Cytochrome P450

Thu, 02 Apr 2026 00:00:00 +0000

The cytochrome P450 (CYP) enzymes are a superfamily of heme-containing monooxygenases found primarily in the endoplasmic reticulum of hepatocytes (liver cells) and enterocytes (intestinal cells). They catalyze the oxidation of a vast range of substrates — pharmaceutical drugs, dietary compounds, endogenous hormones, and environmental toxins — and are responsible for the metabolism of approximately 75% of all clinically used drugs.

The name reflects their biochemistry: “cytochrome” (colored cellular protein), “P450” (the protein absorbs light at 450 nm wavelength when bound to carbon monoxide). Humans have 57 CYP genes organized into 18 families and 44 subfamilies.

First-Pass Metabolism

Thu, 02 Apr 2026 00:00:00 +0000

First-pass metabolism (the first-pass effect) is the metabolic degradation of an orally administered drug as it passes through the intestinal wall and liver before reaching systemic circulation. After absorption from the gut, all blood drains into the portal vein and passes through the liver before entering the general circulation. The liver’s metabolic enzymes — particularly the cytochrome P450 family — can convert a large fraction of the drug into inactive metabolites during this first passage, dramatically reducing the drug’s bioavailability.

Pharmacokinetics on emsenn.net

Cytochrome P450

First-Pass Metabolism