cytochrome P450
Think of cytochrome P450 enzymes as the body's chemical demolition crew. When you swallow a drug, these enzymes — mostly concentrated in the liver — recognize the foreign molecule and begin chopping it up, usually by sticking an oxygen atom onto it. That small change makes the drug easier for your body to handle and eventually flush away.
Chemically, P450s are heme-containing enzymes that catalyze oxidation reactions. They use molecular oxygen and an electron-supplying cofactor to insert one oxygen atom into a substrate (hydroxylation, dealkylation, and related transformations) while the other oxygen atom becomes water. The name 'P450' comes from a spectroscopic quirk: when the heme iron binds carbon monoxide, the enzyme absorbs light strongly at 450 nanometers.
Humans have dozens of P450 isoforms, but only a handful — notably CYP3A4, CYP2D6, CYP2C9, and CYP1A2 — handle the majority of marketed drugs. For a medicinal chemist this family is double-edged: it clears unwanted compounds efficiently, but it also limits how long a useful drug survives in the body, and differences between people in P450 activity (from genetics or other drugs) are a major source of variable and sometimes dangerous drug responses.
Grapefruit juice inhibits intestinal CYP3A4, so drinking it alongside certain statins lets more of the drug survive into the bloodstream and can raise blood levels unexpectedly.
A famous food–drug interaction driven by P450 inhibition.
P450s sit mostly in the membranes of the smooth endoplasmic reticulum, which is why liver microsomes (vesicles formed from that membrane) are the standard tool for studying drug oxidation in the lab.