Cytochrome P450

By Jennifer McDowall

The elimination of foreign compounds (xenobiotics) such as drugs and toxins from the body is an essential process designed to protect against potential toxicity from the foods we eat. The food broken down in the stomach is absorbed by the small intestine and then ferried directly to the liver via the portal vein (see figure below). This allows the liver time to detoxify compounds before they are distributed through the circulatory system. In the liver, there are two main types of metabolism that deal with xenobiotics, and a third that deals with their transport.

· Phase I metabolism results in small chemical changes that make a compound more hydrophilic, so it can be effectively eliminated by the kidneys. These reactions usually involve either adding or unmasking a hydroxyl group, or some other hydrophilic group such as an amine or sulphydryl group, and usually involve hydrolysis, oxidation or reduction mechanisms. Cytochrome P450 enzymes are responsible for most phase I reactions.

· Phase II metabolism takes place if phase I is insufficient to clear a compound from circulation, or if phase I generates a reactive metabolite. These reactions usually involve adding a large polar group (conjugation reaction), such as glucuronide, to further increase the compound’s solubility. Often, the functional groups generated in phase I reactions are required for attachment of the phase II polar groups (though in some cases phase II reactions can occur on their own). Transferase enzymes are responsible for most phase II reactions, e.g. uridine diphosphoglucuronosyl transferase (UGT), N-acetyl transferase (NAT), glutathione S-transferase (GST), and sulphotransferase (ST).

· Phase III involves drug transporters, which influence the effect, absorption, distribution and elimination of a drug. Drug transporters move drugs across cellular barriers, and as such can target sites of accumulation. They are located in epithelial and endothelial cells of the liver, gastrointestinal tract, kidney, blood-brain barrier and other organs.

Cytochrome P450 enzymes are the most important enzymes in Phase I metabolism in mammals, and are primarily responsible for the metabolism (degradation and elimination) of drugs.

Liver portal system: food is digested in the stomach and small intestine, then absorbed in the small intestine, where it is transported to the liver for detoxification before being distributed.

Cytochrome P450, Detox Enzymes

Cytochrome P450 (CYP) enzymes are a superfamily of mono-oxygenases that are found in all kingdoms of life, and which show extraordinary diversity in their reaction chemistry. In mammals, these enzymes are found primarily in the membranes of the endoplasmic reticulum (microsomes) within liver cells (hepatocytes), as well as many other cell types. These enzymes use haem iron to oxidise molecules, often making them more water-soluble for clearance. They achieve this by either adding or unmasking a polar group. In general, the reaction catalysed by these enzymes can be summarised as:

R-H + O₂ + 2e^- + 2H⁺ à R-OH + H₂O

Where R-H is the substrate and R-OH is the oxygenated substrate. The oxygen is bound to the haem in the core of the CYP enzyme. Protons (H⁺) are usually delivered from the cofactor NADH or NADPH through specific amino acids in the CYP enzyme, which relay the protons to the active site, where they are essential for a reductive splitting of the oxygen so a single atom can be added to the substrate. CYP enzymes can receive electrons from a range of different redox partner enzymes.

Mammalian enzymes

Mammalian CYP enzymes can oxidise both xenobiotics and endogenous compounds, and are important for detoxification of foreign substances, as well as for controlling the level of endogenous compounds, such as hormone synthesis and breakdown, cholesterol synthesis and vitamin D metabolism. CYP enzymes are also involved in vascular autoregulation, especially in the brain, and are vital to the formation of cholesterol, steroids and arachidonic acid metabolites. They can also clear the body of metabolic products such as bilirubin, which arises from the breakdown of haemoglobin. There is a high concentration of CYP proteins in the liver, but these enzymes are also found throughout the body, where they often have specialised roles.

Plant enzymes

Plant CYP enzymes are important for the biosynthesis of several compounds, such as hormones, defensive compounds, and fatty acid conjugates.

Bacterial enzymes

Bacterial CYP enzymes are important for several metabolic processes, such as the camphor-hydroxylating catalytic cycle in P. putida, and for the biosynthesis of the antibiotic erythromycin in S. erythraea.

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