, alcohol dehydrogenase and hydrolysis. Then there are the extrahepatic enzymes which through the processes of oxidation, conjugation and hydrolysis are responsible for the biotransformation prior to any drug reaching the systemic circulation.