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Theory of Conversion of Cholesterol to Cholestenone

The conversion of cholesterol into cholesterone is an oxidative process that takes place in bacteria, including bacteria in the human gut. This reaction has been used widely in laboratories for the detection of cholesterol and its reaction products.

  1. Cholesterol and Cholesterone

    • The chemical structures of cholesterol and cholesterone are virtually identical except that cholesterol is an alcohol (contains a hydroxyl, or -OH group) and cholesterone is a ketone (the same oxygen is doubly-bonded to a carbon and has lost its -H). Cholesterone is said to be an "oxidized" form of cholesterol.

    Cholesterol Oxidase

    • Cholesterol is converted into cholesterone by a bacterial protein called cholesterol oxidase. With the exception of its hydroxyl group, cholesterol is hydrophobic--that is, like oil, it does not mix readily with water. Because of this, cholesterol oxidase must extract cholesterol from the cell membrane where it is normally located.

    Cholesterone-Producing Bacteria

    • The bacteria that convert cholesterol into cholesterone live in the digestive tract of most mammals, and cholesterone is found in their waste products. These bacteria use cholesterol as a carbon source and the oxidation reaction as a source of energy. There is some evidence that suggests the cholesterone is also used by the bacteria as a sensor for fungi in the environment.

    Determination of Cholesterol Levels

    • The link between elevated cholesterol and coronary artery disease is now well-established, and, for that reason, cholesterol oxidase has become an important protein in clinical and biological laboratories. The oxidation reaction of cholesterol produces hydrogen peroxide in addition to cholesterone, and hydrogen peroxide is easily detected by standard chemical techniques. The amount of hydrogen peroxide produced in a blood sample incubated with cholesterol oxidase can be used to calculate the total cholesterol level in the blood.

    Detection of Cholesterol Oxidation Products

    • The oxidation of cholesterol in the human body and possibly the ingestion of oxidized cholesterol from animal foods have been implicated in the development of atherosclerosis. For this reason, the ability to detect and quantify cholesterol oxidation products (COPs) has attracted intense research interest. The oxidation machinery in mammalian cells produces oxidation products different from cholesterone. These diverse oxidation products still have the hydroxyl group that differentiates cholesterol from cholesterone and are not readily detected. Researchers and clinicians can use bacterial cholesterol oxidase, which can act on these cholesterol derivatives as well as cholesterol, to convert the hydroxyl to a ketone group and produce hydrogen peroxide. The same methods that are used for cholesterol detection and quantification can then be used to quantify the COPs in a sample.

    Other Applications

    • The cholesterol-to-cholesterone conversion reaction also has been used in studies of cholesterol depletion and as a mechanism for insect pest control. Since it is not metabolized in the human intestinal tract, it has also been proposed as an anti-obesity agent.

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