Cytochrome P450

Cytochrome P450 (CYP) is a superfamily of enzymes found in nearly all living things, from bacteria to plants and animals. Each one holds a heme group, an iron-containing ring like the one in hemoglobin, and most act as monooxygenases: they split a molecule of oxygen and attach one of its atoms to their target, often as a hydroxyl group. The odd name describes the pigment's color: when chemically reduced and bound to carbon monoxide, it absorbs light most strongly at 450 nanometers. Humans have 57 working CYP genes, grouped by sequence similarity into families and subfamilies, which gives names such as CYP3A4 and CYP2D6.

In the body, P450 enzymes help build steroid hormones, bile acids and other lipids, and they are the main tools for clearing foreign chemicals. In the liver, where they sit in the membranes of the endoplasmic reticulum, they make fat-soluble drugs more water-soluble so they can be excreted. Families 1 to 3 handle most medicines, with CYP3A4 and CYP2D6 the busiest.

This explains many drug interactions. One drug can inhibit an enzyme and raise the level of another, as grapefruit juice does with CYP3A4, while others, such as rifampin or St. John's wort, induce the enzymes and speed breakdown. Inherited variants make some people poor or ultrarapid metabolizers, a core topic of pharmacogenomics.