Fortunately, the human body has a well-developed capacity to biotransform most xenobiotics as well as body wastes. An example of a body waste that must be eliminated is hemoglobin, the oxygen-carrying iron-protein complex in red blood cells. Hemoglobin is released during the normal destruction of red blood cells. Under normal conditions hemoglobin is initially biotransformed to bilirubin, one of a number of hemoglobin metabolites. Bilirubin is toxic to the brain of newborns and, if present in high concentrations, may cause irreversible brain injury. Biotransformation of the lipophilic bilirubin molecule in the liver results in the production of water-soluble (hydrophilic) metabolites excreted into bile and eliminated via the feces.
The biotransformation process is not perfect. When biotransformation results in metabolites of lower toxicity, the process is known as detoxification. In many cases, however, the metabolites are more toxic than the parent substance. This is known as bioactivation. Occasionally, biotransformation can produce an unusually reactive metabolite that may interact with cellular macromolecules (e.g., DNA). This can lead to very serious health effects, for example, cancer or birth defects. An example is the biotransformation of vinyl chloride to vinyl chloride epoxide, which covalently binds to DNA and RNA, a step leading to cancer of the liver.