Biotransformation of Drugs: Definition, Types & Process

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  • 0:02 Definition of…
  • 1:16 What Is Cytochrome P450?
  • 2:33 What Do CYPs Do?
  • 3:15 Issues with Biotransformation
  • 5:53 The Future of…
  • 6:28 Lesson Summary
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Lesson Transcript
Instructor: Mary Lou Perin
This lesson covers the body's ability to biotransform, or modify the structure of substances in order to maintain an interior state of dynamic balance. We'll also explore the biotransformation process of drugs.

Definition of Biotransformation

The body is an amazing organism, constantly adjusting to its own interior processes as well as the surrounding environment. All day long, as we go about our normal lives, we are taking in substances that are transformed by the body for its use. We inhale air, from which we absorb oxygen. We take in water, food, chemicals (both good and bad), and drugs (both good and bad). These substances are called xenobiotics because they are not produced by the body but are found in the environment in which we live.

How do they get inside us? We breathe them in (e.g. hydrocarbons from the gasoline we are putting into our car). We ingest them (e.g. medication for diabetes). We absorb them through the skin (e.g. an estrogen patch). Once inside us, they are transported to the liver, where enzymes start the process of breaking them down, known as biotransformation, before putting them to use. In other words, the liver is our recycling plant. In the case of good substrates, they are incorporated into the body's normal functioning. If they are bad, the liver enzymes work to detoxify and rid the body of these substances.

What Is Cytochrome P450?

Cytochrome P450 (CYP450) is the main element in the liver's metabolic process. CYP450 is a large family of proteins named cyto- for 'cell' plus chrome because of the inclusion of a heme or iron factor in the enzyme. The 450 comes from its place on the spectrometer wave length. Spectrometry is a sophisticated lab technique that identifies the chemical signature of unknown compounds.

Occurring in all living things, including viruses, bacteria, animals, plants, and fungi, six of the more than 50 enzymes in this family of proteins metabolize about 90% of all medicines that we consume. These proteins are located in the mitochondria (the powerhouse of the cell) and walls of the human cell. Some enzymes are active in transforming multiple substrates, others in altering only one. These enzymes are essential to the metabolism of vitamin D, hormones, cholesterol, and fatty acids. They function in the metabolism of prostacyclins and thromboxane, which are key to the process of blood clotting. CYPs are also critical in the body's ability to detoxify foreign chemicals.

What Do CYPs Do?

CYPs are responsible for deactivating drugs in several ways:

  1. By oxidizing the substrate (combining with oxygen to change the character or appearance of a substance, e.g. iron combining with oxygen and changing to rust)
  2. By catalyzing the substrate, which means it speeds up a reaction containing the substrate, and 3) By assisting in excreting substrate waste

CYPs can also change an initial drug or pro-drug (which the body may not be able to absorb) into an active form that the body can use. Many nutrients, medications, and herbal therapies are metabolized through the CYP450 enzyme system.

Issues with Biotransformation


Our hereditary uniqueness plays a huge role in biotransformation because there is a genetic variability, called polymorphism, that influences our response to drugs. What this means is that as individuals, we respond differently to substances that we take in. A portion of individuals are poor metabolizers of drugs dependent on the CYP450 enzymes, resulting in disruption in their ability to metabolize common medicines, such as beta blockers, antidepressants, and opioids. In addition, some drugs are known to inhibit liver enzymes thus changing an individual's response to the drug. This variability in response can also be caused by genetic differences in drug transporters and drug receptors.

Drug Interactions

When taken, some medicines act on each other, changing the outcomes. Medicines that cause CYP450 drug interactions can either increase the rate of metabolism of another drug or inhibit the rate of metabolism. If a substantial increase in absorption occurs, then a dose might be lowered to avoid toxicity, but then the individual may not achieve any benefit from the lower dose. On the other hand, if the metabolism of a drug is inhibited, then it may accumulate in the blood and cause toxic effects. These types of interactions may cause unanticipated adverse reactions or therapeutic failures.

The extent to which an inhibitor affects the metabolism of a drug depends upon factors such as the dose and the ability of the inhibitor to bind to the enzyme. If the drug has a narrow safety range, even standard doses may have an adverse effect. Drug interactions can result in having to choose another drug, and if a particular drug is of vital importance to a patient's care, having to change agents can create problems.

Knowledge of the most important drugs metabolized by cytochrome P450 enzymes, as well as the most potent inhibiting and inducing drugs, can help minimize the possibility of adverse drug reactions and interactions. Potential drug interactions are listed by the Federal Drug Administration and on the manufacturer's drug label and/or website.

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