Cytochrome P450 oxidase

Cytochrome P450 oxidase is a generic term for a large number of oxidative enzymes (EC ) important in vertebrate physiology. Most are located in the endoplasmic reticulum of liver cells where they metabolize thousands of endogenous and exogenous toxins, drugs, xenobiotics, and other unneeded and potentially harmful molecules. The name stands for "pigment 450", so named because they absorb light of 450 nm wavelength when bound to carbon monoxide. In most animals, including humans, hepatic cytochrome P450s are the most widely studied of the P450 enzymes.

Mammalian cytochrome P450 oxidases have about 500 amino acids and a heme group at the active site. Most can metabolize multiple substrates, and many can catalyze multiple reactions, which accounts for their central importance in metabolizing the potentially endless variety of foreign molecules. The Human Genome Project identified 63 human genes coding for the various cytochrome P450 enzymes.

A subset of cytochrome P450 enzymes play important roles in the synthesis of steroid hormones by the adrenals, gonads, and peripheral tissue:

• P450scc (also known as P450c11a1) in adrenal mitochondria effects ?the activity formerly known as 20,22-desmolase? (20α-hydroxylase, 22-hydroxylase, cholesterol side chain scission).
• P450c11β in inner mitochondrial membrane of adrenal cortex conducts 11β-hydroxylase, 18-hydroxylase, 18-methyloxidase activities.
• P450c11AS, only in mitochondria of the adrenal zona glomerulosa conducts 11β-hydroxylase, 18-hydroxylase, 18-methyloxidase activities.
• P450c17, in endoplasmic reticulum of adrenal cortex conducts 17α-hydroxylase and 17,20-lyase activities.
• P450c21 in adrenal cortex conducts 21-hydroxylase activity.
• P450aro in endoplasmic reticulum of gonads, adipose tissue, and elsewhere catalyzes aromatase activity.

Genes encoding for the P450 enzymes, and the enzymes themselves, are designated with the abbreviation CYP, followed by an Arabic numeral indicating the gene family, a capital letter indicating the subfamily, and an Arabic numeral for the individual gene. The convention is to italicise when referring to the gene. For example, CYP2E1 is the gene which encodes for the enzyme CYP2E1 which is one of the enzymes involved in paracetamol metabolism.