Chemistry
The Role of Cytochrome P450 in Drug and Toxin Metabolism
Quick fact
Your body has a team of enzymes called cytochrome P450 that can oxidize almost any lipophilic chemical—from caffeine to pollutants—making it more water-soluble for excretion. Genetic variations in these enzymes cause up to 10-fold differences in drug clearance between individuals.
Why this is interesting
You swallow a pill, and within hours it’s mostly gone from your body. But how does your body know exactly what to do with a molecule it has never seen before?
Read the full explanation
Understanding The Role of Cytochrome P450 in Drug and Toxin Metabolism
Think of your body as a factory that must eliminate foreign chemicals (xenobiotics), including drugs and environmental toxins. Many of these molecules are oily and lipophilic, which means they can easily cross cell membranes and accumulate in fat tissue. To get rid of them, your body needs to make them more water-soluble so they can be excreted in urine or bile. That’s where cytochrome P450 (CYP) enzymes come in. These enzymes, located mainly in the liver, act as molecular scissors that add an oxygen atom to the lipophilic molecule. This oxidation reaction introduces a polar ‘handle’ that makes the molecule less lipophilic and prepares it for further processing and excretion. Without this step, many drugs would stay in your body for a very long time.
A deeper explanation
Cytochrome P450 enzymes are a superfamily of heme-containing proteins. The name ‘P450’ comes from their unusual absorption peak at 450 nm when bound to carbon monoxide. The catalytic mechanism involves the reduction of molecular oxygen to water, with the insertion of one oxygen atom into the substrate. This reaction is driven by electrons from NADPH, transferred via a reductase. The active site contains an iron atom in a heme group that binds oxygen, and after a series of electron transfers, forms a highly reactive iron-oxo species that attacks the substrate. This broad reactivity allows CYP enzymes to oxidize a vast array of chemicals, which is why they are the primary enzymes in Phase I metabolism. Because the products are often still somewhat lipophilic, they are frequently substrates for Phase II conjugation reactions—such as glucuronidation—that further increase water solubility and promote excretion. CYP enzymes also play a role in activating some prodrugs (e.g., codeine to morphine) and in activating environmental toxins to reactive intermediates that can cause cellular damage. Variations in CYP genes (polymorphisms) and interactions with other drugs can alter enzyme activity, leading to differences in drug efficacy and toxicity.